Prof. Dr.-Ing. Markus Stommel

Plastics Technology
TU Dortmund University


  • Experimental multi-scale approach to determine the local mechanical properties of foam base material in polyisocyanurate metal panels
    Gahlen, P. and Fröbel, S. and Karbach, A. and Gabriel, D. and Stommel, M.
    Polymer Testing 93 (2021)
    Polyisocyanurate (PIR) foams were examined regarding their local chemical composition using ATR-IR spectroscopy. As a special parameter the PIR: Amide III intensity ratio is to be mentioned, which represents the quantity of the formed PIR groups. Based on the local PIR: Amide III intensity ratio, the mechanical properties (Young's modulus) of the foam base material were analyzed at defined positions by AFM and Nanoindentation. It turned out that the AFM method is only suitable for qualitative analysis, because the values differ strongly from macroscopic measurements. For the measurements using nanoindentation, a new embedding method was developed, which achieves significantly more realistic and reproducible results compared to the embedding method used in the literature and shows a very good agreement with the macroscopic values. In general, it has been shown that a higher PIR: Amide III intensity ratio tends to lead to a higher Young's modulus. Nevertheless, there are other, currently unknown characteristic values which also influence the Young's modulus. © 2020 The Author(s)
    view abstract10.1016/j.polymertesting.2020.106965
  • Prediction of short fiber composite properties by an artificial neural network trained on an rve database
    Breuer, K. and Stommel, M.
    Fibers 9 (2021)
    In this study, an artificial neural network is designed and trained to predict the elastic properties of short fiber reinforced plastics. The results of finite element simulations of three-dimensional representative volume elements are used as a data basis for the neural network. The fiber volume fraction, fiber length, matrix-phase properties, and fiber orientation are varied so that the neural network can be used within a very wide range of parameters. A comparison of the predictions of the neural network with additional finite element simulations shows that the stiffnesses of short fiber reinforced plastics can be predicted very well by the neural network. The average prediction accuracy is equal or better than by a two-step homogenization using the classical method of Mori and Tanaka. Moreover, it is shown that the training of the neural network on an extended data set works well and that particularly calculation-intensive data points can be avoided without loss of prediction quality. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/fib9020008
  • Investigation of process control influence on tribological properties of FLM-manufactured components
    Hesse, D. and Stanko, M. and Hohenberg, P. and Stommel, M.
    Journal of Manufacturing and Materials Processing 4 (2020)
    In recent years, additive manufacturing methods such as Fused Layer Modeling have been continuously improved by industry and research institutions. In many cases, the influence of process control on the mechanical component properties is being investigated. Influencing parameters include the infill and its orientation as well as patterns. Extrusion parameters such as the volume flow, which can be influenced by the speed, the line width, and the layer thickness, and the temperatures, which determine the interlaminar bonding between the lines and layers, are relevant as well. In this contribution, the influence of process control on the tribological properties of cylindrical tribo-test specimens made of polybutylene terephthalate is investigated. Using a reciprocating pin-on-plate tribo-tester, the static and dynamic friction forces as well as the linear wear is determined. The results show a significant influence of the orientation and density of the infill on the tribological properties. Due to the process-specific large degrees of freedom, the advantage of a load-compatible individualisation and consequently the optimisation of tribologically exposed components is given compared to conventional manufacturing processes. © 2020 by the authors.
    view abstract10.3390/jmmp4020037
  • Investigation of the influence of various post-treatment methods on the properties of additive manufactured FDM Nylon 12 samples [Untersuchung des einflusses verschiedener nachbehandlungsmethoden auf die eigenschaften additiv gefertigter FDM Nylon 12 proben]
    Wiedau, L.C. and Hesse, D. and Baumann, J. and Witt, G. and Stommel, M. and Zabel, A. and Kuhlenkötter, B. and Künne, B.
    Zeitschrift Kunststofftechnik/Journal of Plastics Technology 2020 (2020)
    The MERCUR research project Pr-2017-0003 "Analysis of the product development process in the combination of additive and subtractive manufacturing processes for the manufacture of multi-material products" combines different manufacturing and post-treatment processes to connect the respective advantages. The additive fused deposition modeling (FDM) together with the anisotropic component properties resulting from the process are used in this investigation. The influence of chemical and mechanical post-treatment steps on these properties will be investigated. © Carl Hanser Verlag.
    view abstract10.3139/O999.02022020
  • Moisture transport in PA6 and its influence on the mechanical properties
    Sharma, P. and Sambale, A. and Stommel, M. and Maisl, M. and Herrmann, H.-G. and Diebels, S.
    Continuum Mechanics and Thermodynamics 32 (2020)
    Various studies have reported the changes in the mechanical properties and the modification in morphology of polyamide due to the absorption of water. However, the relation between the local water content and the alteration in the properties has not been consolidated in a coupled model yet. In the current work, a simulation model is proposed that can capture the diffusion of water as well as simulate the effect of the local moisture content on the stiffness of polyamide (PA6). To this end, a finite element model was developed by coupling of a nonlinear diffusion model and a viscoelastic material model. The Galerkin finite element method was used to formulate the weak form of the equations for the two physical processes. The coupled nonlinear equations were solved with the help of the Newton method. The diffusion process was studied experimentally with the help of gravimetric measurements. Relaxation tests were conducted on the polyamide specimens that were saturated under different moisture levels. Based on these experimental results, the dependency of the material parameters on the local moisture content was identified and an efficient and stable numerical simulation model has been developed. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00161-019-00815-w
  • RVE modelling of short fiber reinforced thermoplastics with discrete fiber orientation and fiber length distribution
    Breuer, K. and Stommel, M.
    SN Applied Sciences 2 (2020)
    This study presents an analysis of modelling aspects on the effective composite properties of short glass fiber reinforced thermoplastics using representative volume elements (RVE). Although, many investigations were published showing effects of different modelling parameters of RVEs, we further elaborate in this contribution the parameters: influence of fiber packing, fiber shape, bonding of the fibers to the matrix, fiber length distribution and fiber orientation. The knowledge of these influences is used to determine the extent to which the increased modelling accuracy and thus the computational effort leads to an improved RVE’s forecast quality. This objective is achieved by creating and comparing different RVE models of a PBT-GF20 composite. The information required for the RVE models is obtained by experimental characterization of the PBT-GF20 and the PBT matrix material. It can be concluded based on the results of the numerical investigations in conjunction with the experimental tests of the composite that fiber packing, fiber length distribution, fiber orientation and fiber geometry are essential for a precise determination of the effective composite properties. © 2019, The Author(s).
    view abstract10.1007/s42452-019-1890-5
  • Tensor Spines - A Hyperstreamlines Variant Suitable for Indefinite Symmetric Second-Order Tensors
    Kretzschmar, V. and Gunther, F. and Stommel, M. and Scheuermann, G.
    IEEE Pacific Visualization Symposium 2020-June (2020)
    Modern engineering uses optimization to design long-living and robust components. One part of this process is to find the optimal stress-aware design under given geometric constraints and loading conditions. Tensor visualization provides techniques to show and evaluate the stress distribution based on finite element method simulations. One such technique are hyperstreamlines. They allow us to explore the stress along a line following one principal stress direction while showing the other principal stress directions and their values. In this paper, we show shortcomings of this approach from an engineer's point of view and propose a variant called tensor spines. It provides an improved perception of the relation between the principal stresses helping engineers to optimize their designs further. © 2020 IEEE.
    view abstract10.1109/PacificVis48177.2020.1008
  • Use of poly(ethylene naphthalate) as a self-vetoing structural material
    Efremenko, Y. and Fajt, L. and Febbraro, M. and Fischer, F. and Guitart, M. and Hackett, B. and Hayward, C. and Hodák, R. and Majorovits, B. and Manzanillas, L. and Muenstermann, D. and Oz, E. and Pjatkan, R. and Pohl, M. and Radford, D. and Rouhana, R. and Schulz, O. and Štekl, I. and Stommel, M.
    Journal of Physics: Conference Series 1468 (2020)
    Poly(ethylene naphthalate), PEN, is an industrial polyester which has been shown to scintillate in the blue wavelength region. Combined with measurements of a high intrinsic radiopurity, this has sparked interest in the material for use in low-background experiments. © 2020 Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/1468/1/012225
  • Constitutive modeling of the mechanics of polyethylene films in stretch wrapping processes
    Klein, D. and Stommel, M. and Zimmer, J.
    AIP Conference Proceedings 2113 (2019)
    This study contributes to the optimization of lightweight packaging concepts regarding their stability. A very widespread packaging concept is the distribution of goods on a pallet whereas a Polyethylene (PE) stretch film stabilizes the lightweight structure during the shipment. Usually, a stretch wrapping machine applies this stretch film to the pallet. Stiffness and internal stresses of the applied stretch film decisively influence the cohesion of the packaging system but they are difficult to adjust, as the strain history during the wrapping process has a substantial impact on the mechanical behavior of the stretched film on the pallet. Accordingly, the process designer has to reflect the strain history dependent mechanical behavior of stretch film to make optimum use of the packaging material. For this purpose, this study experimentally calibrates a constitutive model of the stretch film. The calibrated model enables us to compute the result of the wrapping process depending on the process parameters during stretch wrapping. We use this result as a base for further numerical and experimental investigations on pallet stability. © 2019 Author(s).
    view abstract10.1063/1.5112661
  • Determination of moisture gradients in polyamide 6 using StepScan DSC
    Sambale, A. and Kurkowski, M. and Stommel, M.
    Thermochimica Acta 672 (2019)
    Polyamides are known to absorb and desorb water to establish an equilibrium with their environment. Water uptake by diffusion leads to a drastic reduction in the strength and stiffness of the material, which is referred as the water-induced plasticizing effect. This effect leads to a shift of the glass transition region towards lower temperatures what can be determined by differential scanning calorimetry (DSC). However, a determination of the glass transition temperature T g is not accurate in samples with inhomogeneous moisture distribution before an equilibrium is reached by diffusion and T g is superimposed by water-specific effects. It is shown that the glass transition can be measured by StepScan DSC even with inhomogeneous moisture distribution within the sample. In addition, a method is developed and validated which uses the low thermal conductivity and the shift of T g to detect a water-saturated sample boundary layer in inhomogeneously conditioned PA 6 samples. © 2018 The Authors
    view abstract10.1016/j.tca.2018.12.011
  • Reduction of ejection forces in injection molding by applying mechanically post-treated CrN and CrAln PVD films
    Tillmann, W. and Stangier, D. and Lopes Dias, N.F. and Gelinski, N. and Stanko, M. and Stommel, M. and Krebs, E. and Biermann, D.
    Journal of Manufacturing and Materials Processing 3 (2019)
    In injection molding, the reduction of ejection forces is a process relevant aspect to improve the production rates. For this purpose, CrN and CrAlN films were sputtered on cylindrical and quadratic AISI H11 cores of an injection mold in order to investigate their influence on the resulting ejection forces to demold polypropylene test components. Within this context, the ejection forces of the PVD coated cores were compared to those of uncoated cores made of AISI H11. For both the cylindrical and quadratic cores, the as-deposited CrN and CrAlN films exhibit higher ejection forces than the uncoated cores due to the increase of the roughness profile after sputtering. It is known that the ejection forces are directly related to the surface roughness. In order to ensure comparable surface conditions to the uncoated surfaces, and to demonstrate the potential of PVD coated mold surfaces when reducing the ejection forces, the coated surfaces were mechanically post-treated to obtain a similar roughness profile as the uncoated cores. The combination of a PVD deposition and post-treatment ensures a significant reduction of the ejection forces by 22.6% and 23.7% for both core geometries. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (
    view abstract10.3390/jmmp3040088
  • Tensor field visualization using fiber surfaces of invariant space
    Raith, F. and Blecha, C. and Nagel, T. and Parisio, F. and Kolditz, O. and Günther, F. and Stommel, M. and Scheuermann, G.
    IEEE Transactions on Visualization and Computer Graphics 25 (2019)
    Scientific visualization developed successful methods for scalar and vector fields. For tensor fields, however, effective, interactive visualizations are still missing despite progress over the last decades. We present a general approach for the generation of separating surfaces in symmetric, second-order, three-dimensional tensor fields. These surfaces are defined as fiber surfaces of the invariant space, i.e. as pre-images of surfaces in the range of a complete set of invariants. This approach leads to a generalization of the fiber surface algorithm by Klacansky et al. [16] to three dimensions in the range. This is due to the fact that the invariant space is three-dimensional for symmetric second-order tensors over a spatial domain. We present an algorithm for surface construction for simplicial grids in the domain and simplicial surfaces in the invariant space. We demonstrate our approach by applying it to stress fields from component design in mechanical engineering. © 1995-2012 IEEE.
    view abstract10.1109/TVCG.2018.2864846
  • Use of poly(ethylene naphthalate) as a self-vetoing structural material
    Efremenko, Y. and Fajt, L. and Febbraro, M. and Fischer, F. and Hayward, C. and Hodák, R. and Kraetzschmar, T. and Majorovits, B. and Muenstermann, D. and Öz, E. and Pjatkan, R. and Pohl, M. and Radford, D. and Rouhana, R. and Sala, E. and Schulz, O. and Štekl, I. and Stommel, M.
    Journal of Instrumentation 14 (2019)
    The discovery of scintillation in the blue regime from poly(ethylene naphthalate) (PEN), a commonly used high-performance industrial polyester plastic, has sparked the interest of the physics community as a new type of plastic scintillator material. This observation in addition to its good mechanical and radiopurity properties makes PEN an attractive candidate as an active structure scintillator for low-background physics experiments. This paper reports on investigations of its potential in terms of production tests of custom made tiles and various scintillation light output measurements. These investigations substantiate the high potential of usage of PEN in low-background experiments. © 2019 The Author(s).
    view abstract10.1088/1748-0221/14/07/P07006
  • Fracture analysis of a metal to CFRP hybrid with thermoplastic interlayers for interfacial stress relaxation using in situ thermography
    Summa, J. and Becker, M. and Grossmann, F. and Pohl, M. and Stommel, M. and Herrmann, H.-G.
    Composite Structures 193 (2018)
    In this work a plane hybrid-structure composed of a metal and a carbon-fiber-reinforced-polymer (CFRP) constituent is introduced. Hereby an interlayer is inserted between the metal and the CFRP constituent, pursuing the task of stress relaxation. In order to study the effect of interfacial stress relaxation several thermoplastics are investigated. In situ passive thermography is used to assess the damage during quasi-static and fatigue mechanical loading. Thus, mechanical properties are correlated with corresponding damage-quantities from non-destructive testing (ndt). These results reveal that transversal cracking and mode-I delamination are the dominant failure processes, which strongly depend on the thermoplastic material. Additional finite element analysis describes strain-energy- and stressconcentrations, which coincide with the observed damage mechanisms and the origins of fracture. © 2018 Elsevier Ltd
    view abstract10.1016/j.compstruct.2018.03.013
  • Influence of surface roughness on the shear strength of direct injection molded plastic-aluminum hybrid-parts
    Bonpain, B. and Stommel, M.
    International Journal of Adhesion and Adhesives 82 (2018)
    The strength of hybrid metal and plastic joints is strongly influenced by the surface roughness of metal. Although many investigations on the change in shear strength of adhesively bonded joints due to roughening have been published, it is not completely understood how different mean roughness indexes Ra of directly joined plastic-metal-parts correlate to the resulting shear strength. This paper describes a schematic roughness - shear strength curve for adhesively bonded specimens and an experimental one for direct injection molded hybrid specimens which is reconciled with the state of the art. Roughening the surface of the metal is realized by grit blasting using fused alumina. After that, the metal is coated by direct injection molded PA 6.6 with 30% short glass fiber and shear strength tests are carried out. It can be concluded that with increasing Ra the shear strength of adhesively bonded specimens increases to a first maximum, then decreases and finally increases again. Direct injection molded samples exhibit a similar trend. The difference is that the second increase is significantly more pronounced, leading to a second maximum which is nearly twice as high as the first one. This difference is explained by the lower strength of the adhesive compared to the plastic and the different pronounced so called surface area effects, notch effects and thickness effects. By increasing Ra they promote cohesive fracture which is detrimental for adhesively bonded but more beneficial for direct injection molded samples. To further explain the experimental results, a FEM model to predict the probability of failure, the expected stress and the failure mode for direct injection molded samples is developed. The FE-analyses confirm the explanatory approach on the joint strength in dependence of the Ra value. © 2018 Elsevier Ltd
    view abstract10.1016/j.ijadhadh.2018.02.003
  • Influence of the stretch wrapping process on the mechanical behavior of a stretch film
    Klein, D. and Stommel, M. and Zimmer, J.
    AIP Conference Proceedings 1960 (2018)
    Lightweight construction is an ongoing task in packaging development. Consequently, the stability of packages during transport is gaining importance. This study contributes to the optimization of lightweight packaging concepts regarding their stability. A very widespread packaging concept is the distribution of goods on a pallet whereas a Polyethylene (PE) stretch film stabilizes the lightweight structure during the shipment. Usually, a stretch wrapping machine applies this stretch film to the pallet. The objective of this study is to support packaging development with a method that predicts the result of the wrapping process, based on the mechanical characterization of the stretch film. This result is not only defined by the amount of stretch film, its spatial distribution on the pallet and its internal stresses that result in a containment force. More accurate, this contribution also considers the influence of the deformation history of the stretch film during the wrapping process. By focusing on similarities of stretch wrappers rather than on differences, the influence of generalized process parameters on stretch film mechanics and thereby on pallet stability can be determined experimentally. For a practical use, the predictive method is accumulated in an analytic model of the wrapping process that can be verified experimentally. This paves the way for experimental and numerical approaches regarding the optimization of pallet stability. © 2018 Author(s).
    view abstract10.1063/1.5034980
  • Intrinsic CFRP-metal-hybrids with rubber interface for the improvement of the damping behaviour
    Pohl, M. and Stommel, M.
    Production Engineering (2018)
    This paper presents an integrated passive damping approach in hybrid metal-CFRP parts for structural applications. In this concept a viscoelastic material is embedded in the joint zone of the hybrid component. To examine the connection strength single-lap-joint specimens were produced and tested and the influence of the used material combinations, different surface structures, and different process parameters i.e. the moment of cross-linking were evaluated. Afterwards, the metal-CFRP hybrids were tested in quasi-static tests to assess their connection strength and failure behaviour. Dynamic cyclic tensile tests with step-wise increased loading conditions were performed to determine the specimens damping behaviour and to estimate their fatigue performance. Finally, these results are compared to a state of the art metal-CFRP hybrid with rivets connecting both materials. © 2018 German Academic Society for Production Engineering (WGP)
    view abstract10.1007/s11740-018-0792-5
  • Kinetic prediction of fast curing polyurethane resins by model-free isoconversional methods
    Stanko, M. and Stommel, M.
    Polymers 10 (2018)
    In this work, the characterisation of reaction kinetics of a methylene diphenyl diisocyanate (MDI)-based fast curing polyurethane resin (PUR) and the mathematical description of its curing process are presented. For the modelling of the reaction process isoconversional methods, which are also called model-free approaches, are used instead of model-based approaches. One of the main challenges is the characterisation of a reactive system with a short pot life, which already starts to crosslink below room temperature. The main focus is the evaluation of the applicability of isoconversional methods for predicting the reaction kinetics of fast curing polyurethane resins. In order to realise this, a repeatable methodology for the determination of time- and temperature-dependent reaction curves using differential scanning calorimetry (DSC) is defined. The cure models defined by this method serve as the basis for process simulations of PUR processing technologies such as resin transfer moulding (RTM) or reactive injection moulding (RIM) and reactive extrusion (REX). The characterisation of the reaction kinetics using DSC measurements is carried out under isothermal and non-isothermal conditions. Within this work isoconversional methods have been applied successfully to experimentally determined DSC data sets. It is shown that the reaction kinetics of fast curing polyurethane resins can be predicted using this methods. Furthermore, it is demonstrated that the time-dependent change of conversion of the considered polyurethane under isothermal curing conditions can also be predicted using isoconversional methods based on non-isothermal DSC measurements. This results in a significant reduction in the experimental effort required to characterise and model the curing process of polyurethanes. © 2018 by the authors.
    view abstract10.3390/polym10070698
  • Predictive quality control of hybrid metal-CFRP components using information fusion
    Berger, D. and Zaiß, M. and Lanza, G. and Summa, J. and Schwarz, M. and Herrmann, H.-G. and Pohl, M. and Günther, F. and Stommel, M.
    Production Engineering 12 (2018)
    The paper presents an approach to determine the durability of hybrid metal-CFRP components combining the results of non-destructive testing (ndt) and finite element simulation The advantage of hybrid metal-CFRP components lies in the use of the properties of the materials used. CFRP parts with higher specific stiffness and strength are combined with metallic joining points, so that established joining processes for metal components can be applied to these lightweight components. In order to further promote the use of these hybrids in industry, it is necessary to guarantee a high level of component reliability through 100% quality control in order to avoid production-related defects. These defects such as delamination or fibre disorientation however vary in shape, size and position and lead to different effects on the part performance and reliability. Therefore the presented approach includes the application of non-destructive testing methods that are applied as in-line quality control measures in order to determine defect characteristics of the inspected parts. Due to the novelty of the component under test it is necessary to evaluate the individual criticality of detected defects and how they affect part performance during the testing procedure. Therefore the acquired ndt-data is used in finite element simulations where defect characteristics are added to the component model and whose effects on part reliability are evaluated. The generation of additional information combining non-destructive testing and simulation is referred to as data fusion. In order to evaluate the validity of the presented approach the determined part performances are compared to experimental mechanic tests. © 2018, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-018-0816-1
  • Viscoelasticity of short fiber composites in the time domain: from three-phases micromechanics to finite element analyses
    Breuer, K. and Schöneich, M. and Stommel, M.
    Continuum Mechanics and Thermodynamics (2018)
    Micromechanical models can be used to calculate the mechanical properties of short glass fiber reinforced thermoplastics. In the present work, a three-step framework is used to validate a three-phases micromechanical model (RDI model) in the time domain, since the analysis of technical components by the finite element method is usually carried out in the time domain. The framework includes mechanical characterization, the implementation of the RDI model and a finite element analysis. The characterization delivers necessary information about the material phases of the composite. A dynamic mechanical analysis is performed to characterize the matrix material in order to obtain the linear viscoelastic properties. The mechanical properties of the matrix–fiber interphase are determined with an inverse calculation. In the second step, the RDI model is used to calculate the frequency depended effective stiffness of the composite. A new developed approach transforms the effective stiffness from the frequency domain into the time domain thus avoiding an explicit inverse Laplace–Carson transformation. In the third step, the RDI model is experimentally validated. © 2018 Springer-Verlag GmbH Germany, part of Springer Nature
    view abstract10.1007/s00161-018-0686-y
  • A coated inclusion-based homogenization scheme for viscoelastic composites with interphases
    Schöneich, M. and Dinzart, F. and Sabar, H. and Berbenni, S. and Stommel, M.
    Mechanics of Materials 105 (2017)
    A coated inclusion-based homogenization scheme is developed for three-phase viscoelastic composites in the Laplace–Carson domain. The interphase between inclusion and matrix is considered as a coating in a composite-like inclusion and shows altered viscoelastic behavior compared to the matrix. The strain concentration equations between the viscoelastic inclusion and the viscoelastic coating are derived with two different models: the double inclusion (denoted DI) model, and the reconsidered double inclusion (RDI) model. Then, the homogenization scheme is based on a modified Mori–Tanaka scheme for three-phase viscoelastic composites, which is validated with the exact analytical formulation in the case of spherical composite inclusions and isotropic behaviors for all constituents. The comparison of the proposed coated inclusion-based homogenization scheme based on the RDI model with the exact analytical solution shows a significant improvement compared to the one based on the DI model in the prediction of effective properties for composites with interphases. Finally, considering experimental dynamic mechanical analyses (DMA) in the frequency domain for a carbon-black filled styrene butadiene rubber from the literature, the effective viscoelastic behavior is estimated with a good accuracy in terms of the storage and loss moduli for different volume fractions of composite inclusions. © 2016
    view abstract10.1016/j.mechmat.2016.11.009
  • Effects of defects in series production of hybrid CFRP lightweight components – detection and evaluation of quality critical characteristics
    Berger, D. and Brabandt, D. and Bakir, C. and Hornung, T. and Lanza, G. and Summa, J. and Schwarz, M. and Herrmann, H.-G. and Pohl, M. and Stommel, M.
    Measurement: Journal of the International Measurement Confederation 95 (2017)
    The field of application of carbon fibre reinforced plastics (CFRP) is expanded successively by the insertion of additional components in manufactured parts. These inserts can be used as mechanical interfaces between CFRP and external forces, which is why it is necessary to ensure that the interface is free of defects. Especially the CFRP component is prone to defects due to process and material properties. Currently there are very limited possibilities to evaluate the quality of this type of innovative combination of components. Therefore this publication presents the optimized operation of both 3D and 2D measurement systems in series production, which are used to detect defects that appear in the production of hybrid metal components in the Resin Transfer Moulding Process. The severity of these defects is characterized in both destructive and non-destructive testing and helps to evaluate the necessity of in-line measurement systems and their allocation in the production process. © 2016 Elsevier Ltd
    view abstract10.1016/j.measurement.2016.10.003
  • Influence of the processing parameters on the fiber-matrix-interphase in short glass fiber-reinforced thermoplastics
    Sambale, A.K. and Schöneich, M. and Stommel, M.
    Polymers 9 (2017)
    The interphase in short fiber thermoplastic composites is defined as a three-dimensional, several hundred nanometers-wide boundary region at the interface of fibers and the polymer matrix, exhibiting altered mechanical properties. This region is of key importance in the context of fiber-matrix adhesion and the associated mechanical strength of the composite material. An interphase formation is caused by morphological, as well as thermomechanical processes during cooling of the plastic melt close to the glass fibers. In this study, significant injection molding processing parameters are varied in order to investigate the influence on the formation of an interphase and the resulting mechanical properties of the composite. The geometry of the interphase is determined using nano-tribological techniques. In addition, the influence of the glass fiber sizing on the geometry of the interphase is examined. Tensile tests are used in order to determine the resulting mechanical properties of the produced short fiber composites. It is shown that the interphase width depends on the processing conditions and can be linked to the mechanical properties of the short fiber composite. © 2017 by the authors.
    view abstract10.3390/polym9060221
  • On the Design, Characterization and Simulation of Hybrid Metal-Composite Interfaces
    Kießling, R. and Ihlemann, J. and Pohl, M. and Stommel, M. and Dammann, C. and Mahnken, R. and Bobbert, M. and Meschut, G. and Hirsch, F. and Kästner, M.
    Applied Composite Materials 24 (2017)
    Multi-material lightweight designs are a key feature for the development of innovative and resource-efficient products. In the development of a hybrid composite, the interface between the joined components has to be considered in detail as it represents a typical location of the initialization of failure. This contribution gives an overview of the simulative engineering of metal-composite interfaces. To this end, several design aspects on the microscale and macroscale are explained and methods to model the mechanical behavior of the interface within finite element simulations. This comprises the utilization of cohesive elements with a continuum description of the interface. Likewise, traction-separation based cohesive elements, i.e. a zero-thickness idealization of the interface, are outlined and applied to a demonstration example. Within these finite element simulations, the constitutive behavior of the connected components has to be described by suitable material models. Therefore, inelastic material models at large strains are formulated based on rheological models. © 2016 Springer Science+Business Media Dordrecht
    view abstract10.1007/s10443-016-9526-z
  • Visualizing gradients of stress tensor fields
    Zobel, V. and Stommel, M. and Scheuermann, G.
    Mathematics and Visualization (2017)
    In some applications, it is necessary to look into gradients of (symmetric) second order tensor fields. These tensors are of third order. In three-dimensional space, we have 18 independent coefficients at each position, so the visualization of these fields provides a challenge. A particular case are stress gradients in structural mechanics. There are specific situations where the stress gradient is required together with the stress to study material behavior. Since the visualization community lacks methods to show these fields, we look at some preliminary ideas to design appropriate glyphs. We motivate our glyph designs by typical depictions of stress in engineering textbooks. © Springer International Publishing AG 2017.
    view abstract10.1007/978-3-319-61358-1_4
  • Estimation of the lifetime of short fiber-reinforced plastic parts in steering systems
    Ferrano, F. and Lipka, A. and Stanko, M. and Stommel, M.
    AIP Conference Proceedings 1779 (2016)
    The paper presents a strain-based approach for the lifetime calculation of short fiber-reinforced plastics which takes into account the elastoplastic and anisotropic behavior of the material. For this, a coupled process-structure simulation is used. The advantages of the new approach are shown by comparison between a linear elastic and isotropic Finite Element (FE) analysis using conservative estimation of the lifetime [1]. Tests of a steering system component subjected to load cycles are the basis. The material used is a glass fiber-reinforced polyamide 66 (PA66 GF30). © 2016 Author(s).
    view abstract10.1063/1.4965520
  • Experimental and numerical analysis of liquid-forming
    Zimmer, J. and Klein, D. and Stommel, M.
    Key Engineering Materials 651-653 (2015)
    The packaging of liquid products is conventionally realized by using two production stages, which are the stretch blow molding and the filling. In the stretch blow molding process, hot polyethylene terephthalate (PET) preforms are inflated by pressurized air into a cavity to form plastic bottles. In a follow-up process, these packages are filled by a separate machine with the desired liquid product. In contrast to that, liquid-forming combines the blowing and filling stages by directly using the liquid product to form a plastic bottle. Through this substitution, two main challenges arise. Firstly, there are significant inertia effects through the liquid mass, leading to additional reaction forces and a spatially inhomogeneous pressure distribution inside the preform. Secondly, the heat transfer between preform and fluid is drastically increased. Because of this cooling effect, a specific combination of forming speed as well as initial preform and liquid temperatures is necessary to avoid thermally induced preform rupture. This is based on the fact that the formability of PET rapidly declines below its glass transition temperature (Tg). Consequently, a process control requires the knowledge of how the process parameters influence the preform cooling. In this paper, a numerical simulation of the liquid-forming process (LF) is introduced including the preform cooling during forming. In addition, the strain-dependent self-heating effect of PET is implemented. Process experiments under different parameter combinations are conducted using simplified bottle geometry. Through a comparison of the results from experiments and from simulation, the influence of process parameters on the temperature drop and thus on thermally induced failure is determined. In this way, process understanding and control are increased. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/
  • Experimental investigation and numerical simulation of liquid supported stretch blow molding
    Zimmer, J. and Chauvin, G. and Stommel, M.
    Polymer Engineering and Science 55 (2015)
    An innovative production process for PET bottles ad containers is analyzed in this article. Liquid Bi-Orientation (LBO) is a liquid supported stretch blow molding (SBM), which combines the separate blowing and filling phases of conventional SBM. The process modification is mainly characterized by forming the bottle using the desired liquid product instead of pressurized air. Consequently, possible improvements evolve regarding production cycle time, energy consumption and machine footprint. To make use of these capabilities, comprehensive process understanding is required, which can be increased using numerical simulation methods. Therefore, in this article, an LBO process model is set-up and experimentally evaluated. The model explicitly considers the fluid-structure interaction between liquid and PET, which significantly influences the PET forming behavior. The key simulation parameters namely the strong rate and temperature dependency of PET and a realistic process parameter determination are also included. The model is evaluated using two different methods to show the reliability of the process prediction. © 2014 Society of Plastics Engineers.
    view abstract10.1002/pen.23961
  • Feature-based tensor field visualization for fiber reinforced polymers
    Zobel, V. and Stommel, M. and Scheuermann, G.
    2015 IEEE Scientific Visualization Conference, SciVis 2015 - Proceedings (2015)
    Virtual testing is an integral part of modern product development in mechanical engineering. Numerical structure simulations allow the computation of local stresses which are given as tensor fields. For homogeneous materials, the tensor information is usually reduced to a scalar field like the von Mises stress. A material-dependent threshold defines the material failure answering the key question of engineers. This leads to a rather simple feature-based visualisation. For composite materials like short fiber reinforced polymers, the situation is much more complex. The material property is determined by the fiber distribution at every position, often described as fiber orientation tensor field. Essentially, the material's ability to cope with stress becomes anisotropic and inhomogeneous. We show how to combine the stress field and the fiber orientation field in such cases, leading to a feature-based visualization of tensor fields for composite materials. The resulting features inform the engineer about potential improvements in the product development. © 2015 IEEE.
    view abstract10.1109/SciVis.2015.7429491
  • Fiber-matrix interphase in applied short glass fiber composites determined by a nano-scratch method
    Schöneich, M. and Zamanzade, M. and Stommel, M.
    Composites Science and Technology 119 (2015)
    The fiber-matrix interphase in composites is defined as the intersection region between fibers and the matrix material. It shows altered matrix material properties. In dependency of the matrix material or the fiber coating, this phase is created by interdiffusion processes at the macromolecular scale driven by thermodynamic forces. Especially for FE simulations of composites and the validation of multi-scale material modeling approaches, the information about the existing interphase becomes important. Thus, the present study analyses the interphase in applied short glass fiber reinforced thermoplastics. For the identification of the interphase thickness, the nano-scratch method provides admissible results. This methodology is improved and adapted to the use in short fiber composite specimens. Thereby, the measured range of interphase thickness represents the inhomogeneity of the interphase. To assure that the measured interphase is not mainly constituted by the sizing of the glass fiber, incineration tests are performed additionally. The comparison of the interphase and the sizing thickness shows a significant thicker interphase. © 2015 Elsevier Ltd.
    view abstract10.1016/j.compscitech.2015.10.004
  • Tensor lines in engineering: Success, failure, and open questions
    Schöneich, M. and Kratz, A. and Zobel, V. and Scheuermann, G. and Stommel, M. and Hotz, I.
    Mathematics and Visualization 40 (2015)
    Today, product development processes in mechanical engineering are almost entirely carried out via computer-aided simulations. One essential output of these simulations are stress tensors, which are the basis for the dimensioning of the technical parts. The tensors contain information about the strength of internal stresses as well as their principal directions. However, for the analysis they are mostly reduced to scalar key metrics. The motivation of this work is to put the tensorial data more into focus of the analysis and demonstrate its potential for the product development process. In this context we resume a visualization method that has been introduced many years ago, tensor lines. Since tensor lines have been rarely used in visualization applications, they are mostly considered as physically not relevant in the visualization community. In this paper we challenge this point of view by reporting two case studies where tensor lines have been applied in the process of the design of a technical part. While the first case was a real success, we could not reach similar results for the second case. It became clear that the first case cannot be fully generalized to arbitrary settings and there are many more questions to be answered before the full potential of tensor lines can be realized. In this chapter, we review our success story and our failure case and discuss some directions of further research. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-15090-1_17
  • Thermo-mechanical simulation of liquid-supported stretch blow molding
    Zimmer, J. and Stommel, M.
    AIP Conference Proceedings 1664 (2015)
    Stretch blow molding is the well-established plastics forming method to produce Polyehtylene therephtalate (PET) bottles. An injection molded preform is heated up above the PET glass transition temperature (Tg∼85°C) and subsequently inflated by pressurized air into a closed cavity. In the follow-up filling process, the resulting bottle is filled with the final product. A recently developed modification of the process combines the blowing and filling stages by directly using the final liquid product to inflate the preform. In a previously published paper, a mechanical simulation and successful evaluation of this liquid-driven stretch blow molding process was presented. In this way, a realistic process parameter dependent simulation of the preform deformation throughout the forming process was enabled, whereas the preform temperature evolution during forming was neglected. However, the formability of the preform is highly reduced when the temperature sinks below Tg during forming. Experimental investigations show temperature-induced failure cases due to the fast heat transfer between hot preform and cold liquid. Therefore, in this paper, a process dependent simulation of the temperature evolution during processing to avoid preform failure is presented. For this purpose, the previously developed mechanical model is used to extract the time dependent thickness evolution. This information serves as input for the heat transfer simulation. The required material parameters are calibrated from preform cooling experiments recorded with an infrared-camera. Furthermore, the high deformation ratios during processing lead to strain induced crystallization. This exothermal reaction is included into the simulation by extracting data from preform measurements at different stages of deformation via Differential Scanning Calorimetry (DSC). Finally, the thermal simulation model is evaluated by free forming experiments, recorded by a high-speed infrared camera. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4918406
  • FE-simulation and validation of liquid-bi-orientation-
    Zimmer, J. and Chauvin, G. and Stommel, M.
    AIP Conference Proceedings 1593 (2014)
    An established method to produce thin walled bottles is Stretch Blow Molding (SBM). Polyethylene terephthalate (PET)-preforms are first heated above their glass transition temperature and subsequently transferred into a closed cavity. In a second step the hot preforms are axially elongated by a stretch rod and simultaneously inflated by pressurized air until a contact with the cavity wall is reached (blowing stage). After a cooling phase, the resulting bottle is ejected and further transferred to a filling station, where the desired liquid content is poured in (filling stage). Alternatively to this sequential procedure, a new process combines the blowing and filling phases. This is done by using the desired liquid content as a pressure medium to inflate the hot preforms. Hence, no separated filling station is required. Moreover the filling time is drastically reduced and the cooling is increased through the heat transfer between hot preform and cold liquid. In the following this process is denoted as liquid-bi-orientation (LBO). Despite of its obvious advantages, LBO is not yet used for industrial series production because SBM is well controlled and established. In this paper the LBO process is investigated by experiments and FE-simulations to obtain a deeper insight and to increase process knowledge. The experiments are conducted at a prototype machine. Hereby, a high speed camera in combination with a transparent cavity enables a recording of the preform deformation. Furthermore, FE-simulations with coupled fluid-structure interactions are conducted to predict the process. In comparison to the high speed video the capabilities of the process model are evaluated. © 2014 American Institute of Physics.
    view abstract10.1063/1.4873741
  • FSI-simulation of Liquid supported Stretch Blow Molding (LBO): Model validation and study of series production scenario
    Zimmer, J. and Stommel, M.
    Key Engineering Materials 611-612 (2014)
    Liquid-Driven Stretch Blow Molding is a new and innovative method to produce PET bottles [1]. In the well-established Stretch Blow Molding (SBM) process, preforms are biaxially deformed by pressurized air into a cavity. The resulting bottles are transferred to a separate machine, where the desired product is filled in. In contrast to that, Liquid-Driven Stretch Blow Molding is characterized by employing the liquid product to deform the material. The former separated blowing and filling steps are thus combined to a single forming stage leading to numerous advantages in energy consumption, cycle time and machine footprint. In this paper, a numerical simulation of the new process is presented. An additional challenge compared to SBM simulations is thereby the consideration of the interaction between liquid and preform. The load application cannot be solely represented by the pressure because the influx behavior as well as gravity and inertia forces influence the preform deformation. A smoothed particle hydrodynamics (SPH) approach is applied to the simulation to incorporate the additional effects. The process model is evaluated by prototype experiments. In addition, a feasibility study shows the applicability of a rotary forming system to the new process. © 2014 Trans Tech Publications, Switzerland.
    view abstract10.4028/
  • Modified mean-field formulations for the improved simulation of short fiber reinforced thermoplastics
    Kaiser, J.-M. and Stommel, M.
    Composites Science and Technology 99 (2014)
    The aim of the contribution is to introduce modified mean-field formulations for the improved simulation of short fiber reinforced thermoplastics. In the first part, the recently proposed second moment incremental formulation for the mean-field homogenization of elastic-plastic composites of Doghri et al. (2011) [1] is modified. A stress concentration factor is introduced, which enables an adequate consideration of stress and strain inhomogeneities in dependence of the fiber orientation. In the second part, special focus is put on the characteristic elastic-plastic behavior of thermoplastics. It is well known, that thermoplastics show a distinct dependence of the volumetric stress in their mechanical elastic-plastic behavior. To account for this dependency, the commonly in mean-field formulations integrated von-Mises plasticity model is replaced by a quadric yield formulation proposed by Kolling et al. (2005) [2]. Conclusive results are achieved with both formulations by comparing simulation and experimental results. © 2014.
    view abstract10.1016/j.compscitech.2014.05.010
  • Tensor visualization driven mechanical component design
    Kratz, A. and Schoeneich, M. and Zobel, V. and Burgeth, B. and Scheuermann, G. and Hotz, I. and Stommel, M.
    IEEE Pacific Visualization Symposium (2014)
    This paper is the result of a close collaboration of mechanical engineers and visualization researchers. It showcases how interdisciplinary work can lead to new insight and progress in both fields. Our case is concerned with one step in the product development process. Its goal is the design of mechanical parts that are functional, meet required quality measures and can be manufactured with standard production methods. The collaboration started with unspecific goals and first experiments with the available data and visualization methods. During the course of the collaboration many concrete questions arose and in the end a hypothesis was developed which will be discussed and evaluated in this paper. We facilitate a case study to validate our hypothesis. For the case study we consider the design of a reinforcement structure of a brake lever, a plastic ribbing. Three new lever geometries are developed on basis of our hypothesis and are compared against each other and against a reference model. The validation comprises standard numerical and experimental tests. In our case, all new structures outperform the reference geometry. The results are very promising and suggest potential to impact the product development process also for more complex scenarios. © 2014 IEEE.
    view abstract10.1109/PacificVis.2014.51
  • β relaxation and low-temperature aging in a Au-based bulk metallic glass: From elastic properties to atomic-scale structure
    Evenson, Z. and Naleway, S.E. and Wei, S. and Gross, O. and Kruzic, J.J. and Gallino, I. and Possart, W. and Stommel, M. and Busch, R.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The slow β relaxation is understood to be a universal feature of glassy dynamics. Its presence in bulk metallic glasses (BMGs) is evidence of a broad relaxation time spectrum that extends to deep within the glassy state. Despite the breadth of research devoted to this phenomenon, its microscopic origin is still not fully understood. The low-temperature aging behavior and atomic structural rearrangements of a Au49Cu26.9Si16.3Ag5.5Pd2.3 BMG are investigated in the regime of the slow β relaxation by employing an ensemble of experimental techniques such as high-intensity synchrotron x-ray scattering, modulated differential scanning calorimetry (MDSC), dynamic mechanical analysis (DMA), impulse excitation, and dilatometry. Evidence of a distinct slow β-relaxation regime is seen in the form of (1) an excess wing of the DMA loss modulus beginning at ~50°C, (2) a crossover effect of elastic modulus with isothermal aging at 50°C, and (3) a broad, nonreversing and largely irreversible sub-Tg endotherm in the MDSC results. Atomic rearrangements occurring at the onset of the measured slow β-relaxation temperature regime were found to be confined mainly to the short-range order length scale while no significant atomic rearrangements occur on the length scale of the medium-range order. Furthermore, evidence is presented that suggests the crossover effect in Young's modulus is due to the evolution of chemical short-range order. These results support the emergent picture of a dynamically heterogeneous glassy structure, in which low-temperature relaxation occurs through atomic rearrangements confined mostly to the short-range order length scale. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.174204
  • Evaluation method for stretch blow moulding simulations with process-oriented experiments
    Zimmer, J. and Detrois, C. and Stommel, M.
    Key Engineering Materials 554-557 (2013)
    In the Stretch Blow Moulding (SBM) process, Poly (ethylene terephthalate) (PET)- preforms are biaxially deformed to produce thin walled bottles. Finite-Element (FE)-Simulations are an important tool to optimise this process in terms of material usage and product performance. Thereby, the implementation of the thermo-mechanical material behaviour of PET plays an important role to achieve realistic simulation results. A common approach for this purpose is to calibrate a material model with stress-strain curves from biaxial stretching experiments. Thin PETsheets are stretched under defined temperatures and strain rates. However, these experiments include process simplifications regarding geometry, heating and deformation parameters. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process. PET-Preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated in free air (free-blow-trial). A IR-camera is used to image the axial and radial temperature distribution on the preform immediately before blowing. The deformation process is recorded via two high speed cameras with a frame rate of 2000/s. The cameras are synchronised with a pressure sensor to consequently calculate reliable stress-strain curves at any points on the preform. In addition FE-simulations of the free blow trials are conducted using a material model calibrated with the simplified stretching experiments of thin PET sheets. Resulting stress-strain-curves from simulations and free-blow-trials are finally compared to evaluate the quality of the material model as well as the underlying testing procedure. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/
  • Influence of the mechanical stress and the filler content on the hydrostatic compression behaviour of natural rubber
    Zimmermann, J. and Stommel, M.
    IOP Conference Series: Materials Science and Engineering 48 (2013)
    The behaviour of natural rubber (NR) compounds under mechanical stress is often reported in literature. An important and widely discussed effect that occurs is the Mullins effect. During the first loading cycles in a tensile test for example, a stress-softening effect is observed. This and other effects on the mechanical behaviour are investigated for different rubber materials with and without different types of fillers and filler contents. Besides, the hydrostatic compression behaviour is affected by the type and content of filler as well, which is shown for an NR with and without waxes and different contents of carbon black (CB) in this contribution. In contrast to the Mullins effect, there is no dependence of the number of loading cycles on the volumetric behaviour determined in hydrostatic compression tests. Furthermore, the influence of the previous stress-softening due to mechanical stress on the compression behaviour is elaborated. Cyclic uniaxial tensile tests are performed to realize the stress-softening in the rubber materials. The subsequent compression tests are compared to compression tests without any pre-stretching to determine the influence of previous mechanical loading on the compression behaviour of natural rubber with different filler contents.
    view abstract10.1088/1757-899X/48/1/012006
  • Inverse determination of modeling parameters to consider inhomogeneities of semicrystalline thermoplastics in structure simulations
    Kaiser, J.-M. and Stommel, M.
    Archive of Applied Mechanics 83 (2013)
    Two semicrystalline thermoplastics, an isotactic polypropylene (iPP, LynedllBasell Moplen HP501L) and a polyethylene-high-density (PE-HD, LynedllBasell Hostalen GC7260), were selected to approve a new approach. The developed approach allows the inverse determination of the amorphous and crystalline mechanical as well as the crystalline geometric constituents' properties. Commonly, these properties are unknown in structure simulations, and hence, the application of micromechanical models to the inhomogeneous microstructure of semicrystalline thermoplastics is restricted. Rather, a homogenous microstructure is assumed, and only one Young's modulus and Poisson's ratio are used in calculations. Thus, the quality and reliability of simulations are limited. In the current study, a new approach was exemplarily conducted for the inverse determination of the required properties by combining a Mori-Tanaka mean field approach with a genetic optimization algorithm. Conclusive results were achieved for both polymers. According to the results, the attained geometric parameters for the crystalline constituents resemble the aspect ratio of the spherulite diameter and the long period of the real crystalline microstructure, and the mechanical properties of the amorphous and crystalline constituents are located within reasonable bounds. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00419-012-0724-3
  • Method for the evaluation of stretch blow molding simulations with free blow trials
    Zimmer, J. and Stommel, M.
    IOP Conference Series: Materials Science and Engineering 48 (2013)
    Finite-Element (FE) simulations are a valuable tool to support the analysis and optimization of production processes. In order to achieve realistic simulation results, a consistent simulation set-up followed by an evaluation through experiments is crucial. Stretch Blow Molding (SBM) is a commonly applied forming method to produce thin walled bottles. Polyethylene terephthalate (PET) preforms are biaxially stretched into a closed cavity to form a bottle. In this process the thermo-mechanical material behavior during forming greatly influences the performance of the end product and consequently plays a key role for a reliable process simulation. To ensure a realistic material representation in the simulation model, an adequate material model is calibrated with stress-strain curves from biaxial tests. Thin PET-sheets are stretched under defined temperatures and strain rates. These representative experiments include process simplifications regarding geometry, heating and deformation parameters. Therefore, an evaluation step subsequent to the simulation set-up is inevitable. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process which enables the crucial evaluation of a process simulation. In the SBM process, the wall thickness distribution of the bottle refers to the preform deformation over time but does not fully define the thermo-mechanical material behavior. In the presented method, PET-preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated into free air (free-blow-trial). An IR-camera is used to obtain the temperature distribution on the preform immediately before blowing. Two high speed cameras are synchronized with a pressure sensor to consequently calculate reliable stress-strain curves at any point on the preform surface. These data is finally compared to results from FE-simulations of the free blow trials.
    view abstract10.1088/1757-899X/48/1/012004
  • The mechanical behaviour of rubber under hydrostatic compression and the effect on the results of finite element analyses
    Zimmermann, J. and Stommel, M.
    Archive of Applied Mechanics 83 (2013)
    Using finite element tools for the dimensioning of rubber components is state of the art. When conducting finite element simulations, a split of the strain energy function, which results in stresses, into a deviatoric and a volumetric part is made. The mechanical behaviour of reinforced natural rubber under hydrostatic pressure is determined to prove the validity of this assumption. Furthermore, a nearly incompressible material behaviour is assumed in simulations of rubber, which may cause an insufficient outcome quality especially for rubber components that are exposed to hydrostatic pressure like highly confined bushings. In this paper, a method is presented to determine the compressibility, or its reciprocal the bulk modulus of rubber. The effect of the bulk modulus of a natural rubber on the simulation results of a bearing is pointed out. The obtained results are compared to test data to show the significance of the value of the bulk modulus for achieving a satisfactory outcome quality. Therefore, reliable information about the in-use behaviour of rubber components is obtained to reduce the costs and the effort in the dimensioning process. © 2012 Springer-Verlag.
    view abstract10.1007/s00419-012-0655-z
  • Visualization and analysis of second-order tensors: Moving beyond the symmetric positive-definite case
    Kratz, A. and Auer, C. and Stommel, M. and Hotz, I.
    Computer Graphics Forum 32 (2013)
    Tensors provide a powerful language to describe physical phenomena. Consequently, they have a long tradition in physics and appear in various application areas, either as the final result of simulations or as intermediate product. Due to their complexity, tensors are hard to interpret. This motivates the development of well-conceived visualization methods. As a sub-branch of scientific visualization, tensor field visualization has been especially pushed forward by diffusion tensor imaging. In this review, we focus on second-order tensors that are not diffusion tensors. Until now, these tensors, which might be neither positive-definite nor symmetric, are under-represented in visualization and existing visualization tools are often not appropriate for these tensors. Hence, we discuss the strengths and limitations of existing methods when dealing with such tensors as well as challenges introduced by them. The goal of this paper is to reveal the importance of the field and to encourage the development of new visualization methods for tensors from various application fields. © 2013 The Eurographics Association and Blackwell Publishing Ltd.
    view abstract10.1111/j.1467-8659.2012.03231.x
  • Influence of hydrostatic pressure and volumetric strain on the mechanical long term behavior of polymers
    Naumann, T. and Stommel, M.
    Journal of Polymer Engineering 32 (2012)
    One of the most crucial issues in developing a material model to describe the long term behaviour of polymers is to represent adequately the load dependency of the considered material. In many publications, it is shown that the free volume affects the mechanical behavior of polymers. For a further investigation of the dependency of the creep behavior on free volume, different experiments are presented in this paper. In one experiment, the creep behavior under tension and shear are compared, to see how the different hydrostatic pressures in these tests influence mechanical behavior. Furthermore, tensile creep tests under different hydrostatic pressures are conducted experimentally. The experiments are conducted on a polycarbonate, a polypropylene and a polymethyl methacrylate. It is shown that the hydrostatic pressure has a significant influence on the creep behavior of all three materials. This effect is related to the change of free volume.
    view abstract10.1515/polyeng-2012-0033
  • Micromechanical modeling and strength prediction of short fiber reinforced polymers
    Kaiser, J.-M. and Stommel, M.
    Journal of Polymer Engineering 32 (2012)
    In this contribution, the embedding and compatibility of commonly used strength criteria in practical engineering design (e.g., Tsai-Hill) into a two-step, mean-field, homogenization approach are investigated. This approach provides the opportunity to account for the heterogeneous microstructure of a polymer composite, caused by the non-unidirectional fiber distribution due to the injection molding process. In a first step, an incremental Mori-Tanaka homogenization scheme is applied to unidirectional sub-domains. In a second step, a Voigt model is used to compute the mechanical composite behavior of an entire domain, which itself is the composition of weighted sub-domains. The chosen two-step approach allows the application of models to predict the strength after both homogenization steps. This leads to two different strength prediction strategies. The selection of certain criteria in combination with the selected level of strength prediction influences the simulation results and the number of material tests necessary for calibration. These two aspects are directly linked to engineering expenses and they are evaluated in a cost benefit analysis. To account for elasto-plasticity, a second-moment formulation is used and extended. The extension allows the direct usage of experimental matrix material data, without having to introduce a virtual matrix as commonly necessary. © 2012 by Walter de Gruyter.
    view abstract10.1515/POLYENG.2011.0605
  • Simulation of the long term behaviour of plastics components
    Stommel, M. and Naumann, T.
    Macromolecular Symposia 311 (2012)
    This paper presents a method to model the mechanical behavior of polymers over a wide time- and load-range by means of finite element analyses. The method includes a material model as well as the determination of material parameters to calibrate the material model. As a special feature of this method the model is calibrated only by using creep data that are commonly available in material data bases. So the procedure improves the simulation of the long time behavior of plastic-components without an additional experimental effort. In combination with time-temperature-superposition principle, the temperature dependency of the long term behavior is represented, too. The simulation results are validated by creep experiments on an example part. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/masy.201000099
  • Synthesis and mechanical properties of organic-inorganic hybrid materials from lignin and polysiloxanes
    Lippach, A.K.W. and Krämer, R. and Hansen, M.R. and Roos, S. and Stöwe, K. and Stommel, M. and Wenz, G. and Maier, W.F.
    ChemSusChem 5 (2012)
    The preparation of silica-containing organic-inorganic hybrid materials composed of kraft lignin, alkoxysilanes, and organic linkers was investigated. 3-Glycidyloxypropyltrimethoxysilane, 3-(triethoxysilyl)propylisocyanate (IPTES), and bis(trimethoxysilyl)hexane were selected as the most promising linkers. The best materials obtained showed improved mechanical and thermal properties compared with lignin itself. The reaction of the hydroxyl groups with IPTES and the sol-gel reaction between the organic linker molecules were studied by attenuated total reflectance FTIR and solid-state 29Si magic-angle spinning NMR spectroscopy. The homogeneous composition was demonstrated by electron microscopy and energy-dispersive X-ray spectroscopy mapping. The mechanical properties were investigated by microindentation and dynamic mechanical thermal analysis. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201200095
  • elastomers

  • finite element method

  • hybrid materials

  • polymers

  • preforming

  • stretch blow molding

  • thermoplastics

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