Abstract
Nitinol tubes were manufactured from Standard Grade VIM-VAR ingots using Tube Manufacturing method “TM-1.” Diamond-shaped samples were laser cut, shape set, then fatigued at 37 °C to 107 cycles. The 50, 5, and 1% probabilities of fracture were calculated as a function of number of cycles to fracture and compared with probabilities determined for fatigue data published by Robertson et al. (J Mech Behav Biomater 51:119–131, 2015). Robertson tested similar diamonds made from the same standard grade of Nitinol as in the current study, two other standard grades of Nitinol, and two high-purity grades of Nitinol expressly designed to improve fatigue life. Robertson’s tubes were manufactured using Tube Manufacturing method “TM-2.” Fatigue performance of TM-1 and TM-2 diamonds were compared: At 107 cycles, strain amplitudes corresponding to the three probabilities of fracture of the TM-1 diamonds were 2–3 times those of the TM-2 diamonds made from the same grade of Nitinol, and comparable to TM-2 diamonds made from the higher-purity materials. This difference is likely a result of the differences in tube manufacturing techniques and effects on resulting microstructures. Microstructural analyses of samples revealed a correlation between the median probability of fracture and median inclusion diameter that follows an inverse power-law function of the form y ≈ x−1.
Highlights
High-cycle fatigue life of Nitinol remains of interest as this unique material is used in an increasing number of Class 3 medical devices as well as for a resurgence of actuator applications [2,3,4,5]
We compare the effects of two tube manufacturing techniques, TM-1 and TM-2, on the highcycle fatigue life of three standard grades and two highpurity grades of superelastic Nitinol used in the manufacture of Class 3 cardiovascular medical devices
The results of the current study suggest that tube manufacturing technique may have a significant effect on fatigue life of a finished superelastic Nitinol component
Summary
High-cycle fatigue life of Nitinol remains of interest as this unique material is used in an increasing number of Class 3 medical devices as well as for a resurgence of actuator applications [2,3,4,5]. With medical devices approved for more demanding cardiovascular applications such as transcatheter aortic and mitral valve repair (TAVR/TMVR), as well as younger cohorts of patients with concomitantly increased required duty cycles, long-term structural integrity of the substrate remains of paramount concern. With these considerations in mind, much attention has been paid to factors affecting the fatigue life of Nitinol to ensure long-term device efficacy. Recent efforts on the part of raw material producers to improve material cleanliness have resulted in notable increases in fatigue life [1, 9] Despite these efforts, there has been little published work exploring the effects of metal processing techniques on fatigue properties
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