Stress relaxation experiments in polycarbonate: A comparison of volume changes for two commercial grades

Stress relaxation experiments were performed on two grades of polycarbonate at room temperature. The tests were performed in uniaxial extension and compression at deformations from the small strain, linear viscoelastic regime to the highly nonlinear viscoelastic regime just below yield. Simultaneous to the control of axial strain, both stress and lateral strain were measured, the latter providing the volume change for 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 greater 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 the propensity to yield vs. fracture.