Uni- and biaxial deformation behaviour of HD-PE dependent on temperature and strain rate at thermoforming conditions
Bernhard Hegemann, P. Eyerer
Institute for Polymer Testing and Polymer Science (IKP), University of Stuttgart
Germany

Keywords: thermoforming, HDPE,

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For a variety of applied simulations in velocity-dependent processes such as crash- and thermoforming simulations, material behaviour and parameters are required to run precise calculations. In the past, parameters for FEM-simulation have been obtained mainly by DMTA measurements run with a frequency and temperature sweep or quasi-static tensile tests. As is well known, DMTA results are only valid for small deformations and tensile tests are only valid for slow processes, whereas in the applications mentioned mostly large deformations with high speeds are applied to polymers.

For thermoforming simulation a “reverse-engineering” method is used to get the necessary parameters for simulation. Fitting processes for material models e.g. K-BKZ require different speeds and temperatures to calculate the relaxation spectra and WLF-parameters. For these parameters a special measurement device similar to impact testing has been developed at IKP and is well described in previous publications.

To specify the measurement range and to reduce the number of trials the mechanical behaviour of HD-PE over a wide range of temperatures and high strain rates has been investigated. Bi-axial tests were performed on the special deformation device. Uni-axial high-speed tensile tests where done to confirm the biaxial results and widen the velocity range.

The results are discussed regarding strain rate and temperature dependence on yield stress and elongation at yield point. They are compared to well known results for amorphous polymers from literature. It is proven that the theoretical material behaviour of amorphous polymers can also be applied on semi-crystalline polymers like HD-PE. Furthermore the effects on reverse-engineering calculations are discussed