Materials and Their Characteristics # 1985 York
Slow crack growth in polyethylene involves one of the most complex fracture processes in the field of fracture mechanics and especially; very little is known about the initiation mechanism. It is generally believed that there is a critical value of the stress intensity, K, below which crack growth does not occur in polymers. For PE at room temperature, cracks grow at the very lowest applied stress because (1) the crystalline region is at 0.7 times the melting point (2) the amorphous region is above Tg and (3) all real materials contain points of stress concentration. The proposition that cracks will grow in PE even at the very lowest stress has very serious consequences hecause there are about 5 x 10° miles of PF aas pipe in the world and as yet there is no well established method for predicting the life time of these pipes.
The time spent during the initiation stage of slow crack growth represents a large part of the total time to failure. This paper is concerned with the morphology and kinetics of the initiation stage of crack growth and their relationship to the rate of subsequent crack arowth.