Modeling of the postyield response of glassy polymers: Influence of thermomechanical history

The continuous development of constitutive equations for the finite strain deformation of glassy polymers has resulted in a number of sophisticated models that can accurately capture the materials' intrinsic behavior. Numerical simulations using these models revealed that the thermal history plays a crucial role in the macroscopic deformation. Generally, macroscopic behavior is assumed not to change during a test, however, for certain test conditions this does not hold and a relevant change in mechanical properties, known as physical aging, can be observed. To investigate the consequences of this change in material structure, the existing models are modified and enhanced by incorporating an aging term, and its parameters are determined. The result is a validated constitutive relation that is able to describe the deformation behavior of, in our case, polycarbonate over a large range of molecular weights and thermal histories, with one parameter set only.