Real Fatigue Stains for Nitinol specimens Under cyclic loading

Fatigue properties of Nitinol have been studied extensively in the literature. Without a clear understanding of the main drivers for fatigue life, it becomes critical to have a consistent treatment of the local strain metric which remains the popular choice for defining fatigue life of Nitinol. Comprehensive knowledge of Nitinol fatigue behavior is complicated because of various factors including the determination of local strains. Typically, Finite element analysis (FEA) is used to evaluate the mean and amplitude strains. These FEA strains are also used to design and execute the fatigue testing of cardiovascular devices and to develop the fatigue life of Nitinol. To validate the FEA model, force-displacement data is usually compared to experimental data from the specimen. This is necessary but not sufficient to validate the FEA procedure to predict true material strains. Further validation of local strains is necessary to derive confidence in the FEA fatigue strain analysis and the subsequent life predictions. We are going to explore the usage of digital image correlation (DIC) technique to experimentally determine local strains of Nitinol specimen using both ramp and fatigue loading conditions. The initial results indicate that the FEA predictions can be accurate for the single-phase regime but can deviate significantly for the mean strain in the mixed-phase regime associated with the stress-induced martensitic phase transformation. This observation has significant implications for the development of accurate life prediction methodology of Nitinol implants based on phenomenological constant life diagram.