Fracture mechanics of shape memory alloys: review and perspectives

Shape memory alloys (SMAs) are inter-metallic alloys displaying recoverable strains that can be an order of magnitude greater than in traditional alloys due to their capacity to undergo a thermal and/or stress-induced martensitic phase transformation. Since their discovery, the SMA industry has been dominated by products for biomedical applications with geometrically small feature sizes, especially endovascular stents. For such products the technological importance of fracture mechanics is limited, with the emphasis being placed on preventing crack nucleation rather than controlling crack growth. However, the successful integration of SMAs into commercial actuation, energy absorption, and vibration damping applications requires understanding and practice of fracture mechanics concepts in SMAs. The fracture response of SMAs is rather complex owing to the reversibility of phase transformation, detwinning and reorientation of martensitic variants, the possibility of dislocation and transformation-induced plasticity, and the strong thermomechanical coupling. Large-scale phase transformation under actuation loading paths, i.e., combined thermo-mechanical loading, and the associated configuration dependence complicate the phenomenon even further and question the applicability of single parameter fracture mechanics theories. Here, the existing knowledge base on the fracture mechanics of SMAs under mechanical loading is reviewed and recent developments in actuation-induced SMA fracture are presented, in terms of the micro-mechanisms of fracture, near-tip fracture environments, fracture criteria, and fracture toughness properties.