An irreversible thermodynamic model for semicrystalline polymers submitted to multisequence loading at large strain

Some relaxation tests under uniaxial tension were performed to characterize the relaxed state of high-density polyethylene (HDPE). These experimental results are necessary to describe and model the mechanical response in large strain by using the distribution of non-linear relaxation (DNLR) approach developed by Cunat et al. [5-7, 10]. After recalling this thermodynamic formalism, we propose a new model to describe the relaxed state corresponding to a stationary non-equilibrium state. This model can be obtained after two distinct internal reorganization "families" are considered. The first one is related to the relaxation mechanisms that depend on the applied strain rate (the viscous contribution of the response). The second is related to the mechanisms that are independent of the strain rate (perfect plastic contribution). In addition, the description of the equilibrium state is based on the physical contribution related to the amorphous chains' conformation changes during the applied extension (of an entropic nature). The interactions between macromolecular chains (of an energetic nature) are neglected at the equilibrium state. An example of multisequence loading simulation is discussed. (C) 2004 Society of Plastics Engineers.