Multiaxial low-cycle fatigue failure mechanism of super-elastic NiTi shape memory alloy micro-tubes

Based on the microscopic observations on the fracture morphology of the super-elastic NiTi shape memory alloy (SMA) micro-tubes subjected to uniaxial and multiaxial low-cycle fatigue loadings, the common features of the fracture surface under both uniaxial and multiaxial fatigue loadings and the special ones appearing only in multiaxial ones are investigated. From the observed results, the multiaxial low-cycle fatigue failure mechanism of the NiTi SMA micro-tubes is investigated. It is found that for the multiaxial fatigue failure of the super-elastic NiTi SMA micro-tubes, besides the common features such as the fatigue striations, dimples and secondary cracks observed in the uniaxial fatigue tests, some special micro-patterns are observed on the fracture surface of the specimens, i.e., V-shaped troughs, scratches, abundant crack-origins, through-walled steps and slanted rupture. It implies that the multiaxial cyclic loadings result in not only a higher damage level at the stage of the crack initiation, but also the multi-directional and multi-planar modes of the crack propagation during the stress-controlled low-cycle fatigue of the NiTi shape memory alloy micro-tubes.