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The influence of strain ratio on bending fatigue properties of TiNi shape-memory alloy thin wires and the process of fatigue crack propagation 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 shorter fatigue life of the superelastic wire. The maximum bending strain of fatigue limit is the martensitic-transformation starting strain. (2) The plane-bending fatigue life curve is expressed by a power function of maximum strain "max and the number of cycles to failure. The smaller the strain ratio, the shorter the fatigue life. (3) In both rotating bending and plane bending, fatigue cracks nucleate on the surface of the wire. One fatigue crack grows preferentially and the fatigue-crack propagated region of fracture surface is fan-shaped. (4) If "max is larger than 1%, "max during the rotating-bending fatigue test becomes a little smaller than that of the initial value. (5) The fatigue crack length can be estimated by measuring increase in electric resistance based on decrease in cross-sectional area due to fatigue crack propagation. (6) The fatigue crack length of the notched wire is expressed by a power function of the number of cycles.

Influence of Strain Ratio on Bending Fatigue Life and Fatigue Crack Growth in TiNi Shape-Memory Alloy Thin Wires