Factors Impacting Nitinol Fatigue

The fatigue response of laser-cut nitinol products is complex and known to be impacted by a wide variety of processing, preconditioning, and use condition factors. Many of these factors have been studied in isolation, but a more comprehensive understanding is limited by variation in test samples and methodologies between studies. In this study, laser-cut diamonds with dimensions and processing representative of, but not identical to, transcatheter heart valve frames were used to initiate a multivariate assessment of factors impacting nitinol fatigue. All diamonds were tested at 60Hz in 37C deionized water out to 100 million cycles. To date, over 600 individual results have been generated across 55 test configurations encompassing a range of crimp strains, mean strains, alternating strains, Af temperatures, and test configurations (diamond ‘push’ vs diamond ‘pull’ testing). Crimp strains from zero to 9% were found to have a linear relationship with fatigue performance, with a beneficial impact of tensile crimp and a detrimental impact of compressive crimp. Increasing Af temperature resulted in a systematic increase in fatigue performance, while the crimp sensitivity trends remained the same. Increasing the mean cyclic strain from 3% to 5% resulted in a systematic decrease in fatigue performance, while again the crimp strain trends remained consistent. The fatigue dataset presented in this study will be further expanded to facilitate development and validation of a nitinol fatigue indicating parameter.