Abstract
The fracture of shape memory alloys is distinct from that of conventional metals, owing to the coexistence and interaction of multiple special features such as martensitic transformation, dislocation-induced plasticity, thermomechanical coupling and others. In this paper, the impact of thermomechanical behavior upon the crack initiation of a NiTi shape memory alloy under Mode I loading is investigated numerically and verified experimentally. A constitutive model incorporating phase transformation, plasticity and thermomechanical coupling is established. Via backward Euler integration and finite-element implementation, the longitudinal strain, martensite volume fraction and temperature field in the vicinity of the crack tip are furnished. The effects of grain size and loading rate on J-integral are revealed. The grain size dependence of crack initiation is non-monotonic. For the samples with grain sizes of 1500 nm, 18 nm and 10 nm, the shielding effect takes place in front of the crack. Additionally, the anti-shielding effect is detected for samples with grain sizes of 80 nm and 42 nm. The parametric study shows that loading rate imposes limited influence on J-integral, which is attributed to a small scale transformation. The decrement of yield stress and the increment of transformation hardening modulus can alleviate the anti-shielding effect and arouse the shielding effect upon crack initiation. The presented results shed light on the design and fabrication of high toughness phase transformable materials.