Fatigue analysis of shape memory alloy helical springs

Estimating the functional as well as structural fatigue life of shape memory alloy (SMA) components has become a challenging issue for researchers. In current work, by considering the effects of large deformations, a rate-dependent energy-based model is developed to predict behavior of SMA helical springs under cyclic mechanical loadings. The effects of strain rate, convective heat transfer coefficient, and mean applied displacement to an SMA spring on its responses are investigated. SMA springs are fabricated through shape setting NiTi wires, and simple tension and fatigue tests are conducted on the fabricated specimens at different temperatures. Fatigue curves are presented in terms of variations in the amount of dissipated energy in stabilized force-displacement response with the number of cycles to failure. The amount of dissipated energy is calculated using the developed model, and the theoretically-predicted fatigue lives are shown to be in a good agreement with experimental results. Therefore, the presented approach is found to be a reliable method to predict lifetime of SMA springs in practice. The fatigue curves confirm the influence of loading frequency on the fatigue life.