Cyclic behavior and fatigue of shape memory alloys

Shape memory alloys (SMAs) exhibit interesting properties when subjected to mechanical or thermal loadings. For instance, they can accommodate large recoverable strains, or recover their shape by simple heating after being inelastically strained.

In many applications, shape memory alloys are subjected to cyclic loadings, which could induce failure of the SMA structure by fatigue. A better understanding of fatigue in SMAs thus seems important in order to further promote the use of these materials.

Among the available literature, some studies suggest that the fatigue of SMA wires varies with the loading frequency,  and explained it by temperature variations. Indeed, the occurrence of the martensitic transformation during a thermal and/or mechanical loading, is accompanied with a production of entropy and heat; this results in a strong thermomechanical coupling, the mechanical response of SMAs being highly dependent on temperature.

Thermomechanical coupling can explain the dependence of SMA behavior on strain rate. In this regard, experimental evidence shows that the dissipated energy evolves non monotonically with the strain rate.