Fatigue analysis of a near-equiatomic pseudo-elastic NiTi SMA

In the present paper, the fatigue behaviour of a near-equiatomic pseudo-elastic NiTi SMA (Shape Memory Alloy) is experimentally examined by means of tensile cyclic loading tests. In situ X-ray Diffraction (XRD) analyses are also performed in order to measure the contents of both austenite and martensite phase during testing. The experimental campaign highlights a stress–strain diagram defined through five different stress–strain functions. Such a diagram is here proposed to be analytically described by a model in two steps: (i) firstly, the domain for each of the above functions is determined by exploiting the experimental measurements of both austenite and martensite phase content; (ii) then, the equations defining each function are deduced by assuming that the fatigued material behaves as a single-spring system or a two-spring system (working in parallel or in series).