Biomedical devices manufactured from Nitinol tube (e.g. Johnson & Johnson's S.M.A.R.T. Stent) invariably experience in vivo loading conditions that cause a variety of fatigue and overload conditions. These loading scenarios can, in the best case scenario lead to minor fracture in the device with no negative consequences to the device longevity, and in the worse case complete failure of the product potentially resulting in harm to the patient. Consequently, it is the primary objective of the current work to provide a comprehensive characterization of the in vitro fatigue-crack growth properties (especially at the all important near-threshold growth rates) and fracture toughness behavior in thin-walled Nitinol tubing typically used in commercial stent manufacture in simulated physiological environment, in order to realize quantifiable engineering parameters for designing against premature failure from overload and/or in vivo fatigue damage in endovascular self-expanding stents.
This work combines standard fracture mechanics test techniques (e.g. fatigue crack-growth curve generation, R-curve testing, etc), X-ray diffraction for texture analysis, and high-spatial-resolution X-ray Microdiffraction at the Advanced Light Source Beamline 7.3.3 at the Lawrence Berkeley National Laboratory. By combining these test techniques, we have learned about the fracture behavior, fatigue characteristics, and the roles of texture and phase transformation on the crack-growth properties of Nitinol tube.
Figure 5: Evolution of a fatigue-induced transformation zone ahead of an atomically sharp crack as determined by X-ray Microdiffraction. Notice the grain-dependant transformation showing suppression of the transformation in grains oriented near the <100> direction.