Toughening due to transformations induced by a crack tip stress field in ferroelastic materials

Abstract

We examined the effects on the SIF (stress intensity factor) of the deviatoric and shear ferroelastic transformations and phase switching near the crack tip for plane strain deformations of ferroelastic-type crystals. Motivated by experimental observations, we confined ourselves to those materials whose paraelastic phase is marked by low anisotropy and therefore can be idealized as isotropic. It was found that the transformed zone ahead of a stationary crack tip contributes nothing to the SIF for single and bi-crystals. However, the transformed wake left behind the steadily growing quasi-static crack tip always reduced the SIF considerably; this toughening effect is very sensitive to the mismatch angles between the crack surface and the principal axes of the transformed material for ferroelastic switching, but not for ferroelastic transformations. The numerical results for the steady-state and quasi-static crack growth were verified by the energy method. The computed results agree qualitatively with the observed values for NiTi-SMA of low anisotropy.