A New Fatigue Model for Titanium-Nickel-Cupper Shape Memory Alloy Subjected to Superelastic Cyclic Deformation

Abstract

We proposed a new model, “series-parallel combined model”, for the fatigue of Ti–Ni–Cu shape memory alloy subjected to superelastic cyclic deformation in order to clarify a fatigue of shape memory alloy due to the cyclic phase transformation. Our model is based on the detailed observation of the stress-strain behavior during the one cycle superelastic deformation, considering together the peculiar fatigue caused by Luders deformation type of cyclic phase transformation. The model was used to predict the fatigue crack origin in the fracture surface. The predictions showed that fatigue life was governed by the failure of the earliest transformed martensite phase. For the purpose of proving the credibility of the model, the fatigue tests were carried out by using our original machine, which was made attentively so as not to change the given nominal strain amplitude and generate the bending deformation of the specimen due to the irrecoverable strain. The fatigue life curve showed the peculiar strain amplitude dependence, and had the peculiar strain amplitude region where the decrease of fatigue life with increasing the given strain amplitude did not occur under either of the conditions able to generate the phase transformation in the parent phase. The fatigue origins of the entire specimens exist in the central region of the fracture surface. The multi-fatigue cracks were observed in the region of origins and ran axially. The transverse cracks among the multi-fatigue cracks propagate from the central region into the surrounding regions. The prediction by our new model was found to agree well with experimental results and the detailed fracture surface observation by electron microscopy.