Preconditioning Effects in Nitinol: FEA Characterization and DIC-Based Fatigue Validation

Preconditioning of a Nitinol implant is a critical procedure prior to delivery of the device to its intended site. This preconditioning, due to assembling, crimping, or other processes, results in a history of deformation in the Nitinol. This strain history can influence the fatigue and performance of the device due to residual stresses (strains) after the preconditioning steps. To accurately predict fatigue behavior, finite element analysis (FEA) must account for this change in the constitutive property while estimating the fatigue strains (or stresses). Additionally, FEA constitutive material models typically use an averaging law for the Austenitic and Martensitic ratio to match the global behavior; however, local strains measured by digital image correlation (DIC) are found to differ. The aim of this paper is to provide a precise characterization of the preconditioning strain for Nitinol implants and evaluate its impact on fatigue strains and implant geometry following preconditioning loads through FEA. Furthermore, we compare both local and global strains measured using DIC with FEA and demonstrate the effect of spatial and temporal resolution on the measured strains to facilitate the model credibility assessment of FEA fatigue life analysis procedures.