Thermo-mechanically coupled functional fatigue of NiTi shape memory alloys under multiaxial cyclic loadings

Abstract

In this work, the rate-dependent and loading path-dependent cyclic deformation behaviors of super-elastic NiTi shape memory alloy (SMA) are investigated under multiaxial cyclic loadings with various loading frequencies. The evolutions of stress–strain response, temperature and dissipation energy with increasing the number of cycles are investigated systematically. It is found that the functional fatigue occurs in the process of cyclic deformation, and such a phenomenon is more significant under non-proportionally multiaxial loadings and with higher frequency. The temperature variation induced by the internal heat generation becomes more obvious with increasing the loading frequency. Moreover, it is revealed that the path-dependent functional fatigue has a positive correlation to a newly defined indicator for different loading paths, i.e., the degree of non-proportionality, and the rate-dependent functional fatigue can be attributed to the thermo-mechanically coupled nature of martensite transformation. The experimental results performed in this work and the revealing deformation mechanisms are helpful to establish a three-dimensional thermo-mechanically coupled cyclic constitutive model of NiTi SMAs.