Impact and dynamic fracture of tough polymers by thermal decohesion in a Dugdale zone

At low temperatures and high loading rates, normally tough crystalline thermoplastics may undergo a transition from ductile tearing to brittle rapid crack propagation (RCP). It is proposed here that RCP — characterised by low toughness, high crack speed (>100 m/s) and a macroscopically smooth fracture surface — occurs by self-sustained melting of a layer, one chain length thick, at each cohesive surface of a crack-tip craze, due to adiabatic heating. Initiation of RCP from a rapidly loaded sharp notch, i.e. impact fracture, requires both the formation of this melt layer, and sufficient crack extension force to propagate it. A schematic linear-elastic analysis based on the Dugdale model accounts both for the measured dynamic fracture resistance, and for the variation of impact fracture resistance with impact speed, in two pipe-grade polyethylenes and in a neat and a rubber-toughened polyacetal. It is concluded that crack initiation resistance, unlike dynamic fracture resistance, cannot be defined as a geometry-dependent material property.