Prof. Dr.-Ing. Frank Walther

Materials Test Engineering (WPT)
TU Dortmund University

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  • Analysis on the mechanical response of composite pressure vessels during internal pressure loading: FE modeling and experimental correlation
    Nebe, M. and Soriano, A. and Braun, C. and Middendorf, P. and Walther, F.
    Composites Part B: Engineering 212 (2021)
    Commercial development of gaseous hydrogen storage in fuel cell electric vehicles is inevitably subjected to reliable and cost-effective design of composite pressure vessels. In this context, certainty in the design process is sought, which is determined by how well the vessel's mechanical response is understood, but more importantly to which accuracy the final collapse can be predicted. As such, a symbiosis of numerical and experimental work appears as a leading path towards robust design methodologies, where both analyses complement and scrutinize each others validity. This research presents the analysis on the mechanical response of composite pressure vessels during internal pressure loading through the correlation of numerical and experimental results on various degrees of complexity. Based on an extensive experimental dataset, a three-dimensional FE model is implemented on a realistic vessel geometry, evaluating its constitutively elastic behavior, and its response under failure and damage progression. Likewise, an established experimental framework is used to derive data by means of contour scans, outer surface strains, airborne acoustic emissions and final burst pressure. The precise recreation of the vessel geometry, together with the detailed analysis approach, permits to show a reasonable agreement between the predicted and the measured structural responses, the sequence of damage onset, and the final collapse occurring in the cylindrical region (<1%). Discrepancies still exist because of the remaining uncertainty concerning the individual layer geometry and the characterization of damage in the helical plies. Altogether, through the alignment of experimental and numerical analyses, this work provides the base for further optimization frameworks, in which an adequate representation of the vessel's meridional thickness profile and material properties stands out as necessary feat to accurately reproduce the mechanical response and final strength in a time- and cost-effective design process. © 2020 Elsevier Ltd
    view abstract10.1016/j.compositesb.2020.108550
  • Application-oriented assessment of the interlayer tensile strength of additively manufactured polymers
    Striemann, P. and Huelsbusch, D. and Niedermeier, M. and Walther, F.
    Additive Manufacturing 46 (2021)
    Extrusion-based additive manufacturing is often characterized with process-property-structure relationships, which lead to superimposed process-related effects. This study aims to separate superimposed effects, which occur due to the change of the process parameter layer height. The mechanical properties, in particular the interlayer tensile strength, are used to characterize the material capability in manufacturing direction z according to ASTM F2971. Different surface textures in the form of idealized, polished specimens and application-oriented, as built specimens complete the experimental design. The investigations highlight a decreasing primary surface profile and a higher material capability with decreasing layer height. The special design of experimental setup enables a retrospective data analysis that separates process-induced effects. Hence, the exact assignment of the proportions of process interactions is disclosed. The study results in a novel approach for characterizing extrusion-based additively manufactured polymer. The basic principle is to replace the common component testing by characterizing idealized material properties and calculating back to application-oriented conditions. The possibility of application-oriented correction factors based on idealized characterized material properties enables the change from component testing to material testing. © 2021 Elsevier B.V.
    view abstract10.1016/j.addma.2021.102095
  • Assessment of galvanostatic anodic polarization to accelerate the corrosion of the bioresorbable magnesium alloy we43
    Wegner, N. and Walther, F.
    Applied Sciences (Switzerland) 11 (2021)
    In the field of surgery, bioresorbable magnesium is considered a promising candidate. Its low corrosion resistance, which is disadvantageous for technical application, is advantageous for surgery since the implant fully degrades in the presence of the water-based body fluids, and after a defined time the regenerating bone takes over its function again. Therefore, knowledge of the corrosion behavior over several months is essential. For this reason, an in vitro short-time testing method is developed to accelerate the corrosion progress by galvanostatic anodic polarization without influencing the macroscopic corrosion morphology. The initial corrosion rate of the magnesium alloy WE43 is calculated by detection of the hydrogen volume produced in an immersion test. In a corresponding experimental setup, a galvanostatic anodic polarization is applied with a three-electrode system. The application range for the polarization is determined based on the corrosion current density from potentiodynamic polarization. To correlate the initial corrosion rate, and accelerated dissolution rate, the corrosion morphologies of both test strategies are characterized by microscopy images, as well as energy dispersive X-ray spectroscopy and Fourier-transform infrared spectroscopy. The results demonstrate that the dissolution rate can be increased in the order of decades with the limitation of a changed corrosion morphology with increasing polarization. With this approach, it is possible to characterize and exclude new unsuitable magnesium alloys in a time-efficient manner before they are used in subsequent preclinical studies. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/app11052128
  • Biomineralization, dissolution and cellular studies of silicate bioceramics prepared from eggshell and rice husk
    Choudhary, R. and Venkatraman, S.K. and Bulygina, I. and Senatov, F. and Kaloshkin, S. and Anisimova, N. and Kiselevskiy, M. and Knyazeva, M. and Kukui, D. and Walther, F. and Swamiappan, S.
    Materials Science and Engineering C 118 (2021)
    The current investigation aims to replace the synthetic starting materials with biowaste to synthesize and explore three different silicate bioceramics. Pure silica from rice husk was extracted by decomposition of rice husk in muffle furnace followed by alkali treatment and acid precipitation. Raw eggshell and extracted silica were utilized for the preparation of wollastonite, diopside and forsterite by the solid-state method. The TG-DSC analysis shows that the crystallization temperature of wollastonite, diopside and forsterite was found to be 883 °C, 870 °C and 980 °C, respectively. The phase purity of wollastonite was attained at 1100 °C whereas diopside and forsterite were composed of secondary phases even after calcination at 1250 °C and 1300 °C respectively. All three materials behaved differently when exposed to the physiological environment, as wollastonite exhibited remarkable apatite deposition within 3 days whereas a distinct apatite phase was noticed on the surface of diopside after 2 weeks and forsterite shows the formation of apatite phase after five weeks of immersion. The rapid dissolution of Mg2+ ion from forsterite lowered the leaching of silicate ions into the simulated body fluid leading to poor apatite deposition over its surface. Chemical composition was found to plays a key role in the biomineralization ability of these bioceramics. Hemolysis and Lactate Dehydrogenase (LDH) release assays were performed to evaluate the hemocompatibility of silicate ceramics cultured at different concentrations (62.5, 125, and 250 μg/mL) with red blood cells and mononuclear leucocytes (MLs) of mice. The hemolytic activity of all the tested bioceramics was insignificant (less than 1%). The interaction between diopside and mouse multipotent mesenchymal stromal cells (MMSCs) caused a negligible increase in the number of apoptosis-associated Annexin V-binding cells whereas forsterite and wollastonite induced an increase in the number of the apoptotic cells only at the concentration of 250 μg/mL. The LDH assay did not show statistically significant changes in the proliferation of MMSCs after treatment with the bioceramics at the tested concentrations when compared to control (p &gt; 0.05). This finding showed that the death of a part of cells during the first 24 h of incubation did not prevent the proliferation of MMSCs incubated with diopside, forsterite and wollastonite for 72 h. © 2020 Elsevier B.V.
    view abstract10.1016/j.msec.2020.111456
  • Comparison of the processability and influence on the microstructure of different starting powder blends for laser powder bed fusion of a fe3.5 si1.5 c alloy
    Strauch, A.L. and Uhlenwinkel, V. and Steinbacher, M. and Großwendt, F. and Röttger, A. and Chehreh, A.B. and Walther, F. and Fechte-Heinen, R.
    Metals 11 (2021)
    This paper examines different blends of starting materials for alloy development in the laser powder bed fusion (LPBF) process. By using blends of individual elemental, ferroalloy and carbide powders instead of a pre-alloyed gas-atomized starting powder, elaborate gas-atomization processes for the production of individual starting powders with varying alloy compositions can be omitted. In this work the model alloy Fe3.5 Si1.5 C is produced by LPBF from different blends of pure elemental, binary and ternary powders. Three powder blends were processed. The base material for all powder blends is a commercial gas-atomized Fe powder. In the first blend this Fe powder is admixed with SiC, in the second with the ternary raw alloy FeSiC and in the third with FeSi and FeC. After characterizing the powder properties and performing LPBF parameter studies for each powder blend, the microstructures and the mechanical properties of the LPBF-manufactured samples were analyzed. Therefore, investigations were carried out by scanning electron microscopy, wave length dispersive x-ray spectroscopy and micro hardness testing. It was shown that the admixed SiC dissolves completely during LPBF. But the obtained microstructure consisting of bainite, martensite, ferrite and retained austenite is inhomogeneous. The use of the lower melting ferroalloys FeSi and FeC as well as the ternary ferroalloy FeSiC leads to an increased chemical homogeneity after LPBF-processing. However, the particle size of the used components plays a decisive role for the dissolution behavior in LPBF. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met11071107
  • Constant temperature approach for the assessment of injection molding parameter influence on the fatigue behavior of short glass fiber reinforced polyamide 6
    Mrzljak, S. and Delp, A. and Schlink, A. and Zarges, J.-C. and Hülsbusch, D. and Heim, H.-P. and Walther, F.
    Polymers 13 (2021)
    Short glass fiber reinforced plastics (SGFRP) offer superior mechanical properties compared to polymers, while still also enabling almost unlimited geometric variations of components at large-scale production. PA6-GF30 represents one of the most used SGFRP for series components, but the impact of injection molding process parameters on the fatigue properties is still insufficiently investigated. In this study, various injection molding parameter configurations were investigated on PA6-GF30. To take the significant frequency dependency into account, tension–tension fatigue tests were performed using multiple amplitude tests, considering surface temperature-adjusted frequency to limit self-heating. The frequency adjustment leads to shorter testing durations as well as up to 20% higher lifetime under fatigue loading. A higher melt temperature and volume flow rate during injection molding lead to an increase of 16% regarding fatigue life. In situ X-ray microtomography analysis revealed that this result was attributed to a stronger fiber alignment with larger fiber lengths in the flow direction. Using digital volume correlation, differences of up to 100% in local strain values at the same stress level for different injection molding process parameters were identified. The results prove that the injection molding parameters have a high influence on the fatigue properties and thus offer a large optimization potential, e.g., with regard to the component design. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/polym13101569
  • Correlation of load drop and crack initiation criteria in fatigue life experiments of metallic materials
    Heckmann, K. and Sievers, J. and Schopf, T. and Lücker, L. and Schmiedt-Kalenborn, A. and Walther, F. and Starke, P. and Acosta, R. and Boller, C.
    Engineering Fracture Mechanics 251 (2021)
    The simulation and assessment of fatigue damage of metallic materials are strongly dependent on the definition of critical damage initiation, i.e. the formation of a crack after a certain number of cycles under fatigue loading. To get this described in an appropriate way classical fatigue analysis needs to be combined with fracture mechanics to obtain a realistic size of an initiating macro crack in a component in the end. Ultimately, this size depends on the procedure of how fatigue life curves are generated from the fatigue tests performed. In this paper strain-controlled fatigue tests and fractographic results have been combined with a computational assessment to better understand the relationship between a traditional load drop criterion characterizing the size of a crack being initiated. It is stated that the location of an initiating crack versus the location of an extensometer applied to the specimen can have an influence on the crack size being initiated and thus the fatigue life for a load drop criterion being fixed. A relationship is proposed to link a crack size dependent on the fatigue test evaluation scheme with the fatigue life, which can be used in engineering assessments. The relevance of practical application cases is discussed. © 2021 Elsevier Ltd
    view abstract10.1016/j.engfracmech.2021.107785
  • Electron beam powder bed fusion of γ‐titanium aluminide: Effect of processing parameters on part density, surface characteristics and aluminum content
    Moritz, J. and Teschke, M. and Marquardt, A. and Stepien, L. and López, E. and Brückner, F. and Macias Barrientos, M. and Walther, F. and Leyens, C.
    Metals 11 (2021)
    Gamma titanium aluminides are very interesting for their use in high‐performance applications such as aircraft engines due to their low density, high stiffness and favorable hightemperature properties. However, the pronounced brittleness of these intermetallic alloys is a major challenge for their processing through conventional fabrication methods. Additive manufacturing by means of electron beam powder bed fusion (EB‐PBF) significantly improves the processability of titanium aluminides due to the high preheating temperatures and facilitates complex components. The objective of this study was to determine a suitable processing window for EB‐PBF of the TNM‐B1 alloy (Ti‐43.5Al‐4Nb‐1Mo‐0.1B), using an increased aluminum content in the powder raw material to compensate for evaporation losses during the process. Design of experiments was used to evaluate the effect of beam current, scan speed, focus offset, line offset and layer thickness on porosity. Top surface roughness was assessed through laser scanning confocal microscopy. Scanning electron microscopy, electron backscatter diffraction (EBSD) and energydispersive X‐ray spectroscopy (EDX) were used for microstructural investigation and to analyze aluminum loss depending on the volumetric energy density used in EB‐PBF. An optimized process parameter set for achieving part densities of 99.9% and smooth top surfaces was derived. The results regarding microstructures and aluminum evaporation suggest a solidification via the β‐phase. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met11071093
  • Experimental and numerical investigation on the impact response of CFRP under 3-point-bending
    Nebe, M. and Schmack, T. and Schaefer, T. and Walther, F.
    Composites Part C: Open Access 4 (2021)
    The strain rate-dependent material characteristic of carbon fiber reinforced plastics (CFRP) is widely known and has been investigated in detail at coupon level. In this study, for the first time the strain rate dependent characteristic of a three-dimensional CFRP structure was investigated. The evolution of the determined strain rate dependency was correlated with the results at coupon level. For this purpose two special 3-point-bending fixtures were developed to obtain the flexural impact response of the investigated T700S DT120 prepreg system at coupon and component (hat profile) level. The rectangular coupon specimens were loaded with quasi-static to intermediate impact velocities from 3.3×10−5 to 10m s−1, while the structural sub components were tested using impact velocities from 3.3×10−5 to 1m s−1. With increasing impact velocities, the experimental tests showed a significant increase in force at first failure and maximum deflection at coupon level. The increases in force were of 52% and 120%, respectively. However, the increase for structural hat profile components was just 12.4% due to a different failure mode. The observed initial failure modes were compressive failure provoked by fiber kinking for the coupon and interlaminar shear failure for the structural component. Regardless of the different failure modes this work proves the necessity of considering the strain rate dependency of a composite material to accurately predict the maximum load capacity of a CFRP structure during a dynamic load event. Additionally, the comparison of the experimental results restults to numerical results revealed weaknesses in the prediction accuracy of the currently used models. © 2020 The Authors
    view abstract10.1016/j.jcomc.2020.100079
  • Failure mode map for E-PBF manufactured Ti6Al4V sandwich panels
    Kotzem, D. and Tazerout, D. and Arold, T. and Niendorf, T. and Walther, F.
    Engineering Failure Analysis 121 (2021)
    In the present work the mechanical properties and failure modes of Ti6Al4V sandwich panels made by electron beam powder bed fusion (E-PBF) were investigated. Analytical models were employed to predict the most probable failure mode of the specimen by adjusting the dimensions of the core and the face sheet. Thereby, different failure modes such as core failure, face wrinkling and face yielding were taken into account in the analytical model. Subsequently, a failure mode map was constructed. Sandwich panels for experimental validation were manufactured by E-PBF. The deformation response of the sandwich panels under three-point bending load was characterized until failure. A good agreement between analytical predictions and experimental data was found. © 2020 Elsevier Ltd
    view abstract10.1016/j.engfailanal.2020.105159
  • Impact of single structural voids on fatigue properties of AISI 316L manufactured by laser powder bed fusion
    Kotzem, D. and Kleszczynski, S. and Stern, F. and Elspaß, A. and Tenkamp, J. and Witt, G. and Walther, F.
    International Journal of Fatigue 148 (2021)
    The laser powder bed fusion of metals (PBF-LB/M) process is already exploited in several industrial applications. The process itself allows to introduce artificial defects which can later be characterized by their influence on the resulting mechanical properties. In this study, the influence of isolated single structural defects (0.3 mm ≤ √area ≤ 1.5 mm) on the fatigue properties is discussed and the √area-parameter model is applied. The obtained results show that the investigated material is highly defect tolerant as artificial defects with √area = 0.3 mm are not crack initiating. Specimens with a defect of √area = 1.0–1.5 mm clearly show crack initiation and propagation starting from the defect, and a fatigue strength estimation tends to be more conservative. © 2021 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2021.106207
  • In situ characterization of polycaprolactone fiber response to quasi-static tensile loading in scanning electron microscopy
    Delp, A. and Becker, A. and Hülsbusch, D. and Scholz, R. and Müller, M. and Glasmacher, B. and Walther, F.
    Polymers 13 (2021)
    Microstructural responses to the mechanical load of polymers used in tissue engineering is notably important for qualification at in vivo testing, although insufficiently studied, especially regarding promising polycaprolactone (PCL). For further investigations, electrospun PCL scaffolds with different degrees of fiber alignment were produced, using two discrete relative drum collector velocities. Development and preparation of an adjusted sample geometry enabled in situ tensile testing in scanning electron microscopy. By analyzing the microstructure and the use of selected tracking techniques, it was possible to visualize and quantify fiber/fiber area displacements as well as local fractures of single PCL fibers, considering quasi-static tensile load and fiber alignment. The possibility of displacement determination using in situ scanning electron microscopy techniques for testing fibrous PCL scaffolds was introduced and quantified. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/polym13132090
  • In Situ Characterization of the Damage Initiation and Evolution in Sustainable Cellulose-Based Cottonid
    Scholz, R. and Delp, A. and Walther, F.
    Minerals, Metals and Materials Series 5 (2021)
    The usage of environmentally friendly materials based on sustainable resources is nowadays more important than ever, especially in technical applications. Cottonid is based 100% on cellulose, therefore sutainable and due to its excellent properties a promising alternative material in terms of eco-friendliness. Within this study, the deformation and damage behavior of two Cottonid variants, an industrial standard as well as the structurally optimized variant M60Z50, is characterized for the first time using innovative in situ testing techniques. Quasi-static tensile tests were comparatively performed in a scanning electron microscope as well as a microfocus computer tomograph, and the development of defects present in the initial condition of the materials were investigated on surface and in volume. In general, in the elastic region, no visible damage initiation on the surface and a decrease of overall void volume within the gauge length could be detected for Cottonid. When reaching the yield strength, cracks initiate on the surface at critical areas, like pores and microcracks, which propagate and assemble until total loss of structural integrity. Further, in the plastic region, an increase in void volume could be shown in the gauge length until final failure. Compared to an industrial standard, M60Z50 exhibits a clearly lower percentage in overall void volume and shows increased mechanical properties, like yield strength and ultimate tensile strength. The structural optimization of M60Z50 seems to result in a more sufficient bonding of the paper layers during the manufacturing process, which improves the deformation and damage behavior under quasi-static loading. © 2021, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-65261-6_77
  • Influence of process parameters on the residual stress state and properties in disc springs made by incremental sheet forming (ISF) [Einfluss der Prozessparameter auf den Eigenspannungszustand und die Eigenschaften von Tellerfedern, hergestellt durch inkrementelle Blechumformung (IBU)]
    Afzal, M.J. and Hajavifard, R. and Buhl, J. and Walther, F. and Bambach, M.
    Forschung im Ingenieurwesen/Engineering Research (2021)
    Disc springs are machine elements that are used when high forces need to be supplied and in limited installation space. They need to fulfil high demands on the stability of the spring characteristics, reliability and lifetime. In corrosive environments, metastable austenitic stainless steels (MASS) disc springs are often used. Tensile stresses that occur during service limit the lifetime of disc springs. Usually, their durability is enhanced by generating favorable compressive residual stresses using shot peening operations. Such operations lead to extra efforts and additional production costs. In this study, the adaptive and targeted generation of residual stresses via incremental sheet forming (ISF) is investigated as alternative to shot peening focusing on EN 1.4310 and EN 1.4401 stainless steel. Previous work has shown that ISF is capable of controlling the radial and tangential stresses in the springs. However, no analysis of the influence of the residual stress state in the rolled sheet strips and the ISF process parameters was performed. The goal of the current work is to analyze the evolution of residual stress during rolling and subsequent incremental forming of disc springs. In order to examine the role of dissipation and temperature increases in the rolling process, sheet blanks rolled at room and elevated temperature are analyzed. The characteristics of the compressive residual stresses induced by ISF are studied for different process parameters. X‑ray diffraction is used to investigate the buildup of these stresses. Using ISF, the generation of compressive residual stresses can be integrated into the forming process of disc springs, and further post-treatment may be skipped. The results show that the residual stress state in the rolled material is crucial, which requires tight control of the rolling temperature. Another result is that ISF is able to yield high compressive residual stresses and improved spring characteristics when small tool diameters and step-down values are used. © 2021, The Author(s).
    view abstract10.1007/s10010-021-00491-w
  • Innovations for the optimal use of weathering steel in steel and composite bridge construction [Innovationen zum optimalen Einsatz des wetterfesten Baustahls im Stahl- und Verbundbrückenbau]
    Ungermann, D. and Hatke, P. and Knyazeva, M. and Walther, F. and Lebelt, P. and Gehrke, J.
    Stahlbau 90 (2021)
    Innovations for the optimal use of weathering steel in steel and composite bridge construction. The research project “Innovations for the optimal use of weathering steel in steel and composite bridge construction” investigated essential aspects for the optimal use of weathering steel in steel and composite bridge construction. In addition to current investigations on the formation of the corrosion-inhibiting surface layer in the current atmosphere, the qualification of two geometry-independent and non-destructive measuring techniques for crack detection underneath the compact surface layer and their verification on real bridge structures was carried out. Furthermore, tests for the optimization of the slip factor of slip-resistant prestressed connections of weathering steel were carried out. Basically, the use of weathering steel offers both economic and ecological advantages compared to organically coated structural steel by the formation of its firmly adhering and corrosion-inhibiting surface layer, especially with regard to the present service life in bridge construction. This article summarises the main research findings to demonstrate the sustainable use and advantages of weathering steel in steel and composite bridge construction under the conditions of use in the current natural atmosphere. © 2021, Ernst und Sohn. All rights reserved.
    view abstract10.1002/stab.202000090
  • Interlayer bonding capability of additively manufactured polymer structures under high strain rate tensile and shear loading
    Striemann, P. and Gerdes, L. and Huelsbusch, D. and Niedermeier, M. and Walther, F.
    Polymers 13 (2021)
    Additive manufacturing of polymers via material extrusion and its future applications are gaining interest. Supporting the evolution from prototype to serial applications, additional testing conditions are needed. The additively manufactured and anisotropic polymers often show a weak point in the interlayer contact area in the manufacturing direction. Different process parameters, such as layer height, play a key role for generating the interlayer contact area. Since the manufacturing productivity depends on the layer height as well, a special focus is placed on this process parameter. A small layer height has the objective of achieving better material performance, whereas a larger layer height is characterized by better economy. Therefore, the capability‐ and economy‐oriented variation was investigated for strain rates between 2.5 and 250 s−1 under tensile and shear load conditions. The test series with dynamic loadings were designed monitoring future applications. The interlayer tensile tests were performed with a special specimen geometry, which enables a correction of the force measurement. By using a small specimen geometry with a force measurement directly on the specimen, the influence of travelling stress waves, which occur due to the impact at high strain rates, is reduced. The interlayer tensile tests indicate a strain rate dependency of additively manufactured polymers. The capability‐oriented variation achieves a higher ultimate tensile and shear strength compared to the economy‐oriented variation. The external and internal quality assessment indicates an increasing primary surface profile and void volume content for increasing the layer height. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/polym13081301
  • Mechanism-oriented characterization of the anisotropy of extruded profiles based on solid-state recycled EN AW-6060 aluminum chips
    Koch, A. and Henkel, T. and Walther, F.
    Engineering Failure Analysis 121 (2021)
    Because of the great potential to reduce the amount of energy, the direct recycling of scrap like aluminum chips by hot extrusion is a hopeful alternative to the usual remelting process. Previous investigations showed that the chips, which are encased by oxide layers, are elongated due to the extrusion process. Therefore, the aim of this study is to test to what extend anisotropic properties, in analogy to fiber-reinforced materials, can be determined. The mechanical properties of cast-based and chip-based specimens with orientations of 0°, 30° and 90° to extrusion direction were characterized by means of mechanical quasistatic and cyclic experiments. It could be shown that quasistatic properties of the 0° orientation are highest for chip-based specimens, whereby the differences to the other orientations are slight. On the other hand, large differences in cyclic creep behavior between the orientations as well as in damage behavior could be determined. © 2020 Elsevier Ltd
    view abstract10.1016/j.engfailanal.2020.105099
  • Microstructure and Fatigue Damage Evolution in Additive-Manufactured Metals Using Enhanced Measurement Techniques and Modeling Approaches
    Awd, M. and Walther, F. and Siddique, S. and Fatemi, A.
    Minerals, Metals and Materials Series 5 (2021)
    Process-induced microstructures have a high impact on the fatigue strength of engineering materials. Advanced materials testing builds the base for the design and manufacturing of reliable, high-performance products for various technical applications. Combining modern analytical and intermittent testing strategies with applied enhanced measurement techniques, i.e., physical instrumentation of testing specimens during loading, allows the characterization of process-structure-property relationships in various fatigue damage stages. Further, in situ mechanical testing in analytical devices like micro-computed tomography (µ-CT) enables the immediate correlation of material’s physical reactions with the applied loading conditions. The focus of the presented studies. Using the proposed technique, the characterization of fatigue damage evolution and progression before failure depending on environmental as well as material specific microstructural characteristics is carried out. Investigations on additively manufactured Al alloys revealed the interaction between porosity and microstructure under very high-cycle fatigue (VHCF) loading conditions. Measurement-based fatigue damage tracking during testing of SLM aluminum alloys revealed the interaction between porosity and microstructure under loading in the very high-cycle fatigue (VHCF) regime. The grain boundary strengthening of the microstructure increased VHCF strength by 33%. © 2021, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-65261-6_68
  • Microstructure and strain rate-dependent deformation behavior of PBF-EB Ti6Al4V lattice structures
    Kotzem, D. and Gerdes, L. and Walther, F.
    Materialpruefung/Materials Testing 63 (2021)
    Additive manufacturing techniques enable the fabrication of new lightweight components with tailored mechanical properties. Considering current application fields, components are often over-dimensioned since a lack of data regarding the mechanical properties under compression or tensile loading at high strain rates is present. In this work, the influence of various strain rates on the mechanical properties of electron beam powder bed fusion Ti6Al4V lattice structures was investigated. In order to capture the damage mechanisms that occurred, a single unit cell plane was considered. In terms of mechanical characterization, high-speed tensile tests at nominal strain rates from 0.025 to 250 s-1 were carried out. By the additional use of a high-speed camera system and subsequent digital image correlation, an investigation of material reactions during shortest test times was enabled. Based on the results, a positive strain rate dependency was identified for yield and ultimate tensile strength for both investigated lattice types. In detail, an increase in ultimate tensile strength of 16 % for BCC- and 20 % for F2CCZ-specimens could be detected. © 2021 Walter de Gruyter GmbH, Berlin/Boston, Germany 2021.
    view abstract10.1515/mt-2020-0087
  • Microstructure formation and mechanical properties of ODS steels built by laser additive manufacturing of nanoparticle coated iron-chromium powders
    Doñate-Buendia, C. and Kürnsteiner, P. and Stern, F. and Wilms, M.B. and Streubel, R. and Kusoglu, I.M. and Tenkamp, J. and Bruder, E. and Pirch, N. and Barcikowski, S. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and Gault, B. and Gökce, B.
    Acta Materialia 206 (2021)
    Oxide dispersion strengthened (ODS) steels are known for their enhanced mechanical performance at high temperatures or under radiation exposure. Their microstructure depends on the manufacturing process, from the nanoparticle addition to the base steel powder, to the processing of the nanoparticle enriched powder. The optimization and control of the processing steps still represent a challenge to establish a clear methodology for the additive manufacturing of ODS steels. Here, we evaluate the microstructure, nanoparticle evolution, and mechanical properties of ODS steels prepared by dielectrophoretic controlled adsorption of 0.08 wt% laser-synthesized yttrium oxide (Y2O3) on an iron-chromium ferritic steel powder (PM2000). The influence of the ODS steel fabrication technique is studied for two standard additive manufacturing techniques, directed energy deposition (DED) and laser powder bed fusion (LPBF). The compressive strength of the ODS steels at 600 °C is increased by 21% and 29% for the DED and LPBF samples, respectively, compared to the DED and LPBF steels manufactured without Y2O3 nanoparticle addition. The Martens hardness is enhanced by 9% for the LPBF ODS steel while no significant change is observed in the DED ODS steel. The microstructure and nanoparticle composition and distribution are evaluated by electron backscatter diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and atom probe tomography, to compare the microstructural features of DED and LPBF manufactured parts. Smaller grain size and more homogeneous distribution with lower agglomeration of Y-O nanoparticles in the LPBF sample are found to be key factors for enhanced mechanical response at 600 °C. The enhanced mechanical properties of the LPBF-processed sample and the more homogeneous nanoparticle dispersion can be linked to results obtained by finite element methods simulations of the melt pool that show two orders of magnitude faster cooling rates for LPBF than for DED. Therefore, this work presents and validates a complete laser-based methodology for the preparation and processing of an ODS steel, proving the modification of the microstructure and enhancement of the high-temperature strength of the as-built parts. © 2020
    view abstract10.1016/j.actamat.2020.116566
  • Microstructure Influence on Corrosion Resistance of Brazed AISI 304L/NiCrSiB Joints
    Penyaz, M. and Otto, J.L. and Popov, N. and Ivannikov, A. and Schmiedt-Kalenborn, A. and Walther, F. and Kalin, B.
    Metals and Materials International (2021)
    The characterization of corrosion resistance, which is essential to estimate the lifetime of brazed joints in corrosive environments, is of central importance for many industrial applications and a basic requirement for the reliable and economic operation of brazed components. High temperature vacuum brazing with thin amorphous-crystalline foils is used for numerous applications such as exhaust gas heat exchangers. In this study one industrial BNi-5a® and two experimental rapidly solidified filler metal foils of Ni7Cr7.5Si4Fe1.5B and Ni20Cr7.5Si4Fe4Mo1.5B wt% were used to braze joints of AISI 304L. In addition, two holding times at 1160 °C were chosen to investigate the effect of the resulting microstructural differences on corrosion resistance. Especially the amount and distribution of borides and silicides within the brazing seam could be changed by the time-dependent diffusion processes, as could be shown by metallographic cross sections. Accelerated intercrystalline corrosion tests were carried out to evaluate the influence of the microstructure on the corrosion depth and damage mechanisms. Additionally, potentiodynamic polarization measurements in synthetic exhaust gas condensate as an application-oriented corrosion medium were performed for a comparative evaluation of corrosion properties and rate. The combination of high chromium-containing filler metal and increased holding time, which led to a more homogeneous microstructure, resulted in a more than five times improved corrosion resistance within both investigations. Graphic Abstract: [Figure not available: see fulltext.]. © 2021, The Korean Institute of Metals and Materials.
    view abstract10.1007/s12540-021-00974-z
  • Microstructure, residual stresses, and strain‐rate‐dependent deformation and fracture behavior of aisi 304l joints brazed with nicrsib filler metals
    Otto, J.L. and Penyaz, M. and Möhring, K. and Gerdes, L. and Schaum, T. and Ivannikov, A. and Schmiedt‐kalenborn, A. and Kalin, B. and Walther, F.
    Metals 11 (2021)
    The knowledge of alloy–process–structure–property relationships is of particular interest for several safety‐critical brazed components and requires a detailed characterization. Thus, three different nickel‐based brazing filler metals were produced with varying chromium and molyb-denum content and were used to braze butt joints of the austenitic stainless steel AISI 304L under vacuum. Two holding times were used to evaluate diffusion‐related differences, resulting in six specimen variations. Significant microstructural changes due to the formation and location of bo-rides and silicides were demonstrated. Using X‐ray diffraction, alloy‐dependent residual stress gra-dients from the brazing seam to the base material were determined and the thermal‐induced residual stresses were shown through simulations. For mechanical characterization, impact tests were carried out to determine the impact toughness, as well as tensile tests at low and high strain rates to evaluate the strain‐rate‐dependent tensile strength of the brazed joints. Further thermal, electri-cal, and magnetic measurements enabled an understanding of the deformation mechanisms. The negative influence of brittle phases in the seam center could be quantified and showed the most significant effects under impact loading. Fractographic investigations subsequently enabled an enhanced understanding of the fracture mechanisms. © 2021 by the authors. Submitted for possible open access.
    view abstract10.3390/met11040593
  • Shear Characterization of Additively Manufactured Short Carbon Fiber-Reinforced Polymer
    Striemann, P. and Bulach, S. and Hülsbusch, D. and Niedermeier, M. and Walther, F.
    Macromolecular Symposia 395 (2021)
    The additive manufacturing (AM) techniques Fused Deposition Modeling (FDM™) or Fused Filament Fabrication (FFF) are all based on material extrusion and are one of the most widely used AM methods. Serial production with this technology requires a deep understanding for the process and the effects of the process parameters on the mechanical properties. In literature tensile, compression, or flexural properties are often considered. However, the basic structure of a FFF part is strongly anisotropic and therefore similar to a composite material. The characterization of a carbon fiber-reinforced polymer (CFRP) involves additional material properties. One of the most informative quality values for CFRP is the interlaminar shear strength (ILSS) in the midplane direction. The ILSS can be handled in an equivalent way for AM parts in form of the interlayer shear strength. The literature shows various approaches, but no existing standard to determine the interlayer shear strength for FFF parts. The aim of the study is a suitable experimental setup for identifying the interlayer shear strength of AM parts. The experimental setup and the specimen geometry in accordance with DIN 65148 are adapted to ensure an interlayer failure mode between two layers. The newly developed test approach is validated with a test series showing a decrease in ILSS of up to 40% by increasing the layer height. © 2021 Wiley-VCH GmbH
    view abstract10.1002/masy.202000247
  • Surface conditioning during deep drilling [Messtechnisches und simulationsbasiertes erfassen von thermomechanischen belastungskollektiven oberflächenkonditionierung beimtiefbohren –teil 2]
    Zabel, A. and Strodick, S. and Schmidt, R. and Walther, F. and Biermann, D. and Wegert, R. and Eisseler, R. and Möhring, H.-C. and Guski, V. and Schmauder, S.
    WT Werkstattstechnik 111 (2021)
    The article deals with aspects of developing methods specifi-cally for surface conditioning in deep hole drilling parallel to machining. This involves metrological and simulation-based approaches for identifying thermo-mechanical process conditi-ons in both BTA and ELB process. Ways for obtaining process data both with sensor technology used in-situ and with FEM simulations performed concomitantly are investigated. These data form the basis of a deep hole process control. The second part presents the work and the results on single lip deep hole drilling. © 2021, VDI Fachmedien GmBbH & Co.. All rights reserved.
    view abstract10.37544/1436-4980-2021-03-26
  • Targeted residual stress generation in single and two point incremental sheet forming (ISF)
    Maqbool, F. and Maaß, F. and Buhl, J. and Hahn, M. and Hajavifard, R. and Walther, F. and Tekkaya, A.E. and Bambach, M.
    Archive of Applied Mechanics 91 (2021)
    The mechanical properties and the operating life of a formed component are highly dependent on the residual stress state. There is always a high magnitude of residual stresses in the components formed by incremental sheet forming (ISF) due to the localized deformation mechanism. Hence, a thorough understanding of the generation of the residual stresses by ISF is necessary. This study investigates the residual stress generation mechanism for two process variants of ISF, i.e., Single Point Incremental Forming (SPIF) and Two Point Incremental Forming (TPIF). This understanding is used to control and targetedly generate the residual stresses to improve the part performance. In this regard, the residual stress state in a truncated cone geometry manufactured using SPIF and disc springs manufactured using TPIF was experimentally analyzed. Validated numerical models for both process variants were developed to study the residual stresses in detail. The residual stress state in SPIF is such that the tool contact side develops tensile residual stresses and the non-contact side undergoes compressive residual stresses. The tool step-down variation was used to control residual stresses and improve the fatigue strength of truncated cones manufactured using SPIF. For TPIF, two different forming strategies were used to analyze the residual stress generation mechanism and the role of major process parameters. The residual stresses for TPIF are pre-dominantly compressive in both directions of forming tool motion. For both process variants of the ISF process, it is shown that the residual stresses can be beneficially utilized to improve mechanical properties of the components. © 2021, The Author(s).
    view abstract10.1007/s00419-021-01935-z
  • Testing procedure for fatigue characterization of steel‐cfrp hybrid laminate considering material dependent self‐heating
    Mrzljak, S. and Schmidt, S. and Kohl, A. and Hülsbusch, D. and Hausmann, J. and Walther, F.
    Materials 14 (2021)
    Combining carbon fiber reinforced polymers (CFRP) with steel offers the potential of utilizing the desired characteristics of both materials, such as specific strength/stiffness and fatigue strength of fiber reinforced polymers (FRP) and impact resistance of metals. Since in such hybrid laminates multiple material layers are combined, a gradual failure is likely that can lead to changes in mechanical properties. A failure of the metal partner leads to an increase in stress on the FRP, which under fatigue load results in increased self‐heating of the FRP. Therefore, a suitable testing procedure is required and developed in this study, to enable a reproducible characterization of the mechanical properties under fatigue load. The resulting testing procedure, containing multiple frequency tests as well as load increase and constant amplitude tests, enabled characterization of the fatigue performance while never exceeding a testing induced change in temperature of 4 K. In addition to the development of the testing procedure, an insight into the manufacturing induced residual stresses occurring in such hybrid laminates, which impacts the load‐bearing capacity, was established using finite element simulation. The gathered data and knowledge represents a basis for future in‐depth investigations in the area of residual stress influence on the performance of hybrid laminates and highlights its importance, since not only the used testing procedure determines the measured fatigue performance. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14123394
  • Thermal influence on the surface integrity during single-lip deep hole drilling of steel components
    Nickel, J. and Baak, N. and Volke, P. and Walther, F. and Biermann, D.
    MM Science Journal 2021 (2021)
    The thermomechanical load on the workpiece surface during the machining process strongly influences its surface integrity and the resulting fatigue strength of the components. In single-lip drilling, the measurement of the mechanical load using dynamometers is well established, but the thermal interactions between the tool and the workpiece material in the surface area are difficult to determine with conventional test setups. In this paper, the development and implementation of an in-process measurement of the thermal load on the bore subsurface is presented. The experimental setup includes a two-color ratio pyrometer in combination with thermocouples, which enable temperature measurement on the tool’s cutting edge as well as in the bore subsurface. In combination, a force measurement dynamometer for measuring the occurring force and torque is used. Thus, the influence of different cutting parameter variations on the thermomechanical impact on the bore surface can be evaluated. © 2021, MM publishing Ltd.. All rights reserved.
    view abstract10.17973/MMSJ.2021_7_2021070
  • Toward tailoring the degradation rate of magnesium-based biomaterials for various medical applications: Assessing corrosion, cytocompatibility and immunological effects
    Hartjen, P. and Wegner, N. and Ahmadi, P. and Matthies, L. and Nada, O. and Fuest, S. and Yan, M. and Knipfer, C. and Gosau, M. and Walther, F. and Smeets, R.
    International Journal of Molecular Sciences 22 (2021)
    Magnesium (Mg)-based biomaterials hold considerable promise for applications in regenerative medicine. However, the degradation of Mg needs to be reduced to control toxicity caused by its rapid natural corrosion. In the process of developing new Mg alloys with various surface modifications, an efficient assessment of the relevant properties is essential. In the present study, a WE43 Mg alloy with a plasma electrolytic oxidation (PEO)-generated surface was investigated. Surface microstructure, hydrogen gas evolution in immersion tests and cytocompatibility were assessed. In addition, a novel in vitro immunological test using primary human lymphocytes was introduced. On PEO-treated WE43, a larger number of pores and microcracks, as well as increased roughness, were observed compared to untreated WE43. Hydrogen gas evolution after two weeks was reduced by 40.7% through PEO treatment, indicating a significantly reduced corrosion rate. In contrast to untreated WE43, PEO-treated WE43 exhibited excellent cytocompatibility. After incubation for three days, untreated WE43 killed over 90% of lymphocytes while more than 80% of the cells were still vital after incubation with the PEO-treated WE43. PEO-treated WE43 slightly stimulated the activation, proliferation and toxin (perforin and granzyme B) expression of CD8+ T cells. This study demonstrates that the combined assessment of corrosion, cytocompatibility and immunological effects on primary human lymphocytes provide a comprehensive and effective procedure for characterizing Mg variants with tailorable degradation and other features. PEO-treated WE43 is a promising candidate for further development as a degradable biomaterial. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ijms22020971
  • Tuneable material properties of Organosolv lignin biocomposites in response to heat and shear forces
    Dörrstein, J. and Schwarz, D. and Scholz, R. and Walther, F. and Zollfrank, C.
    European Polymer Journal 148 (2021)
    Lignin as a renewable biomacromolecule is considered as a sustainable feedstock for the generation of bioplastics and bioplastic composites. However, during thermoplastic processing, high temperature and mechanical forces are known to promote destructive bond-cleavage in a vast majority of macromolecules due to overheating and internal friction between molecular constituents. This study demonstrates that several material properties (e.g. thermal stability, mechanical properties) of biocomposites with a high Organosolv lignin content (≥50 w%) can be tuned by carefully selecting thermal and mechanical energy input during compounding. Organosolv lignin obtained from ensiled grass was shown to undergo in-situ coupling reactions at temperatures below the main degradation point (&lt;200 °C) resulting in an up to 6-fold increase in molecular weight (Mw). The results from Fourier Transform Infrared (FTIR) spectroscopy suggested the formation of ester bonds, which was ascribed to direct esterification reactions, which occur due to energy transfer in the melt phase. When subjecting the extracted lignin to prolonged compounding times, it was shown that higher coupling degrees resulted in a lignin biocomposite with increased stiffness and ultimate tensile stress of about 1800% and 40%, respectively. Temperature was shown to have the highest effect on coupling reactions, whereas the energy transfer by mechanical forces during compounding was lower and followed a non-linear behaviour. In tensile stress–strain curves, biocomposites with high energy input revealed a distinct yield point (16 MPa). Ultra-micro-hardness tests confirmed that the biocomposite became significantly stiffer in response to shear and thermal forces with a reduction in indentation creep Cit of 2.0% to a value of up to 8.5%. Organosolv lignin biocomposites obtained at higher specific energy input (EIN) through thermal energy input and shear forces showed an increased dynamic stiffness Cdyn up to 100%, which was observed by multiple step tests (MST). © 2021 Elsevier Ltd
    view abstract10.1016/j.eurpolymj.2021.110359
  • Advanced test setup for accelerated aging of plastics by visible LED radiation
    Hemmerich, M. and Meyer, J. and Walther, F.
    Materials 13 (2020)
    In this article, a newly developed test setup for the aging of optical plastics by visible radiation (450 nm) is presented. In addition to a comprehensive monitoring of the operating parameters and an efficient cooling of the high-power multiple chips on board the LEDs used, the plastic samples can be fully temperature-controlled, independent of the radiant power of the LED, due to fluid driven thermostatization. The sample surface temperatures and irradiance values were verified by in situ measurements and simulations. To validate the test setup, polycarbonate samples with well-known aging behavior were aged for 1896 h. By spectroscopic IR and UV/vis analysis of the samples at different aging times, known optical aging results of polycarbonate could be observed, which proves the intended operationality of the system. © 2020 by the authors.
    view abstract10.3390/MA13194261
  • Analysis of the functional properties in the bore sub-surface zone during BTA deep-hole drilling
    Schmidt, R. and Strodick, S. and Walther, F. and Biermann, D. and Zabel, A.
    Procedia CIRP 88 (2020)
    The BTA deep hole drilling process is analysed to determine the relationships between process-structure and residual stress as a function of surface conditioning during the machining process. The residual stresses, the hardness and structural changes in the microstructure are used as indicators for a following process control in order to generate an improved component service life or reliability. In the first tests, the deep-drilled samples are examined with regard to residual stresses, bore tolerances, microstructure and hardness in the sub-surface zone. Furthermore, the specimens will be subjected to a heat treatment to establish a stress-free microstructure as a reference state for the magnetic residual stress measurement. © 2020 The Authors.
    view abstract10.1016/j.procir.2020.05.055
  • Biomimetic scaffold fabricated with a mammalian trabecular bone template
    Bulygina, I. and Senatov, F. and Choudhary, R. and Kolesnikov, E. and Kaloshkin, S. and Scholz, R. and Knyazeva, M. and Walther, F. and Anisimova, N. and Kiselevskiy, M.
    Polymer Degradation and Stability 172 (2020)
    This study proposes the method of ultra-high molecular weight polyethylene (UHMWPE) biomimetic scaffold fabrication. Anisotropy is considered to be a distinctive feature of native bone but basically only a 3D-fabricated scaffold structure may be anisotropic, while 3D-printing is not applicable to UHMWPE. We proposed a novel method that suggested a template of native mammalian bone to be used as a negative for UHMWPE scaffold fabrication. This method allows direct replication of the bone's structural features on the micro- and macro-scale. Bone scaffolds obtained using the specified method showed anisotropic structure; the pores' average proportions for scaffold and bone were 770 and 470, and 700 and 500 μm, respectively. According to SEM and CT investigations, the scaffolds' macro- and microstructure mimicked the native bone architecture; this feature distinguishes the proposed method from the other UHMWPE scaffold fabrication techniques. The combination of the hydrophilic surface and the nanorelief affected the adhesion and proliferation of cells: the adhesion of multipotent mesenchymal stromal cells (MMSC) amounted to 40% after 4 h; the proliferation of MMSC was 75% after 48 h. The proposed novel method of fabricating biomimetic scaffolds can be used to obtain bone implants of the complex microstructure and anisotropy from high-melt viscosity polymers which cannot be 3D-printed to be further applied in bone reconstruction. The FT-IR analysis confirmed the occurrence of carboxyl oxidation when the surface of UHMWPE sample was treated with chromic acid. The oxidation index (OI) of the samples was found in the order of etching in chromic acid > sterilization > hot moulding respectively. It can be suggested that the oxidative degradation of UHMWPE can be reduced by optimizing manufacturing conditions and further selection of an appropriate processing method. © 2020 Elsevier Ltd
    view abstract10.1016/j.polymdegradstab.2020.109076
  • Biomimetic UHMWPE/HA scaffolds with rhBMP-2 and erythropoietin for reconstructive surgery
    Senatov, F. and Amanbek, G. and Orlova, P. and Bartov, M. and Grunina, T. and Kolesnikov, E. and Maksimkin, A. and Kaloshkin, S. and Poponova, M. and Nikitin, K. and Krivozubov, M. and Strukova, N. and Manskikh, V. and Anisimova, N. and Kiselevskiy, M. and Scholz, R. and Knyazeva, M. and Walther, F. and Lunin, V. and Gromov, A. and Karyagina, A.
    Materials Science and Engineering C 111 (2020)
    A promising direction for the replacement of expanded bone defects is the development of bioimplants based on synthetic biocompatible materials impregnated with growth factors that stimulate bone remodeling. Novel biomimetic highly porous ultra-high molecular weight polyethylene (UHMWPE)/40% hydroxyapatite (HA) scaffold for reconstructive surgery with the porosity of 85 ± 1% vol. and a diameter of pores in the range of 50–800 μm was developed. The manufacturing process allowed the formation of trabecular-like architecture without additional solvents and thermo-oxidative degradation. Biomimetic UHMWPE/HA scaffold was biocompatible and provided effective tissue ingrowth on a model of critical-sized cranial defects in mice. The combined use of UHMWPE/HA with Bone Morphogenetic Protein-2 (BMP-2) demonstrated intensive mineralized bone formation as early as 3 weeks after surgery. The addition of erythropoietin (EPO) significantly enhanced angiogenesis in newly formed tissues. The effect of EPO of bacterial origin on bone tissue defect healing was demonstrated for the first time. The developed biomimetic highly porous UHMWPE/HA scaffold can be used separately or in combination with rhBMP-2 and EPO for reconstructive surgery to solve the problems associated with difference between implant architecture and trabecular bone, low osteointegration and bioinertness. © 2020 Elsevier B.V.
    view abstract10.1016/j.msec.2020.110750
  • Characterization of damage evolution on hot flat rolled mild steel sheets by means of micromagnetic parameters and fatigue strength determination
    Teschke, M. and Vasquez, J.R. and Lücker, L. and Walther, F.
    Materials 13 (2020)
    In continuous casting processes, inevitable voids (damage) are generated inside the material. The subsequent forming process of hot flat rolling offers the potential of healing these defects by closing the voids and bonding the internal surfaces. In this paper, different forming conditions from hot flat rolling process were characterized with micromagnetic measurement techniques and the influence of the damage evolution on the fatigue behavior was investigated. To characterize the reduction of voids through hot flat rolling processes, nondestructive testing techniques are required. Therefore, micromagnetic measurements such as Barkhausen noise, incremental permeability, and harmonic analysis were carried out, correlated with the number of voids, and compared with each other. The influence of damage evolution of different forming conditions on the fatigue behavior was characterized based on instrumented constant amplitude and multiple amplitude (load increase) tests. A significant increase in fatigue strength due to the hot flat rolling process, which leads to a reduction in the number of voids, was observed. In addition, the fracture surfaces of the specimens were analyzed in the scanning electron microscope. © 2020 by the authors.
    view abstract10.3390/ma13112486
  • Comparison of high-temperature compression and compression-compression fatigue behavior of magnesium alloys DieMag422 and AE42
    Teschke, M. and Koch, A. and Walther, F.
    Materials 13 (2020)
    Due to their high strength-to-weight-ratio, magnesium alloys are very attractive for use in automotive engineering. For application at elevated temperatures, the alloys must be creep-resistant. Therefore, the influence of the operating temperature on the material properties under quasistatic and cyclic load has to be understood. A previous study investigated tensile-tensile fatigue behavior of the magnesium alloys DieMag422 and AE42 at room temperature (RT). The aim of this study was the comparison of both alloys regarding compression, tensile, and compression-compression fatigue behavior. The quasistatic behavior was determined by means of tensile and compression tests, and the tensile-compression asymmetry was analyzed. In temperature increase fatigue tests (TIFT) and constant amplitude tests (CAT), the temperature influence on the cyclic creeping (ratcheting) behavior was investigated, and mechanisms-relevant test temperatures were determined. Furthermore, characteristic fracture mechanisms were evaluated with investigations of the microstructure and the fracture surfaces. The initial material was analyzed in computed tomographic scans and energy dispersive X-ray (EDX) analyses. © 2020 by the authors.
    view abstract10.3390/ma13030497
  • Comparison of the damage evolution in glass fiber-reinforced polyurethane and epoxy in the HCF and VHCF regimes investigated by intermittent in situ X-ray computed tomography
    Hülsbusch, D. and Helwing, R. and Mrzljak, S. and Walther, F.
    IOP Conference Series: Materials Science and Engineering 942 (2020)
    The aim of this work is the comparative characterization of the fatigue and damage behaviors of GFR-polyurethane and GFR-epoxy with application-relevant quasi-isotropic layer setup in the high cycle and very-high cycle fatigue regimes. Therefore, a high-frequency test method based on a resonant testing system (1 kHz) has been further developed and assessed with special consideration of self-heating. In intermittent test procedures, the damage state has been explored by in situ X-ray computed tomography analysis after certain numbers of cycles. It was shown that the overall damage state in the VHCF regime is reduced by a factor of three compared to the HCF regime and accompanied by delayed initiation and propagation of delamination. The latter was proven to be the main reason for a decreased inclination of the S/N-curve in the VHCF regime by 50-60%. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/942/1/012036
  • Damage tolerance evaluation of a unit cell plane based on electron beam powder bed fusion (E-PBF) manufactured Ti6Al4V alloy
    Kotzem, D. and Ohlmeyer, H. and Walther, F.
    Procedia Structural Integrity 28 (2020)
    Electron beam powder bed fusion (E-PBF) enables the fabrication of new and complex light-weight structures within short process times. However, increasing complexity of the producible components leads to complex damage mechanisms and interactions which can not sufficiently be represented by typical material properties such as ultimate tensile strength or fatigue strength. In the current research, the damage tolerance was investigated within a single unit cell plane for a stretch-dominated lattice type under uniaxial cyclic loading based on the E-PBF manufactured Ti6Al4V alloy. During the experiments, a combination of application-specific optical measurement techniques such as digital image correlation and thermography were used to capture occurring material responses. In particular, a clear relationship between local deformation and temperature increase is visible. In comparison to conventionally manufactured material, similar mechanical properties can be achieved in low cycle fatigue range. Furthermore, captured material responses could be used as a basis for future monitoring systems in order to enable a reliable application in safety-relevant components. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2020.10.003
  • Damage tolerance evaluation of E-PBF-manufactured inconel 718 strut geometries by advanced characterization techniques
    Kotzem, D. and Arold, T. and Niendorf, T. and Walther, F.
    Materials 13 (2020)
    By means of electron beam powder bed fusion (E-PBF), highly complex lightweight structures can be manufactured within short process times. Due to the increasing complexity of producible components and the entangled interplay of damage mechanisms, common bulk material properties such as ultimate tensile or fatigue strength are not sufficient to guarantee safe and reliable use in demanding applications. Within this work, the damage tolerance of E-PBF-manufactured Ni-based alloy Inconel 718 (IN 718) strut geometries under uniaxial cyclic loading was investigated supported by several advanced measurement techniques. Based on thermal and electrical measurements, the failure of single struts could reliably be detected, revealing that continuous monitoring is applicable for such complex geometries. Process-induced surface roughness was found to be the main reason for early failure during cyclic loading. Thus, adequate post-processing steps have to be established for complex geometries to significantly improve damage tolerance and, eventually, in-service properties. © 2020 by the authors.
    view abstract10.3390/ma13010247
  • Degradation, bone regeneration and tissue response of an innovative volume stable magnesium-supported GBR/GTR barrier membrane
    Barbeck, M. and Kühnel, L. and Witte, F. and Pissarek, J. and Precht, C. and Xiong, X. and Krastev, R. and Wegner, N. and Walther, F. and Jung, O.
    International Journal of Molecular Sciences 21 (2020)
    Introduction: Bioresorbable collagenous barrier membranes are used to prevent premature soft tissue ingrowth and to allow bone regeneration. For volume stable indications, only non-absorbable synthetic materials are available. This study investigates a new bioresorbable hydrofluoric acid (HF)-treated magnesium (Mg) mesh in a native collagen membrane for volume stable situations. Materials and Methods: HF-treated and untreated Mg were compared in direct and indirect cytocompatibility assays. In vivo, 18 New Zealand White Rabbits received each four 8 mm calvarial defects and were divided into four groups: (a) HF-treated Mg mesh/collagen membrane, (b) untreated Mg mesh/collagen membrane (c) collagen membrane and (d) sham operation. After 6, 12 and 18 weeks, Mg degradation and bone regeneration was measured using radiological and histological methods. Results: In vitro, HF-treated Mg showed higher cytocompatibility. Histopathologically, HF-Mg prevented gas cavities and was degraded by mononuclear cells via phagocytosis up to 12 weeks. Untreated Mg showed partially significant more gas cavities and a fibrous tissue reaction. Bone regeneration was not significantly different between all groups. Discussion and Conclusions: HF-Mg meshes embedded in native collagen membranes represent a volume stable and biocompatible alternative to the non-absorbable synthetic materials. HF-Mg shows less corrosion and is degraded by phagocytosis. However, the application of membranes did not result in higher bone regeneration. © 2020 by the authors.
    view abstract10.3390/ijms21093098
  • Development of an energy-based approach for optimized frequency selection for fatigue testing on polymers – Exemplified on polyamide 6
    Hülsbusch, D. and Kohl, A. and Striemann, P. and Niedermeier, M. and Strauch, J. and Walther, F.
    Polymer Testing 81 (2020)
    Polymers and composite materials show temperature-dependent material properties. Therefore, the frequency resembles a critical part in fatigue testing, due to its influence on the self-heating of the polymeric material and thereby on the number of cycles to failure. The aim of this paper is the development of a testing method, which allows comparable results with varying frequencies. To minimize the self-heating effect on the fatigue behavior, a model was established for selecting optimized frequencies regarding the load-specific temperature increase of the specimen. A new energy-parameter, the induced energy-rate, was introduced and correlated to the load-specific increase in temperature in multiple and constant amplitude tests at ambient conditions. With this approach, it was possible to determine a threshold value for the newly defined induced energy-rate. A stress-specific model was developed and a limit frequency was calculated. The results were verified in multiple and constant amplitude tests and S/N-curves. © 2019
    view abstract10.1016/j.polymertesting.2019.106260
  • Effect of corrosion and surface finishing on fatigue behavior of friction stir welded EN AW-5754 aluminum alloy using various tool configurations
    Chehreh, A.B. and Grätzel, M. and Bergmann, J.P. and Walther, F.
    Materials 13 (2020)
    In this study, fatigue behavior of surface finished and precorroded friction stir welded (FSW) specimens using various tool configurations were comparatively investigated by the load increase method. The FSW using conventional, stationary shoulder and dual-rotational configurations was carried out by a robotized tool setup on 2 mm EN AW-5754 aluminum sheets in butt joint formation. After extraction of the specimens, their weld seam and root surfaces were milled to two different depths of 200 μm and 400 μm to remove the surface and the FSW tool shoulder effects. This surface finishing process was performed to investigate the effect of the surface defects on the fatigue behavior of the FSW EN AW-5754 aluminum alloy sheets. It was found that material removal from the weld and root surfaces of the specimens, increased the fracture stresses of conventional and dual-rotational FSW from 204 to 229 MPa and 196 to 226 MPa, respectively. However, this increase could not be detected in stationary shoulder FSW. Specimens with finished surfaces, which showed superior properties, were used in salt spray and cyclic climate change test to investigate the effect of corrosion on the fatigue behavior of FSW specimens. It was shown that cyclic climate change test reduced the fatigue properties of the base material, conventional, stationary shoulder and dual-rotational FSW approximately 1%-7%. This decrease in the fatigue properties was greater in the case of the salt spray test, which was 7% to 21%. © 2020 by the authors.
    view abstract10.3390/ma13143121
  • Effect of miniaturization and surface roughness on the mechanical properties of the electron beam melted superalloy Inconel®718
    Kotzem, D. and Dumke, P. and Sepehri, P. and Tenkamp, J. and Walther, F.
    Progress in Additive Manufacturing 5 (2020)
    In this work, the Ni-based super alloy Inconel 718 manufactured via electron beam melting is investigated. Typical microstructure of Inconel 718, which was processed by electron beam melting, consists of columnar oriented dendritic structure with strong texture along building direction in hatch region, whereas microstructure differs in contour region, which is supposed to influence mechanical properties in the as-built state. As highly complex geometries are possible to manufacture with additive manufacturing techniques, the influence of miniaturization and surface roughness on microstructural and mechanical properties has to be understood in detail. Therefore, samples were processed with different initial sizes and subsequently tested in as-built and polished condition. Before performing mechanical tests, process-induced microstructure was determined by scanning electron microscope as well as distribution of defects and geometrical deviations by microfocused computed tomography. To characterize the mechanical properties, different testing methods, both tensile and fatigue tests, were carried out. Present investigations show almost similar microstructures in large-scale and small-scale Inconel 718 volumes. However, small-scale volumes show higher number of defects in the form of surface and near-surface defects. Furthermore, as-built specimens show geometrical deviations when compared to initial CAD diameter, which makes the implementation of an average equivalent diameter mandatory. It can be demonstrated that small-scale as-built volumes have reduced mechanical properties, whereby ultimate tensile strength is reduced by 60% and fatigue strength is reduced by 75%, showing that increased defect density and as-built surface roughness have a higher impact on fatigue properties and are the dominating reason for early failure in the as-built state due to multiple crack initiation. © 2019, Springer Nature Switzerland AG.
    view abstract10.1007/s40964-019-00101-w
  • Effect of nanoparticle additivation on the microstructure and microhardness of oxide dispersion strengthened steels produced by laser powder bed fusion and directed energy deposition
    Doñate-Buendia, C. and Streubel, R. and Kürnsteiner, P. and Wilms, M.B. and Stern, F. and Tenkamp, J. and Bruder, E. and Barcikowski, S. and Gault, B. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and Gökce, B.
    Procedia CIRP 94 (2020)
    In this contribution, the effect of nanoparticle additivation on the microstructure and microhardness of oxide dispersion strengthened steels (ODS) manufactured by laser powder bed fusion (L-PBF) and directed energy deposition (DED) additive manufacturing (AM) is studied. The powder composites are made of micrometer-sized iron-chromium-alloy based powder which are homogenously decorated with Y2O3 nanoparticles synthesized by pulsed laser fragmentation in water. Consolidated by L-PBF and DED, an enhanced microhardness of the AM-built ODS sample is found. This increase is related to the significant microstructural differences found between the differently processed samples. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.009
  • Effect of phase formation due to holding time of vacuum brazed AISI 304L/NiCrSiB joints on corrosion fatigue properties
    Otto, J.L. and Penyaz, M. and Schmiedt-Kalenborn, A. and Knyazeva, M. and Ivannikov, A. and Kalin, B. and Walther, F.
    Journal of Materials Research and Technology 9 (2020)
    Whether in turbine components or exhaust gas heat exchangers, vacuum-brazed nickel-based joints are subjected to varying cyclical loads during their applications, often in corrosive environments. The microstructure of the brazed seam, which is determined by the alloy composition and the brazing process parameters, is essential for the service life. In this experimental study a modified BNi-5a foil was produced and used to braze cylindrical AISI 304L butt joints with two different holding times. Using energy dispersive spectroscopy analyses, a direct correlation of the element distribution at the brazing seam with the holding time was detected as a result of diffusion processes. Individual phases were identified, and it could be shown that the longer holding time led to a reduction of borides and silicides as well as to a more even microhardness curve through the seam. The effect of the microstructure on the corrosion fatigue properties was evaluated using multiple amplitude tests by a stepwise increase of the maximum stress amplitude in synthetic exhaust gas condensate. Thereby, improved corrosion fatigue and cyclic deformation behaviors were achieved for the more homogeneous microstructure. Afterwards, topography analyses of the fracture surfaces enabled an understanding of microstructure-dependent damage mechanisms including fatigue crack initiation and propagation. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
    view abstract10.1016/j.jmrt.2020.07.047
  • Electrical resistance-based fatigue assessment and capability prediction of extrudates from recycled field-assisted sintered EN AW-6082 aluminium chips
    Koch, A. and Bonhage, M. and Teschke, M. and Luecker, L. and Behrens, B.-A. and Walther, F.
    Materials Characterization 169 (2020)
    The possibility to directly extrude semi-finished products using a solid-state-recycling process is a promising alternative to the remelting process, which is highly energy-intensive. Therefore, aluminium chips, normally considered as scrap, are used as the basis for the recycling. The recycling process consists of a cold compaction process, a field-assisted sintering (FAST) process to consolidate the chips, and finally a forward rod extrusion process. Compared to approaches which break the oxide layers by applying high shear stresses and deformations, necessary for an adequate welding of the chips, quasistatic and cyclic properties and capabilities are significantly increased. The defect structure of specimens, which was determined by means of computed tomography and which significantly influences the lifetime, could be correlated well with pre-test electrical resistance measurements. Finally, these findings were used to establish a lifetime calculation model based on unique electrical resistance measurements prior to mechanical testing. © 2020 Elsevier Inc.
    view abstract10.1016/j.matchar.2020.110644
  • Erratum to “Service life characterization of orthopedic implant material made of ultra-high molecular weight polyethylene under physiological conditions” [J. Mech. Behav. Biomed. Mater. 104 2020 103617, 1–8] (Journal of the Mechanical Behavior of Biomedical Materials (2020) 104, (S1751616119314122), (10.1016/j.jmbbm.2020.103617))
    Wegner, N. and Scholz, R. and Knyazeva, M. and Walther, F.
    Journal of the Mechanical Behavior of Biomedical Materials 109 (2020)
    The following abstract was missing in the published article. The publisher apologizes for the error. Abstract Due to the demographic change and progress in medical care, the world is facing a particularly strong trend towards an aging population. This trend is accompanied by increasing bone and joint diseases, which often require reconstruction with biomaterials. While the wear behavior of joint implants is well known and has been broadly researched, little attention has been paid to the mechanical stability of orthopedic bone prosthetics such as hydroxyapatite, calcium sulfate, tri-calcium phosphate ceramics, bioactive glasses, or polymer-based substitutes. Biocompatibility is often tested in vivo while there is a general lack of standardized in vitro test methods to determine their mechanical long-term stability, especially under service-relevant cyclic loading, simulating physiological dynamic stresses and environment. Since damage mechanisms change during dynamic loading compared to quasi-static loading, the material can endure lower loads and thus, the characterization under dynamic loading conditions plays an important role. This study represents a validation of a previously developed in vitro short-term method for the characterization of service life of bone prosthetic materials by determining material reaction during cyclic loading in a biomimetic test setup. The investigations were performed on ultra-high molecular weight polyethylene (UHMWPE) as an exemplary prosthetic material. The results confirm the hypotheses previously made and validate the applicability of the short-term method. Furthermore, the strong influence of human body conditions on the mechanical fatigue properties could be shown. In addition to the time-efficient determination of the service life, the advantage of this short-term method is the use of a standardized experimental setup and a standardized sample geometry, ensuring it can be applied to almost any other biomaterial. © 2020 Elsevier Ltd
    view abstract10.1016/j.jmbbm.2020.103828
  • Experimental and analytical analysis on the stacking sequence of composite pressure vessels
    Nebe, M. and Asijee, T.J. and Braun, C. and van Campen, J.M.J.F. and Walther, F.
    Composite Structures 247 (2020)
    The industrialization of fuel cell electric vehicles demands cost efficient storage solutions for hydrogen. While gaseous storage in type IV pressure vessels is currently the most mature technology, further structure optimization needs to be undertaken in order to meet cost requirements. This research investigates the effects of stacking sequence of composite pressure vessels regarding laminate quality, structural deformation and finally burst pressure. Therefore, a known laminate is studied on a subscale vessel geometry with changing stacking sequences. The specimens are pressurized in a specially designed chamber up to burst pressures of 166.11 MPa. Through a multisensor arrangement of stereometric systems, the deformation is tracked up to burst by using 3D digital image correlation. The experimental results show a difference of 67% in burst pressure between the investigated stacking sequences. Experimental cylinder strains and burst pressures are compared to results derived from 3D elasticity theory with implemented first ply failure criterion. Additionally, using X-ray computed tomography and acid digestion tests, insights about the distribution of fiber volume fraction and porosity are provided. For the investigated sequences in this research, the results show the considerable influence of stacking sequence on the laminate quality, the structural deformation and finally the burst pressure of composite pressure vessels. Moreover, it is shown that while the used 3D elasticity approach proved to be a useful tool for the prediction of strains and failure in the cylindrical section, discrepancies between prediction and experiment can arise based on preliminary failure occuring at the cylinder-dome transition. The results therefore emphasize the need for analytical and numerical analysis strategies to consider transition-related effects between cylinder and dome. © 2020 Elsevier Ltd
    view abstract10.1016/j.compstruct.2020.112429
  • Fatigue behavior of conventional and stationary shoulder friction stir welded en aw-5754 aluminum alloy using load increase method
    Chehreh, A.B. and Grätzel, M. and Bergmann, J.P. and Walther, F.
    Metals 10 (2020)
    The load increase method, which is highly efficient in rapidly identifying the fatigue performance and strength of materials, is used in this study to investigate friction stir welded (FSW) EN AW-5754 aluminum alloys. Previous investigations have demonstrated the accuracy and efficiency of this method compared to Woehler tests. In this study, it is shown that the load increase method is a valid, accurate and efficient method for describing the fatigue behavior of FSW weld seams. The specimen tests were performed on 2 mm thick aluminum sheets using conventional and stationary tool configurations. It is shown that an increase in fatigue strength of the FSW EN AW-5754 aluminum alloys can be achieved by using the stationary shoulder tool configuration rather than the conventional one. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10111510
  • Fracture–mechanical assessment of the effect of defects on the fatigue lifetime and limit in cast and additively manufactured aluminum–silicon alloys from hcf to vhcf regime
    Tenkamp, J. and Awd, M. and Siddique, S. and Starke, P. and Walther, F.
    Metals 10 (2020)
    Aluminum–silicon alloys are commonly used in die-cast and additively manufactured (AM) light-weight components due to their good processability and high strength-to-weight ratio. As both processing routes lead to the formation of defects such as gas and shrinkage porosity, a defect-sensitive design of components is necessary for safe application. This study deals with the fatigue and crack propagation behavior of die-cast alloy AlSi7Mg0.3 and additively manufactured alloy AlSi12 and its relation to process-induced defects. The different porosities result in significant changes in the fatigue stress-lifetime (S–N) curves. Therefore, the local stress intensity factors of crack-initiating defects were determined in the high and very high cycle fatigue regime according to the fracture mechanics approach of Murakami. Through correlation with fatigue lifetime, the relationship of stress intensity factor (SIF) and fatigue lifetime (N) could be described by one power law (SIF–N curve) for all porosities. The relationship between fatigue limit and defect size was further investigated by Kitagawa–Takahashi (KT) diagrams. By using El Haddad’s intrinsic crack length, reliable differentiation between fracture and run out of the cast and AM aluminum alloys could be realized. SIF–N curves and KT diagrams enable a reliable fatigue design of cast and AM aluminum alloys for a finite and infinite lifetime. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10070943
  • Gas atomization and laser additive manufacturing of nitrogen-alloyed martensitic stainless steel
    Boes, J. and Röttger, A. and Theisen, W. and Cui, C. and Uhlenwinkel, V. and Schulz, A. and Zoch, H.-W. and Stern, F. and Tenkamp, J. and Walther, F.
    Additive Manufacturing 34 (2020)
    Nitrogen as an alloying element can improve the corrosion resistance and the mechanical properties of stainless steels. Therefore, nitrogen-alloyed martensitic stainless steels, such as X30CrMoN151, have been developed in recent decades and conventional processing of this steel by casting or powder metallurgy is well understood. However, only very few attempts to process nitrogen-alloyed martensitically hardenable stainless steels containing more than 0.2 mass-% of carbon by laser powder bed fusion (L-PBF) have been reported so far. In this study, X30CrMoN15-1 steel powder has been produced from quasi nitrogen-free X30CrMo15-1 steel by gas atomization using N2 as the process gas to introduce nitrogen into the steel. The gas-atomized powder was characterized in terms of nitrogen content, particle size distribution, particle morphology, and flow properties. The powder was then processed by L-PBF under an N2 gas atmosphere, and microstructural investigations were performed on the L-PBF-built samples using scanning electron microscopy and X-ray computed tomography. Additionally, a first impression of the mechanical properties of the L-PBF-built steel in the as-built and quenched and tempered condition was obtained by means of fatigue tests. It was shown that a nitrogen content of 0.16 mass-% could be introduced into the steel during gas atomization. The resulting powder was successfully processed by means of L-PBF, and specimens with a high density were produced. During fatigue testing, a large amount of retained austenite in the as-built condition resulted in a greater damage tolerance of the specimens compared to the heat-treated condition. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101379
  • Humidity-Sensing Material Cottonid – Microstructural Tuning for Improved Actuation and Fatigue Performance
    Scholz, R. and Langhansl, M. and Zollfrank, C. and Walther, F.
    Frontiers in Materials 7 (2020)
    Adaptive actuators are stimuli-responsive materials able to make a direct use of primary energy to produce motion, as it is well known from motile plant structures. Changes in environmental conditions, such as temperature and humidity, trigger passive movements without the need of metabolism or the use of electrical energy. This bioinspired adaptive mechanism is an alternative and sustainable approach in terms of energy conservation in technical applications. The cellulose-based functional material Cottonid is a promising candidate in this context. It is hygroscopic and possesses a process-related structural anisotropy, which results in direction-dependent actuation and fatigue performance. Since Cottonid is a modified natural material, its microstructure is tunable through chemical modification of the cellulose during the manufacturing process. To assess the influence of varying manufacturing parameters on the microstructure, the actuation behavior as well as the mechanical properties, a parameter study was carried out to identify the most promising modifications for stimuli-responsive element production while maintaining mechanical robustness. This was accomplished with respect to variations of the cellulose source, the chemical catalyst for cellulose modification, temperature of the catalyst bath Tcat as well as reaction time treact. Specimens’ microstructures were investigated with scanning electron microscopy, infrared spectroscopy as well as X-ray diffraction. The actuation behavior was characterized over instrumented experiments in a climate chamber in varying environmental conditions, whereas the environmental fatigue behavior was evaluated in tests with superimposed medial and mechanical loading. Obtained results identified Tcat as most influential process parameter onto resulting material actuation properties, which enables a limitation of the possible process window. The findings are used to develop tailored functional materials, where anisotropy and hygroscopicity can be adjusted through the manufacturing process. © Copyright © 2020 Scholz, Langhansl, Zollfrank and Walther.
    view abstract10.3389/fmats.2020.00156
  • Impact of forsterite addition on mechanical and biological properties of composites
    Choudhary, R. and Venkatraman, S.K. and Bulygina, I. and Chatterjee, A. and Abraham, J. and Senatov, F. and Kaloshkin, S. and Ilyasov, A. and Abakumov, M. and Knyazeva, M. and Kukui, D. and Walther, F. and Swamiappan, S.
    Journal of Asian Ceramic Societies 8 (2020)
    The objective of designing a biocompatible and mechanically stable scaffold for hard tissue regeneration was achieved by fabricating diopside/forsterite composites. Superior mechanical strength, slow degradation, excellent antibacterial activity and good cell viability were attained with the increase in forsterite ratio in the composites whereas apatite deposition ability got enhanced as the diopside content was increased. The variation in the rate of apatite deposition on the surface of composites exhibited different surface topography such as nano-structured interconnected fibrous network and globular morphology. The scaffolds after one-month immersion in a physiological environment exhibited good Young’s modulus and compressive strength. Clear and distinguishable prevention of bacterial growth confirms that composites have the potential to inhibit microbial colony formation of nine different clinical pathogens. The composite containing major diopside content was more effective toward S. aureus while the growth of E. coli was inhibited more by the composite containing a higher ratio of forsterite. The interaction of composites with MG-63 cells showed an enhancement in cell viability as the content of forsterite was increased. MTS assay confirmed the cytocompatibility of samples with negligible toxicity effects. © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan.
    view abstract10.1080/21870764.2020.1807695
  • Improvement of Fatigue Strength in Lightweight Selective Laser Melted Alloys by In-Situ and Ex-Situ Composition and Heat Treatment
    Awd, M. and Johannsen, J. and Chan, T. and Merghany, M. and Emmelmann, C. and Walther, F.
    Minerals, Metals and Materials Series (2020)
    Selective laser melting is a powder-bed-fusion process that is applied to different alloys. Thus, it is essential to study what are the different process variables that affect the static, quasi-static, and cyclic mechanical properties. In this contribution, two examples of alloys are introduced: AlSi (AlSi12, AlSi10Mg) and Ti-6Al-4V. The influence of controlled cooling and degassing mechanisms of residual gases is investigated by structural analysis in electron microscopy and X-ray computed tomography. Controlled cooling through platform heating or multi-exposure treatments increased the dendritic width in AlSi alloys and decomposed alpha prime in Ti-6Al-4V. The alteration was a cause for enhanced ductility and slowing of crack propagation. The cyclic deformation is tracked during mechanical testing and is simulated in FE software using a high-throughput methodology to calculate Woehler curves based on Fatemi-Socie damage parameters. The cyclic deformation simulation is in agreement with the experimental data and quantified cyclic damage using Fatemi-Socie parameters. © 2020, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-36296-6_11
  • In situ characterization of damage development in cottonid due to quasi-static tensile loading
    Scholz, R. and Delp, A. and Walther, F.
    Materials 13 (2020)
    Cottonid is a layered material based 100% on cellulose that holds excellent material properties by being completely sustainable. The finite nature of petroleum-based resources nowadays makes these properties significant for technical applications again. To understand how Cottonid reacts to application-oriented mechanical loads and how it fails, development of microstructural damage on the surface and in the volume of Cottonid was studied using innovative in situ testing techniques for the first time. Quasi-static tensile tests were comparatively performed in a scanning electron microscope as well as a microfocus computer tomograph, and the development of defects present in the initial condition of the material was investigated. In the elastic region, no visible damage initiation on the surface and a decrease of overall void volume within the gauge length could be detected. When reaching the yield strength, crack initiation on the surface starts at critical areas, like pores and microcracks, which propagation and assembly could be visualized via scanning electron micrographs. In the plastic region, an increase in void volume could be shown in the gauge length until final failure of the specimen. Innovative material testing techniques presented in this study support lifetime estimation in technical applications and understanding of process-structure-property relations. Particularly, characterization of microstructural damage development due to a mechanical load, which leads to final failure of the specimen, is essential to be able to create material models for lifetime prediction in respect to variable manufacturing or application parameters. © 2020 by the authors.
    view abstract10.3390/ma13092180
  • Influence of aluminum surface treatment on tensile and fatigue behavior of thermoplastic-based hybrid laminates
    Mrzljak, S. and Trautmann, M. and Wagner, G. and Walther, F.
    Materials 13 (2020)
    Hybrid laminates consist of layers of different materials, which determine the mechanical properties of the laminate itself. Furthermore, the structure and interfacial properties between the layers play a key role regarding the performance under load and therefore need to be investigated in respect to industrial applicability. In this regard, a hybrid laminate comprised of AA6082 aluminum alloy sheets and glass and carbon fiber-reinforced thermoplastic (polyamide 6) is investigated in this study with a focus on the influence of aluminum surface treatment application on tensile and fatigue behavior. Four different aluminum surface treatments are discussed (adhesion promoter, mechanical blasting, phosphating, and anodizing), which were characterized by Laser Scanning Microscopy. After the thermal consolidation of the hybrid laminate under defined pressure, double notch shear tests and tensile tests were performed and correlated to determine the resulting interfacial strength between the aluminum sheet surface and the fiber-reinforced plastic, and its impact on tensile performance. To investigate the performance of the laminate under fatigue load in LCF and HCF regimes, a short-time procedure was applied consisting of resource-efficient instrumented multiple and constant amplitude tests. Digital image correlation, thermography, and hysteresis measurement methods were utilized to gain information about the aluminum surface treatment influence on fatigue damage initiation and development. The results show that fatigue-induced damage initiation, development, and mechanisms differ significantly depending on the applied aluminum surface treatment. The used measurement technologies proved to be suitable for this application and enabled correlations in between, showing that the hybrid laminates damage state, in particular regarding the interfacial bonding of the layers, can be monitored not just through visual recordings of local strain and temperature development, but also through stress-displacement hysteresis analysis. © 2020 by the authors.
    view abstract10.3390/ma13143080
  • Influence of anisotropic damage evolution on cold forging
    Langenfeld, K. and Schowtjak, A. and Schulte, R. and Hering, O. and Möhring, K. and Clausmeyer, T. and Ostwald, R. and Walther, F. and Tekkaya, A.E. and Mosler, J.
    Production Engineering 14 (2020)
    This contribution deals with the influence of anisotropic material degradation (damage) within numerical simulations of cold forging. For that purpose, two constitutive frameworks for modeling ductile damage are presented: an isotropic and an anisotropic model. In a first step, both models are calibrated based on a uniaxial tensile test. Then, the forward rod extrusion process is simulated with the isotropic model. The deformation of a characteristic element is transferred to the anisotropic model and the local response is investigated. Both models are compared to one another in terms of the process induced ductile damage. It will be shown, that the magnitude of the induced damage agrees reasonably well, but that the orientation of ductile damage is of major importance. © 2020, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00942-y
  • Influence of cutting parameters on the formation of white etching layers in BTA deep hole drilling [Einfluss der Zerspanparameter auf die Ausprägung von weißen Randschichten beim BTA Tiefbohren]
    Strodick, S. and Berteld, K. and Schmidt, R. and Biermann, D. and Zabel, A. and Walther, F.
    Technisches Messen 87 (2020)
    In this study, the influence of cutting speed and feed rate on surface integrity in Boring Trepanning Association (BTA) deep hole drilling of AISI 4140+QT is investigated. Microstructure and micro-hardness in the subsurface zones of bores are analyzed, using metallographic and micromagnetic methods. It was found that when using high feed rates and cutting speeds, white etching layers (WEL) form at the surface of the bores. These layers are up to three times harder than the substrate material and have a maximum thickness of approx. tWEL ≈ 12 μm. WEL were usually followed by a transitional layer, so that elevated hardness was observed until a depth of dsurf = 35 μm below the surface. Magnetic Barkhausen noise (MBN) analysis proved to be applicable for the fast and reliable detection of WEL. The presented results contribute to gaining a deeper understanding of the complex interrelations between the design of the BTA process, the resulting microstructure in the machined component and the properties of the conditioned surface. Based on discovered correlations, a dynamic process control will be developed for BTA deep hole drilling, which will allow reliably tailoring surface integrity of components to specific demands, like an optimized fatigue performance. © 2020 Walter de Gruyter GmbH, Berlin/Boston 2020.
    view abstract10.1515/teme-2020-0046
  • Influence of powder nitriding on the mechanical behavior of laser-powder bed fusion processed tool steel X30CrMo7-2
    Stern, F. and Grabienski, F. and Walther, F. and Boes, J. and Röttger, A. and Theisen, W.
    Materialpruefung/Materials Testing 62 (2020)
    Additive manufacturing allows for the production of highly complex structures due to its layer-wise local melting of powder material. For this reason, this technique has a high potential for manufacturing extremely lightweight components potential. However, laser based additive manufacturing is still restricted due to the limited amount of processable alloys, especially Fe-based materials. A main object in current research is to expand the varieties for steel that may be used. Additionally, the modification and optimization of steel powder is seen as an interesting aspect for improving the material properties of additively manufactured parts. In this work, secondary hardenable martensitic tool steel X30CrMo7-2 is investigated, starting from the raw powder which is enriched with nitrogen by gas nitriding and subsequently characterized to ensure the usability of the modified powder for laser-powder bed fusion. In a next step, the raw and nitrided powder are used to generate cylindrical specimens to allow for further analysis of the microstructure and for a mechanical characterization of compression behavior. Moreover, a variety of heat treatments is carried out. The higher content of nitrogen leads to an increase in porosity. However, the addition of nitrogen causes an increase in hardness and in the compressive yield point, especially after heat treatment. After tempering, compressive yield stress is increased from 1,111 MPa to 1990 MPa, while for conventional material it is slightly reduced from 1,316 MPa to 1225 MPa. © Carl Hanser Verlag GmbH & Co. KG
    view abstract10.3139/120.111446
  • Influence of specimen position on the build platform on the mechanical properties of as-built direct aged electron beam melted Inconel 718 alloy
    Kotzem, D. and Arold, T. and Niendorf, T. and Walther, F.
    Materials Science and Engineering A 772 (2020)
    Electron powder bed fusion (E-PBF) allows for manufacturing of near-net-shape components of unprecedented complexity. In order to transfer specific benefits into industrial environments, equal properties of any volume built are mandatory. The main objective of this work is to investigate the process-induced material properties in arbitrary build envelope positions. Microstructure, hardness, relative density and fatigue behavior are in focus and thoroughly studied for E-PBF manufactured Inconel 718 alloy. In particular, it can be stated that all specimens have an effective diameter below dimensions set in the initial CAD data. Furthermore, it could be demonstrated that the relative density varies at different positions of the building platform, even for specimens built in direct vicinity. In terms of hardness, specimens at the outer perimeter of the building platform show increased values indicating a higher fraction of strengthening phases. Based on the cyclic tests, a relationship between building platform position and fatigue behavior can be derived. Specimens located in the front of the building platform show superior cyclic properties as compared to specimens in the back of building platform. Notch effects, i.e. process induced topography, are revealed to be the most detrimental influencing factor in the condition tested. © 2019
    view abstract10.1016/j.msea.2019.138785
  • Influence of the Feed Rate in the Single-Lip Deep Hole Drilling Process on the Surface Integrity of Steel Components
    Nickel, J. and Baak, N. and Walther, F. and Biermann, D.
    Lecture Notes in Mechanical Engineering (2020)
    High strength steels like AISI 4140 are commonly used in many technical areas in which the mechanical properties of materials have to meet special requirements, for example, in the case of dynamically loaded parts. In the automotive industry increasing requirements due to lightweight design or energy efficiency lead to increasing demands on the mechanical and dynamic material strength. In response to this development, optimized machining processes are capable of improving the mechanical properties like fatigue performance by influencing the surface integrity of the machined components. In this paper, the influence of the single-lip deep hole drilling process on the surface integrity of quenched and tempered AISI 4140 specimens is analyzed in detail. Under variation of one of the main process parameters, the feed rate, the process output parameters such as cutting forces and the resulting condition of the machined surface and subsurface are determined. In combination with the analysis of the resulting hardness, microstructure and surface conditions of the machined surface, a magnetic Barkhausen noise (MBN) analysis with a custom-built sensor is applied and further developed. With this non-destructive technique, the surface integrity of the bore wall and the fatigue damage over the lifecycle of the part can be analyzed. The correlation of the surface integrity produced by the single-lip deep hole drilling process with the results from the micro-magnetic measurements are used to improve the possibility of predicting a components fatigue performance. © 2020, Springer Nature Singapore Pte Ltd.
    view abstract10.1007/978-981-15-0054-1_21
  • Influence of the process parameters and forces on the bore sub-surface zone in BTA deep-hole drilling of AISI 4140 and AISI 304 L
    Schmidt, R. and Strodick, S. and Walther, F. and Biermann, D. and Zabel, A.
    Procedia CIRP 87 (2020)
    The relationship between the cutting speed, the feed, the resulting process forces during the BTA deep hole-drilling process and the functional properties in the bore sub-surface zone of AISI 4140 and AISI 304 L is analysed. Due to the asymmetric design of the drill head radial forces occur which are supported through guide pads on bore surface. The result is an inner force flow inside the tool that affects a self-guiding effect during the drilling process. Due to this process the bore (sub-)surface zone is impinged with thermal and mechanical loads resulting in hardening, structural changes in microstructure and the occurrence of residual stresses, which can influence the fatigue strength, service life or reliability of the part. Residual stresses are measured using the magnetic Barkhausen noise method. Understanding the relationship between the process forces and functional properties in the bore sub-surface zone is essential for a following process control in order to generate defined bore sub-surface zones. © 2020 The Author(s).
    view abstract10.1016/j.procir.2020.02.010
  • Innovationen zum optimalen Einsatz des wetterfesten Baustahls im Stahl- und Verbundbrückenbau
    Ungermann, D. and Hatke, P. and Knyazeva, M. and Walther, F. and Lebelt, P. and Gehrke, J.
    Stahlbau (2020)
    Innovations for the optimal use of weathering steel in steel and composite bridge construction. The research project “Innovations for the optimal use of weathering steel in steel and composite bridge construction” investigated essential aspects for the optimal use of weathering steel in steel and composite bridge construction. In addition to current investigations on the formation of the corrosion-inhibiting surface layer in the current atmosphere, the qualification of two geometry-independent and non-destructive measuring techniques for crack detection underneath the compact surface layer and their verification on real bridge structures was carried out. Furthermore, tests for the optimization of the slip factor of slip-resistant prestressed connections of weathering steel were carried out. Basically, the use of weathering steel offers both economic and ecological advantages compared to organically coated structural steel by the formation of its firmly adhering and corrosion-inhibiting surface layer, especially with regard to the present service life in bridge construction. This article summarises the main research findings to demonstrate the sustainable use and advantages of weathering steel in steel and composite bridge construction under the conditions of use in the current natural atmosphere. , Ernst und Sohn. All rights reserved.
    view abstract10.1002/stab.202000090
  • Investigation on the influence of damage on the fatigue strength of hot rolled sheet metal
    Liebsch, C. and Möhring, K. and Lohmar, J. and Walther, F. and Hirt, G.
    Production Engineering 14 (2020)
    Continuous casting leads to the formation of voids, which can be considered as an initial state of damage. During hot rolling these voids shall be closed, reducing this initial damage and avoiding its negative influence on the mechanical properties and performance of the produced material. However, hot rolling also influences the microstructure, which in turn affects the performance e.g. the fatigue strength. So far, little research has been published on the separation of those influencing factors. This paper is a first attempt to separate the influence of damage from the microstructure in hot rolling. Numerical simulation is utilized to study void closure throughout the multi-pass process while the evolution of microstructure and damage is investigated experimentally using numerous characterization methods. Two process routes with a large and a small pass reduction have been investigated and comparable microstructures have been achieved. A continuous damage reduction throughout the rolling process has been observed by means of void distribution and density measurements. The large pass reduction showed a slightly reduced damage and an increased fatigue strength in all considered thicknesses, however this could not be traced back to the damage reduction exclusively. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00933-z
  • Laser additive manufacturing of hot work tool steel by means of a starting powder containing partly spherical pure elements and ferroalloys
    Taruttis, A. and Hardes, C. and Röttger, A. and Uhlenwinkel, V. and Chehreh, A.B. and Theisen, W. and Walther, F. and Zoch, H.W.
    Procedia CIRP 94 (2020)
    Until now, additive manufacturing of high-performance materials such as martensitic hardenable tool steels is rarely investigated. This work addresses the introduction of an alternative alloying strategy for hot work tool steel powder, provided for laser powder bed fusion (L-PBF). The focus is on the question whether a powder mixture of spherical iron powder mixed with mechanically crushed ferroalloy particles can be processed by L-PBF, instead of using cost-intensive pre-alloyed gas-atomized powder, and to investigate the material properties associated with it. The particle morphology, packing density and flowability of this L-PBF powder feedstock is compared to gas-atomized spherical pre-alloyed steel powder and the results are correlated to the defect density, the resulting microstructure and the chemical homogeneity. Finally the resulting surface hardness is compared to a conventionally casted material as a reference state. It shows that the L-PBF fabrication of high-dense parts by means of both starting powders is technically feasible. Even though the alternative alloying concept promotes local chemical inhomogeneities within the microstructure, the overall porosity and the appearance of micro cracks are reduced. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.010
  • Load direction-dependent influence of forming-induced initial damage on the fatigue performance of 16MnCrS5 steel
    Moehring, K. and Walther, F.
    Materials 13 (2020)
    Forming processes influence the mechanical properties of manufactured workpieces in general and by means of forming-induced initial damage in particular. The effect of the latter on performance capability is the underlying research aspect for the investigations conducted. In order to address this aspect, fatigue tests under compressive, tensile and compressive-tensile loads were set-up with discrete block-by-block increased amplitudes and constant amplitudes, and performed up to fracture or distinct lifetimes. Aiming at the correlation of the macroscale mechanical testing results at the mesoscale, intensive metallographic investigations of cross-sections using the microscopical methods of secondary electron analysis, energy dispersive spectroscopy and electron backscatter diffraction were performed. Thereby, the correlation of forming-induced initial damage and fatigue performance was determined, the relevance of compressive loads for the cyclic damage evolution was shown, and material anisotropy under compressive loads was indicated. Finally, the need was addressed to perform further investigations regarding crack propagations and crack arrest investigations in order to clarify the mechanism by which initial damage affects cyclic damage evolution. The relevance of the principal stress axis relative to the extrusion direction was emphasized and used as the basis of an argument for investigations under load paths with different stress directions. © 2020 by the authors.
    view abstract10.3390/ma13122680
  • Mechanical properties of thermoplastic-based hybrid laminates with regard to layer structure and metal volume content
    Trautmann, M. and Mrzljak, S. and Walther, F. and Wagner, G.
    Metals 10 (2020)
    Multi layered lightweight material compounds such as hybrid laminates are composed of different layers of materials like metals and unidirectional fibre-reinforced plastics and offer high specific strength. They can be individually tailored for applications like outer cover panels for aircraft and vehicles. Many characteristics especially layer structure, volume contents of the embedded materials as well as layer surface adhesion determine the performance of a hybrid laminate. In this study, the influence of layer structure and metal volume content are evaluated with regard to the mechanical properties of the recyclable hybrid laminate CAPAAL (carbon fibre-reinforced plastics/aluminium foil laminate), which consists of the aluminium alloy AA6082 and a graded structure of glass and carbon fibre-reinforced polyamide 6. Hybrid laminates with different ratios of the fibre-reinforced plastic and numbers of aluminium layers were manufactured by thermal pressing. The consolidation quality of in total four laminate structure variations, including 2/1 and 3/2 metal-to-fibre-reinforced plastic layer structures with fibre orientation variation, were investigated by light microscopy through cross-sections and further on computed tomography. For determination and evaluation of the mechanical properties metrologically instrumented quasi-static tensile and three-point bending tests, as well as tension-tension fatigue tests for the establishment of S-N curves were performed. The results were correlated to the microstructural observations, revealing significant influence by the consolidation quality. The layer structure proved to have a proportional impact on the increase of quasi-static tensile and flexure strength with a decrease in metal volume content. Orienting some of the fibre-reinforced plastic layers in ±45◦ leads to a more evenly distributed fibre alignment, which results in a higher consolidation quality and less anisotropic bending properties. Fatigue results showed a more complex behaviour where not only the metal volume content seems to determine the fatigue loading capability, but also the number of metal-fibre-reinforced plastic interfaces, hinting at the importance of stress distribution between layers and its longevity over fatigue life. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10111430
  • Micro-magnetic damage characterization of bent and cold forged parts
    Samfaß, L. and Baak, N. and Meya, R. and Hering, O. and Tekkaya, A.E. and Walther, F.
    Production Engineering 14 (2020)
    Damage can have a strong impact on the fatigue performance of bulk formed parts for example produced by cold forging and sheet metal formed parts for example produced by bending. One suitable method to detect damage non-destructively in a time-efficient way is the micro-magnetic material characterization. In this paper, the suitability of harmonic analysis of the tangential magnetic field strength for the detection of damage in bent DP800-parts and cold forged 16MnCrS5-parts is discussed. For differently formed parts a correlation between the magnitude of damage and the behavior of the upper harmonics parameters is shown. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00934-y
  • Microstructural damage and fracture mechanisms of selective laser melted Al-Si alloys under fatigue loading
    Awd, M. and Siddique, S. and Walther, F.
    Theoretical and Applied Fracture Mechanics 106 (2020)
    Employment of selective laser melted materials for industrial applications has grown in the recent years owing to improvements in machine systems and scanning techniques to result in near full-density parts; however, there are still limitations when their industrial application, particularly under cyclic loading, is concerned. This study investigates the chain from processing to post-process stress-relief to structural investigations in terms of microstructure and three-dimensional defect analysis by micro-computed tomography, and their corresponding behavior under quasi-static, high-cycle fatigue as well as very high-cycle fatigue for AlSi12 and AlSi10Mg alloys. Microstructure and damage mechanisms have been investigated as a function of in-process and post-process thermal treatments. Their corresponding influence on mechanical behavior revealed that there do exist differences in damage mechanisms in high-cycle fatigue and very high-cycle fatigue where the role of microstructure and small porosity respectively determines the damage mechanism in the two regions. The process parameters determining the required set of material behavior under quasi-static and cyclic loading have been identified. © 2020
    view abstract10.1016/j.tafmec.2020.102483
  • Modeling the residual stresses induced in the metastable austenitic stainless steel disc springs manufactured by incremental sheet forming by a combined hardening model with phase transformation
    Afzal, M.J. and Maqbool, F. and Hajavifard, R. and Buhl, J. and Walther, F. and Bambach, M.
    Procedia Manufacturing 47 (2020)
    A numerical model for predicting the beneficial residual stress induced in metastable austenitic stainless steel (MASS) disc springs through incremental sheet forming (ISF) is developed. An important input for the numerical model is the phase-specific flow curves of the MASS constituents, i.e., austenite γ and martensite α' phase. These curves are determined by using an available CP-FEM framework. The cyclic plasticity effects introduced by the bending/unbending deformation mechanism of the ISF are modeled by using a combined non-linear isotropic/kinematic hardening model. The kinetics of the strain-induced martensite transformation is modeled by using the Olson-Cohen model, whereas the stresses in the whole material are approximated by a mixture rule. The combined non-linear isotropic/kinematic hardening along with Olson-Cohen model and the mixture rule is integrated as a user-material routine (UMAT). The model parameters are calibrated by tensile tests with online Feritscope measurements and tension-compression tests. Afterward, the incremental forming simulations of disc spring manufacturing are performed and the numerically determined residual stress values are compared to the experimental values. The comparison indicates a good match. Hence, the developed numerical strategy provides accurate residual stresses prediction and can be utilized to design the disc spring properties through adjusting residual stresses, which is possible by varying process parameters of the ISF process. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 23rd International Conference on Material Forming.
    view abstract10.1016/j.promfg.2020.04.300
  • Non-destructive characterization of process-induced defects and their effect on the fatigue behavior of austenitic steel 316L made by laser-powder bed fusion
    Stern, F. and Tenkamp, J. and Walther, F.
    Progress in Additive Manufacturing 5 (2020)
    Additive manufacturing (AM) offers a high potential for light weight applications due to its possibility to generate complex structures with a high freedom of design compared to conventional techniques. However, the mechanical characterization of additively manufactured materials has become an essential topic in research over the last years to use AM parts for structural components especially in the automotive and aerospace industry. In the current research, specimens for fatigue tests made of the austenitic stainless steel AISI 316L processed by laser-powder bed fusion technique with three different building orientations were investigated. The gauge sections of the fatigue specimens were scanned by microfocus computed tomography (µ-CT) to detect and identify process-induced defects. The 3D information about the defects gained by µ-CT were compared with the crack-initiating defect on the fractured surfaces. The fatigue data were used for a model-based description of the fatigue strength based on the √area-parameter model. The results depict that the fatigue life is significantly influenced by the orientation of the inherent pores caused by the building direction causing a significant scatter in fatigue life. The µ-CT data allow to estimate the fatigue strength using the √area-parameter model based on the identified critical defects and are in good accordance with the results from the fatigue tests and the data obtained by fractography. Additionally, it could be shown that the √area-parameter model is applicable for additively manufactured 316L steel. It can also give an explanation for the anisotropic fatigue behavior, which supports the assumption that mainly the orientation of the pores and their √area-parameter are influencing the fatigue strength. © 2019, Springer Nature Switzerland AG.
    view abstract10.1007/s40964-019-00105-6
  • Optimization and quality evaluation of the interlayer bonding performance of additively manufactured polymer structures
    Striemann, P. and Hülsbusch, D. and Niedermeier, M. and Walther, F.
    Polymers 12 (2020)
    The application of additive manufacturing changes from prototypes to series production. In order to fulfill all requirements of series production, the process and the material characteristics must be known. The machine operator of additive manufacturing systems is both a component and a material producer. Nevertheless, there is no standardized procedure for the manufacturing or testing of such materials. This includes the high degree of anisotropy of additively manufactured polymers via material extrusion. The interlayer bonding performance between two layers in the manufacturing direction z is the obvious weakness that needs to be improved. By optimizing this interlayer contact zone, the overall performance of the additively manufactured polymer is increased. This was achieved by process modification with an infrared preheating system (IPS) to keep the temperature of the interlayer contact zone above the glass transition temperature during the manufacturing process. Combining destructive and non-destructive testing methods, the process modification IPS was determined and evaluated by a systematic approach for characterizing the interlayer bonding performance. Thereby, tensile tests under quasi-static and cyclic loading were carried out on short carbon flber-reinforced polyamide (SCFRP). In addition, micro-computed tomography and microscopic investigations were used to determine the process quality. The IPS increases the ultimate inter layer tensile strength by approx. 15% and shows at end encyto significantly improved the fatigue properties. Simultaneously, the analysis of the micro-computed tomography data shows a homogenization of the void distribution by using the IPS. © 2020 by the authors.
    view abstract10.3390/POLYM12051166
  • Service life characterization of orthopedic implant material made of ultra-high molecular weight polyethylene under physiological conditions
    Wegner, N. and Scholz, R. and Knyazeva, M. and Walther, F.
    Journal of the Mechanical Behavior of Biomedical Materials 104 (2020)
    view abstract10.1016/j.jmbbm.2020.103617
  • SteBlife, a new approach for the accelerated generation of metallic materials’ fatigue data
    Acosta, R. and Wu, H. and Venkat, R.S. and Weber, F. and Tenkamp, J. and Walther, F. and Starke, P.
    Metals 10 (2020)
    The service life of materials and components exposed to repeated mechanical loads is limited, which is why the understanding of the damage evolution and estimating its fatigue life is of high importance for its technical application. This paper shows how temperature and magnetic field measurement methods can be used to describe the cyclic deformation behaviour of metallic materials and to derive parameters from this, which are used in short–term methods to calculate the fatigue life. Within the SteBLife (stepped–bar fatigue life) approach, only three to five fatigue tests with a stepped fatigue specimen are required to determine a complete S–N or Woehler curve with scatter bands for different failure probabilities. If only a trend S–N curve is required, the number of tests can be reduced to a single fatigue test only. In the framework of this paper, these approaches will be presented for normalised SAE 1045 (C45E) and quenched and tempered SAE 4140 (42CrMo4) steels. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10060798
  • Strain rate-dependent characterization of carbon fibre-reinforced composite laminates using four-point bending tests
    Rojas-Sanchez, J.F. and Schmack, T. and Boesl, B. and Bjekovic, R. and Walther, F.
    Journal of Reinforced Plastics and Composites 39 (2020)
    This research addresses the problem of accurately quantifying the strain rate effect of carbon fibre-reinforced plastics by proposing a method with a simple specimen manufacturing and experiment execution based on four-point bending tests. By easing the strain rate-dependent characterization of carbon fibre-reinforced plastics, less conservative designs can be achieved. The method proposed uses Euler–Bernoulli and Timoshenko’s beam theories to obtain the longitudinal compressive and tensile modulus, compressive strength, shear modulus, and shear yielding point. Transverse properties could not be obtained due to limitations of the fixture employed. A strain-dependent material characterization was done using the proposed method and compared to the characterization of the same material using traditional uniaxial tests. Most of the material properties obtained with different methods correlated within approximately 10%. More work needs to be done to determine how this discrepancy affects simulation results. © The Author(s) 2019.
    view abstract10.1177/0731684419879235
  • Systematic approach for the characterization of additive manufactured and injection molded short carbon fiber-reinforced polymers under tensile loading
    Striemann, P. and Huelsbusch, D. and Mrzljak, S. and Niedermeier, M. and Walther, F.
    Materialpruefung/Materials Testing 62 (2020)
    Material extrusion-based additive manufacturing techniques such as fused deposition modeling or fused filament fabrication are developing from prototyping applications to serial components. The aim of this study is to properly characterize an additively manufactured polymer with the corresponding process-induced defects. To this effect, varied manufacturing orientations of fused filament fabrication were tested with a single-batch material manufactured by injection molding serving as a reference. Scans were carried out via micro-computed tomography to assess the void content and distribution with respect to quality. Local material performance was investigated via quasi-static and cyclic tests under tensile loading. The quasi-static tensile tests indicated a significant reduction of Young's modulus, tensile strength, and strain at fracture for the additively manufactured polymer. The mechanical investigations with cyclic loading intensified this trend of clear reduced mechanical properties due to process-induced defects. The quality assessment revealed void volume contents of the additively manufactured polymer of up to 6.5 % and a void distribution dependent on manufacturing orientation. The results of this study are valuable as design guidelines for highly stressed components and serve as a basis for further characterizations of process-induced defects. © Carl Hanser Verlag, München Materials Testing
    view abstract10.3139/120.111517
  • Torsion plastometer trials to investigate the effect of non-proportional loading paths in caliber rolling on damage and performance of metal parts
    Wang, S. and Dunlap, A. and Möhring, K. and Lohmar, J. and Schwedt, A. and Aretz, A. and Walther, F. and Hirt, G.
    Production Engineering 14 (2020)
    The non-proportional loading path describes a strain-dependent development of stress triaxiality and Lode parameter during metal forming processes. Existing studies suggest a strong dependence of damage evolution on the non-proportional loading path. This work focuses on investigating the influence of non-proportional loading paths observed in hot caliber rolling of the case-hardening steel 16MnCrS5 using laboratory scale experiments. The applied torsion plastometer is highly flexible as it can apply combined loading types (tension, compression and torsion) on notched round specimen and enables deformation at elevated temperature. In this study, two characteristic non-proportional loading paths in caliber rolling and the maximal achievable non-proportional loading path variation were recreated in the torsion plastometer based on both FE simulations and experiments. After deformation, the specimen were further analyzed using Scanning Electron Microscopy (SEM) to quantify the damage. The results indicate an influence of the non-proportional loading path on damage evolution. Furthermore, fatigue tests were employed to characterize the fatigue performance of the deformed specimens. In the torsion plastometer trials carried out no clear correlation of performance and damage was observed. This is most likely due to differences in residual dislocation density after static recrystallization and deviations in the microstructure after hot working. Thus, the superposition of microstructure evolution and damage needs to be considered carefully when testing at elevated temperature. © 2020, The Author(s).
    view abstract10.1007/s11740-019-00949-5
  • Towards deterministic computation of internal stresses in additively manufactured materials under fatigue loading: Part I
    Awd, M. and Labanie, M.F. and Moehring, K. and Fatemi, A. and Walther, F.
    Materials 13 (2020)
    The ongoing studies of the influence of internal defects on fatigue strength of additively manufactured metals adopted an internal crack or notch-like model at which the threshold stress intensity factor is the driving mechanism of fatigue failure. The current article highlights a shortcoming of this approach and offers an alternative based on X-ray microcomputed tomography and cyclic plasticity with a hybrid formulation of Chaboche and Armstrong-Frederick material laws. The presented tessellation and geometrical transformation scheme enabled a significantly more realistic morphological representation of internal defects that yielded a cyclic strain within 2% of the experimental values. This means that cyclic plasticity models have an accurate prediction of mechanical properties without repeating a full set of experiments for additively manufactured arbitrary microstructures. The coupling with a material law that is oriented towards the treatment of cyclic hardening and softening enabled more accurate computation of internal stresses under cyclic loading than ever before owing to the maturity of tessellation and numerical tools since then. The resulting stress-strain distributions were used as input to the Fatemi-Socie damage model, based on which a successful calculation of fatigue lifetime became possible. Furthermore, acting stresses on the internal pores were shown to be more than 450% concerning the applied remote stress amplitude. The results are a pretext to a scale bridging numerical solution that accounts for the short crack formation stage based on microstructural damage. © 2020 by the authors.
    view abstract10.3390/ma13102318
  • Advanced characterization of the cyclic deformation and damage behavior of al-si-mg cast alloys using hysteresis analysis and alternating current potential drop method
    Tenkamp, J. and Bleicher, K. and Klute, S. and Chrzan, K. and Koch, A. and Walther, F.
    Minerals, Metals and Materials Series (2019)
    The demand for lightweight and fatigue-resistant solutions in the automotive and aerospace industries requires the extensive use of light metals like Al-Si-Mg aluminum cast alloys. To ensure a safe design for fatigue-loaded aluminum cast components, the relation between microstructure characteristics of α-Al dendrites (e.g. dendrite arm spacing, microhardness) and interdendritic Al-Si eutectic (e.g. Si eutectic morphology), stress-strain behavior and fatigue lifetime including the number of cycles to crack initiation and crack propagation behavior has to be understood. For this purpose, fatigue tests in the LCF and HCF regime from 1E2 to 1E7 cycles were performed for aluminum cast alloy EN AC-AlSi7Mg0.3. With this, the effect of α-Al microhardness and Si eutectic morphology was investigated on monotonic and cyclic stress-strain behavior by performing tensile, incremental step (IST) and constant amplitude tests (CAT) in the LCF regime. Moreover, microstructure-specific S-N curves were determined in the HCF regime until 1E7 using online hysteresis analysis and alternating current potential drop (ACPD) method for monitoring the deformation and damage evolution. Hereby, computed tomography (CT) analyses were used to evaluate the damage state intermittent and post-mortem in CAT. As a result, ACPD method could be validated as a sensitive and reliable technique for quantitative characterization of stress- and cycle-dependent deformation and damage behavior including cyclic hardening, softening and saturation as well as crack initiation and propagation until failure. © 2019, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-05864-7_23
  • Balancing purification and ultrastructure of naturally derived bone blocks for bone regeneration: Report of the purification effort of two bone blocks
    Barbeck, M. and Jung, O. and Xiong, X. and Krastev, R. and Korzinskas, T. and Najman, S. and Radenkovic, M. and Wegner, N. and Knyazeva, M. and Walther, F.
    Materials 12 (2019)
    The present publication reports the purification effort of two natural bone blocks, that is, an allogeneic bone block (maxgraft®, botiss biomaterials GmbH, Zossen, Germany) and a xenogeneic block (SMARTBONE®, IBI S.A., Mezzovico-Vira, Switzerland) in addition to previously published results based on histology. Furthermore, specialized scanning electron microscopy (SEM) and in vitro analyses (XTT, BrdU, LDH) for testing of the cytocompatibility based on ISO 10993-5/-12 have been conducted. The microscopic analyses showed that both bone blocks possess a trabecular structure with a lamellar subarrangement. In the case of the xenogeneic bone block, only minor remnants of collagenous structures were found, while in contrast high amounts of collagen were found associated with the allogeneic bone matrix. Furthermore, only island-like remnants of the polymer coating in case of the xenogeneic bone substitute seemed to be detectable. Finally, no remaining cells or cellular remnants were found in both bone blocks. The in vitro analyses showed that both bone blocks are biocompatible. Altogether, the purification level of both bone blocks seems to be favorable for bone tissue regeneration without the risk for inflammatory responses or graft rejection. Moreover, the analysis of the maxgraft® bone block showed that the underlying purification process allows for preserving not only the calcified bone matrix but also high amounts of the intertrabecular collagen matrix. © 2019 by the authors.
    view abstract10.3390/ma12193234
  • Barkhausen noise-based fatigue life prediction of deep drilled AISI 4140
    Baak, N. and Nickel, J. and Biermann, D. and Walther, F.
    Procedia Structural Integrity 18 (2019)
    High-strength steels like AISI 4140 are commonly used for high dynamically loaded parts. Increasing demands for lightweight parts with higher performance and efficiency in automotive industry claim improving material properties. An optimised drilling process should enhance the fatigue life of deep-drilled components by the induction of residual stresses in the borehole, without the need for expensive additional processing steps, e.g. autofrettage. Non-destructive testing techniques like magnetic Barkhausen noise analysis offer quick and reliable possibilities to detect and classify material parameters like hardness and residual stresses. The aim of this study is to evaluate and extrapolate the resulting fatigue performance of deep drilled round specimens due to drilling parameters at an early stage of fatigue life. It was shown, that the coercive field strength decreases approximately linear with proceeding fatigue damage. The slope of the degradation coefficient is comparable for different surface layer conditions. This leads to the assumption that the variation of the micromagnetic parameters is caused by fatigue-induced microstructural changes. Therefore, a microstructure-based prediction of the fatigue life by means of micromagnetic measurements can be established. © 2019 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2019.08.164
  • Characterization of the fatigue and damage behavior of extruded AW6060 aluminum chip profiles
    Koch, A. and Wittke, P. and Walther, F.
    Structural Integrity 7 (2019)
    Due to a great potential for conserving resources, the direct recycling of aluminum chips by hot extrusion is a promising alternative to energy-intensive remelting process. The mechanical properties of cast-based and chip-based specimens of AW6060 aluminum alloy were characterized by means of mechanical quasi-static and cyclic experiments. The response of the material was followed by means of hysteresis, thermometric and resistometric measurements, whereby the fatigue strength could be estimated by alternating current potential drop-technique. The effects deriving from defects in the microstructure, in the form of cavities and seam welds between the chips, could be correlated with the fatigue properties by means of optical and scanning electron micrographs. © Springer Nature Switzerland AG 2019.
    view abstract10.1007/978-3-030-13980-3_2
  • Characterization of the fatigue behavior of mechanical and thermal aged austenitic power plant steel AISI 347
    Maci, F. and Jamrozy, M. and Acosta, R. and Starke, P. and Boller, C. and Heckmann, K. and Sievers, J. and Schopf, T. and Walther, F.
    Structural Integrity 7 (2019)
    For the comprehensive understanding of the fatigue mechanisms of mechanical and thermally aged materials in power plant reactors, an innovative, resource- and time-optimized approach based on non-destructive testing methods is used. Beside investigations on AISI 347 austenitic stainless steel under cyclic loading, magnetic, resistometric and electrochemical measurement techniques were applied to monitor the proceeding fatigue behavior. Qualitative values indicate surface passivation effects and microstructural changes, which are directly related to fatigue states. In total strain-controlled increase tests, cyclic investigations for the initial and a mechanically and thermal aged condition were carried out under distilled water environment at ambient temperature. In comparison to the initial state, the aging process shows a significant influence on the fatigue behavior with a reduction of the stress amplitude at failure down to 75%. © Springer Nature Switzerland AG 2019.
    view abstract10.1007/978-3-030-13980-3_9
  • Comparative study of defect-based and plastic damage-based approaches for fatigue lifetime calculation of selective laser-melted AlSi12
    Awd, M. and Siddique, S. and Hajavifard, R. and Walther, F.
    Lecture Notes in Mechanical Engineering (2019)
    Selective laser melting is an additive layer manufacturing process based on powder bed fusion using high-energy laser beams. Process features constitute very finely grained cellular and columnar dendritic microstructure in aluminum alloy AlSi12. Melt pool instabilities produce porosity of the keyhole and metallurgical types. Both structural characteristics are investigated in this study using crack propagation-based and plastic damage-based approaches. The aim is to separate influences of microstructure and defects on the fatigue lifetime concerning the fatigue regime of interest. Process-induced defects for two batches of selective laser-melted AlSi12 were investigated using micro-computed tomography. The influence of platform heating during deposition is a subject of study concerning fatigue strength. The results showed that platform heating reduces the amount of remnant porosity and fatigue strength scatter. Fatigue lifetime calculation, based on crack propagation curves and weakest-link theory, was found consistent with low-cycle fatigue experiments. However, prediction of lifetimes using damage monitoring in a load increase test, as well as a MonteCarlo simulation, produced more relevant results in the low-cycle to high-cycle fatigue range. The difference was consideration of damage in the pre-crack initiation phase in the plasticity-based approach of the load increase test. © Springer Nature Singapore Pte Ltd. 2019.
    view abstract10.1007/978-981-13-0411-8_27
  • Computed tomography-based characterization of the fatigue behavior and damage development of extruded profiles made from recycled AW6060 aluminum chips
    Koch, A. and Wittke, P. and Walther, F.
    Materials 12 (2019)
    The possibility of producing profiles directly by hot extrusion of aluminum chips, normally considered as scrap, is a promising alternative to the energy-intensive remelting process. It has to be taken into account that the mechanical properties depend on the quality of the weld seams between the chips, which arise during the extrusion process. To estimate the influence of the weld seams, quasistatic and cyclic investigations were performed on chip-based profiles and finally compared with cast-based extruded profiles. In order to gain comprehensive information about the fatigue progress, different measurement techniques like alternating current potential drop (ACPD)-technique, hysteresis measurements, and temperature measurements were used during the fatigue tests. The weld seams and voids were investigated using computed tomography and metallographic techniques. Results show that quasistatic properties of chip-based specimens are only reduced by about 5%, whereas the lifetime is reduced by about a decade. The development of the fatigue cracks, which propagate between the chip boundaries, was characterized by an intermittent testing strategy, where an initiation of two separate cracks was observed. © 2019 by the authors.
    view abstract10.3390/ma12152372
  • Corrosion and corrosion fatigue properties of additively manufactured magnesium alloy WE43 in comparison to titanium alloy Ti-6Al-4V in physiological environment
    Wegner, N. and Kotzem, D. and Wessarges, Y. and Emminghaus, N. and Hoff, C. and Tenkamp, J. and Hermsdorf, J. and Overmeyer, L. and Walther, F.
    Materials 12 (2019)
    Laser powder bed fusion (L-PBF) of metals enables the manufacturing of highly complex geometries which opens new application fields in the medical sector, especially with regard to personalized implants. In comparison to conventional manufacturing techniques, L-PBF causes different microstructures, and thus, new challenges arise. The main objective of this work is to investigate the influence of different manufacturing parameters of the L-PBF process on the microstructure, process-induced porosity, as well as corrosion fatigue properties of the magnesium alloy WE43 and as a reference on the titanium alloy Ti-6Al-4V. In particular, the investigated magnesium alloy WE43 showed a strong process parameter dependence in terms of porosity (size and distribution), microstructure, corrosion rates, and corrosion fatigue properties. Cyclic tests with increased test duration caused an especially high decrease in fatigue strength for magnesium alloy WE43. It can be demonstrated that, due to high process-induced surface roughness, which supports locally intensified corrosion, multiple crack initiation sites are present, which is one of the main reasons for the drastic decrease in fatigue strength. © 2019 by the authors.
    view abstract10.3390/ma12182892
  • Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy
    Biswal, R. and Zhang, X. and Syed, A.K. and Awd, M. and Ding, J. and Walther, F. and Williams, S.
    International Journal of Fatigue 122 (2019)
    This study was aimed at investigating the effect of internal porosity on the fatigue strength of wire + arc additive manufactured titanium alloy (WAAM Ti-6Al-4V). Unlike similar titanium alloys built by the powder bed fusion processes, WAAM Ti-6Al-4V seldom contains gas pores. However, feedstock may get contaminated that may cause pores of considerable size in the built materials. Two types of specimens were tested: (1) control group without porosity referred to as reference specimens; (2) designed porosity group using contaminated wires to build the specimen gauge section, referred to as porosity specimens. Test results have shown that static strength of the two groups was comparable, but the elongation in porosity group was reduced by 60% and its fatigue strength was 33% lower than the control group. The stress intensity factor range of the crack initiating pore calculated by Murakami's approach has provided good correlation with the fatigue life. The kink point on the data fitting curve corresponds well with the threshold value of the stress intensity factor range found in the literature. For predicting the fatigue limit, a modified Kitagawa-Takahashi diagram was proposed consisting of three regions depending on porosity size. Critical pore diameter was found to be about 100 µm. © 2019 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2019.01.017
  • Engineering the residual stress state of the metastable austenitic stainless steel (MASS) disc springs by incremental sheet forming (ISF)
    Maqbool, F. and Hajavifard, R. and Walther, F. and Bambach, M.
    Production Engineering 13 (2019)
    The paper presents a novel methodology to form disc springs made of metastable austenitic stainless steel using incremental sheet forming (ISF), which is also used to generate compressive residual stresses. The research aims at replacing the shot peening process which has various disadvantages such as a change in disc spring geometry and replacing it with a method that allows a better control of spring properties. Two different methodologies were developed. Firstly, ISF was used to selectively induce the residual stresses in conventionally formed disc springs. Secondly, ISF was used to form the disc spring and to induce the residual stresses during the actual forming process. Residual stresses were measured using bore-hole-drilling. For both methodologies, ISF induces higher compressive residual stresses in the disc spring and a higher spring force in comparison to conventional disc springs. The changes in the spring properties is due to the formation of deformation-induced martensite, which is characterized by using metallurgical investigations and disc compression test. Hence, the strategies developed using ISF can be employed as an alternative forming process for disc springs with integrated surface treatment. © 2018, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-018-0864-6
  • Experimental characterization of the structural deformation of type IV pressure vessels subjected to internal pressure
    Nebe, M. and Maraite, D. and Braun, C. and Hülsbusch, D. and Walther, F.
    Key Engineering Materials 809 KEM (2019)
    The investigations deal with the experimental characterization of the structural deformation of type IV pressure vessels subjected to internal pressure. For the widespread use of hydrogen technology in transport industries, the development of cost-effective storage systems is a crucial step. State of the art in the field of hydrogen storage are type IV pressure vessels, which consist of a polymeric liner and an enforcing winding of carbon fiber-reinforced plastic (CFRP). For the development of material-optimized and high-safety pressure vessels, the acquisition of reliable experimental data in order to validate numerical simulations is a necessity. In a specially designed test chamber subscale vessels are clamped and subjected to internal pressure. At defined pressure stages the vessel’s deformation is recorded and analyzed. Consequently, the overall structural deformation is assessed with regard to the used structural mass, the burst pressure and the resulting failure. The results can be used for structure optimization purposes as well as for the optimization of numerical simulation models. © 2019 Trans Tech Publications Ltd, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.809.47
  • Experimental investigation and finite element modelling of residual stress control in disc springs made of metastable austenitic stainless steel (MASS) using incremental sheet forming (ISF)
    Maqbool, F. and Hajavifard, R. and Walther, F. and Bambach, M.
    Procedia Manufacturing 29 (2019)
    The current study presents a novel methodology to generate compressive residual stresses in disc springs made of metastable austenitic stainless steel by using incremental sheet forming. Conventional disc spring manufacturing uses a forming operation and a subsequent shot peening treatment to generate residual stresses. Shot peening changes the shape of the springs and the spring characteristics. In this contribution, incremental sheet forming is used to form the disc springs and to control the residual stress in a single set-up. As an alternative, pre-formed disc springs are treated using incremental sheet forming to induce the desired residual stresses. The changes in the spring properties are due to the formation of deformation-induced martensite, which is characterized using metallographic investigations and disc compression tests. A significant enhancement of the spring properties is observed. Further, a numerical model considering the transformation induced plasticity effect is used to simulate the process. The process parameters of incremental sheet forming, i.e., tool diameter and tool step-down, are varied to characterize the influence on the spring properties. The numerical results are compared to the experimental observations. It is found that the proposed forming process can be used for controlling the shape and residual stresses in disc springs. © 2019 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.promfg.2019.02.099
  • Experimental study on the actuation and fatigue behavior of the biopolymeric material Cottonid
    Scholz, R. and Langhansl, M. and Zollfrank, C. and Walther, F.
    Materials Today: Proceedings 7 (2019)
    Saving and reducing the consumption of electrical energy is one of the major future challenges for industry and society. It would be desirable, if an actuator autonomously responds to an environmental stimulus like humidity, temperature or light. In this study, the actuation and fatigue behavior of the macromolecular cellulose-based material Cottonid is characterized. It is hygroscopic and possesses process-related anisotropic mechanical properties, which makes it an efficient adaptive material for humidity-driven actuators. A quantitative and qualitative evaluation of the passive movements of Cottonid-based bilayer structures in reaction to humidity absorption and desorption was performed concerning parameters like angle of deflection and saturation. To assess direction-dependent fatigue performance, specimens were prepared in 0° and 90° according to cellulose micro fibril orientation and cyclically loaded in load increase tests. 0° specimens reached highest stresses at failure whereas 90° specimens showed a pronounced loss of stiffness during the tests. Differences in damage development due to alternating micro fibril orientations could be visualized via computed tomography, which leads to a profound understanding of biomechanics. © 2018 Elsevier Ltd.
    view abstract10.1016/j.matpr.2018.11.112
  • Fatigue and corrosion fatigue behaviour of brazed stainless steel joints AISI 304L/BAu-4 in synthetic exhaust gas condensate
    Schmiedt-Kalenborn, A. and Lingnau, L.A. and Manka, M. and Tillmann, W. and Walther, F.
    Materials 12 (2019)
    As brazed stainless steel components in service often have to withstand cyclic loads in corrosive environments, the corrosion fatigue properties of brazed joints have to be characterised. Application-relevant corrosion fatigue tests in corrosive media are extremely rare for brazed joints and cyclic deformation curves are barely investigated. In this study, fatigue tests of brazed AISI 304L/BAu-4 joints were performed in air and synthetic exhaust gas condensate K2.2 according to VDA 230-214. The fatigue behaviour of the brazed joints was compared to properties of the austenitic base material. Strain, electrical, magnetic, temperature and electrochemical measurement techniques were applied within fatigue and corrosion fatigue tests to characterise the cyclic deformation and damage behaviour of the brazed joints. It was found that the fatigue strength of 397 MPa at 2 × 106 cycles was reduced down to 51% due to the superimposed corrosive loading. Divergent microstructure-related damage mechanisms were identified for corrosion fatigue loadings and fatigue loadings of specimens in the as-received and pre-corroded conditions. The investigations demonstrate the important role of corrosive environments for the mechanical performance of brazed stainless steel joints. © 2019 by the authors.
    view abstract10.3390/ma12071040
  • Fatigue of glued-in rods in engineered hardwood products — part I: experimental results
    Myslicki, S. and Bletz-Mühldorfer, O. and Diehl, F. and Lavarec, C. and Vallée, T. and Scholz, R. and Walther, F.
    Journal of Adhesion 95 (2019)
    Glued-in Rods (GiR) are among the high-performance joining technologies used in timber engineering. Engineering design procedures for GiR almost exclusively regulate softwood and softwood engineered wood products (EWP) under quasi-static loads. Since the use of hardwood is expected to significantly increase due to climate change, and potentially establish itself as predominant in Europe`s forests, GiR in combination with hardwood and corresponding EWP need to be investigated. Timber constructions subjected to cyclic loads are increasingly being used, as for example in timber bridges, high rise buildings, and wind turbine towers. No normative regulation is yet available for fatigue of GiR, which is for most part due to incomplete or completely missing experimental data. This paper is the first of a two part series; it presents fatigue investigations on GiR in combination with hardwood EWP with different types of rods, wood species and adhesives. Further, the embedment length and stress ratio were varried. In total, more than 70 cyclic tests were performed resulting in a high number of SN-curves showing the fatigue characteristics of GiR. The results show that two different damage mechanisms exist: in low cycle fatigue range (LCF), timber and adhesive fracture become dominant, while rod failure is the limiting factor in high cycle fatigue range (HCF). Based upon the documented experimental findings, existing standards for cyclic load on softwood connection technologies were discussed. The compagnion paper will present a design methodology based on a wood damage accumulation, giving a complete picture combined with existing metal fatigue models. © 2019, © 2019 Taylor & Francis Group, LLC.
    view abstract10.1080/00218464.2018.1555477
  • Fatigue of glued-in rods in engineered hardwood products — Part II: Numerical modelling
    Myslicki, S. and Walther, F. and Bletz-Mühldorfer, O. and Diehl, F. and Lavarec, C. and Beber, V.C. and Vallée, T.
    Journal of Adhesion 95 (2019)
    Fatigue behaviour of materials is traditionally performed on probes that ensure that stresses (or strains) are very uniform. This is a situation seldom encountered in most adhesively bonded joints, where stresses usually peak at the end of the overlaps. This paper presents a relatively simple model to predict the fatigue behaviour of glued-in rods (GiR) involving hardwood in previously investigated in an extensive experimental campaign. The model is based on strength and stiffness degradation of the components of the GiR, which were experimentally estimated in small scale tests. Based thereupon, Finite Element Analysis (FEA) was used to estimate the effect of material degradation on the residual strength of the GiR, which was then interpreted as S-N-curves. The influence of several parameters, e.g. strength and stiffness degradation rates and the magnitude of the residual strength threshold were numerically investigated. The result showed that it is possible, using a practitioner adapted numerical model, to predict the fatigue behaviour of GiR, based upon comparatively simple fatigue characterisation on their components. © 2019, © 2019 Taylor & Francis Group, LLC.
    view abstract10.1080/00218464.2018.1555478
  • Improved in vitro test procedure for full assessment of the cytocompatibility of degradable magnesium based on ISO 10993-5/-12
    Jung, O. and Smeets, R. and Hartjen, P. and Schnettler, R. and Feyerabend, F. and Klein, M. and Wegner, N. and Walther, F. and Stangier, D. and Henningsen, A. and Rendenbach, C. and Heiland, M. and Barbeck, M. and Kopp, A.
    International Journal of Molecular Sciences 20 (2019)
    Magnesium (Mg)-based biomaterials are promising candidates for bone and tissue regeneration. Alloying and surface modifications provide effective strategies for optimizing and tailoring their degradation kinetics. Nevertheless, biocompatibility analyses of Mg-based materials are challenging due to its special degradation mechanism with continuous hydrogen release. In this context, the hydrogen release and the related (micro-) milieu conditions pretend to strictly follow in vitro standards based on ISO 10993-5/-12. Thus, special adaptions for the testing of Mg materials are necessary, which have been described in a previous study from our group. Based on these adaptions, further developments of a test procedure allowing rapid and effective in vitro cytocompatibility analyses of Mg-based materials based on ISO 10993-5/-12 are necessary. The following study introduces a new two-step test scheme for rapid and effective testing of Mg. Specimens with different surface characteristics were produced by means of plasma electrolytic oxidation (PEO) using silicate-based and phosphate-based electrolytes. The test samples were evaluated for corrosion behavior, cytocompatibility and their mechanical and osteogenic properties. Thereby, two PEO ceramics could be identified for further in vivo evaluations. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ijms20020255
  • In situ computed tomography for the characterization of the fatigue damage development in glass fiber-reinforced polyurethane
    Hülsbusch, D. and Mrzljak, S. and Walther, F.
    Materialpruefung/Materials Testing 61 (2019)
    Fiber-reinforced polymers show a continuous material degradation under cyclic loading, which is why damage development has to be investigated for an exact assessment of fatigue properties. In order to obtain information on damage in the internal volume, conventional mechanical test methods require accompanying support by further developed techniques. In this study, a methodology for in situ computed tomography has been developed and applied to glass fiber-reinforced polyurethane. Polyurethane has advantages over epoxy in terms of impact strength, damage tolerance and abrasion, which are important for various applications. Fatigue properties, on the other hand, are largely unknown. Optimized imaging parameters for computed tomography have been established in order to obtain detailed 3D volume images suitable for analysis. The 3D volumes of the damage state were recorded according to defined fatigue load steps and used to evaluate and correlate the damage development with the mechanical properties. The results confirm known damage characteristics of fiber-reinforced composites but also show material and structure-related differences in crack formation and propagation. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/120.111389
  • In vivo simulation of magnesium degradability using a new fluid dynamic bench testing approach
    Jung, O. and Porchetta, D. and Schroeder, M.-L. and Klein, M. and Wegner, N. and Walther, F. and Feyerabend, F. and Barbeck, M. and Kopp, A.
    International Journal of Molecular Sciences 20 (2019)
    The degradation rate of magnesium (Mg) alloys is a key parameter to develop Mg-based biomaterials and ensure in vivo-mechanical stability as well as to minimize hydrogen gas production, which otherwise can lead to adverse effects in clinical applications. However, in vitro and in vivo results of the same material often differ largely. In the present study, a dynamic test bench with several single bioreactor cells was constructed to measure the volume of hydrogen gas which evolves during magnesium degradation to indicate the degradation rate in vivo. Degradation medium comparable with human blood plasma was used to simulate body fluids. The media was pumped through the different bioreactor cells under a constant flow rate and 37?C to simulate physiological conditions. A total of three different Mg groups were successively tested: Mg WE43, and two different WE43 plasma electrolytically oxidized (PEO) variants. The results were compared with other methods to detect magnesium degradation (pH, potentiodynamic polarization (PDP), cytocompatibility, SEM (scanning electron microscopy)). The non-ceramized specimens showed the highest degradation rates and vast standard deviations. In contrast, the two PEO samples demonstrated reduced degradation rates with diminished standard deviation. The pH values showed above-average constant levels between 7.4-7.7, likely due to the constant exchange of the fluids. SEM revealed severe cracks on the surface of WE43 after degradation, whereas the ceramized surfaces showed significantly decreased signs of corrosion. PDP results confirmed the improved corrosion resistance of both PEO samples. While WE43 showed slight toxicity in vitro, satisfactory cytocompatibility was achieved for the PEO test samples. In summary, the dynamic test bench constructed in this study enables reliable and simple measurement of Mg degradation to simulate the in vivo environment. Furthermore, PEO treatment of magnesium is a promising method to adjust magnesium degradation. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ijms20194859
  • Influence of process parameters, surface topography and corrosion condition on the fatigue behavior of steel/aluminum hybrid joints produced by magnetic pulse welding
    Mrzljak, S. and Gelinski, N. and Hülsbusch, D. and Schumacher, E. and Boehm, S. and Walther, F.
    Key Engineering Materials 809 KEM (2019)
    In this study, magnetic pulse welded steel/aluminum hybrid joints are investigated with the aim of optimizing the process parameters regarding the fatigue behavior. Changes in discharge current, acceleration distance, welding geometry as well as influences of surface topography and corrosion, are examined regarding fatigue life and damage mechanisms. Instrumented multiple amplitude tests combined with constant amplitude tests are carried out for assessing structure-property-relations in a resource-efficient manner. Stress-induced change in strain and alternating current potential drop measurement are well suited for reliable detection of damage initiation and estimation of the fatigue limit. Results reveal that the fatigue properties primarily depend on the imperfections of the weld seam, which are affected mostly by the discharge current and the surface topography. Corrosion shows to be a relevant factor since it decreases fatigue performance. Suitable process parameters are achieved when the fatigue strength of the weld seam lies above the weaker hybrid joint (aluminum). For S235JR and EN AW-1050A-H14 (Al99.5) a suitable discharge current was found to be 349 kA at an acceleration distance of 1.5 mm. © 2019 Trans Tech Publications Ltd, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.809.197
  • Integrated forming and surface engineering of disc springs by inducing residual stresses by incremental sheet forming
    Hajavifard, R. and Maqbool, F. and Schmiedt-Kalenborn, A. and Buhl, J. and Bambach, M. and Walther, F.
    Materials 12 (2019)
    Disc springs are conical annular discs, which are characterized by a high spring force with a small spring travel and good space utilization. In operation, they must meet high demands on the stability of the spring characteristic and the fatigue strength. Under loading, tensile stresses occur which limit the possible applications of disc springs. Compressive stresses can be generated in the stressed areas by means of shot-peening in order to extend the operating limits for a given yield and fatigue strength. Since the spring geometry and characteristics change during shot-peening, the design of the shot-peening treatment is iterative and cumbersome. The present research proposes an incremental forming process for forming and integrated targeted adjustment of residual stresses in disc springs from metastable austenitic stainless steel (MASS), to achieve improved spring properties and high cyclic strength. The main mechanism of residual stress generation is the transformation of metastable austenite into martensite under the action of the forming tool. Different experimental characterization techniques like the hole drilling method, X-ray diffraction, disc compression tests, optical microscopy and cyclic tests are used to correlate the residual stresses and disc spring properties. A numerical model is developed for simulating the martensite transformation in disc springs manufacturing. The results prove that incremental forming enables process-integrated engineering of the desired compressive residual stresses, entailing a higher spring force of metastable austenitic disc springs in comparison to conventional disc springs. Due to martensite formation, the generated residual stresses are stable under cyclic loading, which is not the case for conventionally manufactured springs. © 2019 by the authors.
    view abstract10.3390/ma12101646
  • Interrupted fatigue testing with periodic tomography to monitor porosity defects in wire + arc additive manufactured Ti-6Al-4V
    Biswal, R. and Zhang, X. and Shamir, M. and Al Mamun, A. and Awd, M. and Walther, F. and Khadar Syed, A.
    Additive Manufacturing 28 (2019)
    Porosity defects remain a challenge to the structural integrity of additive manufactured materials, particularly for parts under fatigue loading applications. Although the wire + arc additive manufactured Ti-6Al-4 V builds are typically fully dense, occurrences of isolated pores may not be completely avoided due to feedstock contamination. This study used contaminated wires to build the gauge section of fatigue specimens to purposely introduce spherical gas pores in the size range of 120–250 micrometres. Changes in the defect morphology were monitored via interrupted fatigue testing with periodic X-ray computed tomography (CT) scanning. Prior to specimen failure, the near surface pores grew by approximately a factor of 2 and tortuous fatigue cracks were initiated and propagated towards the nearest free surface. Elastic-plastic finite element analysis showed cyclic plastic deformation at the pore root as a result of stress concentration; consequently for an applied tension-tension cyclic stress (stress ratio 0.1), the local stress at the pore root became a tension-compression nature (local stress ratio −1.0). Fatigue life was predicted using the notch fatigue approach and validated with experimental test results. © 2019 The Authors
    view abstract10.1016/j.addma.2019.04.026
  • Investigation of the anisotropic cyclic damage behavior of selective laser melted AISI 316L stainless steel
    Stern, F. and Kleinhorst, J. and Tenkamp, J. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 42 (2019)
    view abstract10.1111/ffe.13029
  • Investigations of the corrosion damage process of the brazed joint AISI 304L/BNi-2
    Tillmann, W. and Walther, F. and Manka, M. and Schmiedt, A. and Wojarski, L. and Eilers, A. and Reker, D.W.
    Results in Physics 12 (2019)
    One of the most frequently used material combinations to produce heat exchangers for automotive exhaust systems are vacuum brazed AISI 304L/BNi-2 components. In order to simulate the influence of condensate corrosion on such parts during service, the well-established test procedure for testing the resistance of metallic materials to condensate corrosion in exhaust gas, VDA 230-214, is used in this study. For a test duration of up to 6 weeks, miscellaneous corrosive mechanisms were observed and examined concerning their formation and progression. The corrosive attack starts at the diffusion zone on the base metal side due to the formation of chromium borides and the reduced nickel content. Between week 4 and 5, the greatest material removal was observed, which goes in line with a diameter reduction from 6.5 mm down to 5.8 mm of the load-bearing area. In this regard, the ultimate tensile strength drops down from 253.3 MPa to 147.8 MPa after 6 weeks of corrosive testing. © 2019 The Authors
    view abstract10.1016/j.rinp.2018.12.074
  • Local quasi-static and cyclic deformation behaviour of brazed AISI 304L/BAu-4 joints characterised by digital image correlation
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Welding in the World 63 (2019)
    For a reliable design of brazed components, the degradation of mechanical properties due to the corrosive attack by aggressive operating environments has to be considered. In this study, the effect of a condensate corrosion, which is performed according to VDA test sheet 230-214 up to 6 weeks, on the mechanical behaviour of brazed AISI 304L/BAu-4 stainless steel joints is investigated. A time-dependent reduction of the tensile and fatigue strength values down to 42% of the as-received condition is determined. As standard strain measurements are not appropriate to characterise the local strain distributions of heterogeneous material systems, the optical digital image correlation technique is used to evaluate the local quasi-static and cyclic deformation behaviour of the 50 μm wide brazing seam. A novel triggered image acquisition enables measurements in fatigue tests at a frequency of 10 Hz. The reduction of the virtual gauge length from 12.5 down to 0.5 mm leads to an increase of the total strain and ratcheting strain values, which is more pronounced for higher stresses and enhanced for pre-corroded brazed joints. For a microstructure-related analysis of the damage processes, scanning electron microscopy was used. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-018-00693-x
  • Mechanical and microstructural characterization of Arc-welded inconel 625 alloy
    Kotzem, D. and Beermann, L. and Awd, M. and Walther, F.
    Materials 12 (2019)
    The objective of this work was to verify a relatively new fusion-based additive manufacturing (AM) process to produce a high-temperature aerospace material. The nickel-based superalloy Inconel 625 (IN625) was manufactured by an arc-based AM technique. Regarding microstructure, typical columnar-oriented dendritic structure along the building direction was present, and epitaxial growth was visible. The mechanical behavior was characterized by a combination of quasi-static tensile and compression tests, whereas IN625 showed high yield and ultimate tensile strength with a maximum fracture strain of almost 68%. Even quasi-static compression tests at room and elevated temperatures (6500C) showed that compression strength only slightly decreased with increasing temperature, demonstrating the good high-temperature properties of IN625 and opening new possibilities for the implementation of arc-based IN625 in future industrial applications. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma12223690
  • Mechanism-oriented characterization of the load direction-dependent cyclic creep behavior of the magnesium alloys Mg-4Al-2Ba-2Ca and AE42 at room temperature
    Wittke, P. and Klein, M. and Walther, F.
    Engineering Failure Analysis 103 (2019)
    Due to their low density and high strength-to-weight-ratio magnesium alloys are very attractive for lightweight construction. Especially creep-resistant magnesium alloys can also be applied in the engine area of automobiles and therefore offer a high potential for weight reduction. For a safe and efficient use in such applications at elevated temperatures, a fundamental knowledge of the materials properties under cyclic loading is necessary. The aim of this study is the comparative investigation of the cyclic creep (ratcheting)behavior of magnesium alloys Mg-4Al-2Ba-2Ca and AE42 under different load ratios. Fatigue tests in tensile and compressive load ratios have been performed at room temperature. Fatigue strengths were estimated in load increase tests based on structure-sensitive material reactions. The results were correlated with fatigue lifetimes determined in constant amplitude tests. Furthermore, characteristic deterioration mechanisms were evaluated using a dedicated custom sensor system as well as by concomitant microstructural investigations. © 2019 Elsevier Ltd
    view abstract10.1016/j.engfailanal.2019.04.063
  • Micromagnetic-based fatigue life prediction of single-lip deep drilled AISI 4140
    Baak, N. and Nickel, J. and Biermann, D. and Walther, F.
    Structural Integrity 7 (2019)
    Non-destructive testing based on micromagnetic techniques, for example magnetic Barkhausen noise analysis, are quick and reliable possibilities to detect and classify material parameters like hardness and residual stresses. High-strength steels, like AISI 4140 (42CrMo4 + QT), are commonly used for highly dynamically loaded parts. Increasing requirements on weight, performance and efficiency of automotive industry claim increasing demands on material properties. The aim of this study is to evaluate the surface conditions of deep drilled round specimens due to drilling parameters and to predict the resulting fatigue strength by micromagnetic measurements. Furthermore, modified process parameters should enhance fatigue life without the need for expensive processing steps, e.g. autofrettage. © Springer Nature Switzerland AG 2019.
    view abstract10.1007/978-3-030-13980-3_3
  • Microstructural influences on the fatigue crack initiation and propagation mechanisms in hypo-eutectic Al-Si cast alloys
    Gerbe, S. and Tenkamp, J. and Scherbring, S. and Bleicher, K. and Krupp, U. and Michels, W. and Walther, F.
    Procedia Structural Integrity 23 (2019)
    Al-Si cast alloys are accompanied by different defects and microstructural heterogeneities like porosity, varying secondary dendrite arm spacings (SDAS) and multiple appearances of the eutectic. These issues are well known but technically difficult to prevent for the industry. To improve the light weight performance of cast aluminum alloys the present work deals with influences on crack initiation and propagation mechanisms of such heterogeneities in the high-cycle-fatigue (HCF) and very-high-cycle-fatigue (VHCF) regime for the two hypo-eutectic cast aluminum alloys AlSi8Cu3 (in-series sand castings) and AlSi7Mg0.3 (laboratory gravity die castings). Furthermore, the occurrence of facet-regions and their role in early and late state of fatigue damage evolution is discussed by showing results from modified Kitagawa-Takahashi analysis and ex-situ computed tomography investigation during intermitted HCF experiments. In the early HCF regime crack initiation starts at multiple pores. Due to coalescence of these defects, shear-stress-controlled crack propagation along slip bands leads to the occurrence of facets. In the VHCF regime, porosity provides strong scattering of the number of cycles to failure. A reduction in SDAS increases the fatigue strength significantly. However, in absence of porosity fatigue crack initiation and propagation is controlled by shear stresses on facet-like slip-planes. Additionally, the barrier role against dislocation movement of eutectic regions can be shown by means of these specimens. © 2019 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2020.01.137
  • Microstructure and mechanical properties of as-built and heat-treated electron beam melted Ti–6Al–4V
    Syed, A.K. and Awd, M. and Walther, F. and Zhang, X.
    Materials Science and Technology (United Kingdom) 35 (2019)
    This paper investigates the effect of post-deposition heat treatment on porosity, microstructure, and mechanical properties of Ti–6Al–4V produced via an Electron Beam Melting process. Samples were studied in the conditions of as-built and heat treated at 920°C and 1030°C. The as-built samples were characterised by columnar β grains consists of α+β microstructure with Widmanstätten and colony morphologies were found. Heat treatment resulted in increased α lath width. The yield strength and ultimate tensile strength was greater in the as-built condition than wrought material. Porosity re-growth occurred after heat treatment but it did not affect the tensile properties. Greater ductility after heat treatment was attributed to the larger α lath width which increases effective slip length. © 2019, © 2019 Institute of Materials, Minerals and Mining.
    view abstract10.1080/02670836.2019.1580434
  • Non-destructive testing derived parameters for microstructure-based residual service life assessment of aging metallic materials in nuclear engineering
    Acosta, R. and Boller, C. and Starke, P. and Jamrozy, M. and Knyazeva, M. and Walther, F. and Heckmann, K. and Sievers, J. and Schopf, T. and Weihe, S.
    Materialpruefung/Materials Testing 61 (2019)
    Metallic components in nuclear engineering are exposed to extensive loads such as pressurization and temperature changes which can affect the properties of the material significantly depending on the load spectrum applied. In view of developing a procedure to evaluate the residual service life of metallic components in nuclear power plants aged during service, metastable austenitic steel AISI 347 (German designation: X6CrNiNb18-10) has been considered as an example. To this purpose, total strain-controlled fatigue tests were carried out under different environmental conditions and monitored by continuously measuring thermometric, resistometric, electromagnetic and electrochemical parameters. These parameters provide an information gain in terms of material characterization when compared to conventional strain measurements. Based on these parameters, the short time evaluation procedure StrainLife has been developed, which allows the determination of local S-N curves with a significantly reduced effort as compared with traditional procedures. This method has been implemented into the structural simulation program PROST for the integrity assessment of the components while considering local fatigue properties. This very effective method allows for the determination of local fatigue properties including the strain-specific local scatter of the metallic microstructure properties of the material which has not been possible by traditional means. © Carl Hanser Verlag GmbH & Co. KG
    view abstract10.3139/120.111417
  • Quasi-static characterization of polyamide-based discontinuous CFRP manufactured by additive manufacturing and injection molding
    Striemann, P. and Hülsbusch, D. and Niedermeier, M. and Walther, F.
    Key Engineering Materials 809 KEM (2019)
    Generating serial components via additive manufacturing (AM) a deep understanding of process-related characteristics is necessary. The extrusion-based AM called fused layer manufacturing (FLM), also known as fused deposition modeling (FDM™) or fused filament fabrication (FFF) is an AM process for producing serial components. Improving mechanical properties of AM parts is done by adding fibers in the raw material to reinforce the polymer. The study aims to create a more detailed comprehension of FLM and process-related characteristics with their influence on the composite. Thereby, a short carbon fiber-reinforced polyamide (CarbonX™ Nylon, 3DXTECH, USA) with 12.5 wt.-% fiber content, 7 μm fiber diameter, and 150 to 400 µm fiber length distribution was investigated. To separate process-related characteristics of FLM, reference specimens were fabricated via injection molding (IM) with single-batch material. For the mechanical characterization, quasi-static tensile tests were carried out in accordance to DIN 527-2. Quality assessment including void content and void distribution was performed via micro-computed tomography (CT). The mechanical characterization clarifies effects on mechanical properties depending on process-related characteristics of FLM. CT scans show higher void contents of FLM specimens compared to IM specimens and void orientation dependent on printing direction. FLM shows process-related characteristics which generally strengthen mechanical properties of polymers. Nevertheless, tensile strength of FLM specimens decrease by more than 28% compared to quasi-homogenous IM specimens. © 2019 Trans Tech Publications Ltd, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.809.386
  • Rods glued in engineered hardwood products part I: Experimental results under quasi-static loading
    Grunwald, C. and Vallée, T. and Fecht, S. and Bletz-Mühldorfer, O. and Diehl, F. and Bathon, L. and Myslicki, S. and Scholz, R. and Walther, F.
    International Journal of Adhesion and Adhesives 90 (2019)
    Glued-in Rods (GiR) represent an adhesively bonded structural connection widely used in timber engineering. Up to now, common practice largely focused on softwood. Most structural adhesives have been, accordingly, specifically formulated to perform on softwood, in particular spruce. The increased use of hardwood, and corresponding engineered wood products (EWP), calls for deeper insights regarding GiR for the connection thereof. This paper, the first of a two part series, presents an overview over extensive research carried with 9 adhesives, 3 EWP, and 4 types of rods. Investigations started at component level, by fully characterising all adhesives, EWP, and rods. They were then extended to characterise the behaviour of interfaces, providing by this a methodology for selecting adhesives. Investigations at full scale followed, involving 5 different adhesives, 3 EWP, and 4 rod types. A total of 180 individual samples were tested. The results allowed to draw conclusions about the relationship between performance of GiR connections, and mechanical properties of their components. This relationship, however, has been found to be relatively weak. The companion paper will present a design methodology based on the material properties determined herein, and explain the ambiguous relationship between performance of the GiR and the mechanical properties of the adhesive, wood, and rods © 2018 Elsevier Ltd
    view abstract10.1016/j.ijadhadh.2018.05.003
  • Very high-cycle fatigue properties and microstructural damage mechanisms of selective laser melted AlSi10Mg alloy
    Awd, M. and Siddique, S. and Johannsen, J. and Emmelmann, C. and Walther, F.
    International Journal of Fatigue 124 (2019)
    The influence of cooling rates in selective laser melted AlSi10Mg on fatigue properties is studied. The failure mechanisms during quasistatic and fatigue loading were analyzed using electron microscopy, X-ray computed tomography and ultrasonic fatigue testing systems. It was found that even when densification mechanism was not significantly enhanced, the morphology of defects was adopting more regularly spherical shape which is less critical under structural loading. The controlled cooling introduced microstructural homogeneity and further property enhancement of the microstructure which contributed to higher quasistatic and fatigue strength in this study and compared to the literature on the same alloy. Elimination of semi-coherent Si agglomerates on the melt pool boundaries impeded crack propagation and forced it to adjust to a perpendicular orientation to the columnar dendrites formed instead. The strengthening mechanism was shown effective in quasistatic, servohydraulic and ultrasonic fatigue tests in very high-cycle fatigue. The influence of testing at ultrasonic frequencies was analyzed through the fatigue data and influence was not significant. Analysis of the interrupted fatigue specimens in the X-ray computed tomography revealed a pore-crack interaction mechanism which was hypothesized earlier in the literature. Although impedance of crack propagation and theoretical drop of stress intensity factors at crack tips was expected, it was implied that recommence of crack propagation and the end life of the specimen will still be dependent on the local strength of the microstructure in the vicinity of the pore. © 2019 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2019.02.040
  • Barkhausen noise assessment of the surface conditions due to deep hole drilling and their influence on the fatigue behaviour of AISI 4140
    Baak, N. and Schaldach, F. and Nickel, J. and Biermann, D. and Walther, F.
    Metals 8 (2018)
    In many technical fields, high-strength steels like AISI 4140 are commonly used for highly dynamically loaded parts. Increasing demands on weight, performance and efficiency of the automotive industry lead to increasing demands on material properties. For surface conditioning, optimised machining processes are capable of improving the fatigue performance without increasing the production cost significantly. This paper compares the influence of three different sulphur contents and three different feed rates on the fatigue behaviour of deep hole drilled AISI 4140 fatigue specimens. The specimens were characterised regarding their surface condition, hardness and microstructure, and afterwards they were tested under fatigue loading for performance assessment. These tests were accompanied with Barkhausen noise analysis. The Barkhausen noise signal was detected by a custom-built sensor that is capable of detecting the magnetic values on the bore wall. Using this technique, a load-independent estimation of fatigue damage was established. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met8090720
  • Comparative analysis of the fatigue characteristics of multilayer steel materials and nanolaminates
    Minakov, A.A. and Plokhikh, A.I. and Schmiedt, A. and Walther, F.
    AIP Conference Proceedings 2053 (2018)
    The paper shows the results of fatigue tests of multilayer metallic materials based on stainless austenitic steel (AISI 304) and stainless ferritic steel (AISI 340). It has been found that number of cycles to failure of multilayer steel material increases 3.5 times with an increase in the number of layers of a laminar structure from 100 to 1,400. The fatigue characteristics of the multilayer steel materials and nanolaminates are analyzed. A relationship has been ascertained between the increase of the fatigue life of a multilayer steel material and the deformation mechanisms. © 2018 Author(s).
    view abstract10.1063/1.5084497
  • Corrosion fatigue assessment of extruded magnesium alloys AZ31 and ME20
    Klein, M. and Wittke, P. and Hoppe, R. and Letzig, D. and Walther, F.
    Materialpruefung/Materials Testing 60 (2018)
    The corrosion fatigue behavior of extruded magnesium alloys AZ31 and ME20 was characterized by means of load increase tests in 5 wt.-% NaCl solution. The results were correlated with the respective corrosion behaviors investigated in immersion tests and by means of microstructural investigations after precorrosion. ME20 exhibited a superior corrosion resistance compared to AZ31, probably as a result of an enhanced passivation tendency due to grain refinement because of the presence of rare earth alloying elements. Accordingly, no differences in the cyclic deformation behaviors of ME20 in air and 5 wt.-% NaCl solution were observed in the load increase tests, while AZ31 exhibited a reduction of the estimated fatigue strengths as well as signs of embrittlement due to the mechanical-medial load. © Carl Hanser Verlag, München.
    view abstract10.3139/120.111113
  • Cyclic deformation behavior of a damage tolerant CrMnNi TRIP steel produced by electron beam melting
    Droste, M. and Günther, J. and Kotzem, D. and Walther, F. and Niendorf, T. and Biermann, H.
    International Journal of Fatigue 114 (2018)
    A high alloy CrMnNi TRIP steel has been processed by electron beam melting, a powder-bed based additive manufacturing (AM) technology, to investigate its fatigue properties. The material was characterized by average grain sizes of 32 μm in the as-built and 106 μm in the solution annealed state. Total strain controlled fatigue tests with strain amplitudes in the range of 0.25% ≤ Δεt/2 ≤ 1.2% were performed revealing a similar cyclic deformation behavior and α′-martensite evolution compared to a hot pressed reference material. Moreover, the fatigue lives of the EBM states were surprisingly high in consideration of severe process-induced lack of fusion defects of more than 500 μm revealed by investigations of the fracture surfaces. Thus, the impact of these inhomogeneities was substantially alleviated by the outstanding damage tolerance of the present TRIP steel induced by its high ductility and remarkable hardening capacity. © 2018 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2018.05.031
  • Dataset on the structural characterization of organosolv lignin obtained from ensiled Poaceae grass and load-dependent molecular weight changes during thermoplastic processing
    Dörrstein, J. and Scholz, R. and Schwarz, D. and Schieder, D. and Sieber, V. and Walther, F. and Zollfrank, C.
    Data in Brief 17 (2018)
    This article presents experimental data of organosolv lignin from Poacea grass and structural changes after compounding and injection molding as presented in the research article “Effects of high-lignin-loading on thermal, mechanical, and morphological properties of bioplastic composites” [1]. It supplements the article with morphological (SEM), spectroscopic (31P NMR, FT-IR) and chromatographic (GPC, EA) data of the starting lignin as well as molar mass characteristics (mass average molar mass (Mw) and Polydispersity (D)) of the extracted lignin. Refer to Schwarz et al. [2] for a detailed description of the production of the organosolv residue and for further information on the raw material used for lignin extraction. The dataset is made publicly available and can be useful for extended lignin research and critical analyzes. © 2018 The Authors
    view abstract10.1016/j.dib.2018.01.060
  • Defect-correlated fatigue assessment of A356-T6 aluminum cast alloy using computed tomography based Kitagawa-Takahashi diagrams
    Tenkamp, J. and Koch, A. and Knorre, S. and Krupp, U. and Michels, W. and Walther, F.
    International Journal of Fatigue 108 (2018)
    Modern aluminum cast alloys are promising candidates for highly loaded lightweight components due to their good strength-to-weight ratio. To enable a safe design of aluminum cast components, the microstructure characteristics, e.g. porosity, secondary dendrite arm spacing and Si eutectic morphology, have to be taken into account. This study deals with the influence of different porosity characteristics on the fatigue behavior of A356-T6 aluminum cast alloy. For this purpose, cyclic deformation tests have been carried out to study fatigue damage and fatigue crack growth in the high cycle fatigue (HCF) regime up to 107 cycles. The porosity of each batch was quantified using micro-computed tomography. Results show that large porosity dominates the HCF fatigue behavior. The relationship between porosity characteristics and the fatigue limit as well as S-N curve was assessed by Kitagawa-Takahashi diagrams and a fracture mechanic approach. © 2017
    view abstract10.1016/j.ijfatigue.2017.11.003
  • Development of biomimetic in vitro fatigue assessment for UHMWPE implant materials
    Scholz, R. and Knyazeva, M. and Porchetta, D. and Wegner, N. and Senatov, F. and Salimon, A. and Kaloshkin, S. and Walther, F.
    Journal of the Mechanical Behavior of Biomedical Materials 85 (2018)
    An important research goal in the field of biomaterials lies in the progressive amendment of in vivo tests with suitable in vitro experiments. Such approaches are gaining more significance nowadays because of an increasing demand on life sciences and the ethical issues bound to the sacrifice of animals for the sake of scientific research. Another advantage of transferring the experiments to the in vitro field is the possibility of accurately control the boundary conditions and experimental parameters in order to reduce the need of validation tests involving animals. With the aim to reduce the amount of needed in vivo studies for this cause, a short-time in vitro test procedure using instrumented load increase tests with superimposed environmental loading has been developed at TUD to assess the mechanical long-term durability of ultra-high molecular weight polyethylene (UHMWPE) under fatigue loading in a biological environment. © 2018 Elsevier Ltd
    view abstract10.1016/j.jmbbm.2018.05.034
  • Effects of high-lignin-loading on thermal, mechanical, and morphological properties of bioplastic composites
    Dörrstein, J. and Scholz, R. and Schwarz, D. and Schieder, D. and Sieber, V. and Walther, F. and Zollfrank, C.
    Composite Structures 189 (2018)
    The present study investigates the effects of high-lignin-loading on properties of lignin/polyethylene-co-vinyl acetate (EVA) rubber composites. Results from mechanical (quasi-static and cyclic) and rheological investigations revealed a brittle-ductile transition around a lignin volume fraction of φL = 0.59 coinciding with a twofold increase in steady-shear viscosity. Towards higher lignin contents, a 36% increase in dynamic stiffness Cdyn from φL = 0.59 (Cdyn ≈ 350 N mm−1) to φL = 0.71 (Cdyn ≈ 550 N mm−1) was observed by load increase tests (LIT). In addition, analyses of the ultra-micro-hardness revealed less indentation creep towards high-lignin-loading. At φL = 0.59, a pronounced relaxation endotherm superimposed on the glass transition (Tg) was observed, which was ascribed to molecular confinement to occur at highly loaded composites. At this point, the molecular weight (Mw) of lignin increased considerably. These results were explained by the different role of lignin at high volume fraction, i.e. a change from lignin as mere stiffness-inducing filler to a strength-imparting and fatigue-resistant matrix component which was supported by morphological analysis. © 2017 Elsevier Ltd
    view abstract10.1016/j.compstruct.2017.12.003
  • Experimental and numerical investigation of the strain rate-dependent compression behaviour of a carbon-epoxy structure
    Schmack, T. and Filipe, T. and Deinzer, G. and Kassapoglou, C. and Walther, F.
    Composite Structures 189 (2018)
    The usage of fibre-reinforced composites in automotive body structures is still a rarity. The main goal in body structure development is to design lightweight structures as cost-efficient as possible. This research contributes to the approach of maximal material usage by considering the strength increase of a carbon-epoxy laminate with increasing strain rate. The objective was to substantiate the well-known material characteristic's strain rate dependency from a coupon level to realistic body structure component – experimentally and numerically. Hence, a special compression fixture was developed to obtain all necessary characteristic values of the investigated T700S DT120 prepreg system. The rectangular coupon specimens were loaded with quasi-static to intermediate strain rates (2×10-4 to 70s-1). A second compression fixture was developed to axial load omega cross-sectional specimens with strain rates from 2×10-4 to 5s-1. The experimental tests showed a significant increase of +23% and +21% in compression strength for rectangular coupon specimens and omega cross-sectional components, respectively. Furthermore, the numerical simulation showed the same increase in strength of +21% for omega cross-sectional components. This work has proven the necessity of considering the strain rate dependency of a composite material to accurately predict the maximum load capacity of a structure during a dynamic load event like a crash. © 2017 Elsevier Ltd
    view abstract10.1016/j.compstruct.2017.11.025
  • Frequency-dependent fatigue and corrosion fatigue assessment of brazed AISI 304L/BNi-2 joints in air and synthetic exhaust gas condensate
    Schmiedt, A. and Lücker, L. and Manka, M. and Tillmann, W. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 41 (2018)
    Since brazed components are often cyclically loaded in corrosive environments, the corrosion fatigue behaviour of the joints has to be investigated. Fatigue tests of brazed AISI 304L/BNi-2 joints, relevant for exhaust gas heat exchangers, were performed with specimens in the as-received condition and after pre-corrosion according to VDA 230-214. Additionally, the superimposed corrosion fatigue loading in an exhaust gas condensate was realised using a corrosion cell. Corrosion-induced and deformation-induced microstructural changes were metallographically evaluated. The influence of the test frequency from 1 to 150 Hz on the cyclic deformation and damage behaviour was characterised. In a statistical analysis, the fatigue strength of 210 MPa at 2·107 cycles was determined for the as-received condition with a 50% failure probability. The pre-corrosion as well as the superimposed loading lead to a reduction of the fatigue strength down to 22%. A novel test strategy is suitable for precise fatigue and corrosion fatigue assessments. © 2018 Wiley Publishing Ltd.
    view abstract10.1111/ffe.12902
  • In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements
    Tillmann, W. and Walther, F. and Luo, W. and Haack, M. and Nellesen, J. and Knyazeva, M.
    Journal of Thermal Spray Technology 27 (2018)
    In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography. © 2017, ASM International.
    view abstract10.1007/s11666-017-0673-z
  • Influence of condensate corrosion on tensile and fatigue properties of brazed stainless steel joints AISI 304L/BNi-2 for automotive exhaust systems [Einfluss von Kondensatkorrosion auf die Zug- und Ermüdungseigenschaften der Edelstahllötverbindungen 1.4307/Ni 620 für Automobilabgassysteme]
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Materialwissenschaft und Werkstofftechnik 49 (2018)
    Materials and brazed joints for automotive exhaust systems have to resist the corrosive nature of aggressive exhaust gases as well as static and cyclic loads. In the present study, the influence of condensate corrosion according to VDA 230–214, with an ageing duration of up to 6 weeks, on the tensile and fatigue properties of stainless steel AISI 304L and brazed AISI 304L/BNi-2 joints is investigated. In relation to the as-received condition, the ultimate tensile strength is decreased down to 58 % and a reduction of the fatigue strength at 2⋅106 cycles down to 22 % is determined for brazed specimens, pre-corroded for 6 weeks. In contrast to the brazed stainless steel joints, the condensate corrosion does not influence the tensile properties of the AISI 304L base material. Stress concentrations at the corrosion-dependent circumferential grooves at the brazing seam are evaluated by stress intensity factors, which are well appropriate to characterise the fatigue behaviour depending on the corrosion condition. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201800007
  • Influence of the deep hole drilling process and sulphur content on the fatigue strength of AISI 4140 steel components
    Nickel, J. and Baak, N. and Biermann, D. and Walther, F.
    Procedia CIRP 71 (2018)
    When using quenched and tempered steels in the automotive industry or other industrial applications the fatigue behavior is of elementary importance. The surface and subsurface integrity of machined parts is known as one key factor related to the fatigue strength of components and is strongly influenced by the machining process. Especially for the deep hole single lip drilling, a large part of the cutting force is transferred to the bore wall and the surface and subsurface zone are influenced by the interaction between the tool and the workpiece material. In this study, the approach of inserting residual stresses into the bore wall and influencing the microstructure of the bores subsurface in a way, that the components can withstand a higher mechanical and dynamic load, is investigated in detail. In a close cooperation between the authors the parameters of the single-lip drilling process and the workpiece material are correlated with the produced surface quality and subsurface microstructure as well as the resulting fatigue strength of the components. For the comparative characterization of the fatigue behavior, established test procedures of destructive material testing in combination with new non-destructive test methods, such as the Barkhausen noise analysis, are applied and developed further. The results of the fatigue tests in correlation with the results of the Barkhausen noise analysis are used to design a model for predicting the fatigue strength of drilled components. © 2018 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.procir.2018.05.069
  • Influence of the microstructure on the cyclic stress-strain behaviour and fatigue life in hypo-eutectic Al-Si-Mg cast alloys
    Tenkamp, J. and Koch, A. and Knorre, S. and Krupp, U. and Michels, W. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    Aluminium alloys are promising candidates for energy-and cost-efficient components in automotive and aerospace industries, due to their excellent strength-to-weight ratio and relatively low cost compared to titanium alloys. As modern cast processing and post-processing, e.g. hot isostatic pressing, result in decreased frequency and size of defects, the weakest link depends on microstructural characteristics, e.g. secondary dendrite arm spacing (SDAS), Si eutectic morphology and α-Al solid solution hardness. Hereby, fatigue investigations of the effect of the microstructure characteristics on the cyclic stress-strain behaviour as well as fatigue mechanisms in the low cycle and high cycle fatigue regime are performed. For this purpose, samples of the aluminium cast alloy EN AC-AlSi7Mg0.3 with different Si eutectic morphology and α-Al solid solution hardness were investigated. To compare the monotonic and cyclic stress-strain curves, quasistatic tensile tests and incremental step tests were performed on two microstructure conditions. The results show that the cyclic loading leads to a hardening of the material compared to monotonic loading. Based on damage parameter Woehler curves, it is possible to predict the damage progression and fatigue life for monotonic and cyclic loading in hypo-eutectic Al-Si-Mg cast alloys by one power law. © The Authors, published by EDP Sciences, 2018.
    view abstract10.1051/matecconf/201816515004
  • Mechanical characterization of friction drilled internal threads in AZ91 profiles
    Wittke, P. and Teschke, M. and Walther, F.
    International Journal of Advanced Manufacturing Technology 99 (2018)
    In this study, the influence of friction drilling tool pre-heating on mechanical properties of chipless manufactured internal threads in thin-walled AZ91 magnesium casting alloy profiles is investigated. In this context, the influence of manufacturing processes on microstructure and the resulting fracture behavior during mechanical loading are in focus for the determination of failure mechanisms. Two batches were investigated, whereas specimens were manufactured without and with pre-heating the friction drilling tool before manufacturing. The mechanical properties were determined in tensile and fatigue tests in tensile loading range. The mechanical results were correlated with the profile qualities in form of computed tomography analyses and hardness mappings. Light and electron microscopic investigations of fractured surfaces were performed to analyze the fracture behavior in cyclic tests. Process-related and stress-related work hardening effects were determined at the edge area of the threads. Differences in fracture behavior under quasi-static and cyclic loads were determined. Turns of internal threads connected to the threaded rod were sheared off in tensile tests without visible cracks on the exterior surface of the flat profile specimens, whereas cyclically tested specimens provided fractured surfaces for fractographic failure analyses. Crack initiation at thread root and two stages of crack propagation until complete failure due to overload fracture were investigated. Pre-heating of the friction drilling tool during manufacturing of the threads had no influence on quasi-static and fatigue properties, respectively. © 2018, Springer-Verlag London Ltd., part of Springer Nature.
    view abstract10.1007/s00170-018-2698-y
  • Micro-magnetic and microstructural characterization of wear progress on case-hardened 16MnCr5 gear wheels
    Knyazeva, M. and Vasquez, J.R. and Gondecki, L. and Weibring, M. and Pöhl, F. and Kipp, M. and Tenberge, P. and Theisen, W. and Walther, F. and Biermann, D.
    Materials 11 (2018)
    The evaluation of wear progress of gear tooth flanks made of 16MnCr5 was performed using non-destructive micro-magnetic testing, specifically Barkhausen noise (BN) and incremental permeability (IP). Based on the physical interaction of the microstructure with the magnetic field, the micro-magnetic characterization allowed the analysis of changes of microstructure caused by wear, including phase transformation and development of residual stresses. Due to wide parameter variation and application of bandpass filter frequencies of micro-magnetic signals, it was possible to indicate and separate the main damage mechanisms considering the wear development. It could be shown that the maximum amplitude of BN correlates directly with the profile form deviation and increases with the progress of wear. Surface investigations via optical and scanning electron microscopy indicated strong surface fatigue wear with micro-pitting and micro-cracks, evident in cross-section after 3 × 105 cycles. The result of fatigue on the surface layer was the decrease of residual compression stresses, which was indicated by means of coercivity by BN-analysis. The different topographies of the surfaces, characterized via confocal white light microscopy, were also reflected in maximum BN-amplitude. Using complementary microscopic characterization in the cross-section, a strong correlation between micro-magnetic parameters and microstructure was confirmed and wear progress was characterized in dependence of depth under the wear surface. The phase transformation of retained austenite into martensite according to wear development, measured by means of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) was also detected by micro-magnetic testing by IP-analysis. © 2018 by the authors.
    view abstract10.3390/ma11112290
  • Microstructural characterization of the anisotropy and cyclic deformation behavior of selective laser melted AlSi10Mg structures
    Awd, M. and Stern, F. and Kampmann, A. and Kotzem, D. and Tenkamp, J. and Walther, F.
    Metals 8 (2018)
    The laser-based fusion of metallic powder allows construction of components with arbitrary complexity. In selective laser melting, the rapid cooling of melt pools in the direction of the component building causes significant anisotropy of the microstructure and properties. The objective of this work is to investigate the influence of build anisotropy on the microstructure and mechanical properties in selective laser melted AlSi10Mg. The alloy is comprehensively used in the automotive industry and has been one of the most frequently investigated Al alloys in additive manufacturing. Using specimens produced in three different building orientations with respect to the build platform, the anisotropy of the microstructure and defects will be investigated using scanning electron microscopy and microcomputed tomography. The analysis showed a seven-times higher pore density for the 90°-specimen compared to the 0°-specimen. The scanning electron microscopy revealed the influence of the direction of the cooling gradient on the constitution of the eutectic phase. Mechanical properties are produced in quasi-static and fatigue tests of variable and constant loading amplitudes. Specimens of 0° showed 8% higher tensile strength compared to 90°-specimens, while fracture strain was reduced almost 30% for the 45°-specimen. The correlation between structural anisotropy and mechanical properties illustrates the influence of the building orientation during selective laser melting on foreseen fields of application. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met8100825
  • Model-based correlation between change of electrical resistance and change of dislocation density of fatigued-loaded ICE R7 wheel steel specimens
    Starke, P. and Walther, F. and Eifler, D.
    Materialpruefung/Materials Testing 60 (2018)
    Weight-optimized component design as well as a reliable estimation of the lifetime of metallic materials and components require a comprehensive understanding of fatigue processes and a systematic investigation of the underlying fatigue behavior. This becomes even more important when designing highly loaded components such as wheels of high-speed passenger railway systems. Typically, mechanical stress-strain hysteresis measurements and increasingly different types of temperature and electrical resistance measurements are used to characterize the fatigue behavior and fatigue processes. Here, electrical resistance measurements provide significant information as they allow the detection of microstructural changes, e. g., through changes in dislocation density and structure. In addition, electrical resistance measurements can be considered in load increase and constant amplitude tests with inserted load-free sequences and in service load tests to characterize damage progress. In this paper, characteristic values of the change in electrical resistance were determined for ICE R7 wheel steel specimens and correlated with dislocation density, which was load- and cycle-dependent and determined through transmission electron microscopy. © Carl Hanser Verlag, München.
    view abstract10.3139/120.111202
  • On femtosecond laser shock peening of stainless steel AISI 316
    Hoppius, J.S. and Kukreja, L.M. and Knyazeva, M. and Pöhl, F. and Walther, F. and Ostendorf, A. and Gurevich, E.L.
    Applied Surface Science 435 (2018)
    In this paper we report on the competition in metal surface hardening between the femtosecond shock peening on the one hand, and formation of laser-induced periodic surface structures (LIPSS) and surface oxidation on the other hand. Peening of the stainless steel AISI 316 due to shock loading induced by femtosecond laser ablation was successfully demonstrated. However, for some range of processing parameters, surface erosion due to LIPSS and oxidation seems to dominate over the peening effect. Strategies to increase the peening efficiency are discussed. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.11.145
  • Oxide dispersion-strengthened alloys generated by laser metal deposition of laser-generated nanoparticle-metal powder composites
    Doñate-Buendía, C. and Frömel, F. and Wilms, M.B. and Streubel, R. and Tenkamp, J. and Hupfeld, T. and Nachev, M. and Gökce, E. and Weisheit, A. and Barcikowski, S. and Walther, F. and Schleifenbaum, J.H. and Gökce, B.
    Materials and Design 154 (2018)
    A new method is proposed for producing nanoparticle-metal composite powders for laser additive manufacturing of oxide-dispersion strengthened (ODS) alloys. Different composite powders containing laser-generated Y2O3 and yttrium iron garnet (YIG) nanoparticles were produced and consolidated by Laser Metal Deposition (LMD). The structural properties of the manufactured ODS alloys were analyzed, and their hardness, remnant porosity, and temperature-dependent compression behavior were characterized to study the effect of the composition and size of the nanoparticles on the structural and mechanical properties. While the structural analyses did not show significant differences between the processed samples within the limits of the characterization methods that were used, the temperature-dependent compression behavior showed an increase of up to 22 ± 11% in the high-temperature strength of the specimens that contained only 0.08 wt% of laser-generated nanoparticles. This increase is attributed to the dispersed and deagglomerated nature of the nanoparticles that were used during the powder-preparation step. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.05.044
  • Qualification of selective laser-melted Al alloys against fatigue damage by means of measurement and modeling techniques
    Awd, M. and Johannsen, J. and Siddique, S. and Emmelmann, C. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    Aluminum alloys processed through selective laser melting possess unique features of microstructure, defect morphology and mechanical properties. Constitution of fine cellular dendrites results from the high-cooling rate of the melt pool during the consolidation process. Investigation of the microstructure by scanning electron microscopy identifies supersaturation of Si particles as a secondary strengthening mechanism. On the contrary, platform heating that induces coarser microstructure leads to migration of Si particles from the Al matrix to the eutectic phase. As a result, tensile strength is reduced by ∼3%, while fracture strain is increased by ∼17%. Fine-grained structures exhibit a lower amount of plastic damage accumulation as well as delayed crack initiation as determined by the applied measurement techniques. Finite element models of the investigated configurations are obtained using scans of computed tomography under consideration of process-induced defects. Comparison of modeling and experimental results concluded that dominant fatigue damage mechanisms are related to the loading regime from low-cycle (LCF) to very-high-cycle fatigue (VHCF). Thus, process-inherent features of microstructure and porosity have different quantitative effects concerning the applied load. In VHCF, a material configuration with platform heating possesses an improved fatigue strength by ∼33% at 1E9 cycles, concerning the material configuration without platform heating. © The Authors, published by EDP Sciences, 2018.
    view abstract10.1051/matecconf/201816502001
  • Rods glued in engineered hardwood products part II: Numerical modelling and capacity prediction
    Grunwald, C. and Vallée, T. and Fecht, S. and Bletz-Mühldorfer, O. and Diehl, F. and Bathon, L. and Walther, F. and Scholz, R. and Myslicki, S.
    International Journal of Adhesion and Adhesives (2018)
    The second part of this series of two papers presents the modelling and strength prediction of Glued-in Rods (GiR) experimentally investigated in Part I. Unlike what has been documented in previous publications, significant effort was put into extensive modelling of all components (adhesive, wood, and rods), in particular regarding stress components other than shear. Based upon the material modelling, stresses inside the GiR were estimated through Finite-Element Analysis (FEA), which indicated that transverse tensile strength are at least as significant as shear stresses in their magnitude. Both results mitigate previous research findings that focused on shear-dominated failure mechanisms and neglected transverse tensile strength. Combining the material characterisation with FEA, and reformulating strength in probabilistic terms, then allowed to perform predictions of joint capacities for all 60 experimentally investigated GiR-configurations. The comparison between predicted and experimental values showed a good agreement wit relative difference amounting to –3% for beech GLT, –2% for oak GLT, and +1%, respectively. Unlike Fracture Mechanics and Cohesive Zone Modelling, necessary parameters were solely obtained independently from the GiR itself, and no single parameter had to be back-fitted on the experimental results of the GiR. Results clearly showed that transverse tensile strength of the wood is at least as important as shear strength for joint capacity of GiR, and that longitudinal strength plays a minor role. © 2018 Elsevier Ltd
    view abstract10.1016/j.ijadhadh.2018.05.004
  • Simulation of cyclic deformation behavior of selective laser melted and hybrid-manufactured aluminum alloys using the phase-field method
    Siddique, S. and Awd, M. and Wiegold, T. and Klinge, S. and Walther, F.
    Applied Sciences (Switzerland) 8 (2018)
    Selective laser melting process has already been developed for many metallic materials, including steel, aluminum, and titanium. The quasistatic properties of these materials have been found to be comparable or even better than their conventionally-manufactured counterparts; however, for their reliable applications in operational components, their fatigue behavior plays a critical role, which is dominated by several process-related features, like surface roughness, remnant porosity, microstructure, and residual stresses, which are controlled by the processing features, like imparted energy density to the material, its corresponding solidification behavior, the cooling rate in the process, as well as post-processing treatments. This study investigates the influence of these parameters on the cyclic deformation behavior of selective laser melted as well as hybrid-manufactured aluminum alloys. The corresponding microstructural features and porosity conditions are evaluated for developing correlations between the process conditions to microstructure, the deformation behavior, and the corresponding fatigue lives. From the numerical point of view, damage development with respect to process-induced cyclic deformation behavior is assessed by the phase-field method, which has been identified as an appropriate method for the determination of fatigue life at the respective applied stress levels. Fatigue strength of SLM-processed parts is found better than their cast counterparts, while hybridization has further increased fatigue strength. No effect of test frequency on the fatigue life could be established. © 2018 by the authors.
    view abstract10.3390/app8101948
  • Tensile and fatigue assessments of brazed stainless steel joints using digital image correlation
    Schmiedt, A. and Jaquet, S. and Manka, M. and Tillmann, W. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    For mechanical tests of brazed stainless steel joints, the local deformation behaviour within the small area of the brazing seam is a major concern, because local strains cannot be detected with standard mechanical extensometers. The current study allows a fundamental comprehension of the gauge length influence on the strain measurements of brazed joints with smooth and notch-containing surfaces, under quasi-static and cyclic loadings. Therefore, the optical measurement technique of digital image correlation (DIC) is used within tensile and fatigue tests of brazed AISI 304L/BAu-4 joints in an as-received and pre-corroded condition. A triggered image acquisition of the DIC system is successfully applied to evaluate the local ratcheting fatigue behaviour in the area of the brazing seam at a frequency of 10 Hz. The gauge length influence, analysed in the range of 0.5 to 12.5 mm, is more pronounced with increasing tensile and fatigue stresses and is significantly enhanced for notch-containing surfaces. Instrumented load increase tests with strain, electrical, magnetic and temperature measuring techniques have proven to be appropriate to estimate fatigue properties of the brazed joints with a deviation of 4%. Fatigue and corrosion fatigue damage mechanisms are evaluated by using scanning electron microscopy with secondary and back-scattered electron detectors. © The Authors, published by EDP Sciences, 2018.
    view abstract10.1051/matecconf/201816506003
  • Characterization of hybrid joining techniques for FRP/Steel-structures under combined mechanical and thermal loading
    Hoepfner, M. and Becker, T. and Hülsbusch, D. and Walther, F.
    Key Engineering Materials 742 KEM (2017)
    In order to optimize the design of vibrating screening machines and realize significant weight reductions, the use of hybrid structures is gaining importance. In this context, the joining of FRP and steel and their interactions due to different material properties were investigated. Therefore, quasi-static tests with combined mechanical and thermal loads were carried out. To realize the simultaneous application of physical measurement techniques, e.g. optical and acoustic measurements, and thermal loads, short-wave infrared emitter technique was used instead of thermal chambers. Thus, the mechanical characteristics and acoustic emissions could be determined and assessed. The results show different structural mechanisms of hybrid joining at room and elevated temperatures. The characteristics of failure modes, shear stresses, strains and acoustic emissions could be correlated to determine the damage developments and mechanisms. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.742.358
  • Characterization of residual stresses in austenitic disc springs induced by martensite formation during incremental forming using micromagnetic methods
    Baak, N. and Garlich, M. and Schmiedt, A. and Bambach, M. and Walther, F.
    Materials Testing 59 (2017)
    Disc springs are very common and important machine elements used in almost all fields of mechanical engineering. They are applied when high spring forces are required and only a small mounting space is available, for example, in clutches or rolling bearings. Residual stresses induced by shot peening improve the performance significantly, at the same time shot peening increases the production time and cost. This paper investigates an incremental forming process that allows forming of the spring geometry and generation of residual stresses in a single setup. Accurate measurements of the spring performance are needed to control the product properties, especially when new processes are introduced. The possibility to apply magnetic measurements to control the spring properties is evaluated. A relation between spring characteristics and magnetic Barkhausen noise envelope is established. The findings are supported by residual stress measurements and light optical microscopy, showing martensite islands caused by incremental forming.
    view abstract10.3139/120.111012
  • Comparative characterization of quasi-static and cyclic deformation behavior of glass fiber-reinforced polyurethane (GFR-PU) and epoxy (GFR-EP)
    Hülsbusch, D. and Jamrozy, M. and Frieling, G. and Mueller, Y. and Barandun, G.A. and Niedermeier, M. and Walther, F.
    Materialpruefung/Materials Testing 59 (2017)
    Glass fiber-reinforced polymers (GFRP) are highly suitable for use in transportation industry in order to achieve the targets of energy and resource efficiency. In this context, due to its high specific strength, GFR-epoxy (GFR-EP) has already been implemented in a wide range of applications. However, in cases of energy efficiency and damage tolerance, GFR-EP shows disadvantages compared to GFR-polyurethane (GFR-PU). The aim of this study is the comparative characterization of the quasi-static and cyclic deformation behavior of GFR-PU and GFR-EP with similar layer setup. The mechanical properties have been investigated in instrumented tensile, interlaminar shear strength and compression after impact tests. In addition, the tests were combined with varying temperatures (-30 °C, RT, +70 °C) with respect to aerospace applications to determine the material property development under low and elevated temperatures. In cyclic investigations, the fatigue properties have been estimated by resource-efficient multiple step tests and validated in constant amplitude tests. Hysteresis and temperature measurements were applied in order to investigate the damage processes. It could be shown that polyurethane exhibits improved damage tolerance by significantly reducing delamination area under impact loading, whereas epoxy leads to optimized properties under elevated temperature. Furthermore, epoxy generally underlines higher capabilities under cyclic loading, which is due to void content of polyurethane. © Carl Hanser Verlag GmbH &Co. KG.
    view abstract10.3139/120.110972
  • Comparison of microstructure and mechanical properties of Scalmalloy® produced by selective laser melting and laser metal deposition
    Awd, M. and Tenkamp, J. and Hirtler, M. and Siddique, S. and Bambach, M. and Walther, F.
    Materials 11 (2017)
    The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al3-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (μ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength. © 2017 by the author.
    view abstract10.3390/ma11010017
  • Corrosion fatigue assessment of creep-resistant magnesium alloys DieMag422 and AE42
    Klein, M. and Frieling, G. and Walther, F.
    Engineering Fracture Mechanics 185 (2017)
    Corrosion fatigue behaviors of creep-resistant magnesium alloys DieMag422 and AE42 were characterized by means of lifetime- and process-oriented approaches. Fatigue properties of both materials were assessed in constant amplitude tests at several adjusted corrosion rates. Deterioration and cracking mechanisms were identified and monitored by means of corrosion potential response analysis during constant amplitude tests. Reduction of corrosion fatigue strength could be quantitatively correlated with adjusted corrosion rates. Analysis of corrosion potential response revealed characteristic deterioration and cracking mechanisms for LCF and HCF regime. © 2017 Elsevier Ltd
    view abstract10.1016/j.engfracmech.2017.02.024
  • Development of a stochastic approach for fatigue life prediction of AlSi12 alloy processed by selective laser melting
    Siddique, S. and Awd, M. and Tenkamp, J. and Walther, F.
    Engineering Failure Analysis 79 (2017)
    Parts manufactured by selective laser melting (SLM) process possess unique features in terms of surface roughness, microstructure, residual stresses as well as defect distribution. These defects are responsible for failure of the parts in functional applications. When fatigue loading is applied, these defects are the dominant cause of crack initiation, resulting in scatter of fatigue properties. This scatter occurs due to interacting phenomena like defect size, location as well as the magnitude and type of load. For the purpose of investigating the effect of defects on fatigue life performance of AlSi12 manufactured by selective laser melting, a procedure was developed based on the weakest-link theory and Weibull's probability density function. Using various destructive and non-destructive techniques, defects, including remnant porosity and surface roughness, have been characterized in amount, size and location. Therefore fatigue life prediction, relying on equations constituted from crack propagation properties, was carried out. Predicted fatigue life and Weibull's statistical parameters were used to compare the effect of both defect types on fatigue reliability of AlSi12 produced by SLM. The most probable fatigue life for a sample was interpreted based on Weibull probability density function with respect to maximum probability of occurrence. The prediction of numerous possible values enabled an estimation of fatigue scatter to be made. Thus, the findings of this novel approach enabled conclusions about strength and reliability of different SLM AlSi12 configurations and gave a prelude towards application-oriented design of SLM components. © 2016 Elsevier Inc.
    view abstract10.1016/j.engfailanal.2017.03.015
  • Fatigue and fracture reliability of additively manufactured Al-4047 and Ti-6Al-4V alloys for automotive and aerospace applications
    Siddique, S. and Walther, F.
    Lecture Notes in Mechanical Engineering PartF9 (2017)
    Metal additive manufacturing is at a stage that it can now be used not only for rapid prototyping but for rapid manufacturing of functional components as well. However, for the reliable employment of parts, their mechanical performance is an important parameter not only in terms of their quasistatic strength but their fatigue performance for dynamic applications. Their fatigue performance should be at par with that of conventionally manufactured alloys. There can still be reliability issues for the additively manufactured parts, as the specific issues—remnant porosity, surface finish, residual stresses, and fatigue scatter—are influenced by the selected process parameters. This paper presents the state-of-the-art of fatigue performance for additively manufactured Al-4047 and Ti-6Al-4V and ways to improve and manipulate the part properties. The results show that their mechanical performance is comparable, even better in some cases, to that of conventionally manufactured materials if appropriate processing parameters and post-processing techniques are employed. © Springer Science+Business Media Singapore 2017.
    view abstract10.1007/978-981-10-1771-1_6
  • Forming-induced damage and its effects on product properties
    Tekkaya, A.E. and Ben Khalifa, N. and Hering, O. and Meya, R. and Myslicki, S. and Walther, F.
    CIRP Annals - Manufacturing Technology 66 (2017)
    Damage is caused in the microstructure of metals during forming. Damage is not a failure, but affects the mechanical properties of the component under service loads. This paper explores experimentally the effect of metal forming process parameters on the evolution of damage and the resulting product properties. As a representative for bulk forming processes, cold forging is investigated. It is shown that an increase of the extrusion ratio leads to lower damage and increased fatigue strength. Air bending, as a sheet forming process, is analysed, exhibiting that damage can be influenced by process design such as the superposition of stresses. © 2017
    view abstract10.1016/j.cirp.2017.04.113
  • Front Face Flow Drilling of Lightweight Cast Materials
    Biermann, D. and Walther, F. and Hannich, S. and Wittke, P.
    Procedia Engineering 207 (2017)
    Front face flow drilling, which has been investigated at the Institute of Machining Technology (ISF), TU Dortmund University, represents a new application of the conventional flow drilling process. With this new technique it is possible to form closed holes with diameters, which can exceed the local wall thickness of the profile. By using a subsequent threading operation, it is possible to generate solid joints. In this article investigations regarding the machining of the aluminium cast alloy AlSi10Mg are presented. The feasibility of the front face flow drilling application was analysed for the machining of thin profiles with a wall thickness of tW = 6 mm. Flow drilling tools with a diameter of dFD = 5.4 mm have been used. Feed forces as well as torques were measured during the flow drilling process. In order to generate solid threads, a high quality of the formed holes has to be ensured. To quantify the quality, measurements of the circularity as well as the diameters were carried out. Both aspects can have an influencing effect on the threading operation. Due to the low formability of the used alloys, adjustments of the flow drilling process had to be realised. Besides the variation of process parameters such as peripheral speed and feed velocity, a pre-heating of the tool was conducted to raise the formability of the workpiece material and to increase the process stability. The pre-heating of the tool was accomplished by using a portable induction system with an integrated temperature control. As a result of the investigations, suitable process strategies will be recommended regarding a stable front face flow drilling operation of the lightweight cast alloy AlSi10Mg. Further research regarding front face flow drilling is planned towards an adaptation to magnesium cast alloys. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.858
  • Influence of plasma electrolytic oxidation coatings on fatigue performance of AZ31 Mg alloy
    Klein, M. and Lu, X. and Blawert, C. and Kainer, K.U. and Zheludkevich, M.L. and Walther, F.
    Materials and Corrosion 68 (2017)
    Magnesium and its alloys are attractive for lightweight construction, but suffer often from poor corrosion resistance. Plasma electrolytic oxidation is a promising surface treatment to overcome these limitations. Recently, introduction of particles to the PEO electrolyte has been explored as new strategy to provide a wider range of compositions and new functionalities for PEO coatings. However, this surface treatment can have negative impact on the fatigue strength. In the present study, the influence of PEO coatings with and without particle addition on the corrosion fatigue behavior of AZ31 Mg alloy is investigated. The corrosion fatigue behavior is investigated in load increase tests and constant amplitude tests in 0.5% NaCl solutions. Results are correlated with the corrosion behavior evaluated in polarization and electrochemical impedance spectroscopy measurements. Corrosion tests show significant improvement of the corrosion resistances of PEO-coated specimens. However, the uncoated material exhibits the highest corrosion fatigue strength, whereas a reduction of 7% for the PEO-coated specimen without particles and 27% for the PEO-coated specimen with particles is found. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/maco.201609088
  • Mechanism-oriented characterization of the fatigue behavior of glass fiber-reinforced polyurethane based on hysteresis and temperature measurements
    Hülsbusch, D. and Jamrozy, M. and Mrzljak, S. and Walther, F.
    Key Engineering Materials 742 KEM (2017)
    In order to optimize resource efficiency, glass fiber-reinforced polymers (GFRP) have been implemented in recent years in a wide range of applications in transportation industry. In this context, GFR-epoxy (GFR-EP) is currently being used mainly because of their sufficiently investigated properties and production processes. Polyurethane (PU), however, shows advantages in terms of energy efficiency and damage tolerance. The aim of this study is the characterization of the fatigue behavior of GFR-PU by stepwise exploration of damage development on microscopic level. Therefore, multiple amplitude and constant amplitude tests have been carried out. Hysteresis and temperature measurements were applied in order to investigate the damage processes and correlated with in situ computed tomography (CT) in intermitting tests. The damage development and mechanisms could be characterized and separated. The results confirm known GFRP damage characteristics, whereas also material-specific peculiarities regarding crack development could be revealed. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.742.629
  • Micro-CT defect analysis and hardness distribution of flat-face extruded en AW6060 aluminum chips
    Goerlich, P. and Scholz, R. and Walther, F.
    Materialpruefung/Materials Testing 59 (2017)
    Besides the energy-intensive secondary metallurgical recycling route, aluminum chips can alternatively be extruded to final profiles by extrusion. The mechanical properties of the extruded profiles have a dependency on the quality of the weldments of the chips, which differs locally due to the batch process. To characterize the influence of this dependency on the mechanical properties, round pre-compacted chip blocks consisting of EN AW-6060 were pre-heated for six hours at 550 °C, extruded with flatface dies at a recipient temperature of 450 °C and divided into three zones: Profile, transition and contact zone. A micro-computed tomographic defect analysis was performed on the samples. It has been shown that the profile samples of both geometries have a very low defect quantity and volume, while towards the contact zone the number and volume increases significantly and small delaminations occur on the surface. For the determination of the hardness distribution, a macro hardness mapping was performed. The coarse grain edge in the outer region of the specimens, which has resulted from increased temperature as a result of the recipient friction and shear stress, shows a slightly increased hardness. Round profiles show a concentric hardness decrease and square profiles a linear drop towards the center of the cross section of the specimens. © 2017 Carl Hanser Verlag, München.
    view abstract10.3139/120.111050
  • Microstructure-based assessment of the corrosion fatigue behavior of the creep-resistant DieMag422 and AE42 magnesium alloys
    Klein, M. and Buhr, P. and Walther, F.
    Solid State Phenomena 258 SSP (2017)
    Magnesium alloys offer high potential for lightweight constructions, e.g. in automotive applications. However, their application range is limited due to their low corrosion resistance. In the present study, the influence of corrosion on the microstructure and the depending mechanical properties under cyclic loading were characterized for the creep-resistant DieMag422 (Mg-4Al-2Ba-2Ca) and AE42 magnesium alloys. In this context, fatigue properties in distilled water and sodium chloride solutions were assessed in constant amplitude tests. The results were correlated with corrosion properties of the alloys, which were evaluated by immersion tests. Corrosion- and deformation-induced microstructural changes were observed by light and scanning electron microscopy (SEM), yielding a structure-property-relationship for a comprehensive understanding of mechanical and corrosive deterioration mechanisms. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/SSP.258.530
  • Production-and microstructurebased fatigue assessment of metallic AISI 304/430 multilayer materials produced by hot pack rolling
    Schmiedt, A. and Luecker, L. and Kolesnikov, A.G. and Plokhikh, A. and Walther, F.
    Materialpruefung/Materials Testing 59 (2017)
    Metallic multilayer materials consisting of hundreds or thousands of layers offer a high potential for broad applications in modern technology. A uniform and gradual thinning of layers can be realized by using alloys with different crystal structures in combination with the efficient and high-performance technology of hot pack rolling. However, investigations on fatigue properties, especially to evaluate the influence of the number of layers, are still missing. In the present study, the fatigue behavior of metallic multilayer materials consisting of austenitic and ferritic stainless steels AISI 304 and AISI 430 with 100 and 1400 layers are characterized by applying a time-efficient load increase procedure. Therefore, instrumented stepwise load increase tests were performed to define suitable loading parameters for a convenient comparison of fatigue properties in constant amplitude tests. A benefit of the complex production process leading to 1400 layers was verified concerning the investigated load level in the range of low cycle fatigue with a significant improvement by the factor of 3.5. The alternating current potential drop method for measurements of change in voltage was determined to be most suitable to detect microstructural changes at an early state of fatigue damage for multilayer materials. Microstructures as well as fractured surfaces were investigated using light and scanning electron microscopy to evaluate the results of the two technological manufacturing routes as well as the crack and failure behavior. © Carl Hanser Verlag GmbH &Co. KG.
    view abstract10.3139/120.110976
  • Tensile and fatigue investigations of timber joints with glued-in FRP rods
    Tannert, T. and Zhu, H. and Myslicki, S. and Walther, F. and Vallée, T.
    Journal of Adhesion 93 (2017)
    Glued-in rods are an effective method to form timber connections that are increasingly in the focus of research. Compared to steel rods, fibre-reinforced polymer (FRP) bars provide higher resistance against corrosion, reduced weight, and lower heat conductivity. Despite excellent mechanical performance, high fire resistance, and improved aesthetics, they are, however, not yet widely used due to lack of design regulation. This is particularly true for cyclic loads, where determination of fatigue characteristics depends upon time-intensive experimental procedures. In this research, 50 glued-in FRP rod specimens with different embedment lengths were manufactured and tested in uniaxial tension: a first set under quasi-static load and a second set under cyclic load. For the fatigue tests, a new approach based on a stepwise load increase was used to estimate fatigue strength, a method that aims at reducing the experimental effort usually associated with Woehler curves. Experimental results indicated that quasi-static and fatigue strength increased with the overlap length up to an apparent maximum. The estimated fatigue strength from the load increase tests (LITs) was approx. 41% of the quasi-static strength. Additionally, constant-amplitude tests were performed and the obtained data points fit well into established Woehler curves for timber. © 2016 Taylor & Francis
    view abstract10.1080/00218464.2016.1190653
  • Very high cycle fatigue and fatigue crack propagation behavior of selective laser melted AlSi12 alloy
    Siddique, S. and Imran, M. and Walther, F.
    International Journal of Fatigue 94 (2017)
    Selective laser melting is a maturing additive manufacturing process which has been studied in the recent years for processing different alloys. The focus, however, remained on the processibility, design considerations and static or quasistatic mechanical properties. For the application of the process for functional components, it is required that the process parts perform well in cyclic applications as well. The influence of processing parameters on the resulting part parameters and the corresponding fatigue behavior remains unaddressed. This study investigates the influence of base plate heating and post-process stress-relief on part properties like process-incited defects, which are critical for fatigue loading, for AlSi12 alloy. Investigations have been made for very high cycle fatigue as well as fatigue crack growth. To understand the fatigue phenomenon, characterization of basic material properties is also carried out. Results show that the material defects as well as microstructure is influenced by these parameters. The corresponding fatigue and crack growth behavior is improved by base plate heating which changes the fatigue crack initiation mechanisms as well. Relatively reduced thermal gradients by base plate heating helps improving the fatigue reliability by reducing the fatigue crack initiation from material defects. © 2016 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2016.06.003
  • Application of micro-magnetic testing systems for non-destructive analysis of wear progress in case-hardened 16MnCr5 gear wheels
    Tenkamp, J. and Haack, M. and Walther, F. and Weibring, M. and Tenberge, P.
    Materials Testing 58 (2016)
    Micro-magnetic testing methods are qualified for non-destructive quantification of hardness, hardness depth and residual stresses. Among others they are applied for detection of grinding burn in gear wheels, but an application for wear condition monitoring has not yet been published. In this paper, results of initial research of determination of wear condition in gear wheels by application of micro-magnetic testing systems are presented. For comparison of different wear conditions, gears were loaded for increasing numbers of cycles in a test rig based on FVA information sheet 54/7 and DIN ISO 14635 part 1. Operating conditions were altered by usage of different lubricants. Afterwards, wear conditions were determined by conventional techniques, i.e., measuring change in profile and loss of material. Four measurement principles were evaluated for change in micro-magnetic properties determination, magnetic Barkhausen noise analysis, permeability and eddy-current measurements, as well as harmonic analysis of tangential field strength. A general suitability of micro-magnetic testing approach for characterization of wear condition of gear wheels could be demonstrated by comparison of micro-magnetic properties with common wear indicators. Micro-magnetic properties were not solely influenced by wear condition, as the selected oil, and hence the tribochemical conditions in contact also showed a significant effect on measured values. Therefore, further survey is required for direct correlation of micro-magnetic properties with (micro-) structural material changes.
    view abstract10.3139/120.110924
  • Assessment of Influence of Corrosive Environment on Fatigue Performance of Intrinsic CFRP/Steel Hybrids
    Klein, M. and Hülsbusch, D. and Siddique, S. and Walther, F.
    Materials Today: Proceedings 3 (2016)
    Hybrid structures are very attractive for lightweight applications, e.g. in automotive or aircraft industries. For safe and efficient usage in such applications, high fatigue strength and good corrosion resistance are mandatory with regard to loading under service conditions. In the present study the fatigue performance of two intrinsically produced hybrid structures with different steel constituents in air and salt spray environment was investigated to describe the influence of superimposed corrosion loads. Additionally, the corrosion behavior was characterized in potentiodynamic polarization measurements and afterwards correlated with the fatigue results, leading to a quantitatively describable process-structure-property relationship for corrosion influence on fatigue performance of intrinsic CFRP/steel hybrids. Both hybrid structures exhibited galvanic corrosion, wherein one material combination showed significantly higher corrosion rates leading to worse corrosion fatigue behavior in salt spray environment. © 2016 Elsevier Ltd.
    view abstract10.1016/j.matpr.2016.07.005
  • Characterisation of the mechanical and corrosive properties of newly developed glass-steel composites
    Lyubimova, O. and Gridasova, E. and Gridasov, A. and Frieling, G. and Klein, M. and Walther, F.
    Materiali in Tehnologije 50 (2016)
    This paper presents a preliminary research about a newly developed glass-steel composite created with diffusion welding of glass (C49-1) and carbon steel (St3sp). The main conclusions on the process of forming a diffusion zone during the welding of glass and steel are made. The results of quasi-static and cyclic mechanical tests and corrosion investigations are presented and interpreted on the basis of the microstructure developed during diffusion welding and described in this article.
    view abstract10.17222/mit.2014.305
  • Comparison of fatigue life assessment by analytical, experimental and damage accumulation modelling approach for steel SAE 1045
    Imran, M. and Siddique, S. and Guchinsky, R. and Petinov, S. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 39 (2016)
    Fatigue life assessment for two-phase steel SAE 1045 has been carried out by experimental and simulation techniques. Analytical approach, termed as fatigue lifetime calculation, was employed making use of a load increase testing procedure and constant amplitude tests equipped with measurement techniques – plastic strain amplitude, change in temperature and change in electrical potential difference. The predicted fatigue life has been validated by constant amplitude tests and compared with fatigue life estimation by microstructure-based simulation. Simulation has been carried out over the complete cross section of the specimen. The simulation uses damage accumulation in the gage section of the specimen culminating in the macro-crack propagation, taking into account the inhomogeneous fatigue resistance of the material element. The results show that at the initial intervals of high cycle fatigue range at relatively higher stress amplitudes, the experimental and simulation results are in agreement; whereas in the (high cycle fatigue) region at relatively low stress amplitudes, the simulation results were found more optimistic and the corresponding fatigue scatter is also increased. Each scatter is attributed to the relatively small number of analysed models of the material structure. Scanning electron microscope was used to determine volume fraction of the microstructure for simulation. Fatigue fracture surface analysis shows that crack initiated from internal defect of material and crack propagation is driven by silicon oxide inclusion. © 2016 Wiley Publishing Ltd.
    view abstract10.1111/ffe.12426
  • Corrosion fatigue assessment of creep-resistant magnesium alloy Mg-4Al-2Ba-2Ca in aqueous sodium chloride solution
    Wittke, P. and Klein, M. and Dieringa, H. and Walther, F.
    International Journal of Fatigue 83 (2016)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba-2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with one single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2015 Elsevier Ltd.
    view abstract10.1016/j.ijfatigue.2015.04.001
  • Cyclic Deformation Behavior of Friction Drilled Internal Threads in AlSi10Mg and AZ31 Profiles
    Wittke, P. and Walther, F.
    21st European Conference on Fracture, (ecf21) 2 (2016)
    Metallic lightweight materials are used for enhancing dynamic range, resource optimization and emission reduction in many fields of traffic engineering, whereby aluminium and magnesium components are manufactured by means of welded, adhesive and screw joints. Friction drilling, as forming process with subsequent manufacturing of threads, offers the opportunity to produce an internal thread in lightweight profiles with a usable thread depth larger than the profile thickness, making use of local material expansion. Moreover, the direct manufacturing offers a huge potential for time and cost saving in comparison to conventional thread machining. Microscopic-based characterization of mechanical properties of aluminium AlSi10Mg and magnesium AZ31 internal threads in thin-walled profile specimens was carried out using tensile tests and fatigue tests in tensile loading range. The internal threads were chipless manufactured by means of thread forming. Variations in the geometric process parameter wall thickness were compared. Differences between the AlSi10Mg chill casting alloy and the AZ31 continuous casting alloy in maximum tolerable loads and fatigue limits were correlated with the production-related profile qualities of the profile specimens. The maximum tolerable loads increase linearly with increasing wall thickness of the specimens, whereby AlSi10Mg specimens were about 20-24% lower in the quasi-static range and about 37-47% lower in the cyclic range in comparison to AZ31 specimens due to oval forms of the core holes caused by the friction drilling process. Plastic strain behavior and deformation-induced changes in temperature in load increase tests were evaluated to reliably estimate the fatigue limit of magnesium AZ31 internal threads. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.407
  • Determination of the young modulus of Ti-TiAl3 metallic intermetallic laminate composites by nano-indentation
    Yener, T. and Güler, S. and Siddique, S. and Walther, F. and Zeytin, S.
    Acta Physica Polonica A 129 (2016)
    Nano-indentation is an important technique to determine the Young modulus of multiphase materials where normal tensile tests are not appropriate. In this work, Ti-TiAl3 metallic-intermetallic laminate composites have been fabricated successfully in open atmosphere using commercial purity Al and Ti foils with 250 μm and 500 μm initial thicknesses, respectively. Sintering process was performed at 700 °C under 2 MPa pressure for 7.5 h. Mechanical properties including the Young modulus were determined after manufacturing. The Young moduli of metallic and intermetallic phases were determined as 89 GPa and 140 GPa, respectively. Microstructure analyses showed that aluminum foil was almost consumed by forming a titanium aluminide intermetallic compound. Titanium aluminides grow up through spherical shaped islands and metallic-intermetallic interface is a wavy form in Ti-Al system. Thus, the final microstructure consists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructure and phase characterizations were performed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Hardness of test samples was determined as 600 HV for intermetallic zone and 130 HV for metallic zone by the Vickers indentation method.
    view abstract10.12693/APhysPolA.129.604
  • Direction-dependent mechanical characterization of cellulose-based composite vulcanized fiber
    Scholz, R. and Mittendorf, R. M. and Engels, J. K. and Hartmaier, A. and Kunne, B. and Walther, F.
    Materials Testing 58 (2016)
    Vulcanized fiber is a macromolecular cellulose-based composite material manufactured using the parchmentizing process. The cellulose is produced from the chemical digestion of plant-based raw materials (wood, cotton) or textile waste. Chemical additives used during manufacturing are completely removed. After the process, vulcanized fiber possesses improved properties concerning mechanical strength and abrasion as well as corrosion resistance in comparison to its raw materials. Concerning its economic life cycle assessment, low density, electrical insulating capability and balanced properties, vulcanized fiber has a potential, up to now unused, as a light and renewable structural material for applications in automotive or civil engineering industries. Research activities concerning the mechanical properties are insufficient and existing standards are out-of-date. In this work, for the first time a direction-dependent characterization of the process-related anisotropic mechanical properties of the material is realized with the aim to formulate an adequate material model for numerical simulations in the next step.
    view abstract10.3139/120.110929
  • Electrochemical-controlled Characterization of the Corrosion Fatigue Behavior of Creep-resistant Magnesium Alloys DieMag422 and AE42
    Klein, M. and Walther, F.
    Procedia Engineering 160 (2016)
    Magnesium alloys offer a high potential for lightweight construction, however their application range is limited due to their low corrosion resistance. In the present study the corrosion fatigue behaviors of the creep-resistant alloys DieMag422 and AE42 were characterized and compared. In this context, fatigue properties of specimens in sodium chloride solutions as well as under simultaneous galvanostatic anodic polarization were assessed in constant amplitude tests. The results were correlated with the corrosion behavior of the alloys, which was investigated in instrumented immersion tests. Corrosion- and deformation-induced changes in microstructure were observed by light and scanning electron microscopy, yielding a structure-property relationship for a comprehensive understanding of corrosion fatigue processes. The reduction of the corrosion fatigue strength with increasing corrosion impact could be quantitatively correlated with the adjusted corrosion rates. However, different corrosion morphologies between both materials were found, leading to varying influence on the corrosion fatigue behavior. © 2016 The Author(s).
    view abstract10.1016/j.proeng.2016.08.876
  • Fatigue Assessment of Laser Additive Manufactured AlSi12 Eutectic Alloy in the Very High Cycle Fatigue (VHCF) Range up to 1E9 cycles
    Siddique, S. and Imran, M. and Wycisk, E. and Emmelmann, C. and Walther, F.
    Materials Today: Proceedings 3 (2016)
    Selective laser melting (SLM) is a novel technique in laser additive manufacturing which uses laser energy to melt powder material according to the geometry of the computer aided design (CAD) model provided to the SLM system. The process uses a layer-wise manufacturing process which ensures that the manufactured parts possess good mechanical properties. The process is specifically suitable for complex geometries and customized parts which otherwise would be costly and, even, impossible to be manufactured using conventional manufacturing processes. However, for the application of the SLM process for aerospace applications, their performance needs to be investigated in very high cycle fatigue (VHCF) region. This study aims at determining the VHCF behavior of the AlSi12 alloy manufactured by the SLM process. Fatigue characterization has been carried out at frequencies of 20 Hz for high cycle fatigue (HCF) range and 20 kHz for the VHCF range until 1E9 cycles. Optical and scanning electron microscopes were used for microstructural and fracture analysis. The results show that the SLM parts outperform that of cast materials. However microstructural features as well as process-induced defects need to be controlled for a reliable fatigue performance. © 2016 Elsevier Ltd.
    view abstract10.1016/j.matpr.2016.07.004
  • Influence of process parameters on the quality of aluminium alloy en AW 7075 using Selective Laser Melting (SLM)
    Kaufmann, N. and Imran, M. and Wischeropp, T.M. and Emmelmann, C. and Siddique, S. and Walther, F.
    Physics Procedia 83 (2016)
    Selective laser melting (SLM) is an additive manufacturing process, forming the desired geometry by selective layer fusion of powder material. Unlike conventional manufacturing processes, highly complex parts can be manufactured with high accuracy and little post processing. Currently, different steel, aluminium, titanium and nickel-based alloys have been successfully processed; however, high strength aluminium alloy EN AW 7075 has not been processed with satisfying quality. The main focus of the investigation is to develop the SLM process for the wide used aluminium alloy EN AW 7075. Before process development, the gas-atomized powder material was characterized in terms of statistical distribution: size and shape. A wide range of process parameters were selected to optimize the process in terms of optimum volume density. The investigations resulted in a relative density of over 99%. However, all laser-melted parts exhibit hot cracks which typically appear in aluminium alloy EN AW 7075 during the welding process. Furthermore the influence of processing parameters on the chemical composition of the selected alloy was determined. © 2016 The Authors.
    view abstract10.1016/j.phpro.2016.08.096
  • Separation of surface, subsurface and volume fatigue damage effects in AISI 348 steel for power plant applications
    Klein, M. and Starke, P. and Nowak, D. and Boller, C. and Walther, F.
    Materials Testing 58 (2016)
    A wide range of industries including energy, chemistry, pharmacy, textiles, food and drink, pulp and paper, etc. is using stainless steels. Metastable austenitic steels such as used in power plants and chemical industry are subjected to cyclic mechanical and thermal loading in air as well as under the influence of corrosive media. This paper provides an overview on different nondestructive and electrochemical measurement techniques, which allow differentiating fatigue damage effects in total strain controlled multiple and constant amplitude tests with respect to damage appearance on surface, in subsurface area as well as in volume of specimens or components microstructure. In addition to conventional mechanical stress-strain hysteresis curves, electrical resistance, magnetic and open circuit potential measurements have been applied to characterize the cyclic deformation behavior of the metastable austenitic steel AISI 348 (X10CrNiNb18-9) in laboratory air and in distilled water. Based on these results obtained, the paper provides an outlook on the possibility for an efficient (remaining) fatigue life evaluation approach, which is adapted to the needs of the application areas.
    view abstract10.3139/120.110896
  • Short-time procedure for fatigue assessment of beech wood and adhesively bonded beech wood joints
    Myslicki, S. and Vallée, T. and Walther, F.
    Materials and Structures/Materiaux et Constructions 49 (2016)
    Timber as a structural material has experienced a revival for aspects related to sustainability, positive effect on interior building and increased architectural possibilities. To overcome a series of limitations related to partly obsolete traditional techniques, timber engineering sets high expectations on adhesive bonding as a joining method. Research on adhesively bonded timber joints has proven their superiority over mechanical fasteners in terms of strength and stiffness. Research on methods to design adhesively bonded timber joints under static loads is currently ongoing and significant progress has already been achieved. The topic of fatigue behaviour, however, has only been addressed very rarely, largely because “classical” methods for fatigue assessment based on Woehler curves, is extremely time-consuming. This paper aims to fill this gap by describing and validating an advanced time- and cost-efficient approach to quantify the behaviour of bonded timber joints under fatigue. Therefore the fatigue behaviour of beech wood as base material and adhesively bonded beech wood joints as structural elements were investigated by means of a short-time procedure. In this approach, cyclic tests with a stepwise increase of the stress amplitude were performed while the plastic strain amplitude was measured. The plastic strain amplitude gives reliable signals for an estimation of the fatigue strength. This is validated by traditional cyclic tests with constant stress amplitudes. It could be shown that the testing approach provides reliable results and has a high potential in time and cost savings for wood and adhesively bonded wood joints. © 2015, RILEM.
    view abstract10.1617/s11527-015-0640-4
  • Computed tomography for characterization of fatigue performance of selective laser melted parts
    Siddique, S. and Imran, M. and Rauer, M. and Kaloudis, M. and Wycisk, E. and Emmelmann, C. and Walther, F.
    Materials and Design 83 (2015)
    Components manufactured by maturing additive manufacturing techniques like selective laser melting (SLM) find potential competence in several applications especially in automotive and aerospace industries as well as in medical applications like customized implants. The manufactured parts possess better, or at least comparable, yield strength and tensile strength values accompanied with a reduced fracture strain. Though their fatigue performance in the as-built condition is impaired due to surface roughness, it can be sufficiently improved by post-process surface treatments. Even then, there exists a high fatigue scatter due to remnant porosity. Characterization of remnant porosity is necessary for a reliable component design to be employed for cyclic applications. Computed tomography has been used in this study to evaluate the influence of porosity-incited stress concentration on the corresponding fatigue scatter. Microscopic analysis, tensile tests, fatigue tests with continuous load increase and constant amplitudes as well as finite element analysis have been used for this purpose. Critical pore characteristics and a modification in the process scanning strategy have been recommended so that the components can be reliably used in fatigue-loaded applications. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matdes.2015.06.063
  • High-precision deformation and damage development assessment of composite materials by high-speed camera, high-frequency impulse and digital image correlation techniques
    Myslicki, S. and Ortlieb, M. and Frieling, G. and Walther, F.
    Materialpruefung/Materials Testing 57 (2015)
    Although composite materials like wood, vulcanized fiber and carbon reinforced plastic (CFRP) are already investigated by means of their mechanical properties, the abrupt fracture mechanism as well as the deformation behavior right before and after fracture has not been investigated. However, it is marginally investigated for CFRP because of the quite high fracture speed. The knowledge about the damage evolution as the crack start and propagation can help to improve the strength and sensitivity to fracture by improving the materials structure and to utilize these materials for structural applications. For the investigated materials, fracture happens abruptly as it is the nature of composites and the detailed fracture mechanisms could not be detected by conventional measurement techniques. Therefore, an innovative combination of testing devices is presented which is able to fill this gap. Tensile tests were performed to receive conventional stress-strain curves. At the fracture stage, a high-speed camera recorded the fracture process. This information could be combined with digital image correlation (DIC) to visualize the deformation behavior. At the same time acoustic emission (AE) was used to detect the spectrum of mechanical vibrations which gives information about the released energy due to fracture. The challenging triggering of the high-speed camera was solved for each material individually. By using improved light sources, the recording speed could be set up to 2 million frames per second (Mfps). The investigations show different fracture mechanisms for each composite. Wood and vulcanized fiber were also investigated in different directions due to their anisotropy. © Carl Hanser Verlag, München.
    view abstract10.3139/120.110813
  • Influence of process-induced microstructure and imperfections on mechanical properties of AlSi12 processed by selective laser melting
    Siddique, S. and Imran, M. and Wycisk, E. and Emmelmann, C. and Walther, F.
    Journal of Materials Processing Technology 221 (2015)
    Selective laser melting (SLM) offers high potential for manufacturing complex geometries and custom-made parts due to its unique layer-wise production process. A series of samples of AlSi12 have been manufactured by SLM process to study the effect of process parameters and post-build heat treatment on the microstructure and the corresponding mechanical properties. Optical microscope, scanning electron microscope, quasistatic tests, continuous load increase fatigue tests and constant amplitude fatigue tests have been employed for characterization. A remarkable eutectic microstructure, with dendritic width changing with SLM process parameters, has been observed. Relationship between SLM process parameters, resulting microstructure and the consequent changes in mechanical properties has been discussed. Base plate heating has been found critical in controlling the in-process microstructure. Mechanical properties of SLM parts outperform those of conventionally manufactured alloy, and can be varied as per requirement, by altering the build rate, keeping the process costs in control. Fatigue scatter can also be controlled by heating the base plate during the process. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2015.02.023
  • Influence of the production process on the deformation and fatigue performance of friction drilled internal threads in the aluminum alloy 6060
    Wittke, P. and Liu, Y. and Biermann, D. and Walther, F.
    Materials Testing 57 (2015)
    Aluminum alloys are used for enhancement of dynamic range, resource optimization and emission reduction in many fields of traffic engineering, whereby aluminum components are manufactured by means of welded, adhesive and screw joints. Friction drilling, as forming process with subsequent manufacturing of threads, offers the opportunity to produce an internal thread in lightweight profiles with a usable thread depth larger than the profile thickness, making use of local material expansion. Moreover, the direct manufacturing offers a huge potential for time and cost saving in comparison to conventional thread machining. Microstructural characterization of mechanical properties of EN AW-6060 internal threads, both in profile and bulk material specimens, was carried out using tensile tests and fatigue tests in the tensile loading range. A comparison was made between the manufacturing techniques tapping, thread forming and thread milling. The maximum tolerable loads of the profile specimens are about 50 % lower in the quasi-static range and about 25 % lower in the cyclic range in comparison to bulk material specimens. Formed threads show the best and cut threads the worst mechanical properties which were correlated with the production-related profile qualities and changes in microstructure. Multiple step tests prove that the fatigue limit of aluminum internal threads, validated in single step tests until 10(7) cycles, can be reliably estimated by means of plastic strain.
    view abstract10.3139/120.110712
  • Model-based correlation between the electrical resistance and the dislocation structure of the fatigue-loaded ICE-railway wheel steel R7
    Starke, P. and Walther, F.
    Materialpruefung/Materials Testing 57 (2015)
    A weight-optimized component design and a precise estimation of the fatigue life or even the remaining service life for metallic materials and components requires a comprehensive understanding of fatigue mechanisms as well as the systematic investigation of the fatigue behaviour. This becomes even more relevant in the case of complex and highly loaded geometries of components which are used e. g. in rail/wheel systems of high-speed passenger transport applications. To characterize the fatigue behaviour and the structure-mechanical processes, usually mechanical stress-strain hysteresis and recently various types of temperature and electrical resistance measurements are applied. In particular, electrical resistance measurements gain in information by measuring microstructural changes which are related to changes in dislocation density and structure. In addition, resistive measurement techniques can be applied in constant amplitude tests interrupted by load-free sequences and also in service load tests for the characterization of damage evolution processes. The research work presented in this paper is focused on the load- and cycle-dependent correlation of (specific) electrical resistance values with dislocation density measurements for the ICE-railway wheel steel R7. © Carl Hanser Verlag, München.
    view abstract10.3139/120.110681
  • Vpliv električnega toka pri izdelavi intermetalne zlitine niti s sintranjem, aktiviranim z električnim tokom
    Yener, T. and Siddique, S. and Walther, F. and Zeytin, S.
    Materiali in Tehnologije 49 (2015)
    This study focuses on investigating the fabrication of in-situ intermetallic NiTi composites from a powder mixture containing the mass fractions 50 % nickel powder and 50 % titanium powder. The elemental powders were mixed in the stoichiometric ratio corresponding to the NiTi intermetallic molar proportion of 1: 1, ball-milled and uniaxially compressed under a pressure of 170 MPa. Sintering was then carried out for 15 min in a steel mold using the electric-current-activated sintering method. Electric-current values of 1000 A, 1300 A and 2000 A were used for the sintering while keeping the voltage in the range of 0.9 V to 1.2 V. The phases in the samples were analyzed with XRD and their Vickers hardness was measured as (701 ± 166) HV0.05. Energy dispersive X-ray spectroscopy carried out with a scanning electron microscope (SEM-EDS) showed that the microstructures of the samples consist of different phases such as Ti, Ni2Ti3, NiTi2, Ni3 Ti and TiO2 as a function of electric current. The XRD analysis also supported the SEM-EDS results. The nano-indentation technique was used to determine the elastic modulus of different phases.
    view abstract10.17222/mit.2014.161
  • Chemical-Mechanical Characterization of the Creep-Resistant Mg-Al-Ca Alloy DieMag422 Containing Barium - Quasistatic and Cyclic Deformation Behavior in Different Defined Corrosion Conditions
    Wittke, P. and Klein, M. and Walther, F.
    Materials Testing 56 (2014)
    The influence of corrosion on the microstructure and the depending mechanical properties was investigated for the creep-resistant Mg-Al-Ca alloy DieMag422 containing barium. In order to investigate the corrosion behavior, potentio-dynamic polarization measurements and immersion tests were performed in pH7 without and with sodium chloride. Specimens in defined corrosion conditions were investigated by SEM for microstructure-related assessment of corrosion mechanisms. Strength and strain properties of non-corroded and corroded specimens were compared in tensile tests, underlining a significant decrease of tensile strength and fracture strain with increasing corrosion grade. The fatigue behaviour of the DieMag422 alloy in different corrosion conditions was characterized in multiple step and single step tests by means of mechanical stress-strain-hysteresis, temperature and electrical resistance measurements. Load increase tests allow to estimate the endurance limit and to determine the stress amplitude leading to fracture with one specimen. Fatigue results also showed a significant decrease in the estimated endurance limit and the failure stress with increasing corrosion grade. The applied physical measurement techniques can be equivalently used for the characterization of the fatigue behavior and representation of the actual fatigue state. The thermal and electrical materials responses were proportional to cyclic plastic deformation and provide the opportunity to evaluate the actual fatigue state of components under service loading in terms of condition monitoring. © Carl Hanser Verlag.
    view abstract10.3139/120.110519
  • Corrosion fatigue behaviour of creep-resistant magnesium alloy mg-4al-2ba-2ca
    Wittke, P. and Klein, M. and Walther, F.
    Procedia Engineering 74 (2014)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba- 2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with a single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2014.06.228
  • Effects of defects in laser additive manufactured Ti-6Al-4V on fatigue properties
    Wycisk, E. and Solbach, A. and Siddique, S. and Herzog, D. and Walther, F. and Emmelmann, C.
    Physics Procedia 56 (2014)
    Laser Additive Manufacturing (LAM) enables economical production of complex lightweight structures as well as patient individual implants. Due to these possibilities the additive manufacturing technology gains increasing importance in the aircraft and the medical industry. Yet these industries obtain high quality standards and demand predictability of material properties for static and dynamic load cases. However, especially fatigue and crack propagation properties are not sufficiently determined. Therefore this paper presents an analysis and simulation of crack propagation behavior considering Laser Additive Manufacturing specific defects, such as porosity and surface roughness. For the mechanical characterization of laser additive manufactured titanium alloy Ti-6Al-4V, crack propagation rates are experimentally determined and used for an analytical modeling and simulation of fatigue. Using experimental results from HCF tests and simulated data, the fatigue and crack resistance performance is analyzed considering material specific defects and surface roughness. The accumulated results enable the reliable prediction of the defects influence on fatigue life of laser additive manufactured titanium components. © 2014 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.phpro.2014.08.120
  • Microstructure-based characterization of the corrosion behavior of the creep resistant Mg-Al-Ba-Ca alloy DieMag422
    Klein, M. and Kuhlmann, F. and Wittke, P. and Dieringa, H. and Walther, F.
    Materials and Corrosion 65 (2014)
    The corrosion behavior as well as the microstructure in initial state and after corrosive deterioration were investigated for the newly developed Mg-Al-Ba-Ca alloy DieMag422. In order to investigate the corrosion behavior, potentiodynamic polarization measurements and immersion tests were performed in different NaCl concentrations. The microstructure was observed using light microscopy and combined SEM and EDX studies of the specimens before and after corrosive deterioration. Potentiodynamic polarization measurements and immersion tests showed that DieMag422 has a strong susceptibility to corrosion in NaCl solutions. Light microscope and SEM studies revealed that the difference in the corrosion behavior can be attributed to the different occurrence of the anodic a-Mg phase and the cathodic Ca-rich phase. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201307327
  • Microstructure-oriented fatigue assessment of construction materials and joints using short-time load increase procedure
    Walther, F.
    Materialpruefung/Materials Testing 56 (2014)
    Stress-strain-hysteresis, change in deformation-induced temperature and change in DC-based electrical resistance measurements are applied for the detailed characterization of structural-mechanical processes in construction materials and joints under multiple-step and single-step fatigue loading. Results concerning the influence of joining technologies on austenitic steel AISI304, carbon-fiber reinforced polymers (CFRP) and beech wood materials, of environmental media on magnesium alloys Mg-4Al-2Ba-2Ca (DieMag422) and Mg-10Gd-lNd, and of manufacturing processes on titanium alloy Ti-6A1-4V and wood-plastic composites (WPC) are discussed. The load- and cycle-dependent change in microstructure was investigated by light and electron microscopy and correlated with fatigue properties, to reach a preferably precise description of process structure property relationship in a qualitative and quantitative manner. The time-efficient load increase procedure applied for evaluation of joining, environmental and manufacturing influence on fatigue performance is suitable for production-accompanied usage. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/120.110592
  • Compression-creep response of magnesium alloy DieMag422 containing barium compared with the commercial creep-resistant alloys AE42 and MRI230D
    Dieringa, H. and Huang, Y. and Wittke, P. and Klein, M. and Walther, F. and Dikovits, M. and Poletti, C.
    Materials Science and Engineering A 585 (2013)
    The development of creep-resistant magnesium alloys that avoid the use of rare-earth alloying elements is an important area of research. The creep response of Mg-Al-Ca alloy containing barium (DieMag422) was compared to that of commercially available creep resistant magnesium alloys AE42 and MRI230D. The creep tests were performed between 175°C and 240°C at stresses between 60MPa and 120MPa. From the temperature and stress dependence of the minimum creep rate, the apparent activation energy Qc and the stress exponent n for creep were calculated. The concept of a threshold stress was applied. True stress exponents nt close to 5 were calculated. Microstructural investigations and phase analysis were performed on the as-cast materials as well as after creep. Fine precipitates could be identified that justified application of the concept of threshold stress. The DieMag422 alloy shows an improvement in creep resistance at low stresses compared with the other two alloys AE42 and MRI230D. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.07.041
  • Non-destructive subsurface damage monitoring in bearings failure mode using fractal dimension analysis
    Holweger, W. and Walther, F. and Loos, J. and Wolf, M. and Schreiber, J. and Dreher, W. and Kern, N. and Lutz, S.
    Industrial Lubrication and Tribology 64 (2012)
    Purpose: Bearings in field applications with high dynamic loading, e.g. wind energy plants, suffer from sudden failure initiated by subsurface material transformation, known as white etching cracks in a typical scale of μm, preferably around the maximum Hertzian stress zone. Despite many investigations in this field no precise knowledge about the root cause of those failures is available, due to the fact that failure under real service conditions of wind energy plants differs from what is known from test rig results in terms of contact loading, lubrication or dynamics. The purpose of this paper is to apply Barkhausen noise measurement to a full bearing test ring running under conditions of elastohydrodynamic lubrication (EHL) with high radial preload. Design/methodology/approach: Full bearing tests are carried out by use of DGBB (Deep Grove Ball Bearings) with 6206 specification, material set constant as 100Cr6, martensitic hardening, 10-12 percent maximum retained austenite and radial preload of 3500 MPa. Speed is set 9000 rpm, temperature is self setting at 80°C by test conditions. For tests, synthetic hydrocarbon base oil (Poly-α-Olefine) with a 1 percent amount of molydenum dithiophosphate (organic chain given as 2-ethylhexyl) was used. Findings: Non-destructive fractal dimension analyses by use of Barkhausen noise measurements is of versatile value in terms of recording bearing manufacturing processes, but can also be part of non-destructive condition monitoring of bearings in field applications, where predictive reactive maintenance is crucial for availability of the plant. Research limitations/implications: Barkhausen noise signal recording may also be valuable for case studies related to microstructure changes of steel under operation conditions. Bearings are exposed in plenty of conditions to phenomena such as straying currents, subsequently straying magnetic fields. Hardly anything is known about how microstructure of bearing steel is susceptible to such conditions. This will be part of further studies. Originality/value: Results given in the paper show that sudden bearing failure, according to formation of subsurface material property changes might be driven by activities of dislocations. Since those activities start with sequences of stress field-induced formation of domains, later by formation of low-angle subgrains, and at least phase transformation, recording of the Barkhausen signal would lead to real predictive condition monitoring in applications where a highly dynamic loading of the contact, even with low nominal contact pressure leads to sudden failure induced by white etching. © Emerald Group Publishing Limited.
    view abstract10.1108/00368791211218650
  • Studies of the contribution of alternating electromagnetic fields toward material fatigue in 100Cr6
    Holweger, W. and Wolf, M. and Walther, F. and Trojahn, W. and Mütze, A. and Kunzmann, J. and Schreiber, J. and Mayer, J. and Reichelt, M.
    Industrial Lubrication and Tribology 64 (2012)
    The purpose of this paper is to show how controlled exposure of electromagnetic fields toward bearing steel vulnerates the microstructure. The ability of Barkhausen Noise signal processing is used for detecting phenomena such as dislocation and subgrain formation processes as the beginning of later failures. A Barkhausen noise signal measurement equipment is used for detecting subsurface distress of 100Cr6 as a function of the applied electromagnetic and mechanical stress. Barkhausen noise signal is mathematically processed by use of fractal dimension analysis. The paper cleary reveals significant impact of electromagnetic field in junction with mechanical loading. Electromagnetic impact depends on the magnitude of the field. Research limitations are given by the fact that in real field applications, e.g. wind power plants, bearings are exposed by multiple influences and the methodology is not applicable to those conditions. The methodology can be applied to real field applications in condition monitoring systems. Up to now, no reasonable on-line measurement is in use determining sub surface fatigue phenomena. The paper hence, reveals the possibility to raise condition monitoring into a new perspective. The use of Barkhausen noise signal processing, as presented here, is original with respect to real field applications, such as wind power plants with a high demand in condition monitoring, especially off-shore plants. © 2012, Emerald Group Publishing Limited
    view abstract10.1108/00368791211249629
  • "phybal" a short-time procedure for a reliable fatigue life calculation
    Starke, P. and Walther, F. and Eifler, D.
    Advanced Engineering Materials 12 (2010)
    The reliable calculation of the fatigue life of high-strength steels and components requires the systematic investigation of the cyclic deformation behaviour and the comprehensive evaluation of proceeding fatigue damage. Besides mechanical stress-strain hysteresis measurements, temperature and electrical resistance measurements were used for the detailed characterisation of the fatigue behaviour of the steel SAE 4140 in one quenched and tempered, one normalised, one bainitic and one martensitic condition. To guarantee optimal operation conditions the new fatigue life calculation method ́PHYBAĹ on the basis of generalised Morrow and Basquin equations was developed. It is a short-time procedure which requires the data of only three fatigue tests for a rapid and nevertheless precise determination of S-N (Woehler) curves. Consequently, ́PHYBAĹ provides the opportunity to reduce significantly experimental time and costs compared to conventional test methods. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900344
  • Monotonic and cyclic deformation behaviour of the SiC particle-reinforced aluminium matrix composite AMC225xe
    Smaga, M. and Walther, F. and Eifler, D.
    Advanced Engineering Materials 12 (2010)
    The monotonic and cyclic deformation behaviour of the aluminium matrix composite AMC225xe - i.e., the aerospace grade aluminium alloy AA 2124 reinforced with 25vol.-% ultrafine SiC particles - is characterised in detail on the basis of mechanical stress-strain hysteresis curves as well as temperature and electrical resistance measurements. A pronounced difference in plastic strain response is observed between tension and compression under monotonic and cyclic loading. ln fully reversed stress-controlled constant amplitude tests, negative plastic mean strains developed. The cyclic deformation behaviour ofAMC225xe is characterised by pronounced initial cyclic hardening. The endurance limit is reliably estimated in continuous load increase tests. ln particular, electrical resistance data are used as input parameters for fatigue life calculations analogous to the Basquin equation. Microstructural details are investigated by light and scanning electron microscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900345
  • fatigue

  • magnesium alloys

  • mechanical properties

  • microstructure

  • scanning electron microscopy

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