Craze testing for tough polyethylene

Abstract

It has been generally accepted that all the modes of fracture, including rapid crack growth, quasi-static fracture and slow crack growth in polyethylene are associated with the behaviour of the craze ahead of the crack tip. For recently developed tough pipe grade polyethylene materials, the need for knowledge of the craze behaviour seems particularly important in understanding the fracture behaviour of various modes since, with the low Young's moduli and low yield stresses, large craze zones tend to make it difficult to interpret the test data using a fracture mechanics approach. A novel test method is described in this paper which is designed for craze generation and craze behaviour analysis under plane strain conditions. The test method has been proved to be suitable both for quasi-static fractures and for long term fractures, and hopefully for rapid fractures, in tough polyethylene materials. The test data of yield stress against time to yielding show a clear brittle ductile transition which implies different fracture mechanisms in short term fracture and in long term fracture.