The purpose of this paper is to define a low cycle fatigue criterion for NiTi shape memory alloys (SMAs) in order to perform numerical calculations necessary for designing structures made of SMA and subjected to cyclic loading. For this purpose, fatigue tests are performed in a temperature and deformation regime in which the alloy exhibits pseudoelasticity. A behavior similar to plastic shakedown is observed; during the first cycles a hysteresis loop with a varying size which eventually stabilizes is obtained. Therefore, by analogy with plastic fatigue (low cycle fatigue), it is shown that the dissipated energy of the stabilized cycle is a relevant parameter for the estimation of lifetime.

This paper is aimed at developing a low cycle fatigue criterion for pseudoelastic shape memory alloys to take into account thermomechanical coupling. To this end, fatigue tests are carried out at different loading rates under strain control at room temperature using NiTi wires. Temperature distribution on the specimen is measured using a high speed thermal camera. Specimens are tested to failure and fatigue lifetimes of specimens are measured. Test results show that the fatigue lifetime is greatly influenced by the loading rate: as the strain rate increases, the fatigue lifetime decreases. Furthermore, it is shown that the fatigue cracks initiate when the stored energy inside the material reaches a critical value. An energy-based fatigue criterion is thus proposed as a function of the irreversible hysteresis energy of the stabilized cycle and the loading rate. Fatigue life is calculated using the proposed model. The experimental and computational results compare well.