Polymer materials are known to dilate during plastic deformation. This thesis is a study on some of the mechanisms behind the volume change and how it is affected by triaxiality in stress. The goal was to assess how the current hyperelastic-viscoplastic constitutive material model for thermoplastics made at Structural Impact Laboratory (SIMLab) could be developed further.
The volume change was studied by conducting tension tests on axisymmetric smooth and notched specimens made of high-density polyethylene (HDPE) and polyvinyl chloride (PVC). In order to change the stress triaxiality, the notched specimens had four different notch radii. All tests were monitored by a digital charge-coupled device (CCD) camera. To map the deformations of the specimens, the images were postprocessed in a custom-made digital image correlation (DIC) algorithm that was created in the numerical computing environment and programming language MATLAB. Further, simulations of the tests were run in the finite element software LS-DYNA, using the implemented material model for thermoplastics developed at SIMLab. SIMLab's material model is currently based on the Raghava yield surface and plastic potential. Amodification of the model, employing the Gurson - Tvergaard - Needleman (GTN) yield surface and plasticpotential incorporating the evolution of voids during deformation of the material, was also evaluated.
A relationship between the stress triaxiality and the volume strain during plastic deformations was found from the tests. The stress triaxiality was also found to affect the yield stress, the local strain rate, the radial strain,the equivalent plastic fracture strain and the fracture surface. The tests also suggest that nucleation of voids should be described as strain controlled. Comparing the tests to the simulations it was evident that thevolume change in the materials was not captured properly with the model employing the Raghava potential.The simulations using the GTN potential however, showed far better estimations of the volume strain.Adjustments of the model employing the GTN yield surface and plastic potential are still required to simulatethe strain softening properly.