Determination of the Critical Flaw Size for Crack Growth in Nitinol Material Used for Biomedical Applications Through Focused Ion Beam Notch Fatigue

Fatigue fracture in Nitinol medical devices can occur from near surface intrinsic or extrinsic inclusions in an area of high localized strain such as the apex of a stent strut. This has drove the industry to move towards ultra-pure extra low inclusion (ELI) material to increase the longevity of these devices. Fatigue studies of ELI Nitinol have demonstrated an improvement in the material fatigue behavior by reducing the size and amount of inclusions. Despite these improvements, these are only probabilistic determinations of inclusion size and location to aid in life prediction of Nitinol material used in biomedical devices. If the exact location, geometry, and size of a flaw could be produced, better quantification on the critical flaw size for fatigue fracture, instead of a probabilistic determination, could be found. This paper presents research on the quantification of critical flaw size produced by focused ion beam (FIB) in ELI Nitinol. Crack-like flaws were produced in 0.635mm diameter ELI Nitinol, Af =2.5°C, wire according to the Ragu solution for a semi-elliptical crack in a cylinder with a crack length, a, to half-width, c, ratio of 0.6. This ratio puts the highest stress-intensity factor at the root of the crack or flaw. Several flaws were induced with apparent crack lengths from 3µm to 14µm. Samples were tension-tension fatigue tested at a mean strain of 3% and various strain amplitudes to elicit both survival and fatigue fracture in samples to determine the critical flaw size for fatigue crack growth in biomedical Nitinol.