Effect of Pre-Strain, Temperature and Time on the Shelf Life Behavior of Nitinol

Nitinol as a material is often considered stable at temperatures below 100 °C since these temperatures are significantly lower than the theoretical temperature range of [0.3-0.5] Tm (Tm = absolute melting temperature) at which temperature viscoplastic phenomena become active. However, when considering a constrained device, factors such as the peak pre-strain and the associated stress coupled with the exposed temperature and time may have a significant impact on the mechanical behavior of Nitinol. This in-turn can impact the performance and the shelf life of a constrained device. This effort investigates the effect of constant applied uniaxial strain (at different test temperatures over different exposure times) on the superelastic unloading response of Nitinol. To quantify this behavior on a more practical level, tests were conducted by constraining apical samples to different peak bending strains and exposed to an accelerated aging environment. The results from the testing suggest that the tensile response of the Nitinol is impacted by the strain and the time of dwell at a specific temperature under constrained conditions.