The damage zone ahead of the arrested crack in polyethylene resins

Formation and growth of the crack tip damage zone during slow stepwise crac k propagation in polyethylene resins was studied experimentally. The study focused on the differences between the damage zone in high density polyethy lene (HDPE), that represented traditional single-craze morphology, and the damage zone in more fracture resistant ethylene copolymers (MDPE) under pla in strain conditions. It was shown that improved fracture resistance correl ated with development of an epsilon-shaped damage zone that consisted of th e central craze and an accompanying pair of hinge shear zones of comparable length. The shear zones emanated from the crack tip immediately above and below the central craze where highly stretched material formed a membrane t hat separated the crack tip from the cavitated material in the craze. The r emarkable observation that the shear zones underwent crazing despite the pr esumably unfavorable stress-strain conditions was attributed to a dilatatio nal stress component resulting from partial re-distribution of the load as the main craze opened. Microscopic analysis revealed differences in the cra zed material between the single-craze (HDPE) and the epsilon-shaped (MDPE) morphology. An array of cellular cavities separated by walls of biaxially o riented material in the MDPE craze contrasted with the traditional structur e of uniaxially stretched fibrils in the HDPE craze. The stepwise developme nt and fracture of the damage zone was monitored in time, and the differenc es in kinetics of these processes between the two types of morphologies wer e characterized. (C) 2001 Kluwer Academic Publishers.