Yield and Fracture

Abstract Some polymers yield before they break; others show brittle fracture, but even they show some energy absorption, and rubber-toughened plastics have microscopic features that enhance this. Yield is often accompanied by post-yield softening, causing necking and cold drawing: predictable in terms of Considère’s construction. Yield stress is highly dependent on strain rate and temperature, following the Eyring model of plastic flow. It is also pressure-dependent, modelled by a pressure-modified yield criterion. Brittle fracture follows the Griffith theory and linear elastic fracture mechanics (LEFM), but a unique source of energy absorption in polymers is crazing, that is, local nm-scale cavitation. Fracture toughness increases with entanglement of the molecules. Failure studies of highly ductile polymers require techniques of elastic–plastic fracture mechanics, such as the essential work of fracture test. In rubber-toughened plastics, tiny elastomeric particles are incorporated: they cavitate at crack tips, relieving hydrostatic stress and enabling local crazing or yield to occur, increasing toughness.