Effects of triaxial stress on martensite transformation, stress–strain and failure behavior in front of crack tips in shape memory alloy NiTi

Based on the finite element calculation in the conditions of plane stress and plane strain, effects of triaxial stress on martensite transformation, stress–strain and failure behavior in front of crack tips in shape memory alloy NiTi are analyzed. The results show that in plane strain condition the martensite transformation and the plastic deformation in the fully transformed martensite in front of crack tips are strongly suppressed by the high triaxial stress. The maximum normal stress in front of crack tips in plane strain is higher than that in plane stress, and the equivalent plastic strain in plane strain is much lower than that in plane stress. For the fatigue failure of the superelastic shape memory alloy NiTi in plane strain condition, when the maximum stress-intensity factor Kmax is less than 15 MPa m1/2 ahead of a crack tip, the martensite transformation at the crack tip can be totally suppressed by the high triaxial stress and the fatigue crack will propagate in the untransformed austenite. In plane stress condition, the martensite transformation can occur at very small Kmax due to the low triaxial stress, the fatigue crack will propagate in the transformed martensite in front of crack tips. This analysis result agrees with the experimental result in [A.L. McKrlvey, R.O. Ritchie, Metall. Mater. Trans. 32A (2001) 731–743]. The analysis on the fracture behavior shows that in plane strain condition with high triaxial stress, the crack propagation in fracture process of shape memory alloy NiTi tends to be in an unstable manner. However, in plane stress condition with low triaxial stress, the crack propagation tends to be in stable manner.