A MULTIMODE APPROACH TO FINITE, 3-DIMENSIONAL, NONLINEAR VISCOELASTICBEHAVIOR OF POLYMER GLASSES

In this study a phenomenological constitutive model is proposed to des cribe 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 stre ss is to alter the intrinsic time scale of the material. A quantitativ e three-dimensional constitutive equation for polycarbonate as a model polymer was obtained by approximating the linear relaxation time spec trum by eighteen Leonov modes, all exhibiting the same stress dependen ce. A single Leonov mode is a Maxwell model employing a relaxation tim e that is dependent on an equivalent stress proportional to the Von Mi ses stress. Furthermore, a Leonov mode separates the (elastic) hydrost atic and (viscoelastic) deviatoric stress response and accounts for th e geometrical complexities associated with simultaneous elastic and pl astic deformation. Using a single set of parameters, the multi-mode Le onov model is capable of describing realistic constant strain rate exp eriments, including the strain rate dependent yield behavior. It is al so capable of giving a quantitative description of nonlinear stress-re laxation experiments. (C) 1996 Society of Rheology.