The stress-strain curve of shear-banding polystyrene

Abstract

A zone of diffuse non-Hookean shear strain is shown by birefringence measurements to initiate before, and to propagate ahead of, shear bands in glassy polystyrene. A moiré analysis of the displacements around the propagating shear band packet reveals that the shear band packet, although highly visible, contributes only a minor fraction of the total non-Hookean specimen strain while the diffuse shear zone contributes the major portion. The results also show conclusively that the polymer within the expanding shear zone undergoes strain softening. These facts allow one to properly interpret the existence of a load maximum in constant rate compression tests of polystyrene as due to the expansion and strain softening of the diffuse shear zone, rather than to propagation of the shear band packet or specimen buckling. The load maximum occurs when the shear zone has fully expanded across the specimen width.