Tests and numerical simulations of polymer components: Numerical validation of a constitutive material model for thermo plastics

Abstract

A new constitutive material model intended for thermo plastics, developed at NTNU, was studied in this thesis. The model is implemented as a user-defined material in the finite element code LS-DYNA. For calibration and validation of the model an extensive battery of material and component tests of HDPE and PVC were conducted and reported.

For a material model to gain popularity easy calibration is vital. An easy to use analytical calibration procedure has therefore been developed, eliminating the need for inverse modelling. The calibration procedure has shown good results in numerical simulations of the material tests of the materials studied here. The procedure is, however, based on a simplification of the constitutive relations of the model. The safe use of the procedure will thus have limitations, which should be investigated further.

In order to validate the material model numerical simulations of well defined three-point bending tests, as well as material tests in tension and compression, have been used. The model performs well in material test result reproduction, but does not capture all strain rate effects observed in the material tests. The over all impression is that the model performs well in predicting bending behaviour. A major concern in bending and compression simulations is the plastic potential function used in the model. This predicts dilatation incompression and thus over estimates transverse strains and in turn compressive stresses in bending.