Lionel Gélébart

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• Research Director
• Research Director at Atomic Energy and Alternative Energies Commission

The purpose of the present letter is to propose an efficient, accurate and robust FFT-based solver for transient diffusion in heterogeneous materials with “realistic” BC, taking advantage of two recent advances in terms of boundary conditions and finite difference schemes to overcome their actual limitations (periodic BC and spurious oscillations)....

This work aims at developing a numerical method for conductivity problems in heterogeneous media subjected to mixed Dirichlet/Neumann boundary conditions. The method relies on a fixed‐point iterative solution of an auxiliary problem obtained by a Galerkin discretization using an approximation space spanned by mixed cosine‐sine series. The solution...

The porous structure of snow becomes denser with time under gravity, primarily due to the creep of its ice matrix with viscoplasticity. Despite investigation of this behavior at the macroscopic scale, the driving microscopic mechanisms are still not well understood. Thanks to high‐performance computing and dedicated solvers, we modeled snow elasto‐...

Low-temperature superconductors are widely used in high field accelerator magnets, mostly within Rutherford type cables. We have proposed the “CoCaSCOPE” approach in order to build a numerical model of impregnated low-temperature Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconduc...

Because of their simplicity, efficiency and ability for parallelism, FFT-based methods are very attractive in the context of numerical periodic homogenization, especially when compared to standard FE codes used in the same context. They allow applying to a unit-cell a uniform average strain with a periodic strain fluctuation that is an unknown quan...

FFT based solvers have proven to be efficient computational methods for the numerical simulations of composite materials with complex microstructures and sophisticated material behavior. Nevertheless, these solvers require a regular discretization of the investigated domain and periodic boundary conditions. Local multi-grid methods appear as an app...

Fuel cladding and structural components made of zirconium alloys, used in light and heavy water nuclear reactors, exhibit, during normal operation, significant in-reactor deformation. Fast Fourier Transform (FFT) simulations have been conducted on large grain aggregates to simulate the in-reactor behavior of recrystallized Zircaloy-4. Original cons...

The effect of grain size on the viscoplastic behaviour of M5Framatome zirconium alloy thin sheets was investigated at high temperature under uniaxial tension, using a variety of equiaxed microstructures with controlled grain sizes. In the α phase domain, a Coble diffusional creep regime and a dislocation creep regime were observed, in agreement wit...

The mechanical behavior of a 15 µm-thick chromium coating deposited on a zirconium alloy substrate using a particular physical vapor deposition process is studied at room temperature using several experimental techniques at different scales: biaxial tests (internal pressure + axial tension) with several stress biaxiality ratios on outer-coated tube...

A framework for damage modelling based on the Fast Fourier Transform (FFT) method is proposed to combine the variational phase-field approach with a cohesive zone model. This combination enables the application of the FFT methodology in composite materials with interfaces. The composite voxel technique with a laminate model is adopted for this purp...

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science...

The weakest link theory, sometimes proposed to analyze size effects on the plastic behaviour of single crystals, is introduced in 3D numerical simulations of polycrystals. The approach relies on a random distribution of sources in space and strength associated to a crystal plasticity law with constant per layer Critical Resolved Shear Stresses (CRS...

The ability of softening strain gradient plasticity to simulate intragranular plastic slip localization modes called slip and kink bands, at is investigated at incipient plasticity. A strain gradient crystal plasticity model based on a quadratic energy contribution of the Nye tensor has been implemented within the massively parallel FFT-based solve...

The ability of softening strain gradient plasticity to simulate intragranular plastic slip localization modes called slip and kink bands, at is investigated at incipient plasticity. A strain gradient crystal plasticity model based on a quadratic energy contribution of the Nye tensor has been implemented within the massively parallel FFT-based solve...

Fast Fourier transform based algorithms, relying on the initial idea proposed by Moulinec and Suquet in 1998, are very efficient in the context of periodic homogenization in solid mechanics. The purpose of this short note is to propose a simple modification of these algorithms to extend their application domain from periodic boundary conditions (BC...

Explicitly modeling slip bands is a promising method to simulate the consequences of intragranular plastic slip localization on the behavior of strongly localizing crystals such as irradiated metals. In this study, we propose a very efficient framework to implement this modeling strategy based on a massively parallel FFT-based solver enhanced with...

Crack initiation and propagation in three braided SiC/SiC composite tubes with different braiding angles are investigated by in situ tensile tests with synchrotron micro-computed tomography. Crack networks are precisely detected after an image subtraction procedure based on Digital Volume Correlation. FFT based simulations are performed on the full...

Cracks play an essential role in the degradation of the thermomechanical behavior of ceramic matrix composites. However, characterizing their complex 3D geometries within a complex microstructure is still a challenge. This paper presents a series of procedures, based on X-ray tomographic images, to evaluate the applied 3D strains, including their t...

Cracks play an essential role in the degradation of the thermomechanical behavior of ceramic matrix composites. However, characterizing their complex 3D geometries within a complex microstructure is still a challenge. This paper presents a series of procedures, based on X-ray tomographic images, to evaluate the applied 3D strains, including their t...

We compare two full-field approaches – a crystal plasticity finite element method (CP-FEM) and crystal plasticity fast Fourier transform-based (CP-FFT) method – for a specific crystal plasticity law introduced for neutron-irradiated austenitic stainless steel SA304L currently used in nuclear reactor vessel internals. This particular law is employed...

SiC/SiC composites are considered as promising candidate materials for high temperature applications, such as fuel cladding tubes in fission nuclear plants (i.e. accident tolerant fuel clad for current light water reactors or future high temperature gas-cooled reactors) or as structural components in fusion energy plants. This category of ceramic m...

SiC/SiC composites are considered as promising candidate materials for high temperature applications, such as fuel cladding tubes in fission nuclear plants (i.e. accident tolerant fuel clad for current light water reactors or future high temperature gas-cooled reactors) or as structural components in fusion energy plants. This category of ceramic m...

Thermoplastic polymer composites filled with hollow glass microspheres (HGM) are of growing interest as lightweight materials. In order to better understand the relationship between the microstructure and the mechanical properties, Finite Element simulations based on the microstructure are now commonly performed. However, their computational cost b...

We investigate the ability of local continuum crystal plasticity theory to simulate intense slip localization at incipient plasticity observed experimentally in metals exhibiting softening mechanisms. A generic strain softening model is implemented within a massively parallel FFT solver framework to study intragranular strain localization throughou...

In the case of highly heterogeneous microstructures, such as textile composites herein, conformal FE meshes are difficult to generate for image-based modeling. As an alternative way regular meshing based on the initial image discretization can be used. However, it requires a large number of elements to reduce undesirable effects due to the voxelize...

Damage initiation is an important issue to understand the mechanical behavior of ceramic matrix composites. In the present work, a braided SiC/SiC composite tube was studied by FFT simulation tightly linked with micro-computed tomography (µCT) observations performed during an in situ uniaxial tensile test, which provide both the real microstructure...

The variational phase-field method is an attractive non-local approach of modeling fracture in heterogeneous materials. However, these materials usually require a fine mesh to resolve the fracture process zone. Consequently, the standard finite element solver becomes cumbersome due to the large number of elements in applications with highly heterog...

Les tubes composites SiC/SiC sont étudiés en vue de leur utilisation comme matériau pour le gainage du combustible nucléaire. Ces tubes, élaborés par procédé d'infiltration chimique en phase vapeur (CVI) sur une préforme tressée 2D, présentent un comportement mécanique non linéaire endommageable régi par une microfissuration au sein du matériau. L’...

The plastic behavior of irradiated Reactor Pressure Vessel (RPV) steels is described by constitutive equations capturing the temperature and strain rate sensitivities. The flow stress is decomposed into its fundamental components associated with the microstructure features peculiar to RPV steels, such as carbides, dislocation network and deformatio...

SiC/SiC composite tubes are studied as materials for nuclear fuel cladding. A thorough understanding on the mechanisms of damage to this material requires both an experimental and a numerical study. In situ tensile tests were performed under X-ray tomography at the SOLEIL synchrotron. Post processing methods have been developed to analyze the micro...

The mechanical behavior of 2D-braided SiC/SiC composite tubes depends strongly on the braiding angle. Tubes of three different braiding angles (30°, 45°, 60°) have been observed during in situ tensile tests using synchrotron X-ray tomography. In order to detect the slightly open cracks from heterogeneous microstructures, various image processing te...

Silicon carbide (SiC) is a material of interest for nuclear applications because of its stability at high temperature and under irradiation. In contrast with monolithic ceramics, architected SiC/SiC composites exhibit a quasi-ductile and reproducible behavior. Therefore, they are promising candidates for future fuel cladding tube application. The a...

The mechanical behavior of architectured SiC/SiC composites is driven by different damage mechanisms whose understanding is required for building micromechanics-based models able to reproduce and predict its complexity. The kinematics of the surface, precisely analyzed using DIC at the textile pattern scale, exhibit a fiber realignment unexplained...

The elastic behavior of SiC/SiC composite is investigated at the scale of the tow through a micromechanical modeling taking into account the heterogeneous nature of the microstructure. The paper focuses on the sensitivity of transverse properties to the residual porosity resulting from the matrix infiltration process. The full analysis is presented...

FFT-based solvers introduced in the 1990s for the numerical homogenization of heterogeneous elastic materials have been extended to a wide range of physical properties. In parallel, alternative algorithms and modified discrete Green operators have been proposed to accelerate the method and/or improve the description of the local fields. In this sho...

The purpose of this paper is to experimentally validate a 1D probabilistic model of damage evolution in unidirectional SiC/SiC composites. The key point of this approach lies in the identification and validation at both local and macroscopic scales. Thus, in addition to macroscopic tensile tests, the evolution of microscopic damage mechanisms – in...

We investigate the creep behavior of concrete at mesoscale by considering 3D periodic numerical samples composed of a linear viscoelastic matrix ruled by generalized Maxwell models, in which are distributed 872 polyhedral elastic aggregates of various size and shape. Two different numerical methods are applied: The Finite Element (FE) method with v...

The in situ strain behavior upon ion beam irradiation of a Tyranno SA3 SiC fiber was investigated in real time. For this purpose, a tensile test device suitable for micrometrical samples was developed to allow studies in various irradiation facilities. A 7.44 μm diameter SiC fiber was submitted to both low mechanical loading at 300 MPa and 92-MeV X...

FFT-based methods are used to solve the problem of a heterogeneous unit-cell submitted to periodic boundary conditions, which is of a great interest in the context of numerical homogenization. Recently (in 2010), Brisard and Zeman proposed simultaneously to use Conjugate Gradient based solvers in order to improve the convergence properties (when co...

A new finite element method is proposed to model the evolution of cracks in a 3D lattice under mechanical and thermal fatigue loading. This step-by-step method, based on the separation of initiation and propagation phenomenon, is tuned by fitted entrance parameters which command fatigue laws. It is founded on linear elasticity assumptions, and the...

The evaluation of the reliability of pressure vessels in nuclear plants relies on the evaluation of failure probability models. Micromechanical approaches are of great interest to refine their description, to better understand the underlying mechanisms leading to failure, and finally to improve the prediction of these models. The main purpose of th...

The purpose of the present study is to characterize matrix crack propagation and fiber breaking occurrences within SiC/SiC minicomposite in order to validate later on a multiscale damage model at the local scale. An in situ X-ray microtomography tensile test was performed at the European Synchrotron Radiation Facility (ESRF, ID19 beamline) in order...

In order to evaluate the stress-strain fields within periodic composites, finite element simulations are commonly performed on representative unit-cells. For flat composites, periodic boundary conditions are well established and widely used. In the present paper, the definition of periodic boundary conditions is extended to non-flat unit-cells in o...

Most modellings found in literature for the multiple cracking process of 1D composites can be categorised into three different approaches: a Continuous Approach (CA) that assumes an infinitely long composite, and two random approaches that consider composites of finite length. The Random Strength Approach (RSA) rests on a spatial discretization of...

In polycrystalline elastic simulations, grain boundaries can be considered as volume interphases or as elastic interfaces assuming a displacement jump across the interface. Such an interface description does not account for the in-plane deformation of the interface and Poisson effects cannot be reproduced. The purpose of this Note is to provide an...

Dans le cadre du développement des réacteurs nucléaires du futur, il a été souligné la nécessité de concevoir des réacteurs à spectres de neutrons rapides permettant à la fois une meilleure utilisation des ressources en 238U et également une diminution de la radiotoxicité des déchets. Par ailleurs, des conditions d’utilisation à plus haute températ...

SiC/SiC composites are studied for their potential use in the next generation of nuclear reactors. A multiscale approach is under development to construct a predictive modelling of their complex damageable mechanical behaviour due to their heterogeneous microstructure. This paper focuses on the damage characterisation of the composite at the scale...

Il existe de nombreux modèles pour décrire la fissuration des composites 1D, avec, pour la plupart, les points communs sui-vants : la mécanique est simplifiée en 1D, le comportement des matériaux (fibres et matrice) est élastique, l'interphase est souvent modélisée par une contrainte de cisaillement. Le point délicat de ces modèles consiste à gérer...

On the one hand, considering the improvements of mechanical and thermal behaviours of the last generation of SiC fibers (Hi-Nicalon S, Tyranno SA3); on the other hand, regarding physical and chemical properties and stability under irradiation, SiC/SiC composites are potential candidates for nuclear applications in advanced fission and fusion reacto...

In the context of the development of the next generation of nuclear reactors, SiC/SiC composites are candidate for structural applications. Because of their complex thermo-mechanical behaviour, due to their complex microstructure, a multi-scale approach is under development. An important microstructural parameter of the CVI composite is the complex...

This paper focuses on the characterization of the macroporosity, the porosity among the tows, observed in chemical vapor infiltration composites and on its effect on the thermo-mechanical behavior. The experimental characterization of macroporosity is performed using an X-ray tomography technique. Numerical 3D images are used to describe the distri...

SiC/SiC composites are candidates for structural applications at elevated temperatures in the context of the development of the 4th generation of nuclear reactors. A multiscale approach is under development to construct a predictive modelling of their complex mechanical behaviour due to their heterogeneous microstructure. This approach is based on...

IntroductionExperimentalResultsDiscussionConclusion Acknowledgements

A multiscale modeling, involving molecular dynamics and finite element calculations, of the degradation of the thermal conductivity of polycrystalline silicon carbide due to the thermal (Kapitza) resistances of grain boundaries is presented. Molecular dynamics simulations focus on the <111> family of tilt grain boundaries in cubic SiC. For large ti...

Owing to progress in the manufacturing on SiC fibers, the mechanical and thermal behaviors of SiCf/SiCm composites have been sharply improved. Besides, regarding their physical and chemical properties and their stability under irradiation, SiC/SiC composites are potential candidates for nuclear applications in advanced fission (Generation IV) and f...

L'étape de détermination du comportement "apparent" d'un milieu hétérogène à partir d'un volume élémentaire de taille inférieure à la taille d'un Volume Elémentaire Représentatif (VER) est sensible aux choix des conditions aux limites (CL). Les CL utilisées usuellement sont les CL en contrainte ou en déformation homogène au contour ainsi que les CL...

Une approche multi-échelle a été entreprise afin d'obtenir un modèle prédictif du comportement mécanique des composites SiC/SiC. L'étude du comportement élastique à l'échelle du toron (microstructure poreuse et hétérogène), réalisée sur des microstructures générées à partir des résultats expérimentaux de la caractérisation microstructurale, met en...

The purpose of this paper is to deal with the effect of the complex geometry of the porosity of CVI SiC/SiC composites on the elastic behaviour at the scale of the tow, which is an intermediate scale before the modelling of the woven composite. For that purpose, the developed numerical approach consists in the generation of representative micro-str...

CEA has developed a specific device for tensile tests on fibers at high temperature, up to 1800°C, under secondary vacuum. Two different SiC fibers are examined in this study: Hi-NicalonTM S and TyrannoTM SA3. Microcomposites have been also fabricated to determine SiC CVI matrix elastic properties. This paper presents the obtained results: the evol...

This paper presents a methodology for multiscale coupling between the morphology and texture of a microstructure as has been characterised experimentally, and the results of mechanical strain field analysis. This methodology is based on a coupling between experimental characterisation of the microstructure, in situ and/or ex situ mechanical tests,...

RESUME: Le carbure de silicium (SiC), en raison de ses propriétés réfractaires et de sa bonne compatibilité avec le flux neutronique, est retenu pour constituer les pièces de structure des coeurs des réacteurs nucléaires de future génération. L'influence d'une irradiation aux ions Xe (95 MeV) en température (400°C), aux fluences comprises entre 3,0...

Description Forty-one peer-reviewed papers cover all aspects of the fuel cycle. The Basic Metallurgy and Processing sections represent the beginning of the fuel cycle from component design and manufacture. The BWR Corrosion, PWR Corrosion, and Deformation Mechanism sections represent component performance under nominal service conditions. The Prima...

The purpose of this paper is to present an identification method of the crystalline behavior of a material from a mechanical test performed on a polycrystalline sample. Because of the lack of knowledge about its crystalline behavior, this method is applied to a Zirconium alloy. This identification is based on a finite element modeling of the micros...

In this work a three-scale model of the mechanical behaviour of titanium aluminides is presented. The first scale transition between the macroscopic length scale and the scale of an individual α2, γ or lamellar α2 + γ grain is made using elastically self-consistent transformation field analysis (TFA) with anisotropic elasticity. The constitutive eq...

Pour certaines conditions de vitesse de refroidissement, les alliages de fonderie a base TiAl presentent une microstructure biphasee [Ti 3 Al (α 2 ) + TiAl (y)] constituee de petits grains monophases y et de grains lamellaires biphases (α 2 + γ); la distribution radiale des grains lamellaires genere alors une macrostructure dont la taille est comme...

The overall and local tensile responses of an α iron multicrystalline sample are investigated in order to derive the plastic constitutive equations for the constituent single crystals. The macroscopic stress–strain curve and some statistical characteristics of the strain field measured on the sample surface are compared with their simulated counter...

Our goal is to address the elastic stiffness of polycrystalline materials as the grain size decreases down to the nanometric scale. Tensile tests on W/Cu multilayers exhibiting various (nanometric) thicknesses have been carried out under synchrotron radiation. Analyses of X-ray diffraction data provide the average axial elastic strain in the two ty...