Considerations on Tension-Tension Fatigue Predictions for Nitinol

Lifetime predictions of components that experience millions-to-billions of in vivo cyclic mechanical motions are critical for the design and optimization of medical devices manufactured from Nitinol. The increasing use of Nitinol for aerospace actuators and elastocoloric devices also require robust fatigue predictions. As such, in vitro stress‐life, or strain‐life fatigue data are routinely determined for the material and combined with full component testing to develop design and life‐prediction strategies for the safe use of the device. This investigation reports on the tension-tension fatigue of ø0.25mm wire to 10M cycles. The effects of pre-strain (6-10.5%), mean strain (1-4.5%), strain amplitude (0.2-0.6%) and ∆T (test temperature-A_f = 5-17˚) are summarized and are used to develop an empirical model based on stress amplitude. This model incorporates the effects of cyclic hysteresis and modulus as well as upper and lower plateau stresses in accord with expected distributions of austenite and martensite.