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Nitinol superelastic stents have been widely used to treat the vascular stenosis due to its excellent mechanical behavior and biocompatibility. However, there exist conflicts between the functional properties and mechanical properties of the stent. An optimization method has been employed to deal with the conflictions with the consideration of the whole implementation process of the stent in this paper. A straight vascular with tumor inside is considered. A commonly used NiTinol superleastic stent with diamond shape strut is employed. The vascular wall tissue and stenotic plaque are also treated as hyperelastic materials. Softwares Isight, ABAQUS and Solidworks are utilized to perform the optimization job. It can be seen that the stresses are high at the areas around the fillets of the stent due to the stress concentration from a primary analysis. Therefore, the two fillets radius, thickness and radius of the stent are chosen as four optimization variables. The optimization object is to decrease the maximum stress of stent and increase the displacement of the plaque. After the optimization, the maximum stress can be decreased by 8.2 %, which implies that the stent's work life can be increased. The stenosis of the blood vessel can be decreased from 56 % to 40.0 %.

In fatigue experiments of NiTi shape-memory alloys (SMAs), TiC inclusions have been found to cause cracks. Based on bending-rotation fatigue (BRF) experiments, which have evolved as one standard method to study the structural fatigue of superelastic NiTi wires, the influence of TiC inclusions on the fatigue behavior of NiTi SMAs has been analyzed quantitatively in this article. Aurichio's superelastic model was implemented into the finite element (FE) code ABAQUS. One specimen without inclusion and seven specimens with inclusions, at different distances with respect to the neutral axis of the wire specimens, have been analyzed. The stress distributions at the cross sections are nonlinear, and there is a stress plateau in the cross section when the phase transformation occurs. The stress distribution in the cross section of the specimen without inclusion is not only dependent on the load, but also dependent on the loading path and loading history. On the other hand, the maximum stress of the specimen without inclusion is not always at the surface, which is due to the phase transformation behavior of SMAs. The existence of the inclusions changes the stress distributions in the cross section. The maximum stress is dependent on the position of the inclusions, the load, and the loading path. It has been found that the maximum stresses increase as the distance from the inclusion to the neutral axis increases. When the inclusion is at the specimen surface, the maximum stress is the highest among all the studied cases. Such high stresses caused by the inclusions can easily induce fatigue cracks. The simulation can explain the fatigue behavior of BRF experiments and provide a deep insight into the fatigue fracture mechanism of SMAs.

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