The transition from ductile to slow crack growth failure in a copolymer of polyethylene

The failures of ethylene-hexene copolymer single-edge notch tensile specimens were observed under a constant tensile load. The notch opening was measured against time over a range of stress. Three failure modes were observed: ductile, brittle and transitional. The microscopic changes at the notch tip were correlated with each of the modes of failure. Early in the test the ultimate mode of failure can be predicted from the microstructural changes in the notch. In the transition region, the lifetime increases as the stress increases because the blunting of the notch offsets the effect of the applied stress-stress field. The ductile failure is controlled by the macroscopic creep behaviour and the brittle failure occurs by slow crack growth that starts at a craze.