Evolution of internal damage during actuation fatigue in shape memory alloys

Shape Memory Alloys (SMAs) are subject to multiple types of fatigue including structural fatigue due to cyclic load application as well as functional fatigue due to cyclic phase transformation. Due to the thermomechanical nature of the phase transformation, the area of functional fatigue can be further subdivided into pseudoelastic fatigue, which is fatigue due to stress induced phase transformation, or actuation fatigue which is fatigue due to repeated thermally induced phase transformation. In the area of actuation fatigue, many works have attempted to predict the actuation fatigue lifetime, however the evolution of the microstructure during the actuation fatigue lifetime has yet to be examined. This work utilizes X-ray computed microtomography to examine the evolution of internal damage during actuation fatigue and has found that internal damage evolves in a non-linear manner unique to SMAs. Specifically, internal damage is shown to nucleate quickly during initial actuation cycling, followed by a steady state growth and leading to rapid coalescence at the end of the actuation fatigue lifetime. This experimentally observed non-linear internal damage growth has been introduced into a SMA constitutive model and the model is shown to give good lifetime predictions both for uniaxial as well as multiaxial loading conditions.