Crack Growth under Actuation Loading in a NiTiHf High Temperature Shape Memory Alloy

There is an increasing demand to compact and lightweight actuators in aerospace and automotive industries. The solid-state actuators are promising candidates for replacing some existing actuators or making it possible to put actuators where it was impossible or impractical with current commercially available methods. Shape memory alloys (SMAs) have the highest work output for a specific volume among all active materials. They can create high forces and large strokes, as well as providing multifunctionality, which is beneficial due to the reduced complexity, weight and volume. Therefore, it is important to characterize SMAs for fatigue and fracture. The SMA actuators operate by cycling temperature at a bias load—thus, to get representative fatigue life or crack growth behavior thermal cycling must be considered. However, running actuation fatigue tests can be time consuming, with high overhead costs. Therefore, a unified methodology is proposed to capture the driving forces in both mechanical and actuation fatigue. In this work, compact tension specimens of ternary Ni50.3Ti29.7Hf20 have been tested for both actuation and mechanical fatigue crack growth. Under mechanical fatigue, the specimen is cycled isothermally at a temperature that both forward and reverse transformation occur upon unloading and loading, respectively. In actuation fatigue, the specimen is heated and cooled at constant load to upper and lower cycle temperatures to ensure complete transformation within each cycle. These results aim to relate the actuation and mechanical crack growth rates, and provide insight into crack growth mechanisms in SMAs.