A rate-dependent endochronic approach to thermoplastic materials: temperature and filler volume fraction dependence

An endochronic viscoplastic approach, derived from the theory of finite viscoplasticity based on isomorphisms, is presented. The model allows to characterize viscoplastic material behaviour with equilibrium hysteresis using a rate-independent elastoplastic model with an endochronic flow rule and a logarithmic elastic law in parallel connection with a non-linear Maxwell model. The volumetric strains are assumed to be purely elastic. The proposed model describes the non-linear mechanical behaviour of thermoplastic materials such as filled polytetrafluoroethylene (PTFE). The temperature and filler volume fraction dependencies of the model parameters have been identified by a Levenberg-Marquardt algorithm. In contrast to previous suggestions, the proposed model is capable to describe both the non-linearities during strain controlled loading and the non-linearities during stress controlled unloading in a unified approach. The results of the numerical simulations are in fair agreement with the experimental data.