Fracture of NiTi under constant-displacement thermomechanical loading

The literature on fatigue and fracture of shape memory alloys (SMAs) has been mainly within the purview of mechanical (isothermal) loading conditions. By the expansion of SMA applications in various engineering fields, investigations on thermomechanical fatigue and fracture have gained momentum in recent years. These studies mainly focus on constant force (isobaric) loading path, an idealization of more complex loading paths that SMA actuators may undergo. Other thermomechanical loading paths, such as constant displacement loading, have yet to be explored. This work investigates the mechanical response of NiTi SMA under constant displacement thermomechanical loading. To this aim, single edge notch tension specimens, harvested from thin NiTi strips, are pulled and then subjected to thermal sweep. Fracture under constant displacement loading is observed for the first time. Following this intriguing observation, finite element analyses are performed to understand and quantify mechanical fields near the notch. The effect of various parameters, including the applied displacement, thermal cycling range, and phase transformation characteristics, is investigated. This work is considered a step towards understanding fatigue and fracture of SMAs under more complex thermomechanical loading paths.