Abstract
This paper presents an analysis of low cycle fatigue life under strain control (ea-Nf curve) of NiTi wires submitted to bending-rotation fatigue. Fatigue tests were carried out on stable austenitic, superelastic, biphasic and stable martensitic NiTi wires, with strain amplitudes from 0.6% to 12%. An austenitic stainless steel wire was also tested for comparison, with strain amplitudes from 0.4% to 1.1%. The resulting ea-Nf curves together with data from the literature are compared. Fatigue life of the NiTi wires were always longer than that of the austenitic stainless steel in all investigated conditions. For strain amplitudes up to 4%, ea-Nf curves for biphasic and superelastic wires are consistent with those values reported in the literature, closely approaching the stable austenitic wire’s curve. For higher strain amplitudes, it is found that fatigue life of superelastic and biphasic wires increases with strain until they approach the fatigue life curve of stable martensite wire. This unusual behavior results in a “Z-shaped” curve for high strain values. It is possibly linked to the inhibition of martensitic transformation ahead of the crack tip, caused by the volume reduction inherent to that phase transformation, until this transformation occurs all over the material.
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