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Tscharnuter D.,Polymer Competence Center Leoben | Gastl S.,Polymer Competence Center Leoben | Pinter G.,Institute of Materials Science and Testing of Plastics
Conference Proceedings of the Society for Experimental Mechanics Series | Year: 2013

The nonlinear viscoelastic behavior of POM was characterized in tensile and compression tests. Digital image correlation strain measurements were used the determine the axial and transverse strain, thus providing the necessary data for three-dimensional modeling. The nonlinear viscoelastic model of Schapery was chosen to describe the time-dependent mechanical behavior. A parameter identification procedure using nonlinear optimization is presented. It is shown that the Schapery model is capable of describing the nonlinear viscoelastic relaxation behavior in tension and compression. © The Society for Experimental Mechanics, Inc. 2013. Source


Tscharnuter D.,Polymer Competence Center Leoben | Jerabek M.,Borealis | Major Z.,Johannes Kepler University | Pinter G.,Institute of Materials Science and Testing of Plastics
European Polymer Journal | Year: 2011

In the modeling of the mechanical response of a polymer over a large strain range, the nonlinear viscoelastic and viscoplastic behavior must be considered. For many polymers, nonlinear behavior is observed at low loads, e.g. by a stress-dependence of the creep compliance for stresses above 2 MPa in case of the polypropylene used in this study. Additionally, plastic deformation has been observed at strains below the yield point for several polymers. In this study, the irreversible deformation by cavitation and shear yielding of polypropylene are characterized in the pre-yield regime in uniaxial tensile tests using digital image correlation. The recovery of strain after unloading at a prescribed strain level is measured and used to identify the evolution of the plastic strain during uniaxial tension. An experimental technique for simultaneous determination of the true stress-true strain curve and the degree of stress whitening, which relates to the amount of cavitation, is introduced and the initiation of cavitation is compared to the plastic deformation detected in strain recovery at various temperatures. © 2010 Published by Elsevier Ltd. Source


Tscharnuter D.,Polymer Competence Center Leoben | Jerabek M.,Polymer Competence Center Leoben | Jerabek M.,Borealis | Major Z.,Institute of Materials Science and Testing of Plastics | And 3 more authors.
Mechanics of Time-Dependent Materials | Year: 2011

Advanced evaluation methods for the determination of linear viscoelastic material functions have been developed already forty years ago, but it is still common practice to use the so-called ten-times rule. The rule is that a sufficiently long time needs to pass after a constant load or strain is applied so that the material response is close to the response of step excitation. A substantial amount of experimental data in the short-term range has to be discarded to obtain valid results. Renewed attention has been given to improving this situation during the last few years. In this paper, the application of modern evaluation techniques to the determination of the relaxation modulus of isotropic homogeneous linear viscoelastic materials is presented. A new method that is applicable to general strain histories is introduced and applied to different loading types. © 2010 Springer Science+Business Media, B. V. Source


Tscharnuter D.,Polymer Competence Center Leoben | Jerabek M.,Polymer Competence Center Leoben | Jerabek M.,Borealis | Major Z.,Institute of Materials Science and Testing of Plastics | And 3 more authors.
Mechanics of Time-Dependent Materials | Year: 2011

Due to the viscoelastic behavior of polymers mechanical properties are strongly affected by the loading history. To obtain the time-dependent Poisson's ratio without further data manipulation, stress relaxation tests have to be carried out. Only few results for viscoelastic materials have been published to date, but the theory of Poisson's ratio in the framework of linear viscoelasticity has received some attention with respect to loading histories other than relaxation, i.e. creep and constant rate of strain tests. The main objective of this work is to compare the potential of different testing methods to determine Poisson's ratio. Transverse and axial strain have been measured in relaxation tests, creep experiments and displacement rate controlled tensile tests. Relaxation tests are evaluated accounting for the finite loading time and the results are compared with those of tensile creep and displacement rate controlled tensile tests. An optimization based method to determine linear viscoelastic material functions developed previously is applied to calculate Poisson's ratio. © 2010 Springer Science+Business Media, B. V. Source


Leevers P.S.,Imperial College London | Horsfall I.,Cranfield University | Rager A.,Imperial College London | Rager A.,BMW AG | And 5 more authors.
Polymer Testing | Year: 2014

To support the selection of polymers for applications subject to impact, industry needs an international standard method for evaluating high-rate fracture resistance. As another step towards establishing one, three generic thermoplastics were tested by five laboratories in a round-robin programme. Strain-gauged single edge-notched bend specimens of high density polyethylene, poly(vinyl chloride) and poly(methyl methacrylate) were subjected to impact at speeds from 0.2 to 27 m/s, and the crack tip loading time to fracture initiation, ts, was measured. All three polymers indicated an inverse 4/3-power dependence of ts on impact velocity, as predicted for an adiabatic decohesion mechanism - which is able to account for ts in terms of bulk properties. Fracture toughness was calculated from failure time using a velocity dependent 'key curve' correction, assuming a constant elastic modulus. The scatter in toughness was significant, particularly for the more brittle materials, and appeared to be independent of test speed. The findings justify an emphasis on fracture initiation time as the primary measured parameter, and guide our proposals for further refining the method.© 2013 Elsevier Ltd. All rights reserved. Source

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