The Effect of Small Cracks on the Bending Fatigue of Superelastic Nitinol

Previously, the small crack growth threshold in superelastic Nitinol was explored for the case of Mode I fracture mechanics axial tension-tension fatigue. Biomedical devices made of superelastic Nitinol however are fatigued in unidirectional bending due to the apex and diamond configurations associated with final device geometries. As a result, the application and effect of small cracks in pure bending fatigue in Nitinol have yet to be understood. Additionally, little is known about how the presence of small starter cracks might affect the fatigue limit and scatter in a Nitinol S-N curve. Lastly, the majority of Nitinol fatigue performed in bending is on specimens where the geometry requires finite element simulation to predict target mean and alternating strain conditions.

For this study, we aimed to perform four-point bending fatigue on superelastic Nitinol strip with and without the introduction of a starter crack in the specimen. Four-point bending fatigue is ideal for bending fatigue test conditions closer to the first principal mechanics solutions. For this testing, starter cracks of a single size were induced through focused ion beam (FIB) milling. Specimens were fatigue cycled on the lower plateau at different alternating strains at 3% mean strain and after a relevant pre-strain to a runout condition of 10 million cycles.