Influence of stored elastic strain energy on fatigue behaviour of NiTi shape memory alloy thermal actuator wire

Influence of stored elastic strain energy, Eelse, on thermo-mechanical fatigue behaviour of NiTi shape memory alloy (SMA) thermal actuator wire was investigated. Two near equi-atomic NiTi SMA wires obtained from different sources were evaluated for quasi-static and functional fatigue properties. Study showed that the wires had similar chemical composition, transformation temperatures and static mechanical properties. However, the functional fatigue behaviour of the wires upon thermo-mechanical cycling (TMC) was found to be significantly different. Under a variable TMC stress in the range 150-450 MPa and 4% recovery strain, one of the wires showed better stability, and substantially higher fatigue life (~30000 cycles) than the other (~3500 cycles). Thermodynamic and microstructural analyses indicated that the wide variation in fatigue response of the wires was due to difference in magnitude of Eelse in the material. It is observed that at a given temperature above austenite start temperature (As), the wire with higher stored Eelse, generated about 70-100 MPa higher recovery stress than that of the wire with lower stored Eelse. As a consequence, the maximum temperature, Tmax, necessary for generation of preset peak stress during reverse (martensite→austenite) transformation, was always less in the former wire than that of the latter. This in turn was responsible for wide variations in thermo-mechanical fatigue behaviour of the two wires upon TMC.