EFFECTS OF INITIAL ANISOTROPY ON THE FINITE STRAIN DEFORMATION-BEHAVIOR OF GLASSY-POLYMERS

Solid phase deformation processing of glassy polymers produces highly anisotropic polymer components as a result of the massive reorientatio n of molecular chains during the large strain forming operation. Indee d, the polymer preform used as the starting material is usually anisot ropic owing to its prior deformation history. The process end product has often been fashioned for a particular application, i.e. to possess an increased flow strength along a particular axis, thereby exploitin g the orientation induced anisotropy effects. The fully three-dimensio nal issues involved in the use of glassy polymer components include an isotropic flow strengths, limiting extensibilities, and deformation pa tterns. These characteristics have been altered by the initial forming operation but are obviously not expected to be enhanced in all direct ions. The presence of anisotropy in structural components may also lea d to premature failure or unexpected shear localization. In this repor t the effects of initial deformation and the associated anisotropies a re investigated through uniaxial compression tests on preoriented poly carbonate (PC) and polymethylmethacrylate (PMMA) specimens. The evolvi ng anisotropy is monitored by testing materials preoriented by various amounts of strain and under different states of deformation. The tens orial nature of the anisotropic material is characterized by examining the preoriented material response in three orthogonal directions. A m odel for the large strain deformation response of glassy polymers has been shown by ARRUDA and BOYCE [in press] to be well predictive of the evolution of anisotropy during deformation in initially isotropic mat erials. Here the authors evaluate the ability of the model developed i n ARRUDA and BOYCE [in press] to predict several aspects of the anisot ropic response of preoriented materials. Using material properties det ermined from the characterization of the isotropic material response a nd a knowledge of the anisotropic state of the preoriented material, m odel simulations are shown to accurately capture all aspects of the la rge strain anisotropic response including flow strengths, strain harde ning characteristics, cross-sectional deformation patterns, and limiti ng extensibilities. Although anisotropy has been shown to evolve with temperature and strain rate in BOYCE, ARRUDA and JAYACHANDRAN [in pres s] and also state of deformation in ARRUDA and BOYCE [in press], we su bmit an experimental observation that the subsequent deformation respo nse of preoriented polymers may be predicted using only a measure of o ptical anisotropy, and not the prior strain or thermal history. Optica l anisotropy, as measured for example by birefringence, therefore repr esents a true internal variable indicative of the evolution of anisotr opy with inelastic strain, state of strain, and temperature.