Fatigue life assessment of Ni-Ti stent-like devices: investigation of a fracture mechanics approach

Nickel-Titanium (Ni-Ti) alloys are considered the top choice for producing self-expanding cardiovascular devices, enabling minimally invasive surgical procedures. These devices include peripheral stents used in the treatment of atherosclerosis in peripheral arteries. The cyclic loads experienced by peripheral stents throughout their lifespan, primarily due to leg movements, are responsible for fatigue failures and significant drawbacks. Phenomenological approaches relying on combined in vitro and in silico methods were investigated in past years to evaluate device safety under several loading conditions. However, the pseudoelastic behavior of the material limits the applicability of standard fatigue criteria, and the specific failure modality still remains debated. Since fracture initiation occurs from manufacturing-related defects, damage-tolerant methods commonly applied in other structural fields might constitute a potential complementary tool. This work aims to investigate the suitability of a fracture mechanics approach for fatigue life prediction of Ni-Ti stent-like devices. Fatigue tests at different strain levels were performed on surrogate samples having the same strut dimension and texture of stents. Fracture surfaces were inspected through scanning electron microscopy to define a statistical distribution of the defect size at the fracture initiation site. To properly fit a crack growth law, crack propagation tests were performed on standard samples. The energetic cyclic J-integral parameter was assumed as crack driving force and the crack propagation law was integrated from initial to final defect size to predict the fatigue life. Promising results compared to experimental observations were obtained, demonstrating the potential role of fracture mechanics in studying Ni-Ti struts fatigue failure.