THE ROLE OF SHEAR STRESSES AND SHEAR STRAINS IN TRANSFORMATION-TOUGHENING

The effect of shear stresses and shear strains on microstructural development in transformation-toughened materials is studied with a procedure that treats the particles as a distribution of small, circular, transforming spots that surround the tip of a semi-infinite stationary crack. Microstructural simulations were conducted by incrementing a set of remote applied stresses consistent with a mode I K-field on a cracked plane containing untransformed spots. When a critical stress criterion was satisfied at a spot centre, that spot underwent a uniform stress-free transformation strain. Microstructures at crack-growth initiation show that the action of shear strains and the influence of shear stresses have an important effect on the size and shape of the region of transformed spots. The presence of shear can produce transformation zone shapes which are radically different from those predicted by the conventional continuum model of a dilatational strain triggered by a critical level of mean stress.