A THEORY FOR CLEAVAGE CRACKING IN THE PRESENCE OF PLASTIC-FLOW

A theory is proposed for cleavage cracking surrounded by pre-existing dislocations. Dislocations are assumed not to emit from the crack fron t. It is argued that the pre-existing dislocations, except for occasio nal interceptions with the crack front, are unlikely to blunt the majo r portion of the crack front, so that the crack front remains nanoscop ically sharp, advancing by atomic decohesion. The fracture process the refore consists of two elements: atomic decohesion and background disl ocation motion. An elastic cell, of size comparable to dislocation spa cing or dislocation cell size, is postulated to surround the crack tip . This near-tip elasticity accommodates a large stress gradient, match ing the nanoscopic, high cohesive strength to the macroscopic, low yie ld strength. Consequences of this theory are explored in the context o f slow cleavage cracking, stress-assisted corrosion, fast running crac k, fatigue crack growth, constraint effects, and mixed mode fracture a long metal/ceramic interfaces. Computational models and experiments to ascertain the range of validity of this theory are proposed.