A CONSTITUTIVE MODEL FOR THE NONLINEAR VISCOELASTIC VISCOPLASTIC BEHAVIOR OF GLASSY-POLYMERS

Two features of the glassy state of an amorphous polymer, which play a key role in determining its mechanical properties, are the distribute d nature of the microstructural state and the thermally activated (tem poral) evolution of this state. In this work, we have sought to captur e these features in a mechanistically motivated constitutive model by considering a distribution in the activation energy barrier to deforma tion in a thermally activated model of the deformation process. We thu s model what is traditionally termed the nonlinear viscoelastic behavi or as an elastic-inelastic transition, where the energetically distrib uted nature of inelastic events and their evolution with straining is taken into account. The thermoreversible nature of inelastic deformati on is modeled by invoking the notion of strain energy stored by locali zed inelastic shear transformations. The model results are compared to experimental data for constant true strain rate uniaxial compression tests (nonmonotonic) at different rates and temperatures; its predicti ve capabilities are further tested by comparison with compressive cree p tests at different stress levels and temperatures.