Finite Element Framework for Fatigue Performance Assessment of Superelastic Nitinol Used in Medical Devices

Fatigue life analysis of superelastic nitinol in medical device applications is challenging due to a number of confounding factors. Chief among them is the lack of a clear definition of a nitinol fatigue constant life diagram. It is equally difficult to convert the complex loading and anatomic boundary conditions that the device is subjected to in service and through delivery into the material fatigue strain at the worst-case location. Although ascertaining accurate statistical information on the fatigue life strength distribution is time-consuming and technically demanding, lack of a standard definition and consensus on the proper analysis procedure to derive the governing fatigue stress and strain quantity is the most fundamental issue. Consequently, controversies persist concerning the existence of a fatigue endurance limit, the nature of the constant lifeline, the effect of precondition strain and mean strain on nitinol fatigue, and the stress-life-reliability relationship based on statistical analysis of fatigue life data. Although the local strain state is believed to be the major driver of fatigue for nitinol, the correlation between far field boundary conditions and the local material strain state is often misinterpreted or grossly simplified. This paper seeks to examine fatigue-governing parameters and their impacts on the fatigue strength and life distribution to provide a basic framework for nitinol fatigue life prediction. Specifically, the cyclic strain quantities derived using different finite element approaches are investigated. In particular, the focus is to clarify the influence of finite element strain calculation on the cyclic material strain dependence of nitinol fatigue constant life for the determination of global load parameters. The overall goal is to establish a finite element strain calculation 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.