Crack formation and pathways in Nitinol biomedical devices
Alan R. Pelton, G.RAU Inc., USA
Maximilien Launey, G.RAU Inc., USA
William LePage, The University of Tulsa, USA
Nickel-Titanium (NiTi) has become the material of choice for advanced therapeutic devices for structural heart and other endovascular diseases. Medical devices are crimped into delivery systems that induce a single strain excursion with values of up to ~12%. Upon deployment into the disease site, these crimp strains are (partially) recovered and then the devices are subjected to hundreds of millions of fatigue cycles due to in vivo anatomic and physiological motions. Over the past several years, an understanding of the fatigue crack characteristics of NiTi is beginning to emerge. Insights in crack path characteristics have come from in-depth studies with ultrahigh spatial resolution synchrotron X-ray microdiffraction (XRµD), optical digital image correlation (DIC), scanning electron microscope DIC (SEM-DIC) as well as scanning/transmission electron microscopy (S/TEM). These methods are used in combination with fracture mechanics techniques to measure in situ three-dimensional strains, phases and crystallographic alignment ahead of a growing fatigue crack. Individual austenite (B2 crystal structure) grains experience local strains of less than 1.5% whereas the martensite (B19’ crystal structure) accommodates up to ~6% strain. Furthermore, the plastic region ahead of the crack is composed of deformed (i.e., detwinned martensite). The micromechanical transformation process depends upon the material texture, grain size, grain orientation, as well as inclusion (Ti4Ni2Ox) size and volume fraction. These factors directly influence the transformation zone size/shape as well as the crack path in biomedical NiTi.