Impact fatigue behavior of superelastic NiTi shape memory alloy wires

Superelastic cyclic behavior of NiTi wires was studied by cyclically deforming several samples at impact strain rates on the order of 10 s−1, using an instrumented tensile-impact device. The stress–strain cycles were performed up to different maximum strain ratios in order to study the cyclic impact fatigue for the incomplete, complete stress-induced martensitic transformation, and for the elastoplastic deformation of the martensitic phase. Impact results show that the stress-induced martensitic transformation stresses decrease with the number of cycles, although this decrease is lower than that obtained in cycling fatigue experiments performed at low strain rates. This suggests that the increment of the dislocation density, responsible for the stress reduction, depends on the strain rate applied during deformation. Moreover, the near-adiabatic condition fulfilled when deforming at high strain rates, promoting higher stress-induced martensitic transformation stresses during cycling at impact than the ones reached at low strain rates. In addition, the dissipated energy per cycle is also affected by the strain rate, being at impact 10–15% lower than the values obtained when the deformation occurs in isothermal conditions, that is at strain rates lower than 10−4 s−1.