EVOLUTION OF PLASTIC ANISOTROPY IN AMORPHOUS POLYMERS DURING FINITE STRAINING

The large strain deformation response of amorphous polymers results pr imarily from orientation of the molecular chains within the polymeric material during plastic straining. Molecular network orientation is a highly anisotropic process, thus the observed mechanical response is s trongly a function of the anisotropic state of these materials. Throug h mechanical testing and material characterization, the nature of the evolution of molecular orientation under different conditions of state of strain is developed. The role of developing anisotropy on the mech anical response of these materials is discussed in the context of asse ssing the capabilities of several models to predict the state of defor mation-dependent response. A three-dimensional rubber elasticity sprin g system that is capable of capturing the state of deformation depende nce of strain hardening is used to develop a tensorial internal state variable model of the evolving anisotropic polymer response. This full y three-dimensional constitutive model is shown to be successfully pre dictive of the true stress vs. true strain data obtained in our isothe rmal uniaxial compression and plane strain compression experiments on amorphous polycarbonate (PC) and polymethylmethacrylate (PMMA) at mode rate strain rates. A basis is established for providing the polymer de signer with the ability to predict the flow strengths and deformation patterns of highly anisotropic materials. A companion paper by ARRUDA, BOYCE, and QUINTUS-BOSZ [in press] shows how the model developed here in is used to predict various anisotropic aspects of the large strain mechanical response of preoriented materials. Additional work has been done to extend the model to include the effects of strain rate and te mperature in ARRUDA, JAYACHAN-DRAN, and BOYCE [in press].