Low-cycle fatigue life of superelastic NiTi wires

This paper analyzes low-cycle fatigue life under strain control (εa–Nf curve) of NiTi wires in bending-rotation tests. These were carried out on stable austenite, superelastic, and stable martensite wires, with strain amplitudes from 0.6% to 12%. For strain amplitudes up to 4%, εa–Nf curves of superelastic wires are close to those values reported in the literature, and close to that of the stable austenite wire. For higher strain amplitudes, the fatigue life of superelastic wires increases with strain and approaches the fatigue life of stable martensite wire. This unusual behavior causes a “Z-shaped” curve. Fatigue crack characteristics were studied by scanning electron microscopy. Simple finite element models of the test were also developed. The results suggest that the abnormal shape of the superelastic wire curve is associated to the changes in fatigue properties that occurs when the superelastic material transform to martensite.