Contribution à l'étude des couplages dans les interfaces, les polymères et les milieux poreux.

Research topics covered: We present the global approach that has been implemented within IFPEN in collaboration with the University of Poitiers, around the formulation of multiphysical couplings in polymer materials, and by extension and more generally, in porous media subject to mass transfer, plasticity and damage effects. Certain couplings, from other collaborations, between plasticity and damage are also be approached under the prism of a bi-phasic viscoplastic approach and cohesive zones.Thématiques de recherche abordées : Nous présentons la démarche globale qui a été mise en place au sein d’IFPEN en collaboration avec l’Université de Poitiers, autour de la formulation de couplages multiphysiques dans les matériaux polymères, et par extension et de façon plus générale, dans les milieux poreux soumis à des effets de transfert de masse, de plasticité et d’endommagement. Certains couplages, issues d’autres collaborations, entre plasticité et endommagement sont également abordés sous le prisme d’une approche bi-phasique viscoplastique et celui des zones cohésives

Topology Optimization of Fluidic Pressure Loaded Structures using the Biot-Darcy Method

In many applications, design problems involving structures experience fluidic pressure loads. During topology optimization (TO) of such design problems, these loads adapt their direction and location with the evolution of the design, which poses various challenges. A novel approach to optimize a relaxed formulation of such design problems is presented to provide a continuous and consistent treatment of design-dependent pressure loads. Its effect is to allow for micro-perforated composite as admissible designs. The porosity of each finite element is related to its density variable using a regular function, yielding a smooth transition between the solid and void phases. A design-dependent pressure field is established using Biot-Darcy's law and the associated PDE is solved using the finite element method. The approach provides a computationally inexpensive evaluation of load sensitivities using the adjoint-variable method. Since it places no assumption on the number of holes cut within the domain, it can be seen as a topology optimization algorithm. Numerical results are presented for various two dimensional problems. We seek minimizers of the sum of the elastic compliance, fluid-elastic compliance and of the weight of a solid structure under fluidic pressure loads

A consistent model coupling adhesion, friction, and unilateral contact

International audienceA model considering both unilateral contact, Coulomb friction, and adhesion is presented. In the framework of continuum thermodynamics, the contact zone is considered as a material boundary and the local constitutive laws are derived by choosing two specific surface potentials : the free energy and the dissipation potential. Because of the non regular properties of these potentials, convex analysis is used to derive the local behavior laws from the state and the complementary laws. The adhesion is characterized by an internal variable β, introduced by Frémond, which represents the intensity of adhesion. The continuous transition from a total adhesive condition to a possible pure frictional one is enforced by using elasticity coupled with damage for the interface. Non penetration conditions and Coulomb law are strictly imposed without using any penalty. The variational formulation for quasistatic problems is written as the coupling between an implicit variational inequality, a variational inequality, and a differential equation. An incremental formulation is proposed. An existence result under a condition on the friction coefficient is given. A numerical method is derived from the incremental formulation and various algorithms are implemented : they solve a sequence of minimization problems under constraints. The model is used to simulate a micro-indentation experiment conducted to characterize the behavior of fiber/matrix interface in a ceramic composite. Identification of the constitutive parameters is discussed

Influence de la concentration de gaz sur la croissance de cavités dans le PVDF soumis à une décompression de CO2.

Certaines structures polymères en PVDF utilisées dans l'industrie pétrolière sont soumises à de fortes décompressions de CO2, induisant un endommagement sous forme de mousses. Lors d'une décompression, la croissance sous forme de cavités de défauts initiaux dans le polymère dépend des interactions gaz-polymère. Le processus de croissance de la cavité dépend de la vitesse de diffusion du gaz dans la cellule et de la rigidité de la solution polymère gaz. Nous proposons un modèle diffuso-viscoplastique couplé à une loi de cavitation afin de prédire l'évolution d'un taux initial de cavités dans le matériau. Une étude paramétrique du modèle, implémenté dans le logiciel AbaqusTM, montre que la prise en compte d'une dépendance avec la concentration dans le module dYoung modifie fortement la croissance des cavités lors de la décompression

Solveur par éléments finis pour modéliser les transferts conducto-radiatifs dans des milieux hétérogènes à des échelles discrètes et continues

International audienceL'article compare trois modèles conducto-radiatifs distincts (continu/équation du transfert radiatif, continu/Rosseland, discret) qui peuvent être utilisés pour modéliser le transfert de chaleur à haute température au sein de céramiques réfractaires poreuses. Les 3 modèles sont traités dans un solveur basé sur la méthode des éléments finis. Les non-linéarités inhérentes sont, ici, traitées à l'aide d'une méthode de point fixe couplée à une méthode de linéarisation Newton-Raphson. En se basant sur des lois d'homogénéisation semi-empiriques, il apparaît que l'approche continue/équation du transfert radiatif donne des résultats satisfaisants, et cela même lorsque les transferts radiatifs prédominent. Le modèle homogénéisé à échelle continue est, ici, un bon candidat pour être utilisé à l'avenir dans des problèmes d'optimisation topologique des céramiques en conditions d'usage

Impact of the extinction behaviour of sic lattices on their conductive-radiative heat transfers

International audienceRefractory architected ceramics constitute a class of highly porous media which gains in importance for enginnering high-temperatures applications. From a thermal modelling viewpoint, one of the main challenges, is to finely describe the transport of thermal radiation which plays a major role in the dtermination of their thermal performances. Such a consideration is today crucial for developing topology optimization processes in order to define the best 3D geometries for a given set of objectives. To conduct these methodologies, it is important to quickly solve the radiative transfer equation at the continuous scale while taking into account as accurately as possible the meso-texture desbribing the 3D solid network. This goes back to assume that the physical statements governing this equation are valid, the porous architectures having to behave as an equivalent Beerian medium. However, when non Beerian behaviour is highlighted, the latter framework needs to be revised. To go one step further with this issue, numerical 3D geometries with a regular arrangement of cubic cells are generated with an homemade software. The analysis of their extinction cumulative distribution function curves obtained with the Radiative Distribution Function Identification method allows us to check whether or not the attenuated thermal radiation follows a Beerian behaviour. The effects of the textural parameters will then be discussed. Finally, preliminary practical considerations will be given for describing their conductive radiative behaviour when they are enclosed between a hot and a cold plate through both a continuous and the discrete scale methodologies, using a stabilized vectorial Finite Element Method solver

Diffuso-Kinetics and Diffuso-Mechanics of Carbon Dioxide / Polyvinylidene Fluoride System under Explosive Gas Decompression: Identification of Key Diffuso-Elastic Couplings by Numerical and Experimental Confrontation

The work aims at identifying the key diffuso-elastic couplings which characterize a numerical tool developed to simulate the irreversible ‘Explosive Decompression Failure’ (XDF) in semi-crystalline polymer. The model proposes to predict the evolution of the gas concentration and of the stress field in the polymer during the gas desorption [DOI: 10.1016/j.compositesa.2005.05.021]. Main difficulty is to couple thermal, mechanical and diffusive effects that occur simultaneously during the gas desorption. The couplings are splitting into two families: indirect coupling (i.e., phenomenology) that is state variables (gas concentration, temperature, and pressure) dependent. direct coupling, (i.e., diffuso-elastic coupling) as polymer volume changes because of gas diffusion; The numerical prediction of the diffusion kinetics and of the volume strain (swelling) of PVF2 (polyvinylidene fluoride) under CO2 (carbon dioxide) environment is concerned. The prediction is carried out by studying selected combinations of couplings for a broad range of CO2 pressures. The modeling relevance is evaluated by a comparison with experimental transport parameters analytically identify from solubility tests. A pertinent result of the present study is to have demonstrated the non-uniqueness of the coefficients of diffusion (D) and solubility (Sg) between the diffuso-elastic coupling (direct coupling) and indirect coupling. Main conclusion is that it is necessary to consider concomitantly the two types of couplings, the indirect and the direct couplings