Crack tip stress distribution and stress intensity factor in shape memory alloys

The crack tip stress-induced martensitic transformation and the resulting stress distribution in a nickel-titanium (NiTi)-based shape memory alloy have been analysed. In particular, the effects of temperature, within the stress-induced transformation regime, in single edge crack specimens have been studied by a recent analytical model, finite element simulations and experimental measurements. The results of the analytical model have been compared with those obtained from finite element simulations, carried out by using a special constitutive model for shape memory alloys, and good agreements have been observed. Furthermore, full field numerical results have been used to better understand the evolution of the volume fraction of martensite in the crack tip region. Finally, experimental measurements have been carried out, by using single edge crack specimens obtained from commercially available NiTi sheets by electro-discharge machining. The results have been analysed by linear elastic fracture mechanics theory, and the analytical model has been used to calculate modified stress-intensity factors for shape memory alloys. A slightly increase of the critical stress-intensity factor has been observed with increasing the testing temperature, and this result is in accordance with the model calculations, which indicate a toughening effect, i.e. a reduction of the crack tip stress intensity factor.