High Cycle Fatigue Evaluation of Laser Cut Nitinol Coupons at Varying Mean Strains

To investigate the impact of varying mean strain on high cycle fatigue strength, testing was conducted on 240 laser cut Nitinol fatigue coupons. These coupons were similar, although not identical, in geometry, raw material, processing, and preconditioning to commercial laser cut nitinol medical devices such as transcatheter aortic valve replacements (TAVR). Twenty-four coupons were tested at each of ten different strain conditions to 100 million cycles. Validated FEA was used to identify each test level; strain amplitude was varied from 0.3% to 1.0% and mean strain was varied from 1.5% to 6.0% across the test matrix. The resulting data shows trends that in some cases corroborates, and other cases deviates from recently published literature on the mean strain effect in Nitinol. At mean strain levels typically observed in superelastic TAVR applications, the fatigue strength in this study is considerably greater than reported elsewhere for Nitinol laser cut components. The overall fatigue strength displays a Goodman-like behavior, with performance dropping sharply at high mean strains, which are not typically observed in superelastic TAVR applications.