Molecular chain plasticity model similar to crystal plasticity theory based on change in local free volume and FE simulation of glassy polymer

In this paper, the new concept of a "molecular chain slip system" is analogically proposed on the basis of crystal plasticity theory for metals, and a molecular chain plasticity model that can reproduce the large deformation behaviors of glassy polymers is presented. In this model, the independent rotation of slip systems is allowed, which is a major difference from conventional crystal plasticity theory. Moreover, an inelastic response law based on probabilistic theory considering the change in the local free volume is adopted as a hardening law so as to express the nonlinear viscoelastic response. It is shown that the present model has many advantages over those in previous works. For example, the molecular chain network model based on J₂-flow theory and Argon's hardening law cannot directly express the deformation-induced orientation of molecular chains, the propagation of a high-strain-rate shear band and the nonlinear viscoelastic response before the initial yielding, which is an inelastic behavior peculiar to polymers. A finite element (FE) simulation based on the present model is carried out for polymethyl methacrylate (PMMA) under plane-strain tension. The large deformation and nonlinear viscoelastic behaviors of glassy polymers, e.g., the neck propagation with a shear band and the orientation of molecular chains, are represented.