The fracture toughness of tough polyethylenes by a novel high pressure technique

Abstract

The failure of two grades of polyethylene has been studied using the technique of torsion under superposed hydrostatic pressure. The behaviour of unnotched samples of both grades was ductile at all pressures and strain-rates. However, at sufficiently high pressures both grades of polyethylene (including a tough copolymer) failed in a brittle manner when a surface notch was exposed to a suitable pressure fluid. Measurement of fracture stresses for various notch depths lead to a value for a critical stress intensity factor at each of several pressures. A linear extrapolation was then used to estimate the critical stress intensity factor at atmospheric pressure to be 1.11±0.05 MN m−3/2 and 1.68±0.08 MN m^−3/2 for the homopolymer and copolymer, respectively. An independent measurement at atmospheric pressure for the homopolymer using compact tension geometry yielded a value of 1.28±0.02 MN m^−3/2 confirming the accuracy of the extrapolation procedure and that the effect of the environment on the behaviour was not substantial.