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A comprehensive, multi-method experimental characterization of fracture is conducted on shape memory alloy NiTi that exhibits superelasticity due to austenite-to-martensite stress induced phase transformation. This characterization includes (i) load-based measurement of critical stress intensity factor (K max ) using ASTM standard E399, (ii) measurement of crack tip opening displacement (CTOD) per ASTM standard E1290, (iii) the digital image correlation (DIC) characterization of the transformation zone as well as the displacement-field based measurement of K max from the DIC data. Samples have also been tested at T=100 °C to suppress the martensitic transformation to investigate transformation toughening. The experimental investigation is complemented with finite element (FE) analysis that uses Auricchio-Taylor-Lubliner constitutive model. A direct observation with DIC revealed a small scale transformation (K-dominance). K max of the transforming material is higher than that of the transformationsuppressed material tested at 100 °C, suggesting transformation toughening. At 100 °C, the material becomes quite brittle with a very small crack-tip plastic zone when the transformation mechanism is blocked. By measures of critical CTOD, the gap widens even more between the superelastic and transformation-suppressed cases, particularly because of the side effect that, in this very interesting material, material modulus increases with temperature. Evaluating the transformation zone from the DIC strains with reference to the uniaxial stress-strain curve, an equivalent strain form is proposed in conjunction with the plane stress FE prediction.

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Crack growth in shape memory alloys under mode-I isothermal loading is simulated using finite element analysis. The experimental data refers to a compact tension (CT) experiment on nearequiatomic nickel-titanium (NiTi). The constitutive model used is calibrated from uniaxial experiments on the same material system. The simulation is run in a three-dimensional CT specimen modeled in Abaqus finite element suite using the virtual crack closure technique and an experimentally determined fracture toughness value. The numerical results provide a quantitative description of the observed stable crack growth in terms of load-displacement curve, deformation distribution, and size and shape of the stress-induced transformation zone close to the crack tip.