THERMAL CYCLIC DEFORMATION AND DEGRADATION OF SHAPE MEMORY EFFECT IN Ti-Ni ALLOY

Abstract The most basic single axial cyclic motion (i.e. fatigue) tests by the alternative heating up and cooling down of shape memory Ti-Ni50.2at%wires were performed. Heating up was done by direct current method or temperature controllable water-bath and the characteristics on the cyclic motion and its degradation in shape memory effect (SME) are discussed. The degradation of SME can be divided into two factors; the cyclic deformation by thermoelastic transformation ΔD and the gradual inelasatic elongation U of Ti-Ni wire. The stress dependency of each SME degradation factor, ΔD or ΔU, and the variation of transformation temperature with thermal cycles are discussed. Lastly, nondestructive estimation of total thermal fatigue life of wire is tried from the view point of the initial changing rate of these degradation parameters.