Experimental and Computational Rotary Bend Fatigue to Characterize Very High Cycle Fatigue of Nitinol

The use of superelastic nitinol in implants continues to grow as physicians, scientists, and engineers design more novel medical devices to utilize its unique characteristics. As many of these devices are expected to be long-term implants, it becomes critically important to increase our understanding of nitinol fatigue mechanisms beyond 10⁷ or 10⁸ cycles as well as to develop techniques that can reduce the burden of very high cycle fatigue tests. The primary objective of this work is to critically assess ‘Fatigue to Fracture’ testing techniques as outlined in ASTM F3211. Toward that end, we characterized the material fatigue life of electropolished nitinol wires in rotary bend out to 10⁹ cycles. Scanning electron microscopy (SEM) was used to evaluate the material inclusion content as well as fatigue initiation sites. Finite element analysis (FEA) was employed to predict the volume of highly strained material and the volume and surface area of material undergoing phase transformation at each alternating strain condition. Results revealed fatigue fractures beyond 10⁷ cycles with many occurring between 10⁸ and 10⁹ cycles. This data will be used as an input into statistical modeling for predicting the fatigue life of a surrogate device per ASTM F3211. As increasing numbers of nitinol implants are required to remain durable after years or decades of cyclic loading, these results suggest that common assumptions about endurance limits at very high cycles may not always be reliable. We expect these results to be of high value since such long-term fatigue data are rarely published.