Numerical Analysis of the Balloon Dilatation Process Using the Explicit Finite Element Method for the Optimization of a Stent Geometry

Endovascular stent surgery is a minimally invasive surgical procedure to treat disorders of the circulatory system as blockage of blood vessels caused by the build up of plaque (fatty deposits, calcium deposits, and scar tissue) in the arteries, a condition called atherosclerosis. Nearly all of the medium-sized and large blood vessels in the body's vascular system can be accessed by a catheter system. This fact has contributed to a rapid increase in the performance of endovascular stent surgery. Before implantation the stent is crimped onto a balloon which results in a diameter reduction. The balloon catheter with the collapsed stent is placed in the narrowed artery. In the blood vessel the stent is dilated through inflation of the balloon. Then, the balloon catheter is deflated, leaving the stent in place to hold the artery open. The catheter and the guide wire are removed. Deflation of the balloon leads to a certain amount of recoil, reinforced by the outer pressure of the blood vessel. The remaining mean stresses at this state are the initial condition for a possible fatigue calculation. Due to heartbeat induced blood pressure oscillation the stent is exposed to high cycle fatigue loading on one side and to low cycle fatigue loading due to daily body movement on the other side. The introduction of new stent materials and the optimization of the stent design can be supported by Finite Element Simulation. Within this study the load steps crimping, balloon dilatation and recoil will be investigated. The correct modelling of the described load steps with the finite element method demands the use of an isotropic-kinematic hardening model as changes in the loading direction appear. Therefore the Chaboche model for a combined isotropic-kinematic hardening has been used in the virtual development process with the FEM-code. The Chaboche model is an optional hardening model in the general user material MF_GenYld developed by MATFEM. Additionally it has to be taken into account that the maximum fracture strain is a function of the stress state. The algorithm CrachFEM can be coupled with MF_GenYld for a prediction of fracture initiation during dilatation. As a prerequisite for the simulation of the balloon dilatation process a folded structure of the balloon has to be generated. For the given balloon geometry – with conical shapes at both ends - analytical folding tools for airbags exhibit problems in mapping the unfolded structure to the folded structure. Therefore the balloon folding process has been simulated directly with FEA. The explicit FEA code LS-DYNA in MPP version has been used for all analyses in this project. The explicit-dynamic integration method has significant advantages for FEA model with a great number of DOFs (CPU times increases only linearly with number of DOFs) and complex contact conditions (i.e. contact of tools and stent, possible self contact in the final phase of crimping and contact between stent and balloon).

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