Characterization of Fatigue Life of Ultrafine Grained NiTi Superelastic Wires Under Uniaxial Loading

The role of the stress induced martensitic transformation on the fatigue life of Ni-rich ultrafine grained NiTi superelastic wires has been investigated. Two types of experiments have been considered to elucidate the question. In the first class, the stress range was kept in between the forward and reverse plateaus levels, thus avoiding evident martensitic stress induced transformation. Strain controlled experiments were performed in wires in fully austenite, fully stress induced martensite and austenite–martensite coexistence zone with nearly half of the wire length transformed to martensite. In the second type of experiments, pseudoelastic cycling was performed. In this case the stress induced austenite to martensite transformation is involved. The obtained results indicate a definite deleterious effect of the stress induced transformation on the fatigue life. Additional important aspects related with fatigue life of NiTi wires like fretting fatigue effects in the grip–specimen contact region and the relative importance of the crack initiation and crack propagation stages on the total fatigue life are also presented.