A rotating-bending fatigue testing machine was newly developed for the evaluation of the fatigue strength of a shape memory alloy wire. The rotating-bending fatigue tests were successfully performed for the TiNi shape memory alloy wire at various temperatures in water. The results were summarized as follows. If the maximum strain amplitude was in the regions of the rhombohedral-phase (R-phase) transformation, the fatigue life was longer than 107 cycles. If the maximum strain amplitude was in the region of the R-phase transformation completion and the martensitic transformation, the fatigue life became shorter as the strain amplitude increased. If the fatigue tests were performed in air, the temperature of the material rose and the transformation stress increased under cyclic deformation, yielding short fatigue life. The higher the ambient temperature, the shorter the fatigue life of the wire. The oxide film formed on the surface of the Wire did not affect the low cycle fatigue.
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TOBUSHI et al. (1996) studied this question.
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