The damage zone ahead of the arrested crack in polyethylene resins

Formation and growth of the crack tip damage zone during slow stepwise crack propagation in polyethylene resins was studied experimentally. The study focused on the differences between the damage zone in high density polyethylene (HDPE), that represented traditional single-craze morphology, and the damage zone in more fracture resistant ethylene copolymers (MDPE) under plain strain conditions. It was shown that improved fracture resistance correlated with development of an epsilon-shaped damage zone that consisted of the 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 that separated the crack tip from the cavitated material in the craze. The remarkable observation that the shear zones underwent crazing despite the presumably unfavorable stress-strain conditions was attributed to a dilatational stress component resulting from partial re-distribution of the load as the main craze opened. Microscopic analysis revealed differences in the crazed material between the single-craze (HDPE) and the epsilon-shaped (MDPE) morphology. An array of cellular cavities separated by walls of biaxially oriented material in the MDPE craze contrasted with the traditional structure of uniaxially stretched fibrils in the HDPE craze. The stepwise development and fracture of the damage zone was monitored in time, and the differences in kinetics of these processes between the two types of morphologies were characterized.