Material Characterization for Predicting Impact Performance of Plastic Parts

Finite Element Codes are useful tools for predicting the structural performance of plastic components. Through the use of these tools conceptual designs can be assessed and mature designs can be optimized; thereby, shortening the costly build and test cycle. In the past predictions were most useful in predicting the load-displacement response of the component. This could be accurately done by accurately modeling the geometry and boundary conditions and by knowing the modulus of the material. As plastics have increasingly been used in more demanding applications such as load bearing automotive components, other nonlinear deformation processes and failure mechanisms become important. In plastics the yield stress is typically very strain rate sensitive and can also be pressure dependent as well. To accurately predict impact performance it is important to characterize the rate and sometimes the pressure dependence of the material and to incorporate this into the finite element material model. An even more important consideration is the actual failure event. Will the material behave ductiley or brittlely? Under what conditions will it behave ductilely or brittlely? In order to answer these questions, the strain rate, temperature and stress triaxiality of the applications needs to be known as well as the affect of these parameters upon the material performance. Once the expected failure mode is ascertained, the next step is to determine the actual failure criteria of the material. Determining the failure criteria of the material is more involved; however, it is critical for determining whether the part can meet the impact requirements of the application. This paper will outline a material characterization approach for establishing material deformations models, anticipated failure modes, and ductile and brittle failure criteria.