Fatigue and Fracture of Small Cracks in Superelastic Nitinol

The present study develops a hybrid total life and damage tolerance fatigue methodology for the determination of a small crack growth threshold in Nitinol. Axial tension-tension/Mode I fatigue testing was performed on superelastic Nitinol wire specimens with starter cracks milled into the specimens using focused-ion-beam (FIB). Fatigue testing was performed in displacement control at various strain amplitudes as is typical with Nitinol fatigue given its superelastic properties. The simplistic geometry of the test specimen and axial loading conditions also allowed for the calculation of stress amplitude and initial stress intensity factor range. A logistic regression analysis was used to determine strain/stress-based fatigue limits. The small crack growth threshold for superelastic Nitinol was obtained for the first time from physically small cracks with sizes comparable to native inclusions. Lastly, a fractographic analysis was performed to examine crack initiation and propagation from the FIB-milled starter cracks and to identify relevant fractographic features. While the proposed methodology is centered around the use case of Nitinol medical devices, it could be adapted for other materials and applications where small crack initiation and propagation play an important role in fatigue lifetime prediction.