ADVANCED MATERIALS & PROCESSES | OCTOBER 2023 40 FEATURE EXPLORING NITINOL’S BILLION CYCLE FATIGUE LIFE The goal of a new research study is to understand the mechanism of Nitinol ultra-high cycle fatigue and to eventually predict when fracture will occur. Brian T. Berg Boston Scientific, St. Paul, Minnesota Kenneth I. Aycock, Wayne M. Falk, and Jason D. Weaver U.S. Food and Drug Administration, Silver Spring, Maryland Nitinol’s remarkable qualities including recoverable high strain capability, strength, and biocompatibility enable its use in medical devices from dental arch wires to catheter-delivered heart valves[1]. In these life- enhancing and life-supporting applications, fractures could lead to therapy failure, pain, disability, or death. To aid in assessing Nitinol device durability, researchers from Medtronic, Boston Scientific, and the U.S. Food and Drug Administration (FDA) have been measuring and modeling Nitinol fatigue. Though additional experiments are still in progress, scientists are beginning to publish the initial results[2]. ROTARY BEND FATIGUE EXPERIMENTS AND STATISTICS In a recently published study by the authors of this article, researchers conducted high-speed rotary bend fatigue testing per ASTM E2948-16A of standard purity Nitinol wire to one billion cycles, a number approximately equal to 25 years of heartbeats[3]. One notable characteristic of the results is the near complete absence of fractures between approximately 100,000 cycles and 100 million cycles (105 to 108 cycles) (Fig. 1). Thus, although many material tests are run and published to 10 million cycles, the appealing idea of an infinite fatigue life based on 10 million cycle data is wrong. Given the apparent bimodal fracture behavior with some fractures occurring at low cycle (below ~100,000 cycles) and some fractures occurring at ultra-high cycle (greater than ~100 million cycles), researchers used a novel competing-failure-mode model to statistically characterize the results (Fig. 1). Ongoing work continues to probe what appears to be distinct fatigue mechanisms occurring between low cycle and ultra-high cycle fatigue in standard purity Nitinol at cycle counts approaching 2 billion. COMPUTATIONAL MODELING To better characterize the test setup for guided rotary bend fatigue, the researchers conducted finite element analysis (FEA) based on digital photos to estimate actual boundary conditions. The simulations allowed the team to investigate variations in mechanics along the wrapped wire length and also to estimate the volume and surface area undergoing phase transformation with each wire rotation[2]. Tensile testing per ASTM F2516-18 was performed Fig. 1 — Experimental fatigue data with competing failure model overlaid. Low cycle fatigue behavior exhibits a typical logarithmic trend with decreasing strain amplitude. However, that trend reaches a threshold strain below which fatigue life extends dramatically. Fatigue fractures may still occur below this threshold. This research study shows a dependence of increasing life with decreasing strain amplitude in the ultra-high cycle region. Often in engineering practice, the test is not carried out beyond 107 cycles experimentally; in this case, the second slope would not be detected—leading some to mistakenly assert an infinite fatigue limit. 8
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