Thermo-mechanical behavior of shape memory alloy made stent- graft by multi-plane model

Constitutive law for shape-memory alloys subjected to multi-axial loading, which is based on a semi-micromechanical integrated multi-plane model capable of internal mechanism observations, is generally not available in the literature. The presented numerical results show significant variations in the mechanical response along the multi loading axes. These are attributed to changes in the martensitic variants nucleated in response to the directionality of the applied loading, as well as to micro-structural texture/fabric present in the multi-planes showing different orientations at any single point through the material. Numerical simulations suggest that the characterization and modeling of the microstructure is of paramount importance in understanding the phenomenology of the thermo-mechanical behavior of shape-memory alloys that are used in manufacturing of stents. The Niti-S Biliary Stent is a self-expanding uncoated tubular prosthesis designed to maintain patency of bile duct strictures caused by malignant tumors. It consists of a selfexpanding thermo-mechanical metal stent. The biliary stent is made of Nickel Titanium alloy (Nitinol) wire, which expands at body temperature. The stent is deployed with supplied introducers for percutaneous and endoscopic use. The existing endoprostheses differ in several aspects, such as shape design and materials. The Niti-S Biliary Stent (NNN) is only indicated for the palliation of malignant structures in the biliary. This paper aims to propose a capable multi-plane thermo-mechanical model predicting relevant information to understand the principles of stent-grafts behavior and even to develop new method for the correct use of this device. Hence, the use of a stent-graft is based on different characteristics are predicted, and the significant features of an ideal device can be pointed out. Additionally, the materials currently in use to fabricate this type of prosthesis controlled and checked and consequently new materials may be suggested.