ADVANCED MATERIALS & PROCESSES | OCTOBER 2023 42 FEATURE analysis of fatigue fractures with lifetimes that are orders of magnitude apart may originate at inclusions of the same size/shape, even though they are subjected to the same macroscopic stress state. It is unknown whether the phase transformation adjacent to propagating cracks plays a role, but the interaction of phase transformation with the underlying microstructure deserves further investigation. CONCLUSION This research used a multifaceted approach to investigate the fatigue life of medical grade Nitinol to one billion cycles for the first time in open literature and it is the authors’ hope that this initial study of ultra-high cycle fatigue of Nitinol spurs more investigations. Future work for the research team includes studying fatigue life to two billion cycles, characterizing the ultra-high cycle fatigue life of high purity Nitinol, and investigating the effect of mean strain on ultra-high cycle fatigue. It is hoped that results from these efforts will inform a better understanding of the mechanism of Nitinol ultra-high cycle fatigue, resulting in new or revised standards and guidance documents as well as regulatory science tools[7]. From the published work investigating rotary bend fatigue of standard purity Nitinol, the primary conclusions are as follows. Fatigue fractures of Nitinol occur beyond 108 cycles and may be relatively common depending on the applied alternating strain. In guided rotary bend fatigue, alternating strain can vary along the wrapped wire length and it appears that low cycle fatigue may be linked to the presence of cyclic phase transformation. Metallographic analysis of Nitinol and characterization of transverse inclusion dimensions may provide a means to estimate fatigue life. The competing failure model provided a reasonable fit of Nitinol fatigue life over the large cycle range evaluated. ~SMST Notes This article is based on the authors’ publication[2]. Interested readers should consult the publication for additional detail on methodologies used and observed results. The findings and conclusions in this article have not been formally disseminated by the U.S. FDA and should not be construed to represent any agency determination or policy. The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health and Human Services. For more information: Brian Berg, senior research fellow, Boston Scientific, 4100 Hamline Ave., St. Paul, MN, 55113, 763.494.2677, brian.berg@bsci.com, www.bostonscientific.com. References 1. M.J. Mahtabi, N. Shamsaei, and M.R. Mitchell, Fatigue of Nitinol: The State-of-the-Art and Ongoing Challenges, J. Mech. Behav. Biomed. Mater., Vol 50, p 228-254, 2015. 2. J.D. Weaver, et al., Rotary Bend Fatigue of Nitinol to One Billion Cycles, Shape Mem. Superelasticity, 18 Jan. 2023, p 1-24, DOI: 10.1007/s40830-022-00409-7. 3. ASTM E2948-16A, Standard Test Method for Conducting Rotating Bending Fatigue Tests of Solid Round Fine Wire, ASTM International. 4. ASTM F2516-18, Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials, ASTM International. 5. Y. Murakami, Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, Academic Press, 2019. 6. ASTM E2283-08, Standard Practice for Extreme Value Analysis of Nonmetallic Inclusions in Steel and Other Microstructural Features, ASTM International. 7. Catalog of Regulatory Science Tools to Help Assess New Medical Devices, retrieved Aug. 28, 2023. https://www. fda.gov/medical-devices/science-and-research-medical- devices/catalog-regulatory-science-tools-help-assess-new- medical-devices. 10 Fig. 4 — Comparison of metallographic inclusion distribution to fracture surface inclusion distribution. The extreme value extrapolation ASTM E2283-08 predicts larger inclusions for larger areas at risk (high stress regions) in rotary bend versus inclusions found in individual metallographic areas[6].
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