R. Van Impe

Numerical Study of the Uniformity of Balloon-Expandable Stent Deployment

Stents are small tubelike structures, implanted in coronary and peripheral arteries to reopen narrowed vessel sections. This endovascular intervention remains suboptimal, as the success rate is limited by restenosis. This renarrowing of a stented vessel is related to the arterial injury caused by stent-artery and balloon-artery interactions, and a local subsequent inflammatory process. Therefore, efforts to optimize the stent deployment remain very meaningful. Several authors have studied with finite element modeling the mechanical behavior of balloon-expandable stents, but none of the proposed models incorporates the folding pattern of the balloon. We developed a numerical model in which the CYPHER stent is combined with a realistic trifolded balloon. In this paper, the impact of several parameters such as balloon length, folding pattern, and relative position of the stent with respect to the balloon catheter on the free stent expansion has been investigated. Quantitative validation of the modeling strategy shows excellent agreement with data provided by the manufacturer and, therefore, the model serves as a solid basis for further investigations. The parametric analyses showed that both the balloon length and the folding pattern have a considerable influence on the uniformity and symmetry of the transient stent expansion. Consequently, this approach can be used to select the most appropriate balloon length and folding pattern for a particular stent design in order to optimize the stent deployment. Furthermore, it was demonstrated that small positioning inaccuracies may change the expansion behavior of a stent. Therefore, the placement of the stent on the balloon catheter should be accurately carried out, again in order to decrease the endothelial damage.

Elastic and Elastic-Plastic Buckling of Liquid-Filled Conical Shells

SUMMARY The best way of interpreting experimental and numerical results regarding buckling of liquid-filled conical shells is discussed. The stabilizing effect of the normal pressure exerted by the fluid is emphasized. Design equations developed previously for buckling in the elastic range are recalled. A test set-up designed to investigate buckling in the elastic-plastic range is described. It required the use of mercury as the loading medium. The test results are discussed. Results obtained numerically and the experimental results match reasonably well. The test results substantiate a design equation for buckling in the elastic-plastic range which was put forward as a conjecture several years ago.

Design of cold-formed steel purlins attached to roof sheeting based on Eurocode 3: Effect of gravity load

In the design of cold-formed steel purlins based on Eurocode 3, the lateral bending moment in the free flange of the purlin, when in tension, is assumed to be zero due to flange curling and second order effects. To investigate the validity of this design assumption, non-linear analytical and finite element models for analyzing how the gravity load affects the maximum mid-span stresses in cold-formed steel purlins attached to the roof sheeting are presented in this paper. The second-order effect of tensile stresses induced by the gravity load in the free flange of purlins is investigated by emphasizing on Z and C sections. The results obtained from the two models are compared with Eurocode 3, and they are included in this article. It is observed that for shorter purlin span lengths the design assumption underestimates the maximum mid-span stresses.

Stresses in and buckling of unstiffened cylinders subjected to local axial loads

Abstract Local force introduction in cylinders has received relatively little attention in the past and the designer often finds himself lost by the lack of design regulations or experimental evidence. This research activity, starting at the Ghent University, examines this stability problem and its purpose is to develop simple recommendations in order to assist designers in the steel industry. A short overview of the available literature on this particular topic is brought into the present contribution, together with our preliminary experimental observations and theoretical analyses with respect to unstiffened cylindrical shells subjected to local forces.

Mechanical behaviour of a freely expanding stainless steel stent

The use of the finite element method (FEM) in the design process of medical devices has gained increasing importance in the last decade, e.g. in the design of stents, where the FEM may offer insights into its complex mechanical behavior. Through FEM, the important role of plasticity in a stainless steel (SS) stent expansion process can be investigated (De Beule et al. (2005)). The present study aims at assessing the stent’s resistance to radial displacements during free expansion.