Growth of crazes in glassy polymers

Abstract

Craze extension measurements have been made: (α) in unoriented commercial glassy polystyrene and polymethyl-methacrylate under uni-axial tension at both room temperature and – 20°C; and (6) in a special grade of unoriented polystyrene free of any additives, under a large variety of multi-axial states of stress, also at both room temperature and – 20°C. In all cases, an asymptotic constant velocity behaviour was observed in which the velocity correlated only with the maximum principal tensile stress in the field. A theoretical model is presented for craze growth in which craze matter is produced by a mechanism of a repeated interface convolution at the craze tip. This model accounts for all the features of the observed behaviour. It is shown, however, that at very high stresses nearing that of general yield, another mechanism based on independent pore formation at the craze tip will give a faster growth rate and should be preferred in that range where wide-spread intrinsic crazing can occur, or where cracks propagate.