Etj Edwin Klompen

A multi‐mode approach to finite, three‐dimensional, nonlinear viscoelastic behavior of polymer glasses

In this study a phenomenological constitutive model is proposed to describe the finite, nonlinear, viscoelastic behavior of glassy polymers up to the yield point. It is assumed that the deformation behavior of a glassy polymer up to the yield point is completely determined by the linear relaxation time spectrum and that the nonlinear effect of stress is to alter the intrinsic time scale of the material. A quantitative three‐dimensional constitutive equation for polycarbonate as a model polymer was obtained by approximating the linear relaxation time spectrum by eighteen Leonov modes, all exhibiting the same stress dependence. A single Leonov mode is a Maxwell model employing a relaxation time that is dependent on an equivalent stress proportional to the Von Mises stress. Furthermore, a Leonov mode separates the (elastic) hydrostatic and (viscoelastic) deviatoric stress response and accounts for the geometrical complexities associated with simultaneous elastic and plastic deformation. Using a single set of...

Processing-induced properties in glassy polymers : development of yield stress in polycarbonate

Abstract A new method is employed to directly predict the development of yield stress distributions in injection molded products of glassy polymers. The approach is based on the results of a study on the temperature dependence of the evolution of yield stress during annealing of polycarbonate below Tg. In combination with the process-related thermal history, derived from numerical simulations of the injection molding process, an estimate of the yield stress distribution throughout a product can be obtained. Computed yield stresses of injection molded plates prove to be in excellent agreement with the experimental values, including their dependence on mold temperature and plate thickness.