FATIGUE BEHAVIOR CHARACTERIZATION OF NITINOL FOR PERIPHERAL STENTS

Nitinol stents are nowadays widely used for the treatment of occlusions in peripheral arteries. However, the expansion of this indication has also highlighted some complications. In particular, the patient daily activities expose the peripheral arteries to large and cyclic deformations which may cause long-term failure of the device and consequently re-occlusion of the artery. Accordingly, the assessment of the stent fatigue rupture is of primary importance to assure the effectiveness of stenting procedure. However the fatigue behavior characterization of Nitinol for peripheral stent is a quite difficult problem because of the complexity of the in vivo solicitations the stent is subjected to and the strong nonlinearity in the material response. In this paper we approached the problem in two steps: (i) in the first step the study of the stent solicitations under realistic (physiological) conditions was performed through the use of numerical simulations which allowed sophisticated patient-specific models of the stenting procedure; (ii) in the second step, the previous results were used for the design of an experimental campaign and the following execution of the tests for the material mechanical characterization and fatigue life study. The tests were performed on Nitinol specimens derived from the same tubes used for producing a commercial peripheral stent and created following the same procedure employed for the device. As a consequence of the small dimension of the specimens, a preliminary design of the experimental test set-up was also required. The obtained results allowed a sufficiently accurate characterization of the stent material fatigue behavior in the range of interest.