Effect of Mean Strain and Pre-Strain on Fatigue Strength of Superelastic Nitinol

Fatigue life analysis of superelastic nitinol in medical device applications is challenging due to the lack of accurate statistical information on the fatigue life strength distribution and a clear definition of Nitinol fatigue constant life diagram.  The concept of fatigue endurance limit and the effect of mean strain on fatigue in the form of Goodman, Soderberg or Gerber relationship has been questioned in light of recently published experimental data. The present paper seeks to examine the fatigue governing parameters and their impact on the fatigue strength and life distribution in order to provide a basic framework for the Nitinol fatigue life prediction. Specifically, the cyclic strain quantities derived using different finite element approaches and their effects on mean strain dependence are investigated and verified experimentally by fatigue testing of nitinol specimens in a simulated physiological environment. The results will be analyzed using various statistical distribution functions. In particular, the focus is to clarify the influence of finite element strain calculation on the mean-strain dependence of Nitinol fatigue constant life for rationalizing the most suitable life prediction model. The overall goal is to present a guideline on finite element strain calculation and a statistical framework for predicting the durability performance of structure-critical medical devices with a confidence level meeting the requirements of international standards, regulatory guidance and clinical practice for patient safety.