Understanding the Deformation and Fracture of Nitinol Endovascular Stents Using In Situ Synchrotron X-ray Microdiffraction

Fig. 1: Maps of the deviatoric strain of the B2 austenite along the vertical y axis (εyy) from x-ray diffraction analysis. The global displacements for these maps are shown in the upper left corner of each image. The dark lines illustrate the approximate location of the edges of the diamond strut. The red color indicates tensile strain whereas blue indicates compressive strain; note also the presence of a white neutral axis. The maximum local strain in austenite was measured to be ±1.5%. Above this strain, austenite transforms to martensite, which for this analysis is not analyzed, but can be inferred from where the austenite disappears. Martensite generally initiates at the surfaces with the highest applied deformation and begins to move toward the center of the diamond strut, as indicated by the arrows in 3mm, 5mm, and 6mm maps. However, it is observed that, even at 6mm deformation, there is a region of strainstabilized retained austenite along the center of the strut that resists transformation. Consequently, the martensite transformation front moves down along Understanding the Deformation and Fracture of Nitinol Endovascular Stents Using In Situ Synchrotron X-ray Microdiffraction