Experimental study and life prediction on fatigue failure of NiTi shape memory alloy under multi-axial one-way shape memory cyclic loadings

In this paper, the thermo-mechanical coupling fatigue experiments of NiTi shape memory alloy (SMA) microtubes involving one-way shape memory effect (OWSME) are first carried out under the uniaxial and multiaxial stress-controlled cyclic loadings with some specific loading paths and stress levels. Experimental results show that the OWSME cyclic degradation and fatigue lives of NiTi alloy micro-tubes are dependent on the prescribed loading paths and stress levels; and the multi-axial loadings accelerate the reorientation-induced ratchetting, resulting in more obvious OWSME cyclic degradation in NiTi SMA micro-tubes. Based on the experimental results, the dissipation energy is defined as the damage variable, and the damage evolution equations of mechanical loading and thermal loading parts are established from the correspondent experimental results by considering the influence of loading path. Then, combined with the established failure criterion, the fatigue lives of the NiTi alloy presented during the multi-axial OWSME cyclic loadings are predicted. It is demonstrated that all the predicted lives are located within the twice scatter band, and most of them are within the scatter band of 1.5 times; the predicted lives of the proposed multi-axial fatigue life-prediction model agree with the experimental ones well.