Crazing and fracture associated with the interaction of shear bands in polystyrene

Abstract

The deformation and fracture behaviour of sheet polystyrene compressed edge-on between flat dies has been studied at room temperature. Using this test procedure it is possible to produce shear bands and crazes in the same specimen. Fracture occurs by nucleation of a crack in a craze which lies in the plane normal to the plane of the sheet and parallel to the compression axis. The amount of shear band formation depends on the prior heat treatment. No shear bands were observed in quenched material and larger compression displacements were required to nucleate crazes and fracture. Intense shear bands were formed in slowly cooled material and their interaction resulted in easier craze nucleation and fracture. Quench ageing studies were made between 293° and 360°K. There was a smooth increase of the yield stress with ageing temperature but the displacement to fracture showed an abrupt decrease which corresponded with the ageing treatment required for shear band formation. Fracture surface studies have confirmed previous work and in addition have shown that favourably oriented shear bands provide a preferred path for crack propagation.