Dr.-Ing. Till Clausmeyer

Institute of Forming Technology and Lightweight Components
TU Dortmund University

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  • Methods for measuring large shear strains in in-plane torsion tests
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology 287 (2021)
    The in-plane torsion test achieves true strains far beyond 1.0 for sheet metals, especially using specimens with circular grooves. The accurate measurement of these high strains is a challenge for the conventional digital image correlation (DIC). Thus, the determination of flow curves is limited and fracture strains for very ductile materials cannot be measured. A new grooved specimen is introduced to avoid strain localization. Shear stress and shear strain along a defined area in the groove are constant so that strains can be measured independently of the DIC system setting without error due to strain localization. Furthermore, three methods for the measurement of very high shear strains in the in-plane torsion test are presented: Firstly, the limit of the optical strain measurement is extended by multiple renewal of the digital image correlation (DIC) pattern on the samples. Secondly, the shear strain for the planar specimen is calculated exactly from the rotation angle curve. Lastly, a new incremental method is presented. This method enables to determine shear strains for plane and grooved specimen exactly by only measuring the torque and the angle of rotation. All methods were applied for three steel sheet materials namely DP1000, CP1000 and DC04. The equivalent strain in a grooved in-plane torsion test of sheet steel DC04 was determined as 3.3 with the new incremental method. Such high strains far exceed conventional methods for determining the flow curve. © 2019 The Author(s)
    view abstract10.1016/j.jmatprotec.2019.116516
  • Adiabatic blanking of advanced high-strength steels
    Schmitz, F. and Winter, S. and Clausmeyer, T. and Wagner, M.F.-X. and Tekkaya, A.E., (1)
    CIRP Annals 69 (2020)
    Adiabatic blanking of advanced high-strength steels with initial flow stresses above 1300 MPa is investigated. The blanked edge exhibits a unique S-shape. Localisation and properties of shear bands are analysed in shear-compression tests. A shear-compression stress state before separation leads to blanked edges without fracture zone, burr, or roll-over. Numerical modelling predicts the characteristic shape of the blanked edge satisfactorily. Physics-based models reveal that the strain rate sensitivity of the workpiece material is the key parameter affecting the width and the surface hardness of the shear band. Stress triaxiality and strain rate sensitivity determine the minimum size of radii in complex parts blanked without failure. © 2020 CIRP
    view abstract10.1016/j.cirp.2020.03.007
  • 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
  • Micromechanical modeling of DP600 steel: From microstructure to the sheet metal forming process
    Vajragupta, N. and Maassen, S. and Clausmeyer, T. and Brands, D. and Schröder, J. and Hartmaier, A.
    Procedia Manufacturing 47 (2020)
    This study proposes a micromechanical modeling scheme to predict relevant mechanical behavior of DP600 steel for the sheet metal forming process. This study can be divided into two parts which are the prediction of the advanced anisotropic initial yield function by means of microstructure-based simulations and the investigation of microstructure changes during the sheet metal forming process. Firstly, based on the quantitative microstructure characterization of DP600 steel by EBSD analysis, the obtained statistical information of important microstructural features is used to generate a microstructure model with the help of an advanced dynamic microstructure generator (ADMG), which combines a particle simulation method with radical Voronoi tessellation. In the next step, finite element simulations with a non-local crystal plasticity model for the individual grains are conducted. With the help of these simulations, the crystal plasticity parameters are adapted to match the experiments. The resulting parameterized microstructure model of DP600 steel is then applied to various loading conditions to investigate the corresponding mechanical responses. For the second part, macroscopic simulations of the bending process are performed and local deformation fields of the location of interest are captured and imposed as boundary conditions on the microstructure model to study the changes in the microstructural features. © 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.2020.04.347
  • Numerical investigation of damage in single-step, two-step, and reverse deep drawing of rotationally symmetric cups from DP800 dual phase steel
    Nick, M. and Feuerhack, A. and Bergs, T. and Clausmeyer, T.
    Procedia Manufacturing 47 (2020)
    Damage evolution in deep drawing processes has an influence on the service properties of manufactured components. Knowledge and control of the damage state will therefore allow a reduction in safety factors and, consequently, in sheet thickness. To investigate the damage after deep drawing of rotationally symmetric cups, a LEMAITRE type damage model calibrated in previous works was used in the Finite Element (FE) simulation of the deep drawing process. Three process variants were investigated: single-step deep drawing, two-step deep drawing, and reverse deep drawing. The single-step deep drawing process was used as a reference to evaluate the damage evolution in two-step and reverse deep drawing. In both two-step and reverse deep drawing, the overall damage values did not decrease when compared to single-step deep drawing. However, in reverse deep drawing specifically, the location of maximum damage in the cup wall was moved from an area near the cup floor towards the edge of the cup. In real-life components, this control over the location of maximum damage will allow a tailoring of the damage state in the component to the expected service loads, thereby increasing service life or allowing a reduction in sheet thickness, respectively. © 2020 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.promfg.2020.04.195
  • On mesh dependencies in finite-element-based damage prediction: application to sheet metal bending
    Sprave, L. and Schowtjak, A. and Meya, R. and Clausmeyer, T. and Tekkaya, A.E. and Menzel, A.
    Production Engineering 14 (2020)
    The properties of a local and a regularised gradient-enhanced continuum damage model are highlighted and both types of models are applied to the simulation of an air bending process. Constitutive relations are summarised for both Lemaitre-type models and a brief description of their implementation into Abaqus user material subroutines is given. With (several) material parameters obtained from a basic parameter identification process, an air bending experiment is simulated with different mesh densities. By means of the damage evolution as well as the distribution of representative damage and hardening variables, the mesh dependence of the local model in contrast to the mesh independence of the gradient-enhanced model is analysed for two air bending processes with different die width. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00937-9
  • Potential and status of damage controlled forming processes
    Hirt, G. and Tekkaya, A.E. and Clausmeyer, T. and Lohmar, J.
    Production Engineering 14 (2020)
    In modern process design of metallic components, the influence of the metal forming process on the component properties can be taken into account. However, damage occurring concurrent to forming cannot be accounted for yet. Due to the complex multi-scale, multi-mechanism nature of damage, it is very challenging to predict its evolution through any metal forming process chain. In order to enable such damage controlled forming processes in the future three research questions need to be addressed in detail: How are the mechanisms governing the damage initiation and evolution in metals best characterized? How can the damage mechanisms be described and the damage evolution be predicted using models? How do forming processes influence the damage evolution? Answering these questions and considering damage during process design will in the long term lead to improved lightweight components that do not require conventional safety factors, as their performance is already fully known. © 2020, The Author(s).
    view abstract10.1007/s11740-019-00948-6
  • Prediction and analysis of damage evolution during caliber rolling and subsequent cold forward extrusion
    Schowtjak, A. and Wang, S. and Hering, O. and Clausmeyer, T. and Lohmar, J. and Schulte, R. and Ostwald, R. and Hirt, G. and Tekkaya, A.E.
    Production Engineering 14 (2020)
    Damage in the sense of voids influences material as well as product properties and thus is important for the performance of formed components. In this work, the influence of caliber rolling prior to a cold forward extrusion process in terms of damage evolution is investigated. To this end, an uncoupled Lemaitre-type damage model considering the effects of strain-rate and temperature dependent plasticity is employed. The damage-related parameters are identified in an inverse manner based on notched tensile tests. Numerical investigations of the forming processes show that damage increases throughout the process chain. The simulations are validated in terms of density measurements and microscopic investigations. The experimental measurements and numerical simulations are in good qualitative agreement. It is shown that the influence of rolling on subsequent cold forward extrusion is negligible for this experimental setup. For other process parameters, however, the influence could be significant. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00935-x
  • Prediction of ductile damage in the process chain of caliber rolling and forward rod extrusion
    Clausmeyer, T. and Schowtjak, A. and Wang, S. and Gitschel, R. and Hering, O. and Pavliuchenko, P. and Lohmar, J. and Ostwald, R. and Hirt, G. and Erman Tekkaya, A.
    Procedia Manufacturing 47 (2020)
    Many metal forming processes involve several steps, which influence the shape and properties of the final component. The previous manufacturing process of the semi-finished component influences the properties of extruded components. The authors analyze the evolution of damage and voids in the sequence of caliber rolling to cold forward rod extrusion. The analysis is performed with the help of a variant of the Lemaitre model, microstructural analysis of the void area fraction and density measurements. The numerical analysis of this process chain makes use of a fully three-dimensional simulation approach. Even though the damage distribution is non-axisymmetric in caliber rolling, the distribution is almost axisymmetric after cold forward extrusion. The shoulder opening angle in extrusion is varied to compare model predictions with experiments. The decrease in density is largest for the largest shoulder opening angle and smallest for the smallest shoulder opening angle. The simulations agree qualitatively well with the experiments in term of the measured void area fraction and the density changes. The quantitative comparison reveals differences of one to two orders of magnitude. © 2020 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.promfg.2020.04.201
  • Shifting value stream patterns along the product lifecycle with digital twins
    Dittrich, M.-A. and Schleich, B. and Clausmeyer, T. and Damgrave, R. and Erkoyuncu, J.A. and Haefner, B. and de Lange, J. and Plakhotnik, D. and Scheidel, W. and Wuest, T.
    Procedia CIRP 86 (2020)
    The concept of digital twins promises high potentials for product design, manufacturing, user experience and recycling. Thus, digital twins have received increasing interest in academia and industry. However, the actual benefits of digital twins remain in many cases unclear. This article aims to summarize selected recent developments in this field and demonstrate use cases from different phases of the product lifecycle. For that purpose, examples from the design, manufacturing, use and recycling phase are presented. In a subsequent discussion, ideas for new value stream patterns using digital twins are envisioned and research questions are derived. © 2019 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.01.049
  • Strain hardening under large deformation for AA5182
    Zhang, C. and Wang, Y. and Shang, H. and Wu, P. and Fu, L. and Lou, Y. and Clausmeyer, T. and Tekkaya, A.E. and Zhang, Q.
    IOP Conference Series: Materials Science and Engineering 967 (2020)
    In this study, an aluminium alloy of AA5182 is taken as the research object to study strain hardening under large plastic deformation. Tensile tests are done for four specimens, including dog-bone specimens, notched specimens, specimens with a central hole and in-plane shear specimens. Bulging tests are also conducted to measure strain hardening under balanced biaxial tension. In addition, an experimental method called in-plane torsion test is also used for shear loading. At least three experiments are completed for each type of specimens along the rolling direction (RD), diagonal direction (TD), and transverse direction (DD). The stroke of each tests is measured by a digital image correlation (DIC) system, and the load-stoke curves were obtained for the tests. Combined with an inverse engineering method, the strain hardening properties are calibrated for the alloy under different loading conditions of shear, uniaxial tension, plane strain tension, and balanced biaxial tension. The strain hardening under various loading conditions is compared and modelled by various yield functions to evaluate their performance. It is concluded that inverse engineering approach is a simple but powerful method to obtain the stress-strain curve up to large plastic deformation. It is also observed that it needs to develop yield functions to model yielding behaviour under complex loading conditions. © 2020 Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/967/1/012030
  • Testing of formed gear wheels at quasi-static and elevated strain rates
    Clausmeyer, T. and Gutknecht, F. and Gerstein, G. and Nürnberger, F.
    Procedia Manufacturing 47 (2020)
    Geared components can be manufactured from sheet metals by sheet-bulk metal forming. One relevant load case in service are overload events, which might induce elevated strain rates. To determine the characteristic hardening and fracture behavior, specimens manufactured from the deep-drawing steel DC04 were tested with strain rates ranging from 0.0001 to 5 s−1. The gear wheels manufactured by sheet-bulk metal forming are tested at crosshead velocities of 0.08 mm/s and 175 mm/s. The tests are analyzed by measuring deformed geometry and hardness. While the tensile tests results show obvious strain-rate dependency, the hardness measurements show no strain-rate depended effect. The analyses are complemented by finite-element-simulations, which assess the homogeneity of deformation and point out the mechanisms of failure. Both coupled and uncoupled ductile damage models are able to predict the critical areas for crack initiation. The coupled damage model has slight advantages regarding deformed shape prediction. © 2020 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.promfg.2020.04.191
  • Effect of plastic strain and ductile damage on elastic modulus of multiphase steel and its impact on springback prediction
    Münstermann, S. and Sparrer, Y. and Yao, Y. and Lian, J. and Meya, R. and Clausmeyer, T. and Tekkaya, A.E.
    AIP Conference Proceedings 2113 (2019)
    The springback behavior of cold formable steel of grade DP 1000 is assessed experimentally and numerically. Bending tests according to the VDA test specification are interrupted at three characteristic roller displacements, and the unloading characteristics are investigated. The tests are simulated with four different material models: i.) elastic plastic simulation with constant elastic modulus, ii.) elastic plastic simulation with plastic strain-dependent elastic modulus, iii.) damage mechanics simulation with constant elastic modulus, iv.) damage mechanics simulation with plastic strain-dependent elastic modulus. To provide the required input data for these simulations, the effect of plastic strain on the elastic modulus is studied based on uniaxial tensile tests, whereas the possible effect of ductile damage evolution on springback properties is numerically captured by the modified Bai-Wierzbicki model. The studies reveal that consideration of plastic strain effects on the elastic modulus adds a significant amount of accuracy to the numerical simulations, whereas the consideration of ductile damage does not improve the simulation results as much. This observation has to be related to the fact that steel DP 1000 is characterized by late damage initiation with rapid subsequent damage accumulation. © 2019 Author(s).
    view abstract10.1063/1.5112739
  • Investigation of evolving yield surfaces of dual-phase steels
    Hou, Y. and Min, J. and Guo, N. and Lin, J. and Carsley, J.E. and Stoughton, T.B. and Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology (2019)
    The aim of this paper is to describe the evolving yield behavior of dual-phase steels during plastic deformation characterized for ten loading paths using a series of mechanical tests including uniaxial tension, uniaxial compression, in-plane torsion and cruciform biaxial tension with the aid of digital image correlation techniques for strain measurement. Large plastic strains in the gauge area of cruciform specimens tested were enabled by a laser deposition process to strengthen the arms in order to measure deformation behavior of the sheet without arbitrarily thinning the gauge section. Experimental yield loci were determined for three dual phase steels with different strength levels up to equivalent plastic strains of ˜0.11 for DP590, ˜0.07 for DP780, ˜0.05 for DP980, respectively. Several existing anisotropic yield criteria under both associated flow rule (AFR) and non-associated flow rule (non-AFR) were applied to describe the anisotropic yield behavior of these DP steels. A comparative study was preformed to validate prediction accuracy of yield criteria with experimental measurements including yield loci, yield stresses and rφ -values under uniaxial tension in seven orientations as well as yield stresses and rb -value under equi-biaxial tension. The results show that non-AFR significantly improved prediction accuracy of both stresses and r-values simultaneously. Under non-AFR, an order of two in the yield stress function is sufficient to accurately predict flow stresses. The evolution of both yield stress and plastic potential surfaces of DP steels were illustrated by changing parameters in the yield criterion as functions of equivalent compliance λ¯. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116314
  • Damage mechanisms and mechanical properties of high-strength multiphase steels
    Heibel, S. and Dettinger, T. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Materials 11 (2018)
    The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties. © 2018 by the authors.
    view abstract10.3390/ma11050761
  • Evaluation of micro-damage by acoustic methods
    Gerstein, G. and Briukhanov, A. and Gutknecht, F. and Volchok, N. and Clausmeyer, T. and Nürnberger, F. and Tekkaya, A.E. and Maier, H.J.
    Procedia Manufacturing 15 (2018)
    Several methods for the determination of integral damage in sheet metal are reviewed and investigated at the example of the ferritic steel DC04. A novel method to determine damage in cylindrical rings is proposed. Such a method is useful for the measurement of damage in components manufactured by sheet-bulk forming. In sheet-bulk-forming or precision forging sheets with a thickness in the range of 1-5 mm are processed by making use of an intended three-dimensional material flow to form toothed components such as gears. Damage leads to the modification of physical properties, such as Young's modulus and the related resonance frequency. Young's moduli were determined by various methods such as tensile tests and the frequency of natural oscillations of rectangular samples as well as cylindrical rings. Additionally, the change in the propagation velocity of ultrasonic waves in rectangular bars was examined as a damage criterion and reference measurements of damage by electron microscopy were carried out. The damage values obtained by electron microscopy are consistent with the results of the other investigated methods. © 2018 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.promfg.2018.07.273
  • Experimental setup to characterize flow-induced anisotropy of sheet metals
    Gutknecht, F. and Gerstein, G. and Traphöner, H. and Clausmeyer, T. and Nürnberger, F.
    IOP Conference Series: Materials Science and Engineering 418 (2018)
    For many metals, a transient variation of the yield stress can be observed when changing the orientation of a load-path. Such behavior affects the manufacturing process itself, e.g. by increasing forming forces, altered material properties or springback of the manufactured components. Hence, the aim of this work is to develop a novel experimental setup to characterize hardening effects due to flow-induced anisotropy for sheet metals. The proposed experiment consists of two subsequent forming operations. Initially, a hydraulic bulge test is conducted, followed by torsion of the hemispherical preformed sheet. Such approach captures the effects of flow-induced anisotropy like cross hardening as could be proved for the example of the conventional deep-drawing steel DC04. The benefits of the presented setup are (i) high plastic strains in the pre-loading step and (ii) determination of several combinations of pre- and subsequent loading. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/418/1/012085
  • Influence of cutting tool stiffness on edge formability
    Levin, E. and Larour, P. and Heuse, M. and Staupendahl, D. and Clausmeyer, T. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 418 (2018)
    For the evaluation of the forming behaviour of cut edges of AHSS the Hole Expansion Test (HET) standardized in the ISO 16630 is generally used. However, the observed Hole Expansion Ratio (HER) is prone to significant scatter. One reason for this scatter is the subjective observation of the through thickness crack by the machine operator. Additionally, the ISO is not very specific in its description of the test setup and material preparation, actually allowing great variations in the cutting process. Although the nominal cutting clearance is specified, a low stiffness of the punching machine or tool can cause a non-uniform clearance along the circumference. Additionally, an oblique position of the punch can cause an angled or eccentric sheared hole, resulting in additional crack initiation sites on the shear cut surface. FE simulations are used to investigate the stiffness effects of a C-frame press design. A comparative round robin analysis based on ISO16630 was performed by cross-testing both cutting and hole expansion setups for a high strength hot rolled steel grade. Significant differences were registered in the HER values, whereby the cutting process was identified to have the largest influence on HER variation. The homogeneity of the burnished zone along the circumference was observed to give valuable information about the cut edge quality and subsequent level of HER values. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/418/1/012061
  • Influence of manufacturing processes on material characterization with the grooved in-plane torsion test
    Traphöner, H. and Heibel, S. and Clausmeyer, T. and Tekkaya, A.E.
    International Journal of Mechanical Sciences (2018)
    In-plane torsion tests offer advantages such as a proportional loading path and a homogeneous stress and strain distribution when characterizing the material behavior in the state of in-plane shear. The use of a grooved specimen is mandatory for the characterization of the damage behavior. The manufacturing of the groove by turning, milling, and electrical discharge machining for a DP600, DP1000, and CP1000 showed a strong influence on the experimentally measurable strain values at failure. Fine machining by milling displayed good results for all tested materials. The notch-effect in turned grooves led to an early fracture initiation. A straightforward parameter identification scheme for the Hosford-Coulomb fracture criterion as proposed by [16] was used to show the sensitivity of fracture curves on the experimental database. © 2018 Elsevier Ltd.
    view abstract10.1016/j.ijmecsci.2017.12.052
  • Material characterization for plane and curved sheets using the in-plane torsion test – An overview
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology 257 (2018)
    The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains beyond 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to truely ideal simple shear during the entire process. The full-field strain and stress analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without loosing the shear state. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves of sheets with curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape sheets were performed. Finally, first results for the determination of the local strength at arbitrary positions of a sheet component are presented. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.02.030
  • Modelling of the blanking process of high-carbon steel using Lemaitre damage model
    Isik, K. and Yoshida, Y. and Chen, L. and Clausmeyer, T. and Erman Tekkaya, A.
    Comptes Rendus - Mecanique 346 (2018)
    This paper presents a methodology to model a blanking process using a continuum mechanical damage model. A variant of the Lemaitre model, in which the quasi-unilateral conditions are taken into consideration to modify the damage behavior under compressive stress states, is selected as material model. S45C high-carbon steel is analyzed experimentally. To characterize the damage behavior of the material, notched round bar tensile tests with three different notch radii (6 mm, 10 mm, and 20 mm) using image analysis are performed. Using digital image processing, the strain at the deformation zone can be computed for the load–strain curves. Those curves are used as an objective function to determine the parameters of the Lemaitre damage model. The experimental results are compared with the results of the FE analysis of the tensile test. The identified model parameters are used in numerical investigations of axisymmetric blanking. The effect of the model's extension to quasi-unilateral damage evolution is discussed. The crack progress in high-carbon steel sheet during blanking and the final sheared part morphology are predicted and compared with the experimental results. Sheared surface and burr height obtained by the analysis coincide with the results of the blanking experiment. © 2018 Académie des sciences
    view abstract10.1016/j.crme.2018.05.003
  • Analysis of dislocation structures in ferritic and dual phase steels regarding continuous and discontinuous loading paths
    Gerstein, G. and Clausmeyer, T. and Gutknecht, F. and Tekkaya, A.E. and Nürnberger, F.
    Minerals, Metals and Materials Series Part F6 (2017)
    In sheet-bulk metal forming processes the hardening behavior of the material depends on the sequence of deformation steps and the type of deformation. Loading path changes induce transient hardening phenomena. These phenomena are linked to the formation and interaction of oriented dislocation structures. The aim of this study is to investigate the effect of continuous and discontinuous loading path changes on the dislocation microstructure in ferritic and ferritic-martensitic dual-phase steel, respectively. For the experiments a biaxial test stand was used, which permits to continuously change the load from tension to shear. In the ferrite single-phase steel transmission-electron microscopy reveals a reduced evolution of oriented dislocation structures for continuous loading path changes compared to discontinuous loading path changes. This evolution is further decreased in dual-phase steel compared to the ferritic steel. Microstructural results for the ferritic steel are accompanied by simulation results with a transient hardening model. © The Minerals, Metals & Materials Society 2017.
    view abstract10.1007/978-3-319-51493-2_20
  • Experimental analysis of anisotropic damage in dual-phase steel by resonance measurement
    Gerstein, G. and Clausmeyer, T. and Isik, K. and Nürnberger, F. and Tekkaya, A.E. and Bruchanov, A.A. and Maier, H.J.
    International Journal of Damage Mechanics 26 (2017)
    The ductile damage in deformed dual-phase steel sheets (DP600) was investigated based on measurements of the degradation of the direction-dependent Young's modulus. The study focuses on the material-induced damage anisotropy in such advanced high-strength steel. The elastic properties in the direction of applied loading of the deformed sheets were determined by measuring the resonance frequency of rectangular samples. The material was investigated in the as-delivered condition and after annealing at 220 for 48 h. Tensile strains of up to 10% were applied after annealing. Tensile tests were performed in different directions with respect to the rolling direction to determine the evolution of damage in different directions. The comparison of the obtained results with the electron micrographs shows that the damage in the steel sheets occurs in the form of nano and micro damages near the grain boundary and interfaces of phases. The maximum decrease of the Young's modulus in the transverse direction was observed for the largest applied deformation of 10% tensile strain in the transverse direction. An efficient calculation method to obtain information on the distribution of anisotropy in the plane of the sheet was applied. This calculation method relies on an efficient representation of the material's texture. In order to assess the influence of texture, the texture was determined experimentally. © 2017 SAGE Publications.
    view abstract10.1177/1056789516650245
  • Failure assessment in sheet metal forming using a phenomenological damage model and fracture criterion: Experiments, parameter identification and validation
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    In this contribution microstructural and macroscopic experimental findings are used to calibrate the phenomenological damage model GISSMO (Generalized Incremental Stress State dependent Model) and the fracture criterion FFL/SFFL (Fracture Forming Limit line/ Shear Fracture Forming Limit line) to assess failure in sheet metal forming simulation. It is shown that macroscopic failure in a commercial dual-phase steel is initiated through a combination of ductile damage mechanisms and local shear banding at a characteristic, stress-state dependent optical or tactile measureable fracture strain. The parameter identification for both models is based on ductile fracture experiments representing characteristic stress states. GISSMO is calibrated inversely using experimental stress-strain curves, optical measured fracture strains and simulation data. The FFL and SFFL are constructed in a direct manner with the optically and tactilely measured fracture strains. Both models are validated comparatively on a cross-die cup showing ductile fracture with slight necking. The use of the fracture criterion in combination with a direct method of determining the fracture lines based on tactile strain measurements leads to an overestimation of the instant of fracture initiation. With the inversely identified parameters of the phenomenological damage model the onset of fracture initiation can be accurately predicted. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.1065
  • High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process
    Winter, S. and Schmitz, F. and Clausmeyer, T. and Tekkaya, A.E. and Wagner, M.F.-X.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    In the automotive industry, advanced high strength steels (AHSS) are widely used as sheet part components to reduce weight, even though this leads to several challenges. The demand for high-quality shear cutting surfaces that do not require reworking can be fulfilled by adiabatic shear cutting: High strain rates and local temperatures lead to the formation of adiabatic shear bands (ASB). While this process is well suited to produce AHSS parts with excellent cutting surface quality, a fundamental understanding of the process is still missing today. In this study, compression tests in a Split-Hopkinson Pressure Bar with an initial strain rate of 1000 s-1 were performed in a temperature range between 200 °C and 1000 °C. The experimental results show that high strength steels with nearly the same mechanical properties at RT may possess a considerably different behavior at higher temperatures. The resulting microstructures after testing at different temperatures were analyzed by optical microscopy. The thermo-mechanical material behavior was then considered in an analytical model. To predict the local temperature increase that occurs during the adiabatic blanking process, experimentally determined flow curves were used. Furthermore, the influence of temperature evolution with respect to phase transformation is discussed. This study contributes to a more complete understanding of the relevant microstructural and thermo-mechanical mechanisms leading to the evolution of ASB during cutting of AHSS. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012026
  • Influence of Different Yield Loci on Failure Prediction with Damage Models
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Physics: Conference Series 896 (2017)
    Advanced high strength steels are widely used in the automotive industry to simultaneously improve crash performance and reduce the car body weight. A drawback of these multiphase steels is their sensitivity to damage effects and thus the reduction of ductility. For that reason the Forming Limit Curve is only partially suitable for this class of steels. An improvement in failure prediction can be obtained by using damage mechanics. The objective of this paper is to comparatively review the phenomenological damage model GISSMO and the Enhanced Lemaitre Damage Model. GISSMO is combined with three different yield loci, namely von Mises, Hill48 and Barlat2000 to investigate the influence of the choice of the plasticity description on damage modelling. The Enhanced Lemaitre Model is used with Hill48. An inverse parameter identification strategy for a DP1000 based on stress-strain curves and optical strain measurements of shear, uniaxial, notch and (equi-)biaxial tension tests is applied to calibrate the models. A strong dependency of fracture strains on the choice of yield locus can be observed. The identified models are validated on a cross-die cup showing ductile fracture with slight necking. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012081
  • Influence of Different Yield Loci on Failure Prediction with Damage Models
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Physics: Conference Series 896 (2017)
    Advanced high strength steels are widely used in the automotive industry to simultaneously improve crash performance and reduce the car body weight. A drawback of these multiphase steels is their sensitivity to damage effects and thus the reduction of ductility. For that reason the Forming Limit Curve is only partially suitable for this class of steels. An improvement in failure prediction can be obtained by using damage mechanics. The objective of this paper is to comparatively review the phenomenological damage model GISSMO and the Enhanced Lemaitre Damage Model. GISSMO is combined with three different yield loci, namely von Mises, Hill48 and Barlat2000 to investigate the influence of the choice of the plasticity description on damage modelling. The Enhanced Lemaitre Model is used with Hill48. An inverse parameter identification strategy for a DP1000 based on stress-strain curves and optical strain measurements of shear, uniaxial, notch and (equi-)biaxial tension tests is applied to calibrate the models. A strong dependency of fracture strains on the choice of yield locus can be observed. The identified models are validated on a cross-die cup showing ductile fracture with slight necking. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012081
  • Material characterization for plane and curved sheets using the in-plane torsion test-an overview
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains up to 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to ideal simple shear during the entire process. The application range of the plane torsion test has been constantly expanded by different approaches and a more sophisticated evaluation has been developed. The full-field analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without inhomogeneities. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves for curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape were performed. Finally, first results for the local determination of the strength' arbitrary locations of a component are presented. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.964
  • Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012076
  • Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012076
  • Modeling of ductile fracture from shear to balanced biaxial tension for sheet metals
    Lou, Y. and Chen, L. and Clausmeyer, T. and Tekkaya, A.E. and Yoon, J.W.
    International Journal of Solids and Structures 112 (2017)
    A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0. mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr-Coulomb model (), and two micromechanism-inspired criteria proposed recently (). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments. © 2017.
    view abstract10.1016/j.ijsolstr.2016.11.034
  • Stress state dependency of unloading behavior in high strength steels
    Sumikawa, S. and Ishiwatari, A. and Hiramoto, J. and Yoshida, F. and Clausmeyer, T. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    Accuracy of springback prediction strongly depends on whether the unloading behavior of the material is properly considered in the material model or not. It is known that the stress-strain relationship for steel sheet during unloading is nonlinear. For accurate springback prediction, the nonlinear unloading behaviors should be observed not only under uniaxial, but also under multi-axial stress state, and properly considered in FEsimulations. In this study, unloading stress-strain curves of high strength steels under four stress states: uniaxial tension, plane strain tension, biaxial tension and shear, were experimentally obtained in several material tests. From the obtained curves with different prestrains, the average elastic moduli were calculated and converted into the equivalent modulus using the isotropic Hooke's law. Moreover, the nonlinearity of the unloading stress-strain curve was evaluated by the instantaneous stress-strain slope. The average elastic modulus and the nonlinearity obviously differ by stress states, prestrains and types of steel. The investigated steels show the stress state dependency of the unloading behavior. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.758
  • Analysis of shear cutting of dual phase steel by application of an advanced damage model
    Gutknecht, F. and Steinbach, F. and Hammer, T. and Clausmeyer, T. and Volk, W. and Tekkaya, A.E.
    Procedia Structural Integrity 2 (2016)
    Shear cutting is still the most preferred process in industry for separation of sheets. An enhanced fully-coupled Lemaitre model is applied for the description of the material behaviour. The local damage model considers the influence of shear and compression-dominated stress states on the propagation of damage. A time-efficient approach for parameter identification is used to obtain proper material parameters from different tensile and torsion tests. Shear cutting experiments for dual phase steel are performed to validate the simulation model. An accurate prediction of the cutting force is obtained with the process model. Furthermore, it is shown that the triaxiality at fracture has to be considered in combination with the predicted geometry to determine the characteristics of the cutting surface, i.e. the burnish and the fracture zone. Copyright © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.prostr.2016.06.215
  • Analysis of shear cutting of dual phase steel by application of an advanced damage model
    Gutknecht, F. and Steinbach, F. and Hammer, T. and Clausmeyer, T. and Volk, W. and Tekkaya, A. E.
    21st European Conference on Fracture, (ecf21) 2 (2016)
    Shear cutting is still the most preferred process in industry for separation of sheets. An enhanced fully-coupled Lemaitre model is applied for the description of the material behaviour. The local damage model considers the influence of shear and compression-dominated stress states on the propagation of damage. A time-efficient approach for parameter identification is used to obtain proper material parameters from different tensile and torsion tests. Shear cutting experiments for dual phase steel are performed to validate the simulation model. An accurate prediction of the cutting force is obtained with the process model. Furthermore, it is shown that the triaxiality at fracture has to be considered in combination with the predicted geometry to determine the characteristics of the cutting surface, i.e. the burnish and the fracture zone. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.215
  • Damage characterization of high-strength multiphase steels
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 159 (2016)
    High-strength steels show an entirely different material behavior than conventional deep-drawing steels. This fact is caused among others by the multiphase nature of their structure. The Forming Limit Diagram as the classic failure criterion in forming simulation is only partially suitable for this class of steels. An improvement of the failure prediction can be obtained by using damage mechanics. Therefore, an exact knowledge of the material-specific damage is essential for the application of various damage models. In this paper the results of microstructure analysis of a dual-phase steel and a complex-phase steel with a tensile strength of 1000 MPa are shown comparatively at various stress conditions. The objective is to characterize the basic damage mechanisms and based on this to assess the crack sensitivity of both steels. First a structural analysis with regard to non-metallic inclusions, the microstructural morphology, phase identification and the difference in microhardness between the structural phases is carried out. Subsequently, the development of the microstructure at different stress states between uniaxial and biaxial tension is examined. The damage behavior is characterized and quantified by the increase in void density, void size and the quantity of voids. The dominant damage mechanism of the dual-phase steel is the void initiation at phase boundaries, within harder structural phases and at inclusions. In contrast the complex-phase steel shows a significant growth of a smaller amount of voids which initiate only at inclusions. To quantify the damage tolerance and the susceptibility of cracking the criterion of the fracture forming limit line (FFL) is used. The respective statements are supported by results of investigations regarding the edge-crack sensitivity. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/159/1/012013
  • Evaluation of Void Nucleation and Development during Plastic Deformation of Dual-Phase Steel DP600
    Isik, K. and Gerstein, G. and Clausmeyer, T. and Nürnberger, F. and Tekkaya, A.E. and Maier, H.J.
    Steel Research International 87 (2016)
    This paper presents investigations on the characterization of ductile damage and identification of the porosity-related material model parameters in a dual-phase steel DP600. As a modeling reference for the damage evolution, a variant from the Gurson model family is taken. The micromechanical investigations related to nucleation and growth of voids have been carried out. In order to show the void-volume evolution during the deformation, post-mortem scanning electron microscope (SEM) analysis of a notched tensile test is used. Using the ion beam slope cutting methodology to prepare the specimens for SEM analysis, the microstructure can be observed in 2D including the voids. In this way, for the dual-phase steel, characteristic damage behavior upon deformation due to interaction of martensite and ferrite can be investigated. The minimum void size (areal) that can be measured is 0.05 µm2. This resolution of the measurements provides the detection of the newly nucleated voids. For the related material parameters, void-size relevant criterion is applied to determine the newly nucleated voids at a certain plastic strain. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201500483
  • Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming
    Isik, K. and Gerstein, G. and Schneider, T. and Schulte, R. and Rosenbusch, D. and Clausmeyer, T. and Nürnberger, F. and Vucetic, M. and Koch, S. and Hübner, S. and Behrens, B.-A. and Tekkaya, A.E. and Merklein, M.
    Production Engineering 10 (2016)
    Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming. © 2016, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-016-0656-9
  • Investigations of ductile damage in DP600 and DC04 deep drawing steel sheets during punching
    Isik, K. and Gerstein, G. and Gutknecht, F. and Clausmeyer, T. and Nürnberger, F. and Maier, H.J. and Tekkaya, A.E.
    Procedia Structural Integrity 2 (2016)
    The paper presents numerical and microstructural investigations on a punching process of 2 mm thick steel sheets. The dual phase steel DP600 and the mild steel DC04 exhibit different damage and fracture characteristics. To distinguish the void development and crack initiation for both materials, interrupted tests at varied punch displacements are analyzed. The void volume fractions in the shearing zone are identified by scanning electron microscopy (SEM). The Gurson model family, which is recently extended for shear fracture, is utilized to model the elastoplastic behavior with ductile damage. The effect of the shear governing void growth parameter, introduced by Nahshon and Hutchinson (2008), is discussed. © 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.087
  • Investigations of ductile damage in DP600 and DC04 deep drawing steel sheets during punching
    Isik, K. and Gerstein, G. and Gutknecht, F. and Clausmeyer, T. and Nurnberger, F. and Maier, H. J. and Tekkaya, A. E.
    21st European Conference on Fracture, (ecf21) 2 (2016)
    The paper presents numerical and microstructural investigations on a punching process of 2 mm thick steel sheets. The dual phase steel DP600 and the mild steel DC04 exhibit different damage and fracture characteristics. To distinguish the void development and crack initiation for both materials, interrupted tests at varied punch displacements are analyzed. The void volume fractions in the shearing zone are identified by scanning electron microscopy (SEM). The Gurson model family, which is recently extended for shear fracture, is utilized to model the elastoplastic behavior with ductile damage. The effect of the shear governing void growth parameter, introduced by Nahshon and Hutchinson (2008), is discussed. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.087
  • Numerical investigation of blanking for metal polymer sandwich sheets
    Gutknecht, F. and Übelacker, D. and Clausmeyer, T. and Erman Tekkaya, A.
    MATEC Web of Conferences 80 (2016)
    Metal polymer sandwich sheets consist of materials with drastically different mechanical properties. Due to this fact and because of high local gradients in the cutting zone during the blanking process, traditional process strategies and empirical knowledge are difficult to apply. A finite-element simulation of the shear cutting process is used to predict the necessary force and the geometry of the cutting surface. A fully-coupled ductile damage model is used for the description of the material behaviour. This model considers the influence of shear and compression-dominated stress states on the initiation of damage. Experimental tensile and compression test data is used for the identification of material parameters. The results of the blanking simulation are compared with experimental data. Furthermore, the evolution of the stress state is analysed to gain understanding of the underlying physics. Finally this model enables the prediction of core compression and other quantities such as the acting stresses and corresponding triaxilities, which provide valuable information for the development of analytical models. © The Authors, published by EDP Sciences, 2016.
    view abstract10.1051/matecconf/20168016002
  • Comparison of two models for anisotropic hardening and yield surface evolution in bcc sheet steels
    Clausmeyer, T. and Svendsen, B.
    European Journal of Mechanics, A/Solids 54 (2015)
    The purpose of the current work is the investigation and comparison of aspects of the material behavior predicted by two models for anisotropic, and in particular cross, hardening in bcc sheet steels subject to non-proportional loading. The first model is the modified form (Wang et al., 2008) of that due to Teodosiu and Hu (1995, 1998). In this (modified) Teodosiu-Hu model (THM), cross hardening is assumed to affect the yield stress and the saturation value of the back stress. The second model is due to Levkovitch and Svendsen (2007) and Noman et al. (2010). In the Levkovitch-Svendsen model (LSM), cross hardening is assumed to affect the flow anisotropy. As clearly demonstrated in a number of works applying the THM (e.g., Boers et al., 2010; Bouvier et al., 2005, 2003; Hiwatashi et al., 1997; Li et al., 2003; Thuillier et al., 2010; Wang et al., 2008) and the LSM (e.g., Clausmeyer et al., 2014, 2011b; Noman et al., 2010), both of these are capable of predicting the effect of cross hardening on the stress-deformation behavior observed experimentally in sheet steels. As shown in the current work, however, these two models differ significantly in other aspects, in particular with respect to the development of the yield stress, the back stress, and the yield surface. For example, the THM predicts no change in the shape of the yield surface upon change of loading path, in contrast to the LSM and crystal plasticity modeling of bcc sheet steels (Peeters et al., 2002). On the other hand, the LSM predicts no hardening stagnation after cross hardening as observed in experiments, in contrast to the THM. Examples are given. © 2015 Elsevier Masson SAS. All rights reserved.
    view abstract10.1016/j.euromechsol.2015.05.016
  • Enhancement of Lemaitre model to predict cracks at low and negative triaxialities in sheet metal forming
    Isik, K. and Doig, M. and Richter, H. and Clausmeyer, T. and Erman Tekkaya, A.
    Key Engineering Materials 639 (2015)
    Advanced high strength steels are still one of the best alternatives for light weight design in the automotive industry. Due to their good performances like high strength and high energy absorption, those steel grades are excellent for body in white components. Because of their restricted ductility, which sometimes leads to the formation of cracks without or low necking during forming operations, conventional forming limit diagrams may fall short. As a remedy, an enhanced variant of the Lemaitre continuum mechanical damage model (CDM) is presented in this work. Previous model extensions of the Lemaitre model improved the damage prediction for the shear and compression dominated stress states by introducing an additional weighting factor for the influence of compression on damage evolution, the so called crack closure parameter h. However, the possible range of the fracture behavior predicted by such models for low and negative stress triaxialities is limited. In this work, the Lemaitre CDM has been enhanced by considering the maximal shear stress to predict the fracture occurrence under shear. Previous models for the effect of void closure on damage evolution are reviewed and a novel model enhancement taking into account the maximal shear stresses is described. The determination of the damage model parameters is presented for a dual phase steel. For this particular material, the response of model enhancement on the failure prediction is discussed for a test part. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.639.427
  • Enhancement of Lemaitre model to predict cracks at low and negative triaxialities in sheet metal forming
    Isik, K. and Doig, M. and Richter, H. and Clausmeyer, T. and Tekkaya, A. E.
    Key Engineering Materials 639 (2015)
    Advanced high strength steels are still one of the best alternatives for light weight design in the automotive industry. Due to their good performances like high strength and high energy absorption, those steel grades are excellent for body in white components. Because of their restricted ductility, which sometimes leads to the formation of cracks without or low necking during forming operations, conventional forming limit diagrams may fall short. As a remedy, an enhanced variant of the Lemaitre continuum mechanical damage model (CDM) is presented in this work. Previous model extensions of the Lemaitre model improved the damage prediction for the shear and compression dominated stress states by introducing an additional weighting factor for the influence of compression on damage evolution, the so called crack closure parameter h. However, the possible range of the fracture behavior predicted by such models for low and negative stress triaxialities is limited. In this work, the Lemaitre CDM has been enhanced by considering the maximal shear stress to predict the fracture occurrence under shear. Previous models for the effect of void closure on damage evolution are reviewed and a novel model enhancement taking into account the maximal shear stresses is described. The determination of the damage model parameters is presented for a dual phase steel. For this particular material, the response of model enhancement on the failure prediction is discussed for a test part. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.639.427
  • Modeling and finite element simulation of loading-path-dependent hardening in sheet metals during forming
    Clausmeyer, T. and Güner, A. and Tekkaya, A.E. and Levkovitch, V. and Svendsen, B.
    International Journal of Plasticity 63 (2014)
    A recent material model considering the evolution of plastic anisotropy in interstitial free steels is validated for the forming process of the channel die, a complex part. In the model the evolution of the intra-granular microstructure is represented by tensor-valued internal variables. The model accounts for the cross hardening behavior observed in rheological tests of interstitial free steels. A novel cross hardening indicator which is directly derived from the constitutive model is proposed. This cross hardening indicator is a quantitative measure for the occurrence of cross hardening in the forming process of complex parts. A correlation between the occurrence of cross hardening and larger values of the stored (elastic) energy is observed. The influence of cross hardening on the forming process is investigated, in particular, the drawing forces and the geometric deviations due to springback. The influence of cross hardening on the forming process of the channel die geometry is small. The influence of cross hardening on the more complex S-Rail geometry is larger due to larger plastic deformation and more severe loading path changes. The concept of the proposed transient hardening indicator should be applicable to other models for the evolution of plastic anisotropy. A possible use of the cross hardening indicator would be the efficient choice of the material model in the context of sheet metal forming simulations. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijplas.2014.01.011
  • Determination of average dislocation densities in metals by analysis of digitally processed transmission-electron microscopy images
    Husser, E. and Clausmeyer, T. and Gershteyn, G. and Bargmann, S.
    Materialwissenschaft und Werkstofftechnik 44 (2013)
    This paper describes an effective and simple procedure to derive information about the dislocation density distribution in metals by applying standard techniques of digital image processing on gray scaled microstructure images obtained from transmission-electron microscopy. In a representative transmission-electron microscopy image, two local dislocation density values were investigated by classical methods and were used as input parameter for further processing. A correlation between dislocations and image intensities is assumed such that dark areas in microstructural images are seen as a dense concentration of dislocations. Then, the contrast is increased for each transmission-electron microscopy photography. In the next step, posterization, a gradation of tone, is applied to these images. From this, a pixel weighted average distribution of gray level correlated dislocation densities is obtained as well as an average value for a given set of images. The implementation of the several processing steps is done in Matlab employing graphical user interfaces. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201300027
  • Experimental characterization of microstructure development during loading path changes in bcc sheet steels
    Clausmeyer, T. and Gerstein, G. and Bargmann, S. and Svendsen, B. and Van Den Boogaard, A.H. and Zillmann, B.
    Journal of Materials Science 48 (2013)
    Interstitial free sheet steels show transient work hardening behavior, i.e., the Bauschinger effect and cross hardening, after changes in the loading path. This behavior affects sheet forming processes and the properties of the final part. The transient work hardening behavior is attributed to changes in the dislocation structure. In this work, the morphology of the dislocation microstructure is investigated for uniaxial and plane strain tension, monotonic and forward to reverse shear, and plane strain tension to shear. Characteristic features such as the thickness of cell walls and the shape of cells are used to distinguish microstructural patterns corresponding to different loading paths. The influence of the crystallographic texture on the dislocation structure is analyzed. Digital image processing is used to create a "library" of schematic representations of the dislocation microstructure. The dislocation microstructures corresponding to uniaxial tension, plane strain tension, monotonic shear, forward to reverse shear, and plane strain tension to shear can be distinguished from each other based on the thickness of cell walls and the shape of cells. A statistical analysis of the wall thickness distribution shows that the wall thickness decreases with increasing deformation and that there are differences between simple shear and uniaxial tension. A change in loading path leads to changes in the dislocation structure. The knowledge of the specific features of the dislocation structure corresponding to a loading path may be used for two purposes: (i) the analysis of the homogeneity of deformation in a test sample and (ii) the analysis of a formed part. © 2012 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-012-6780-9
  • Modeling induced flow anisotropy and phase transformations in air hardening steels
    Barthel, C. and Klusemann, B. and Denzer, R. and Clausmeyer, T. and Svendsen, B.
    Key Engineering Materials 504-506 (2012)
    In this work a material model for hardening development in sheet metals during forming processes involving loading path changes is formulated. In particular, such hardening development is due to the formation and interaction of dislocation microstructures in the material, resulting in an evolution in the size, center and shape of the yield surface. Such yield surface evolution is accounted for in the current model with the help of an evolving structure tensor. The model is intended for an air hardening steel and takes therefore thermomechanics into account in particular phase transformations from ferrite to austenite and from austenite to martensite. As numerical examples a tension shear test and a heating-cooling sequence are simulated. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.443
  • Modeling of anisotropy induced by evolution of dislocation microstructures on different scales
    Clausmeyer, T. and Bargmann, S. and Svendsen, B.
    AIP Conference Proceedings 1353 (2011)
    Many fcc and bcc metals subjected to non-monotonic loading are known to exhibit different kinds of anisotropic hardening. This is due to evolution of (and interaction in) the dislocation microstructure depending on loading type. One purpose of the current work is the investigation of such evolution and interaction in single crystals as well as its effect on their hardening behavior. A single-crystal model accounting for the effects of a change in loading path on the critical shear stress for glide at the glide-system level is developed. On the basis of the crystal plasticity approach an efficient engineering scale model exploiting the insights gained on the lower scales is introduced and applied to simulations of forming processes. © 2011 American Institute of Physics.
    view abstract10.1063/1.3589502
  • Phenomenological modeling of anisotropy induced by evolution of the dislocation structure on the macroscopic and microscopic scale
    Clausmeyer, T. and van den Boogaard, T. and Noman, M. and Gershteyn, G. and Scharper, M. and Svendsen, B. and Bargmann, S.
    International Journal of Material Forming 4 (2011)
    This work focuses on the modeling of the evolution of anisotropy induced by the development of the dislocation microstructure. A model formulated at the engineering scale in the context of classical metal plasticity and a model formulated in the context of crystal plasticity are presented. Images obtained by transmission-electron microscopy (TEM) show the influence of the strain path on the evolution of anisotropy for the case of two common materials used in sheet metal forming, DC06 and AA6016-T4. Both models are capable of accounting for the transient behavior observed after changes in loading path for fcc and bcc metals. The evolution of the internal variables of the models is correlated with the evolution of the dislocation structure observed by TEM investigations. © 2011 The Author(s).
    view abstract10.1007/s12289-010-1017-4
  • Experimental characterization and modeling of the hardening behavior of the sheet steel LH800
    Noman, M. and Clausmeyer, T. and Barthel, C. and Svendsen, B. and Huétink, J. and van Riel, M.
    Materials Science and Engineering A 527 (2010)
    In complex forming processes, sheet metal undergoes large plastic deformations involving significant induced flow anisotropy resulting from the development of persistent oriented (planar) dislocation structures. The aim of the present work is the formulation and identification of a phenomenological model which accounts for the effect of the evolution of this oriented dislocation microstructure on the anisotropic hardening behavior. The model accounts for changes in the size, center, and shape, of the yield surface associated with isotropic, kinematic, and cross hardening, respectively. Identification of the model for the ferritic sheet metal steel LH800 is carried out with the help of shear, reverse shear, and tension-shear tests. The identified model has been validated using it to predict the stress-strain behavior of the material along different tension-shear loading paths and comparison with analogous experimental results. The results and in particular the comparison of theoretical predictions with experimental results clearly demonstrate the need of including cross hardening effects in the modeling of sheet metals like LH800. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.msea.2009.12.013
  • finite element method

  • fracture

  • mechanical properties

  • metal forming

  • microstructure

  • steel

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