Laurent Orgéas

Computational comparison of the bending behavior of aortic stent-grafts

Secondary interventions after endovascular repair of abdominal aortic aneurysms are frequent because stent-graft (SG) related complications may occur (mainly endoleak and SG thrombosis). Complications have been related to insufficient SG flexibility, especially when devices are deployed in tortuous arteries. Little is known on the relationship between SG design and flexibility. Therefore, the aim of this study was to simulate numerically the bending of two manufactured SGs (Aorfix--Lombard Medical (A) and Zenith--Cook Medical Europe (Z)) using finite element analysis (FEA). Global SG behavior was studied by assessing stent spacing variation and cross-section deformation. Four criteria were defined to compare flexibility of SGs: maximal luminal reduction rate, torque required for bending, maximal membrane strains in graft and maximal Von Mises stress in stents. For angulation greater than 60°, values of these four criteria were lower with A-SG, compared to Z-SG. In conclusion, A-SG was more flexible than Z-SG according to FEA. A-SG may decrease the incidence of complications in the setting of tortuous aorto-iliac aneurysms. Our numerical model could be used to assess flexibility of further manufactured as well as newly designed SGs.

Shear and compression behaviour of sheet moulding compounds

At the present time, the rheology of sheet moulding compounds (SMC) during forming is not well known. In order to provide better experimental data, an experimental program was carried out as a result of the development of a new rheometer especially dedicated to this kind of material. Homogeneous simple compression and simple shear tests are presented. They allow the evaluation of the influence of the main parameters on the SMC behaviour: strain rate, temperature and fibre fraction. It is shown that the SMC can be considered as a strongly anisotropic non-linear viscous medium.

Rheology of highly concentrated planar fiber suspensions

The rheology of highly concentrated fibers suspended in power-law fluids is investigated by upscaling the physics at the fiber scale. A deterministic upscaling technique is used, namely the homogenization method for periodic discrete structures. This micro-macro approach is used to carry out a quantitative study of concentrated fiber suspensions with planar fiber orientation, performing “numerical rheometry experiments” on a set of representative elementary volumes of fiber suspensions. The simulations underline the significant influence of the fiber volume fraction and orientation, as well as of the non-Newtonian properties of the suspending fluid on the resulting macroscopic rheological behavior. The predictions of the model are compared with experimental results obtained on an industrial thermoset short fiber-bundle polymer composite (SMC).

Microstructural effects on the flow law of power-law fluids through fibrous media

In this work, the flow of power-law fluids through anisotropic fibrous media is revisited, upscaling the fluid flow at the pore scale with the homogenization method of multiple scale expansions for periodic structures. This upscaling technique permits a quantitative study of the seepage law by performing numerical simulation with simple two-dimensional periodic arrays of circular solid inclusions. The significant role of the solid fraction, the fluid rheology and the porous media anisotropy on the resulting macroscopic flow law is underlined from the simulation.

Reversible dilatancy in entangled single-wire materials

Designing structures that dilate rapidly in both tension and compression would benefit devices such as smart filters, actuators or fasteners. This property however requires an unusual Poisson ratio, or Poisson function at finite strains, which has to vary with applied strain and exceed the familiar bounds: less than 0 in tension and above 1/2 in compression. Here, by combining mechanical tests and discrete element simulations, we show that a simple three-dimensional architected material, made of a self-entangled single long coiled wire, behaves in between discrete and continuum media, with a large and reversible dilatancy in both tension and compression. This unusual behaviour arises from an interplay between the elongation of the coiled wire and rearrangements due to steric effects, which, unlike in traditional discrete media, are hysteretically reversible when the architecture is made of an elastic fibre.

In-plane conduction of polymer composite plates reinforced with architectured networks of Copper fibres

Model composite plates composed of highly conductive slender copper fibres impregnated with a poorly conductive and transparent PMMA matrix were processed with different fibrous architectures, i.e. with various controlled fibre contents and orientations. Their microstructure was characterised using both optical observations and X-ray microtomography. Their in-plane thermal conductivity was measured by using a specific testing apparatus combined with an inverse modelling method. Results point out the strong link between the anisotropy of the in-plane conductivity and of the microstructure. The role of the fibre content on the conductivity is also emphasised. An analytical conduction model which accounts for the influence of the fibre content, the orientation, the aspect ratio and the thermal resistances at fibre-fibre contacts, was proposed and its predictions were compared with the experimental results. Using only one fitting parameter, namely the conductance at fibre-fibre contacts, this model shows a good prediction of all the experiments.

Severe Bending of Two Aortic Stent-Grafts: An Experimental and Numerical Mechanical Analysis

Stent-grafts (SGs) are commonly used for treating abdominal aortic aneurysms (AAAs) and numerical models tend to be developed for predicting the biomechanical behavior of these devices. However, due to the complexity of SGs, it is important to validate the models. In this work, a validation of the numerical model developed in Demanget et al. (J. Mech. Behav. Biomed. Mater. 5:272–282, 2012) is presented. Two commercially available SGs were subjected to severe bending tests and their 3D geometries in undeformed and bent configurations were imaged from X-ray microtomography. Dedicated image processing subroutines were used in order to extract the stent centerlines from the 3D images. These skeletons in the undeformed configurations were used to set up SG numerical models that are subjected to the boundary conditions measured experimentally. Skeletons of imaged and deformed stents were then quantitatively compared to the numerical simulations. A good agreement is found between experiments and simulations. This validation offers promising perspectives to implementing the numerical models in a computer-aided tool and simulating the endovascular treatments.

Mechanical behaviour of a fibrous scaffold for ligament tissue engineering: Finite elements analysis vs. X-ray tomography imaging

The use of biodegradable scaffolds seeded with cells in order to regenerate functional tissue-engineered substitutes offers interesting alternative to common medical approaches for ligament repair. Particularly, finite element (FE) method enables the ability to predict and optimise both the macroscopic behaviour of these scaffolds and the local mechanic signals that control the cell activity. In this study, we investigate the ability of a dedicated FE code to predict the geometrical evolution of a new braided and biodegradable polymer scaffold for ligament tissue engineering by comparing scaffold geometries issued from FE simulations and from X-ray tomographic imaging during a tensile test. Moreover, we compare two types of FE simulations the initial geometries of which are issued either from X-ray imaging or from a computed idealised configuration. We report that the dedicated FE simulations from an idealised reference configuration can be reasonably used in the future to predict the global and local mechanical behaviour of the braided scaffold. A valuable and original dialog between the fields of experimental and numerical characterisation of such fibrous media is thus achieved. In the future, this approach should enable to improve accurate characterisation of local and global behaviour of tissue-engineering scaffolds.

Modelling microstructure effects on the conduction in fibrous materials with fibre?fibre interface barriers

Conduction in fibrous materials made of highly conductive fibres and immerged in a poorly conductive matrix is modelled above the percolation threshold. Firstly, limits of discrete approaches generally used to tackle this problem are determined on elementary fibrous microstructures by comparing discrete solutions with full finite element calculations. Then, more complex 3D fibrous microstructures are numerically generated and a discrete element code is used to analyse the influence of the fibre content, aspect ratio, orientation as well as the quality of fibre–fibre contacts on the effective conductivity tensor. Lastly, a semi-empirical analytical expression is proposed to model numerical results.

3D analysis of paper microstructures at the scale of fibres and bonds

The evolution of paper microstructure parameters, such as porosity and fibre orientation, as a function of papermaking conditions is most often studied at a macroscopic scale. However, modelling the physical and mechanical properties of papers using upscaling approaches requires understanding the deformation micro-mechanisms that are induced by papermaking operations within the structure of paper fibrous networks for individual fibres and fibre-to-fibre bonds. We addressed this issue by analysing three-dimensional images of model papers. These images were obtained using X-ray microtomography. The model papers were fabricated by varying forming, pressing, and drying conditions. For each image, this analysis enabled an unprecedented large set of geometrical parameters to be measured for individual fibres (centreline, shape and inclination of the fibre cross sections) and fibre-to-fibre bonds (inter-bond distance, number of bonds per unit length of fibre, bond surface area) within the fibrous networks. The evolution of the as-obtained microstructure parameters was analysed as a function of papermaking conditions. All results were in accordance with the data available in the literature. A key result was obtained for the evolution of the number of fibre-to-fibre contacts per fibre as a function of the network density. A representative number of contacts was obtained using relatively small imaged volumes. These volumes must only contain enough fibre segments the cumulated length of which is of the same order as the mean fibre length. These results were also used to validate microstructure models for the prediction of the number of fibre-to-fibre contacts within fibrous networks.