ON THE DUCTILE TO BRITTLE TRANSITION IN MARTENSITIC STAINLESS-STEELS - MECHANISMS, MODELS AND STRUCTURAL IMPLICATIONS

A new model of quasi-cleavage fracture toughness and the quasi-cleavage to microvoid coalescence transition is described. Quasi-cleavage occurs when stresses above a critical level encompass a critical area ahead of a crack tip. The underlying mechanism is a stable-to-unstable growth transition of a process zone crack. High toughness can be associated with the quasi-cleavage, particularly in cases involving shallow cracks and small size scales. A mixed quasi-cleavage-microvoid coalescence fracture mode transition occurs when the strains in the process zone exceed a critical level. Coupling these material properties with finite element calculations provides a physical basis for predicting the effects of size, geometry, loading rate and specified failure conditions relevant to actual structures using data from subsized specimens.