Rudy Van Impe

Virtual optimization of self-expandable braided wire stents

At present, the deployment of self-expandable braided stents has become a common and widely used minimally invasive treatment for stenotic lesions in the cardiovascular, gastrointestinal and respiratory system. To improve these revascularization procedures (e.g. increase the positioning accuracy) the optimal strategy lies in the further development of the stent design. In the context of optimizing braided stent designs, computational models can provide an excellent research tool complementary to analytical models. In this study, a finite element based modelling strategy is proposed to investigate and optimize the mechanics of braided stents. First a geometrical and finite element model of a braided Urolume endoprosthesis was built with the open source pyFormex design tool. The results of the reference simulation of the Urolume stent are in close agreement with both analytical and experimental data. Subsequently, a simplex-based design optimization algorithm automatically adjusts the reference Urolume geometry to facilitate precise positioning by reducing the foreshortening with 20% while maintaining the radial stiffness. Therefore, the proposed modelling strategy appears to be a promising optimization methodology in braided stent design.

Plasticity in the Mechanical Behaviour of Cardiovascular Stents during Stent Preparation (Crimping) and Placement (Expansion)

In Western countries, cardiovascular disease is the most common cause of death, often related to atherosclerosis which can lead to a narrowing of the arteries. To restore perfusion of downstream tissues, an intravascular stent (i.e. a small tube-like structure) can be deployed in the obstructed vessel. The vast majority of stents are balloon expandable and crimped on a folded balloon to obtain a low profile for deliverability and lesion access. Several studies have exploited the finite element method to gain insight in their mechanical behaviour or to study the vascular reaction to stent deployment. However, to date – to the best of our knowledge – none of them include the balloon itself in its actual folded shape. Furthermore, literature on the effect of the crimping process on the expansion behaviour of the stent is even scarcer. Our numerical results - accounting for the presence of the balloon in its actual folded shape - correspond very well with data provided by the manufacturer and consequently our approach could be the basis for new realistic computational models of angioplasty procedures. The plastic deformation, prior to the stent expansion and induced by the crimping procedure, has a minor influence on the overall expansion behaviour of the stent but nevertheless influences the maximum von Mises stress and nominal strain. The maximum von Mises stress drops from 440 N/mm² to 426 N/mm² and the maximum nominal strain value lowers from 0.23 to 0.22 at the end of the expansion phase when neglecting the presence of the residual stresses. Depending on the context in which to use the developed mathematical models, the crimping phase can be discarded from the simulations in order to speed up the analyses.

Plasticity as a Lifesaver in the Design of Cardiovascular Stents

A common treatment to restore normal blood flow in an obstructed artery is the deployment of a stent (i.e. small tube-like structure). The vast majority of stents are crimped on a folded balloon and laser cut from 316L stainless steel tubes. Although, several numerical studies (exploiting the Finite Element Method) are dedicated to the mechanical behaviour of balloon expandable stents, there seems to be no consensus regarding the mechanical properties to describe the inelastic material behaviour of SS316L. Moreover, as the typical dimensions of stent struts (e.g. 100 μm for coronary stents) are of a similar order of magnitude as the average grain size in stainless steel (i.e. 25 μm), continuum approaches relying on macroscopic material properties may be questionable. In addition, an experimental study on stainless steel stent strut specimens showed a size-dependency of the failure strain. In this study the impact of the magnitude of the yield stress on the stent expansion behavior is examined. An increase in the yield stress (from 205 N/mm² to 375 N/mm²) results in an increase of the pressure (from about 0.3 N/mm² to approximately 0.4 N/mm²) which the clinician needs to exert for the balloon to unfold and to reach its cylindrical expanded shape. Furthermore, the effect of the size dependency behavior of the material is studied by monitoring the nominal strain during stent expansion. The maximum value of the nominal strain in the expanded stent (e.g. εn = 23 %) does not exceed the critical value of the failure strain, (i.e. εn = 33 %), moreover the critical values are nowhere exceeded in the whole stent during the expansion. Our numerical results - accounting for the presence of the balloon in its actual folded shape - correspond very well with pressure/diameter data supplied by the manufacturer. Consequently, this study shows that the free expansion of new generation balloon-expandable stents can be studied accurately with computational analysis based on the Finite Element Method (FEM) and relying on macroscopic material properties. In this context, there is no need to implement a size-based constitutive material model, but before accepting the results of the study, one should check in any case the maximum strain against the limit as shown above.

Experimental investigation of the influence of temperature on local bridging behaviour in laminated glass elements in post-breakage state

The assessment of the post-breakage performances of laminated safety glass elements used in construction needs to take into account the sensitivity to the temperature of the mechanical behaviour and properties of the product, in particular of the interlayer material. After a general problem statement, an overview is given of tests at different scales, with typical observations and results for two different interlayer materials. Through the different presented experimental scales, the mechanisms ruling the bridging behaviour in cracked laminated safety glass are explained, highlighting the difficulty of assessing the mechanical properties to be used in practical design calculations.

Stringer Stiffened Cylinders on Local Supports – The Plastic Buckling Behavior

The buckling behavior of locally supported cylinders is a topic that is still the subject of many investigations. Adequate design rules have yet to be incorporated into the codes. In this contribution, the plastic buckling behavior of a stiffened cylinder on local supports is studied. Our research consists of two principal components, i.e. experiments on scale models and numerical simulations. Here the numerical simulations are discussed. Firstly the effect of the yield stress is investigated. In order to find a design rule, a large parametrical study has to be performed. In this contribution, the characteristics and the results of this study are described. The simulations show that for a radius to thickness ratio of the cylinder equal to 250, the optimal stiffener dimensions correspond with a plastic buckling phenomenon and a failure stress that is larger than the yield stress. For larger values of the ratio, the elastic instability of the stiffeners precedes the plastic buckling and this shows that the stiffener configuration is not suitable for these values of the radius to thickness ratio.

Glass Structures and Plasticity: Contradiction or Future?

Under normal serviceability temperatures, glass behaves like a linear elastic material, which will break suddenly when tensile stresses exceed a critical value. This does not necessarily mean that fracture occurs without any visual warnings. Especially when appropriate tempered glass is used, glass beams show a considerable deformation capacity. Still, there is no plasticity involved since all deformations are linear. The main safety concept is to create an overall structural “plasticity” for the glass beam as a whole, rather than for the individual material. This is usually done by building laminates: individual glass leafs alternated with soft transparent interlayers are composed as one coherent structural element. Currently, polyvinyl butyral is the most used interlayer material. Experiments with other transparent interlay materials have been carried out at different research institutes, in order to introduce more plasticity in the structural behaviour of glass beams. Not all attempts are as successful, but some results are promising. It is the authors’ opinion that the ability to create an overall structural plasticity will strongly influence the breakthrough of load bearing glass constructions. In the available literature however, the authors lack an overall picture of the current state of the art. For this reason, an attempt is made to present the actual research and different aspects of “plasticity in glass constructions” in this paper.

TORSIONAL STIFFNESS OF LAMINATED GLASS ELEMENTS IN STRUCTURAL APPLICATIONS — INFLUENCE OF A ELASTO-VISCOPLASTIC IONOMER INTERLAYER ON THE PRE-BREAKAGE BEHAVIOUR

To investigate the mechanical behaviour of laminated glass with the elasto-viscoplastic ionomer interlayer SentryGlas® by DuPont, an experimental program was executed on full-scale glass laminates. The results of this research improved the insight in the complex mechanical behaviour of this stiff interlayer. More specifically, a clear influence of load duration and temperature could be observed for both creep and relaxation setups during bending experiments. The stiffness reduction due to the load duration and the temperature raise was most significant for the smaller specimens - with a span of 1 m - while for the largest specimen - with a span of almost 3 meter - the reduction remained less than 25 % of the monolithic bending stiffness, even after a load duration of 48 hours at 65 °C.

Finite Element Stent Design: Parametric Modeling of Braided Wirestents Using PyFormex

Selfexpandable stent(graft)s are supporting tubular mesh devices used for the treatment of occlusive diseases and for the ‘exclusion’ of aneurysms. Wirestents are a class of flexible stents braided from a set of ultra fine wires and currently manufactured in a wide range of materials (e.g. phynox, nitinol, polymers) and compositions (single or multilayer). For design purposes as well as for studying the mechanical behavior of such a device by finite element simulations, a geometrical model using 1D elements will usually be appropriate. However, the computer model will contain a very large number of such elements, and building the geometrical model using classical CAD methodologies may become laborious. Consequently, literature dedicated to the mechanical behavior of braided wirestents is (very) scarce and the stent(graft)s are simplified as virtual single sheets [1].Copyright

Buckling of Stringer Stiffened Cylinders on Local Supports

Summary Cylinders on discrete supports are prone to local instability. This failure phenomenon can be avoided by reinforcing the cylindrical wall by means of stringer stiffeners in combination with ring stiffeners. The goal of our research is to find design rules for this kind of shell structures. For this purpose, a numerical model was developed. In this contribution, the numerical model is validated by verifying the correspondence with the results of experiments on scale models. Two experiments are here discussed. The corresponding failure patterns are examples of the two possible patterns that were found in all the experiments. For the numerical simulations of these experiments, the measured geometrical imperfections and the real stress-strain relationship of the steel were incorporated into the numerical model. The results of the simulations show that the correspondence between experiments and simulations is sufficient.

ON THE IMPERFECTIONS OF CYLINDRICAL SHELLS ON LOCAL SUPPORTS

Times Cited: 0 1st International Conference on Computational Methods (ICCM04) Dec 15-17, 2004 Singapore, SINGAPORE Natl Univ Singapore, Dept Mech Engn