Crack-closure based method for modelling the fatigue behavior of superelastic NiTi

One remarkable trait reportedly exhibited by NiTi is the improvement of its fatigue performance with increasing mean strain; this is in stark contrast to typical engineering materials where a decrease in fatigue behavior is expected. Clarification into this phenomenon, however, still remains incomplete in literature. In this study, the computational methodology developed by Bruzzi et al. (2002) is adapted to predict the complex fatigue behavior of superelastic NiTi. This defect tolerant approach correlates local crack-tip driving force conditions of an initial small crack with an experimental long crack growth rate curve, using crack closure. Computationally derived predictions of fatigue life are attained for stable austenitic, stable martensitic and superelastic NiTi specimens for a multitude of loading conditions and initial defect sizes. Predicted stress-life curves (S-N curves) are compiled and converted to the more relevant strain-based constant life diagram. Further insight is therefore achieved into the individual contribution of the stable austenitic and stable martensitic phases on the unique fatigue behavior of superelastic NiTi. In this manner, this study offers a possible quantitative explanation toward the experimentally exhibited increased fatigue performance of superelastic NiTi.