The Effect of Fatigue Cycling off the Upper and the Lower Plateau Stresses in Superelastic Nitinol

Several total life fatigue studies of superelastic Nitinol explore fatigue cycling off the lower plateau stress (LPS) after a pre-strain to 6% or more. This is typically done to simulate a vascular device that is loaded into the catheter and then deployed where full expansion is limited by the injured vessel or valve. This fatigue methodology is a balance between clinically relevant conditions and exploring fundamental structural fatigue properties of Nitinol. There are several studies in the literature however that explore Nitinol fatigue properties cycling off the upper plateau stress (UPS). These studies explore either pre-straining to the mean cycling condition, or pre-straining, fully unloading, and then moving to the mean condition to cycle. Additionally, some Nitinol vascular devices may see full unloading after deployment and then ballooning where they would also cycle off the UPS.

For this study, we aim to perform four-point bending fatigue on superelastic Nitinol sheet specimens under three test conditions: pre-strain to 3% and cycle, pre-strain to 6% unload to 3% and cycle, lastly pre-strain to 6% unload fully then load to 3% and cycle. The goal of our study is to understand the fundamental differences from cycling off the UPS vs the LPS under these representative clinical/fundamental structural fatigue conditions. We aim to use four-point bending fatigue as the test condition due to its first principle nature where the load is directly applied to produce a wide uniform bending strain.