Papers by Jean-michel Bergheau
Materials Science Forum, Jan 5, 2021
The aim of this paper is to present a model that predicts the transition from internal to externa... more The aim of this paper is to present a model that predicts the transition from internal to external oxidation. This variant is based on the simultaneous resolution of the diffusion equations and the equilibrium equation that stems from the assumption of the local instantaneous thermodynamic equilibrium. It accounts for the possible formation of large precipitates fractions that may act as diffusion barriers. This effect is modeled by introducing a dependence of the diffusion coefficients upon the mass fraction of precipitates. As a counterpart, it is generally impossible to solve the non-linear equations of the model analytically. Thus, a semi-analytical and a finite elements models are presented.
Computational Materials Science, Jun 1, 2022
Recent works have paved the way to theoretical predictions of the conditions governing the transi... more Recent works have paved the way to theoretical predictions of the conditions governing the transition from internal to external oxidation of metals and alloys: such conditions directly result from Wagner (1959)'s classical analytical model, provided that it is made to incorporate a heuristic decrease of diffusion coefficients upon the fraction of oxides, aimed at representing their "barrier effect" upon diffusion. The aim of this paper is to extend these works by removing some of the very restrictive hypotheses introduced by Wagner (1959). First, the formulation initially limited to small fractions of oxides is extended to arbitrarily large fractions. Even in their modified form, the equations are still solvable entirely analytically, albeit with a change of the predicted value of the "critical" fraction of oxides, above which internal oxidation must give way to external oxidation. The new value is in better agreement than previous ones with the scarce available experimental estimates. Second, the formulation is extended to finite-instead of infinitesimal-values of the solubility product governing local chemical equilibrium between the oxide and the chemical elements dissolved in the metallic matrix. The nonlinear equations of the diffusion/precipitation problem then become much more complex and amenable only to some hybrid analytical/numerical solution. The results, although interesting, raise a number of issues essentially tied to the basic hypothesis made of instantaneous local thermodynamic equilibrium. It is finally shown, using a simplistic, prototype kinetic model of oxide precipitation, that relaxation of this hypothesis should permit to solve at least some of these issues.
Fusion Engineering and Design, 2019
For ITER divertor, plasma facing components are made with tungsten as armor material. In previous... more For ITER divertor, plasma facing components are made with tungsten as armor material. In previous papers, it has been shown that plasma facing components are prone to crack, appearing in tungsten block during thermal cyclic heat loading. In order to predict component lifetime, a numerical simulation is proposed in this paper. With regard to previous studies, tungsten (raw and recrystallized) real mechanical behaviors are taken into account. To be used as inputs for numerical simulations, compressive tests at different temperatures and strain rates were realized on raw and recrystallized tungsten. Raw tungsten tests reveal a linear elastic and ideal plastic behavior that is sensitive to strain rate. Concerning recrystallized tungsten, an elastic-viscoplastic behavior is observed on the entire explored temperature range (up to 1150 °C), that can be described by an elastic-plastic model with kinematic hardening. Manson-Coffin relationships are used to estimate the lifetime. When taking into account real mechanical behaviors for raw tungsten and recrystallized tungsten, we show that lifetime estimation is mainly driven by recrystallized thickness in the component, by the ductile to brittle transition temperature and finally by strain rate.
HAL (Le Centre pour la Communication Scientifique Directe), Oct 1, 2006
International audienc
HAL (Le Centre pour la Communication Scientifique Directe), 2006
HAL (Le Centre pour la Communication Scientifique Directe), Oct 25, 2015
To measure and analyze the "cold front" passing through a material dipped in ice water. 2. Theory... more To measure and analyze the "cold front" passing through a material dipped in ice water. 2. Theory: heat conduction Consider a rectangular solid at room temperature. Place its bottom surface in contact with ice water. Heat will flow from the solid into the water. For simplicity, neglect heat flow from air into the solid. (This approximation is based on the fact that air conducts heat less effectively than water.) Under this assumption, the temperature will not vary in the horizontal directions. The temperature will only vary in the vertical direction, z. So we want to determine T(z,t), the temperature of every point within the solid at all times. T(z,t) satisfies the heat conduction equation: 2 2
HAL (Le Centre pour la Communication Scientifique Directe), May 18, 2016
HAL (Le Centre pour la Communication Scientifique Directe), May 18, 2015
-Cette étude présente une nouvelle méthodologie visant à augmenter le temps d'usinage simulé en c... more -Cette étude présente une nouvelle méthodologie visant à augmenter le temps d'usinage simulé en coupe orthogonale .Le but est d'étudier ultérieurement l'évolution de l'usure des outils de coupe dans le cas de l'usinage de l'acier 42CD4. Un modèle lagrangien est donc développé sous le code de calcul ABAQUS simulant la formation du copeau. Une approche baptisée de « modélisation verticale » est développée dans le but d'augmenter le temps de coupe simulé tout en gardant un temps de calcul CPU modéré.
Friction Stir Welding (FSW) is a welding technique which since its invention in 1991 is of great ... more Friction Stir Welding (FSW) is a welding technique which since its invention in 1991 is of great interest to the industry for its many advantages. Despite being widely used, its physical foundations and its relation to the technological parameters of the process are not known in detail. Numerical simulations are a powerful tool to achieve a greater understanding of the physics of the problem. Although several approaches can be found in the literature for FSW, all of them present different limitations that restrict their applicability to the industry. This paper presents a new solution strategy that combines a meshless method, the Natural Element Method (NEM), with a solution separated representation making use of the Proper Generalized Decomposition (PGD), for creating a new powerful updated-Lagrangian method for addressing the 3D model while maintaining a 2D computational complexity.Copyright © 2014 by ASME
Numerical Heat Transfer Part A-applications, Jun 25, 2020
Engineering Fracture Mechanics, Apr 1, 2015
Computer Methods in Biomechanics and Biomedical Engineering, Jul 30, 2014
Human skin is the protective envelope of the human body. It has a stratified structure consisting... more Human skin is the protective envelope of the human body. It has a stratified structure consisting of four main layers: the stratum corneum, the viable epidermis, the dermis and the hypodermis. Each layer has its own features contributing to the in vivo overall answer of the skin to external chemical, biological, mechanical and thermal influences. Studies have shown that skin soft tissues are non-homogeneous, anisotropic and nonlinear viscoelastic materials subjected to a pre-stress in vivo. On the basis of the phenomenological THMPC approach of heterogeneous media (Jouanna et al. 1996), the tri-phasic skin model considered in the work of Abellan et al. (2013) has been extended in the work by Abellan et al. (2014). It incorporates a solid phase with four solid materials, a fluid phase with four fluid materials and an ionic component under ambient constant conditions. Although not negligible, electrical effects are not taken into account in this model. The driving forces for transport are the gradients of the chemical potentials of the fluids and of the ions, coupled with the stress states of the solids. In the mechanical model, skin is considered as a stratified material with four layers modelling the four outer layers of skin: the stratum corneum, the viable epidermis, the dermis and the hypodermis. All layers of the skin model are supposed to be made of fluid-saturated materials. Furthermore, each layer is seen as a different solid material within the solid phase and it is described by its own linear viscoelastic Kelvin–Voigt behaviour law. In the fluid phase, the four fluids model the four interstitial fluids of the skin layers. Each fluid is seen as a Newtonian viscous fluid described by its own behaviour law. This contribution proposes a comparison between four viscoelastic models for the solids and the way these models influence the computed flows of fluids and ions as well as the characterisation of the viscoelastic mechanical properties of skin layers. 2. Theoretical background
Computational and Mathematical Methods in Medicine, 2013
This paper proposes a triphasic model of intact skin in vivo based on a general phenomenological ... more This paper proposes a triphasic model of intact skin in vivo based on a general phenomenological thermohydromechanical and physicochemical (THMPC) approach of heterogeneous media. The skin is seen here as a deforming stratified medium composed of four layers and made out of different fluid-saturated materials which contain also an ionic component. All the layers are treated as linear, isotropic materials described by their own behaviour law. The numerical simulations of in vivo indentation test performed on human skin are given. The numerical results correlate reasonably well with the typical observations of indented human skin. The discussion shows the versatility of this approach to obtain a better understanding on the mechanical behaviour of human skin layers separately.
ISTE eBooks, Jan 26, 2010
HAL (Le Centre pour la Communication Scientifique Directe), May 16, 2022
La simulation numérique des procédés thermomécaniques (soudage, fabrication additive, etc.) à l'é... more La simulation numérique des procédés thermomécaniques (soudage, fabrication additive, etc.) à l'échelle macroscopique permet d'étudier l'influence des paramètres du procédé sur la qualité des pièces fabriquées. Les modèles numériques purement thermiques (sans prise en compte des écoulements dans le bain fondu) sont souvent utilisés pour simuler ces procédés à l'échelle de la pièce. Ces simulations présentent l'avantage d'être rapides. Cependant, ces modèles purement thermiques ne peuvent généralement pas prédire correctement la morphologie du bain fondu (surtout sa profondeur), car ils ignorent les effets de l'écoulement du fluide dans le bain de fusion, en particulier en cas de pénétration profonde. Pour remédier à cette limitation, cet article présente une nouvelle modélisation de la source de chaleur dont la distribution en espace s'adapte progressivement afin d'obtenir la bonne morphologie du bain fondu. L'approche proposée est appliquée et validée sur différents résultats expérimentaux issus de la littérature. Mots clés -simulation par éléments finis ; source de chaleur ; morphologie du bain fondu ; procédé SLM.
HAL (Le Centre pour la Communication Scientifique Directe), May 18, 2015
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Public Domain
HAL (Le Centre pour la Communication Scientifique Directe), Aug 24, 2009
La compréhension des modes de rupture des points soudés d'aciers à Très Haute Résistance (THR) es... more La compréhension des modes de rupture des points soudés d'aciers à Très Haute Résistance (THR) est un enjeu industriel important. Des essais de caractérisation locale sont développés, ainsi que l'observation in situ de la rupture d un point soudé. La quantification du comportement et de la rupture des différentes zones d'un point soudé devra permettre une meilleure compréhension de la soudabilité des aciers THR.
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Papers by Jean-michel Bergheau