Stress raisers and fracture in shape memory alloys: review and ongoing challenges

Shape memory alloys (SMAs) are able to recover large inelastic strains due to their thermal-/stress-induced phase transformation between austenite and martensite. Stress raisers can either initially exist in SMA components as the manufacturing-induced micro-defects, or may nucleate upon monotonic/cyclic loading, for instance, due to decohesion of the second particles or local cyclic plastic deformations. Furthermore, from a physical point of view, there is a problem why SMAs can withstand tens of millions of cycles if they deform elastically but only thousands of cycles if the martensitic transformation is involved in their cyclic deformation under the stress, even if the martensitic transformation is reversible. One of the possibilities is the nucleation and propagation of cracks from the stress raisers since the evolution of the transformation and local mechanical gradients are completely different at the high-stress zones at stress raisers than that being experienced within the elastic bulk. Thus, the successful implementation of SMA elements into engineering applications requires understanding and analysis of the role of the stress raisers in fracture and fatigue crack growth properties of shape memory alloys. The linear and non-linear Fracture Mechanics theories, commonly used to describe the fracture processes in typical structural alloys, need to be enhanced to capture the complex deformation mechanisms characterizing SMAs. In the present paper, first, the latest progress made in experimental, numerical, and theoretical analyses on the role of the stress raisers in the fracture parameters of SMAs are reviewed and discussed under both pure mechanical and thermomechanical loading conditions. Then, the state-of-arts in fatigue crack growth are addressed. In the end, summary and future topics are outlined.