THEORY OF SLOW, STEADY-STATE CRACK-GROWTH IN POLYMER GLASSES

Cracks in polymer glasses can grow slowly preceded by a craze, a narrow zone of plastic cavitation. The craze widens by drawing more polymer from its surfaces into its fibrils but the fibrils themselves fail by local creep. When the crack tip moves at velocity v the loading at the crack tip can be described by a local stress intensity factor K which is the sum of the 'apparent' stress intensity factor K//A and a plastic contribution K//p (usually negative). K//p is found to be minus KP(K)/v where P(K) is an integral over the craze boundary displacement law. Fibril failure by local creep leads to a power law, v varies directly as K**m. From these relations K and v can be determined as a function of K//A. The plot of K vs. K//A is multiple-valued with a stable branch (at high K) and an unstable branch (at low K) separated by a minimum value of K//A which represents a threshold for stable, steady state crack growth. There is also a v threshold, below which cracks will not grow steadily. These predictions, the form of the v-K//A curve and implications for slip-stick crack growth are compared with recent experiments.