Study on identifying a consistent metric for comparison of fatigue performance of superelastic Nickel Titanium across different geometries and loading conditions.

In this study, 50.8 at.% superelastic Ni-Ti material was compared for differences in fatigue performance and its quantification based on geometry and testing conditions. Two different loading conditions and specimen geometries cut from the same tube raw material, namely dogbone in tension-tension fatigue and stent cell in bending fatigue are compared. Digital Image Correlation (DIC) was used to calibrate and improve confidence of the material model used in the Finite Element Analysis (FEA) by comparing the local strains (full-field) in tension and 3-point bending experiments. The full field calibration work was done using both optical DIC and in-situ Scanning Electron Microscopy (SEM) DIC. The details of the novel SEM-DIC technique is also described. The fatigue indicating parameters (FIP) evaluated include strain-based, stress-based, and stress-strain-based metrics. These included the traditionally used strain amplitude, Smith-Watson-Topper parameter (SWT) and Fatemi-Socie (FS), the latter two are stress-strain-based metrics. The results presented here show that the SWT FIP shows improved transferability to geometry and loading condition. SWT showed an 18% difference between dogbone and stent cell fatigue at 50% fracture probability compared to 38% difference for the traditional strain amplitude. The limitations of FEA and its influence on FIP evaluations are also discussed.