Invited Talk: Development of Constant Life Diagram for Superelastic Nitinol: Impact of Mean Strain and Pre-Conditioning on Fatigue

Fatigue life analysis of superelastic nitinol in medical device applications is challenging due to uncertainties in boundary conditions and in the stress analysis associated with complex geometries. This is further compounded by the lack of accurate statistical information on the life and strength distribution of the material and a framework for the constant life diagram. The concept of fatigue endurance limit and the mean strain effect in the form of Goodman relationship is examined in light of recent experimental data that is in disagreement with traditional theories. The present paper examines the fatigue governing parameters and their impact on the fatigue strength and life distribution and provides a basic predictive framework.

In this work, a simple bending specimen geometry is used. FEA simulations using superelastic material constitutive law and fully loading-unloading hysteresis are used to compute the cyclic stress and strain. The effect of mean strain and the pre-conditioning strain is investigated experimentally. The effect of preconditioning is further explored by measurement of residual stress resulting from loading beyond elastic range. Validation of the complete deformation history is achieved by comparing the FEA results with DIC strain maps.

Fatigue testing of nitinol specimens is performed in a simulated physiological environment. Results are analyzed using various statistical distribution functions to arrive at the most suitable life prediction model. We present a statistical framework for predicting the durability of structure-critical medical devices with a confidence level meeting the requirements of international standards, regulatory guidance and clinical practice for patient safety.