Micromechanics and kinetics of deformation zones at crack tips in polycarbonate

Abstract

Plane stress deformation zones are grown from cracks produced by electron beam irradiation in thin films of polycarbonate (PC) bonded to ductile copper grids. The kinetics of zone growth in both length and width are followed by optical microscopy while v _f, the ratio of zone thickness to film thickness, is followed by optical interference and transmission microscopy measurements. These data allow the zone surface displacement and stress profiles to be computed at various times during growth. There is a stress concentration at the zone tip which relaxes to a uniform stress over the rest of the zone up to near the crack tip. Both the tip stress concentration and uniform zone stress decrease as zone growth proceeds. The zone surface displacement rate follows these changes in stress. It is highest just behind the zone tip where the stress concentration exists and is constant in the uniform stress zone. It decreases markedly with time. While the Dugdale model does not predict the details of the zone micromechanics, in particular the zone tip stress concentration, it does predict qualitatively the correct trends. The crack propagates into the oriented polymer in the deformation zone in the form of a characteristic half diamond shape, which is the analogue of diamond cavities previously observed in the fracture process of oriented bulk polymers.