Estimates and bounds of dynamic permeability of granular media

International audienceThis paper presents a study of dynamic permeability of porous media combining homogenization of periodic media (HPM) and the self-consistent method (SCM). By taking advantage of the physical principles identified with HPM, the application of SCM leads to the determination of two physically admissible dynamic permeability assessments, both different from that given by the cell model. A comparison with numerical modeling demonstrates the fairly good reliability of the three estimates for granular media consisting of a periodic array of spherical grains. Furthermore, the self-consistent values enable exact bounds for the dynamic permeability of a wide class of porous media to be derived with a clear identification of their microstructure (grain and fluid size distribution)

Periodic homogenization and consistent estimates of transport parameters through sphere and polyhedron packings in the whole porosity range

International audienceThis paper presents a study of transport parameters (diffusion, dynamic permeability, thermal permeability, trapping constant) of porous media by combining the homogenization of periodic media (HPM) and the self-consistent scheme (SCM) based on a bicomposite spherical pattern. The link between the HPM and SCM approaches is first established by using a systematic argument independent of the problem under consideration. It is shown that the periodicity condition can be replaced by zero flux and energy through the whole surface of the representative elementary volume. Consequently the SCM solution can be considered as a geometrical approximation of the local problem derived through HPM for materials such that the morphology of the period is "close" to the SCM pattern. These results are then applied to derive the estimates of the effective diffusion, the dynamic permeability, the thermal permeability and the trapping constant of porous media. These SCM estimates are compared with numerical HPM results obtained on periodic arrays of spheres and polyhedrons. It is shown that SCM estimates provide good analytical approximations of the effective parameters for periodic packings of spheres at porosities larger than 0.6, while the agreement is excellent for periodic packings of polyhedrons in the whole range of porosity

Contribution à la prédiction de la rupture des Anévrismes de l'Aorte Abdominale (AAA)

L'objectif de ce travail est de contribuer à une meilleure prédiction de la rupture des Anévrismes de l'Aorte Abdominale. Pour ce faire, des simulations par éléments finis ont été mises en oeuvre sur des anévrismes modèles dans des conditions proches de la réalité physiopathologique, i.e. en tenant compte de l'anisotropie de la paroi anévrismale, du caractère poreux du thrombus et des Interactions Fluide-Structure. Dans la première partie, une étude statique en l'absence du thrombus a permis de mettre en évidence l'influence de la géométrie de l'anévrisme et du comportement mécanique (isotrope ou anisotrope) de la paroi sur la distribution des contraintes, i.e. la rupture potentielle de l'anévrisme, ainsi que sur l'évolution du module de Peterson. Dans la seconde partie, une modélisation poro-hyperélastique du thrombus est proposée, en s'appuyant sur des données expérimentales de la littérature. La prise en compte de ce comportement et des Interactions Fluide-Structure montre que la pression intra-thrombus demeure du même ordre de grandeur que la pression intra-luminale, conformément à des mesures in vivo réalisées par ailleurs. Enfin, nous montrons que ceci n'est pas en contradiction avec une réduction du risque de rupture potentielle de l'anévrisme.The aim of this work is to contribute to a better prediction of the Abdominal Aortic Aneurysm rupture (AAA). For that purpose, finite elements simulations have been performed on idealized AAA models under physiopathological like conditions, by taking into account the aneurysmal wall anisotropy, the intra-luminal thrombus porosity and the Fluid-Structure Interactions. In the first part, the influence of the aneurysm geometry and its wall properties (isotropic or anisotropic hyperelasticity) on the wall stress distribution and the Peterson's modulus has been studied in a static analysis and without taking into account the thrombus. In the second part, based on the experimental results from the litterature, a porohyperelastic model has been proposed for the thrombus. By considering such behavior for the thrombus and the Fluid-Structure Interactions, we observe that the intra-thrombus pressure is the same order as the intra-luminal pressure, which is consistent with in vivo measurements. Our results show that despite this unchanged pressure, the maximum wall stress decreases leading to a decrease of the aneurysm potential rupture.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Influence du comportement mécanique des artères sur la prédiction de la rupture des anévrismes de l’aorte abdominale

L'objectif de ce travail est de mettre en évidence l'influence du comportement mécanique des artères sur la distribution spatio-temporelle des contraintes au sein de la paroi anévrismale dues aux forces hémodynamiques. Pour cela, des simulations numériques tenant compte des interactions fluide-structure ont été réalisées sur des anévrismes modèles. Le comportement mécanique des artères est successivement considéré comme hyperélastique isotrope et anisotrope

Towards New Aortic Tissues Analogue Materials: Micro-mechanical Modelling and Experiments

Human abdominal aortic tissue is a complex cylindrical soft sandwich structure, arranged in three different concentric layers. Within these layers, distribution and arrangement of all components display a double-helix architecture of wavy fibres, characterised by distinctive preferred orientations. The macroscopic mechanical behaviour of human healthy abdominal aorta (AA) and aneurysmal (AAA) tissues is highly non-linear, anisotropic and essentially hyperelastic. The global objective of this work is to design and process new artificial hy- perelastic and anisotropic membranes mimicking the macroscopic histological and mechanical features of AA and AAA tissues. These materials will be then used to build more realistic phantoms of AAA for in vitro experiments. The aim of the present study is (i) to develop a theoretical framework able to predict the optimal microstructure and mechanical behaviour of such AA/AAA analogues, and (ii) to provide experimental validation of micro-mechanical modelling

Patient-specific simulation of stent-graft deployment within an abdominal aortic aneurysm

In this study, finite element analysis is used to simulate the surgical deployment procedure of a bifurcated stent-graft on a real patient's arterial geometry. The stent-graft is modeled using realistic constitutive properties for both the stent and most importantly for the graft. The arterial geometry is obtained from pre-operative imaging exam. The obtained results are in good agreement with the post-operative imaging data. As the whole computational time was reduced to less than 2 hours, this study constitutes an essential step towards predictive planning simulations of aneurysmal endovascular surger

Simulation du déploiement d'endoprothèses dans des anévrismes iliaques tortueux

National audienceLe traitement des anévrismes par voie endovasculaire avec pose d'une endoprothèse (EP) est une technique de choix face à la chirurgie ouverte conventionnelle, mais elle reste à fiabiliser. Dans cette étude, une simulation complète par éléments finis de la pose d'EP est proposée afin d'évaluer et comparer les performances mécaniques de cinq dispositifs du marché. Les résultats confirment l'importance de la flexibilité des EPs et offrent une avancée notable dans la simulation de la chirurgie endovasculaire

Comparison between the mechanical behavior of the human healthy AA and commercial prostheses under various mechanical loadings

International audienceStandard chirurgical treatment of abdominal aortic aneurysm (AAA) involves the placement of tubular synthetic aortic prostheses. Most of these implants are made up of polyester textiles or porous expanded polytetrafluoroethylene. Normalized tests are dedicated to their assessment (ISO7198:1998). However, such experiments are not sufficient to characterize the complete mechanical performance of these implants (Le Magnen et al., 2001) and to ensure their mechanical compatibility with the host artery. Thus, the design of mechanically compatible vascular prostheses still remains a challenge. Within this context, a full comparison of the mechanical behavior of the human healthy abdominal aorta (AA) with commercial prostheses is proposed. An original numerical database on the mechanical behavior of human AA subjected to various mechanical loadings is first built and then compared with experimental data obtained from mechanical tests performed on prostheses

Patient-specific numerical simulation of stent-graft deployment: Validation on three clinical cases.

International audienceEndovascular repair of abdominal aortic aneurysms faces some adverse outcomes, such as kinks or endoleaks related to incomplete stent apposition, which are difficult to predict and which restrain its use although it is less invasive than open surgery. Finite element simulations could help to predict and anticipate possible complications biomechanically induced, thus enhancing practitioners' stent-graft sizing and surgery planning, and giving indications on patient eligibility to endovascular repair. The purpose of this work is therefore to develop a new numerical methodology to predict stent-graft final deployed shapes after surgery. The simulation process was applied on three clinical cases, using preoperative scans to generate patient-specific vessel models. The marketed devices deployed during the surgery, consisting of a main body and one or more iliac limbs or extensions, were modeled and their deployment inside the corresponding patient aneurysm was simulated. The numerical results were compared to the actual deployed geometry of the stent-grafts after surgery that was extracted from postoperative scans. We observed relevant matching between simulated and actual deployed stent-graft geometries, especially for proximal and distal stents outside the aneurysm sac which are particularly important for practitioners. Stent locations along the vessel centerlines in the three simulations were always within a few millimeters to actual stents locations. This good agreement between numerical results and clinical cases makes finite element simulation very promising for preoperative planning of endovascular repair

Finite Element Analysis of the Mechanical Performances of 8 Marketed Aortic Stent-Grafts

International audiencePurpose: To assess numerically the flexibility and mechanical stresses undergone by stents and fabric of currently manufactured stent-grafts. Methods: Eight marketed stent-graft limbs (Aorfix, Anaconda, Endurant, Excluder, Talent, Zenith Flex, Zenith LP, and Zenith Spiral-Z) were modeled using finite element analysis. A numerical benchmark combining bending up to 180° and pressurization at 150 mmHg of the stent-grafts was performed. Stent-graft flexibility, assessed by the calculation of the luminal reduction rate, maximal stresses in stents, and maximal strains in fabric were assessed. Results: The luminal reduction rate at 90° was ‹<20% except for the Talent stent-graft. The rate at 180° was higher for Z-stented models (Talent, Endurant, Zenith, and Zenith LP; range 39%-78%) than spiral (Aorfix, Excluder, and Zenith Spiral-Z) or circular-stented (Anaconda) devices (range 14%-26%). At 180°, maximal stress was higher for Z-stented stent-grafts (range 370-622 MPa) than spiral or circular-stented endografts (range 177-368 MPa). At 90° and 180°, strains in fabric were low and did not differ significantly among the polyester stent-grafts (range 0.5%-7%), while the expanded polytetrafluoroethylene fabric of the Excluder stent-graft underwent higher strains (range 11%-18%). Conclusion: Stent design strongly influences mechanical performances of aortic stentgrafts. Spiral and circular stents provide greater flexibility, as well as lower stress values than Z-stents, and thus better durability