Fracture of thermally activated NiTi shape memory alloy wires

This paper presents a study on the fracture of NiTi SMA wire undergoing thermal cycling, activated through resistive heating, under a constant load referred to as thermo-mechanical cycling (TMC). In general, TMC results in initiation of a large number of surface cracks on the SMA wire perpendicular to the wire axis. After initiation, these cracks propagate simultaneously inside the material under cyclic strain. But, the important observation in this study is the generation of cracks in the bulk of the material during TMC, preferentially at the core region of the wire. It has been shown that generation of cracks at the core region is due to strain inhomogeneity during TMC as a consequence of variation in the volume fraction of austenite and martensite phases across the wire cross section. This strain inhomogeneity is attributed to temperature and stress gradients that can exist across the cross section of the wire during thermal cycling. Study shows that nucleation of cracks and their growth at the core region of the wire plays an important role in the fracture of thermally activated SMA wire.

► Micro-mechanism of fracture in NiTi SMA wires upon thermo-mechanical cycling (TMC). ► Generation of cracks within the material, preferentially at the core region, during TMC of NiTi wires. ► Distinction between isothermal and thermo-mechanical fatigue fracture in NiTi SMA wires.