STRESS-RELAXATION EXPERIMENTS IN POLYCARBONATE - A COMPARISON OF VOLUME CHANGES FOR 2 COMMERCIAL GRADES

Stress relaxation experiments were performed on two grades of polycarb onate at room temperature. The tests were performed in uniaxial extens ion and compression at deformations from the small strain, linear visc oelastic regime to the highly nonlinear viscoelastic regime just below yield. Simultaneous to the control of axial strain, both stress and l ateral strain were measured, the latter providing the volume change fo r the samples. The volume change measurements in tension show initial dilatation of the samples followed by a volume relaxation that at the largest strains in one grade of polycarbonate leads to densities great er than those of the undeformed sample. In the case of the compression measurements, the volume decreases upon deformation, but, rather than relaxing back towards the undeformed volume, the samples continuously densify. The differences in the tensile dilatation for the two grades of polycarbonate suggest that the volume behavior may be related to t he propensity to yield vs. fracture.