Effects of shear stresses on crack-growth microstructures in transformation-toughened ceramics

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Abstract
In accordance with a prior study, transformation toughening of zirconia-reinforced ceramics is simulated by treating regions of particles surrounding quasi-statically advancing cracks as Mode-I symmetric distributions of circular spots that undergo a ‘supercritical’ dilatant transformation when a critical stress criterion is satisfied. Three stress criteria are considered: mean stress, maximum in-plane shear stress, and an equal mixture of these two. Simulations with the mean-stress criterion show good agreement with results of previous continuum-based analyses, as the profile of the transformed region ahead of the tip approximates a partial cardioid. In contrast, simulations for the other criteria display claw-like transformed regions extending far ahead of the crack tip. This difference in behaviour is conjectured to arise from the presence of direct spot-to-spot interaction terms in the shear stress and may account for large (i.e. mm size) transformed regions seen in some materials.

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