Study on identifying a consistent metric for comparison of fatigue performance of two grades of NiTi across different geometries and loading conditions

NiTi is widely used in medical devices such as structural valves and stents. Of particular interest is to better understand fatigue performance of NiTi containing implantable devices.

In this study we compare two different grades of 50.8 at % Ni-Ti, Euroflex-SAES VIM/VAR Gen II and small inclusion GRau Inc. HCF for their differences in fatigue performance. Two different loading conditions and specimen geometries cut from the tube raw material, namely dogbone in tension-tension fatigue and stent cell in bending fatigue will be explored. The results show a different relative difference in fatigue performance between the two grades based on the selected geometries and loading conditions. We use a combination of Digital Image Correlation (DIC) and Finite Element Analysis (FEA) to understand the local deformation in order to identify the optimum metric to consistently represent this difference amongst the two geometries and loading conditions chosen. We will present the fatigue data alongside the full field calibration work undertaken to understand the same. The full field calibration work was done using both optical DIC and in-situ Scanning Electron Microscopy (SEM) DIC. We will compare the results and the technique used in both these scenarios. We will use FEA to compute and compare a few different fatigue indicating parameters including the strain amplitude that is commonly used to quantify fatigue performance of NiTi.

In the FEA framework we will present the results of the two commonly used models, the elastic-superelastic and the thermodynamics-based model, namely the MAT30 and MAT291 in LS-DYNA.