Stress-Induced Martensitic Transformation in the Crack Tip Region of a NiTi Alloy

The evolution of stress-induced martensitic transformation in front of the crack tip in a NiTi alloy is analyzed in this investigation, by two-dimensional finite element simulations of single edge-crack specimens. In particular, the transformation start and finish contours, i.e., the boundaries of the transformation zone, were obtained by using plasticity concepts, and the effects of the temperature were taken into account by using the Clausius-Clapeyron relation. Furthermore, comparisons between numerical and analytical results, obtained by Irwin's modified linear elastic fracture mechanics relations, were carried out. These comparisons show that a good agreement in terms of the martensite start and finish sizes is obtained; moreover, the analytic approach could be able to describe the stress field in the crack tip region outside the phase transformation zone, i.e., in the austenitic region, but a proper equation to estimate the effective crack length should be found. To this aim, further studies should be carried out.