Phase transformation volume amplitude as a low-cycle fatigue indicator in nickel-titanium shape memory alloys

Transformation-induced plasticity due to the motion of martensite fronts is a key mechanism for damage accumulation during cyclic loading of NiTi shape memory alloys (SMAs). Intuitively, a larger motion of martensite fronts should result in a larger accumulation of defects, and lead to a shorter low-cycle fatigue life. We validate this hypothesis through fatigue tests and surface strain-field measurements. This result is agnostic of the tensile or bending loading mode. We also demonstrate that the fatigue lifetime intrinsically depends on a combination of two factors: The transformation volume amplitude and the number of inclusions interacting with the phase transformation domains.