A kinetic criterion for quasi-brittle fracture

A new criterion is proposed for quasi-brittle fracture based on the recognition that quasi-static fracture toughness measurements are merely a limiting case of dynamic fracture. It is found that a unique relationship exists between the energy stored ahead of a loaded crack or blunt notch and the velocity of a crack injected into the material by the failure of a microstructural element lying in the zone of elevated stress. The criterion establishes a minimum velocity required for the continued propagation of the injected crack as a function of the crack tip opening displacement at the instant of crack nucleation. Comparison of the predictions of the model with existing data on various steels and an aluminum alloy show excellent correlation. The implications of the model for phenomena such as the ductile-to-brittle transition, microstructural effects on the resistance to unstable crack propagation and grain boundary embrittlement/strengthening by solute segregation are also explored.