Ultrahigh-Resolution In Situ Diffraction Characterization of the Local Mechanics at a Growing Crack Tip in Nitinol

Despite the recognized importance of the in vivo fatigue properties of Nitinol, there is still limited understanding of how fatigue cracks propagate in this material. This study represents an initial approach to gain such understanding via a combination of fracture-mechanics testing and synchrotron x-ray (micro) diffraction, by providing insight into the role of transformational and local strain fields on the progression of fracture in a stent-like material structure. The results presented here compare the fracture-mechanics predicted transformation-zone size and shape with the actual zones measured by micro-diffraction. Tests were conducted with compact-tension specimens, laser-cut from Nitinol tube that was shape-set flat; this configuration mimics the microstructure and texture observed in Nitinol medical devices. Fatigue cracks were grown ex situ at near-threshold conditions (ΔK = 3 MPa√m) to a crack length to sample width ratio of a/W = 0.5. Specimens were then loaded in situ with a miniature straining rig to various stress intensities for multiple fatigue cycles. Thousands of local diffraction patterns (1 μm 2 spot area) spanning hundreds of micrometers surrounding the crack tips, were combined to produce contour maps of phase volume and local strain. The differences in monotonic and cyclic loading conditions can be deduced from these tests and can be used to differentiate in vivo single-event versus cumulative-damage fractures.