| Popis: |
Some crystalline thermoplastics which show high resistance to slow tearing are susceptible to brittle rapid crack propagation. These polymers may also lose much of their impact fracture resistance with increasing impact speed. The thermal decohesion model attributes both phenomena to micro-scale melting resulting from a high plastic work rate within a cohesive zone. The model provides quantitative predictions to which some polymers conform very well whilst other, quite similar, polymers do not. This difference is here attributed to the differing tendency for micro-scale necking to stabilise within a craze. On this basis, the rate-dependence of impact resistance in five polyethylenes, differing in structure and molecular weight, can be characterised using a criterion based on bulk true stress vs. true strain data. |
