Thermomechanical modeling and experimental investigation of transformation-induced creep and stress relaxation in shape memory alloy wires

Creep and relaxation phenomena are being observed in shape memory alloys, not only at high temperatures but also at room temperature, due to their martensitic transformation. Transformation-induced creep and stress relaxation in shape memory alloys occur due to temperature variations during loading and unloading cycles. In this work, a one-dimensional fully coupled thermomechanical model was employed to develop a continuum framework for studying these behaviors in shape memory alloy wires. A decrease or increase in stress was observed during forward or reverse transformation at a constant amount of strain, showing the stress relaxation and stress recovery, respectively. Similarly, the model predicts that strain increases or decreases when stress is held fixed in the course of forward or reverse transformation, meaning the phenomena of creep and creep recovery, respectively. This model provides the ability of investigating the effects of different ambient temperatures, strain rates, applied stresses and strains, and wire radii on the creep and relaxation responses of shape memory alloys. Relaxation and creep experiments at different ambient temperatures and loading or unloading rates were also done on NiTi wires, and the theoretical predictions were shown to be in a good agreement with the empirical observations.