The upper bound of low cycle fatigue life of pseudoelastic polycrystalline NiTi shape memory alloys

The coupling between transformation-induced plasticity (TRIP) and low cycle fatigue of pseudoelastic shape memory alloys (SMAs) is investigated and an upper bound of low cycle fatigue life is found. Through combined in-situ infrared and digital image correlation observations on NiTi sheets, we reveal that the low cycle fatigue of SMAs is governed by the transformation band fronts with the highest temperature where TRIP is maximized. The local state at the band front is in fact determined by a coupling effect of the applied loads and thermal conditions, the former driving the transformation with heat release while the latter determining the heat transfer efficiency. This coupling leads the fatigue lives to distribute along a series of isotherms. As the coupling tends to be infinitesimal, local TRIP is limited to a minimum level, thus resulting in an upper bound of low cycle fatigue life. This agrees well with the experiments.