Shape memory alloys (SMAs) are capable of recovering large deformations through martensitic phase transformation, a reversible transformation between austenite and martensite, driven by stress and/or temperature variations. Owing to their superior properties, SMAs like NiTi are increasingly being utilized in various applications where the successful integration of components requires a comprehensive understanding of their crack growth behavior and fracture mechanics. Investigating the fracture of SMAs is often posed with significant challenges because of the transformation-induced complexities in their thermomechanical response. In this study, a new test methodology for measuring the fracture toughness of SMAs using J-integral as the fracture criterion is proposed that accounts for the transformation/reorientation-induced changes in the apparent elastic properties. A comprehensive set of experiments is carried out to measure the fracture toughness of NiTi and NiTiHf SMAs. To investigate different microstructural phases, various testing temperatures are considered: below the martensite finish temperature, Mf; above the martensite start temperature, Ms; and above the austenite finish temperature, As. At these temperatures, the material either remains in the martensite state throughout the loading, or transforms from austenite to martensite close to the crack tip, or remains always in the austenite state. Fracture toughness values are obtained and conclusions concerning their temperature dependence are drawn. For NiTi specimens, stable crack growth is observed, and the critical J-values 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. Unstable crack growth is observed in NiTiHf specimens due to the limited presence of dissipation mechanisms acting near the crack tip. Crack growth under mechanical and actuation loading (thermal cycling under bias load) is investigated via finite element analysis. Crack growth simulations are run in a three-dimensional model in Abaqus finite element suite using the virtual crack closure technique and the experimentally determined fracture toughness values. The numerical results provide a quantitative description of the observed stable crack growth in terms of thermomechanical response and evolution of the transformation zone. A unified methodology is proposed for fatigue crack growth in SMAs under mechanical and actuation loading paths by employing the range of J-integral as the driving force for crack growth. The methodology is applied to understand the mechanisms contributing to crack growth in the presence of thermal and mechanical induced phase transformation in NiTiHf. The resistance of the material to crack growth is characterized by measuring the crack growth rates corresponding to the range of the applied driving force. The actuation crack growth rates under actuation loading are compared to those of the mechanical crack growth.
