The influence of strain ratio on bending fatigue properties of TiNi shape-memory alloy thin wires and the process of fatigue crack growth were investigated. The results obtained are summarized as follows. (1) The martensitic transformation stress of a superelastic thin wire is higher than that of a shape memory wire, resulting in short fatigue life of the superelastic wire. The maximum bending strain of fatigue limit is the martensitic-transformation starting strain. (2) The plane-bending low-cycle fatigue life curve is expressed by a power function between maximum strain εmax and the number of cycles to failure. The smaller the strain ratio, the shorter the fatigue life. (3) In plane bending, fatigue crack initiates on the surface of the wire. One fatigue crack grows preferentially and the fatigue-crack propagated region is fan-shaped. (4) If εmax is larger than 1%, εmax becomes a little smaller than that of the initial value during the rotating-bending fatigue test. (5) The fatigue crack length of the notched wire is expressed by a power function of the number of cycles.
