Effect of Thickness on Fracture Toughness of NiTi Shape Memory Alloy

SMAs are increasingly being used as solid-state actuators where repeatable phase transformation in the presence of high stresses results in crack initiation and propagation. Therefore, successful applications of SMAs such as NiTi under various thermomechanical loading paths requires a complete understanding of their fracture behavior. Because of the complex fracture mechanics associated with martensitic transformation/reorientation, applying the existing fracture mechanics theories and standard test methods/requirements to SMAs is not straightforward. For instance, the requirement to ensure a thickness-independent fracture toughness, proposed for conventional metals, may be different for SMAs in which the underlying fracture mechanisms differ. In this study, to investigate the thickness dependence of the fracture toughness of NiTi, compact tension specimens with various thicknesses were tested under mode-I. Stable crack growth was observed in all specimen regardless of the thickness. The resistance curves were obtained from load-displacement data and the J-integral values were determined using a recently proposed methodology for SMAs. Critical values were obtained and compared to reveal the thickness dependence of the fracture toughness. The strains at different stages of loading were measured using digital image correlation technique to evaluate the zone of nonlinear deformation mechanism near the crack tip. The results are expected to suggest modifications towards relaxing the existing thickness requirements for fracture toughness measurement in SMAs.