Characterizing Pre-Stress in Nitinol Cardiovascular Implant

Pre-stress, or mechanical pre-conditioning, of Nitinol cardiovascular implants occurs during loading and deployment of these devices. The effect of this mechanical pre-conditioning on the long-term fatigue performance of device has been qualitatively reported in the literature. It has also been observed that pre-stress imparts preferential effect on fatigue life with respect to compressive and tensile pre-stress. To elucidate the relationship between pre-stress and fatigue performance, it is essential to characterize the complex nonlinear hysteric deformation process and the resulting residual stress from loading and deployment of the device. In this investigation, finite element analysis (FEA) with strain-hardening super elastic constitutive models has been used to characterize the detailed deformation process and compute the resulting residual stress under a given loading condition. In addition, several experimental techniques have explored to accurately measure the residual stress for comparison and model validation. Both the micro-focused x-ray diffraction (fXRD)and focused ion beam isolation (FIBI) with precision digital image correlation show great promise for characterizing the residual stress. Furthermore, the latter experimental technique can be further adapted to determined applied stress using in-situ loading stages. In this study we will be presenting detailed analytical method as well as preliminary experimental results for the determination of residual stress. Similar implementations of the material model in different commercially available software will be compared. Finally, these predicted values will be compared with experimentally determined quantities based on fXRD and FIBI methods.