Nitinol, a Nickel-Titanium shape memory alloy, is widely used in self-expanding heart stents for its superelasticity and biocompatibility. It is able to recover large strains up to 8% making it well suited for biomedical applications. For self expanding heart stents the metal is pre-shaped to fit within the artery wall these stents are then crimped into a catheter and later deployed allowing the stent to expand within the artery. During this process the stent is subjected to significant prestrain, followed by cyclic loading from the heartbeat once implanted. Understanding the fatigue behavior of Nitinol is critical to long term reliability of the device. Unlike most metals Nitinol’s fatigue life improves when applying a higher prestrain. This makes it difficult to determine an accurate model for the metals fatigue life. To gain insight into these trends, we used digital image correlation (DIC) to analyze full-field strain data. Tensile testing was conducted with initial prestrains ranging from 5% to 9%, while images were captured for later correlation.
Our experimental data demonstrated that the energy dissipated at higher prestrains lead to less localization during cycling. We hypothesize that this reduction in localization contributes to the observed increase in fatigue strength at higher prestrains. We aim to better understand the relationship between prestrain and fatigue performance so that, in the future, we can develop a predictive model capable of estimating the fatigue life of Nitinol-based biomedical devices based on the prestrain experienced during the initial loading cycle.