Numerical simulation of low-velocity impact loading of polymeric materials

Simulation of ductile polymers subjected to impact loading has become an important topic [1, 2], especially for automotive components related to passenger and pedestrian safety. The aim of our work is to establish and validate numerical models for impact response, and this presentation will focus on a study of impact on injection molded polypropylene plates. SAMP-1 (Semi-Analytical Model for Polymers) [3] was selected as constitutive model in LS-DYNA. This model takes tabulated data from experiments as input, and includes strain rate effects, pressure sensitive plasticity, plastic volume dilatation and damage. As the material behavior is complex, and data for large strains are needed, a major task is to obtain reliable data from material tests. Uniaxial tension and uniaxial compression tests were performed to calibrate the constitutive model. Three-dimensional digital image correlation (3D-DIC) [4] with two cameras and stereo vision was used to determine full-field displacements during uniaxial tensile tests, in order to quantify plastic volumetric strains and to obtain true stress-strain curves (the isochoric assumption is invalid for the present material). Uniaxial compression tests were made with short specimens in order to avoid buckling. Falling weight impact of plates (centrally loaded, circular clamping) and bars (three-point bending), and quasi-static three-point bending of bars, were simulated. Measured force vs. displacement, and permanent deformation of plates, were compared to numerical predictions. Figure 1 shows results for falling weight impact of a 4 mm thick plate. SAMP-1 is suitable for such materials, but improved material data are needed for e.g. strain rate effects and stress state effects.