Stress relaxation and stress recovery of TiNi shape memory alloy in stress-controlled subloop loading

Abstract

The transformation-induced stress relaxation and stress recovery of TiNi shape memory alloy (SMA) in the stress-controlled subloop loading were investigated based on the local variation in temperature and transformation band on the surface of the SMA tape in the tension test. The results obtained are summarized as bellows. (1) In the loading process, temperature increases due to the exothermic martensitic transformation (MT) till the holding strain under a constant stress rate and thereafter temperature decreases during holding the strain constant, resulting in stress relaxation due to the MT. (2) In the unloading process, temperature decreases due to the endothermic reverse transformation till the holding strain under a constant stress rate and thereafter temperature increases during holding the strain constant, resulting in stress recovery due to the reverse transformation. (3) Stress varies markedly in the initial stage followed by gradual change during holding the strain constant. (4) If stress rate is high till the holding strain in the loading and unloading processes, both stress relaxation and stress recovery are large. (5) It is important to take account of these behaviors in the design of SMA elements since the force of SMA elements can vary under constant strain even if temperature is kept constant around SMA elements.