Fracture toughness of NiTi–Towards establishing standard test methods for phase transforming materials

Abstract

A new test methodology for measuring the fracture toughness of shape memory alloys using the critical value of J-integral as the fracture criterion is proposed. The method relies on the ASTM standard method for measuring the fracture toughness of conventional ductile materials extended to account for the martensitic transformation/martensite orientation-induced changes in the apparent elastic properties. A comprehensive set of nominally-isothermal fracture experiments is carried out on near-equiatomic NiTi compact tension specimens at three distinct temperatures: (i) below the martensite-finish temperature, Mf; (ii) between the martensite-start temperature, Ms, and the martensite desist temperature, Md, above which the stress-induced martensitic transformation is suppressed; and (iii) above Md. At these temperatures, the material either remains in the martensite state throughout the loading (martensitic material, case (i)) or transforms from austenite to martensite close to the crack tip (transforming material, case (ii)) or remains always in the austenite state (austenitic material, case (iii)), respectively. The critical J-values for crack growth, i.e., the fracture toughness, reported in all three cases, result in extrapolated stress intensity factors that are much higher than the corresponding values reported in literature on the basis of linear elastic fracture mechanics. Moreover, contrary to literature, the fracture toughness of martensitic and transforming materials is found to be approximately the same while the fracture toughness of stable austenite is considerably higher. This mechanics-aided test method can be potentially utilized for measuring the fracture toughness of martensitically transforming materials beyond shape memory alloys.