Whole-life transformation ratchetting and fatigue of super-elastic NiTi Alloy under uniaxial stress-controlled cyclic loading

The whole-life transformation ratchetting and fatigue failure (including functional fatigue and structural fatigue) of super-elastic NiTi shape memory alloy were observed by uniaxial stress-controlled cyclic loading tests and at room temperature. The effects of peak stress, mean stress and stress amplitude were discussed, and then the interaction of transformation ratchetting and fatigue was then investigated. It is concluded that the whole-life transformation ratchetting of super-elastic NiTi shape memory alloy depends greatly on the stress levels and loading modes, and the dissipated energy in each cycle progressively decreases with the increasing number of cycles, which represents a functional degradation. The results also show that the structural fatigue life of the NiTi alloy presented under the stress-controlled cyclic loading is dependent on the peak stress, mean stress and stress amplitude, and the transformation ratchetting shortens the fatigue life apparently. The fatigue life decreases with the increasing dissipated energy, and a balance between dissipated energy and structural fatigue life should be reasonably considered in the design of damping devices of the NiTi alloy.