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International audience; Several experimental studies, carried out on anisotropic rocks, have evidenced that even though strains, due to isotropic loading and/or internal fluid pressure, are strongly anisotropic, the resulting Biot’s tensor is almost isotropic. Those results were found on two different rocks: a clay rock (France—Bure argillite) and a sandstone from the Vosges region (France). Such (a priori) surprising results led us to develop micromechanical modelling in which anisotropy comes either from an anisotropic solid matrix (and isotropic pore space) or from an anisotropic pore space (and isotropic solid matrix). The obtained results have shown that for both cases the Biot’s tensor is virtually isotropic or presents a very weak anisotropy. This unambiguously supports the fact that a strongly anisotropic porous material is compatible with experimental measurements of isotropic (or quasi isotropic) Biot’s tensor

National audience; This paper presents an enhanced micromechanical analytical model to predict the transverse elastic properties of unidirectionnal (UD) composite materials. Unlike the elastic longitudinal constants, the transverse effective properties does depend on the spatial fibre distribution. This model allows to take into account the effect of local morphological fluctuations, mainly induced by random fibre arrangements, on the effective transverse properties. For that purpose, a “n-phase” Generalized Self consistent Scheme (GSCS) has been extended by using a Morphologically Representative Pattern (MRP) approach. As an example, a two bi-phasic pattern-based model is combined with some recent data of the literature (ultrasonic methods) to explore the broad predictive ability of the model. A full range of fibre volume fractions with fibre packing effects can be covered by the proposed method. Analytical solutions based on two morphological parameters (to which is added the overall volume fraction of fibers) are provided to predict the transverse shear modulus and the transverse bulk modulus of any UD composite material. The two moduli are shown to be linked together. Very simple analytical calculations make possible parametric studies less expensive than numerical or experimental characterizations.; Cet article présente un modèle analytique micromécanique amélioré permettant de prédire les propriétés élastiques transverses des matériaux composites unidirectionnels (UD). Contrairement aux propriétés longitudinales, les propriétés effectives transverses dépendent beaucoup de la répartition spatiale des fibres. Le modèle présenté ici permet de prendre en compte l’effet des fluctuations morphologiques locales, principalement induites par les distributions aléatoires des fibres, sur les propriétés transverses effectives. Pour cela, un modèle Auto-Cohérent Généralisée (ACG) “à n phases” a été étendu avec une approche par Motifs Morphologiques Représentatifs (MMR). À titre d’exemple d’application, un modèle à deux motifs de deux phases a été combiné à des données expérimentales de la littérature (méthodes par ultrasons) pour explorer la capacité prédictive globale du modèle. La méthode proposée couvre une gamme complète de fractions volumiques de fibres avec des fibres qui peuvent se regrouper. Des solutions analytiques basées sur deux paramètres morphologiques (qui s’ajoutent à la fraction volumique en fibres du composite) sont fournies pour prédire le module de cisaillement transverse et le module de compressibilité plan de tout matériau composite UD. Ces deux modules sont liés l’un à l’autre. Des calculs analytiques très simples rendent les études paramétriques possibles moins coûteuses que les caractérisations numériques ou expérimentales.

International audience; This paper presents an enhanced micromechanical analytical model to predict the transverse elastic properties of unidirectionnal (UD) composite materials. Unlike the elastic longitudinal constants, the transverse effective properties does depend on the spatial fibre distribution. This model allows to take into account the effect of local morphological fluctuations, mainly induced by random fibre arrangements, on the effective transverse properties. For that purpose, a “n-phase” Generalized Self consistent Scheme (GSCS) has been extended by using a Morphologically Representative Pattern (MRP) approach. As an example, a two bi-phasic pattern-based model is combined with some recent data of the literature (ultrasonic methods) to explore the broad predictive ability of the model. A full range of fibre volume fractions with fibre packing effects can be covered by the proposed method. Analytical solutions based on two morphological parameters (to which is added the overall volume fraction of fibers) are provided to predict the transverse shear modulus and the transverse bulk modulus of any UD composite material. The two moduli are shown to be linked together. Very simple analytical calculations make possible parametric studies less expensive than numerical or experimental characterizations; Cet article présente un modèle analytique micromécanique amélioré permettant de prédire les propriétés élastiques transverses des matériaux composites unidirectionnels (UD). Contrairement aux propriétés longitudinales, les propriétés effectives transverses dépendent beaucoup de la répartition spatiale des fibres. Le modèle présenté ici permet de prendre en compte l’effet des fluctuations morphologiques locales, principalement induites par les distributions aléatoires des fibres,sur les propriétés transverses effectives. Pour cela, un modèle Auto-Cohérent Généralisée (ACG) “à n phases” a été étendu avec une approche par Motifs Morphologiques Représentatifs (MMR). À titre d’exemple d’application, un modèle à deux motifs de deux phases a été combiné à des données expérimentales de la littérature (méthodes par ultrasons) pour explorer la capacité prédictive globale du modèle. La méthode proposée couvre une gamme complète de fractions volumiques de fibres avec des fibres qui peuvent se regrouper. Des solutions analytiques basées sur deux paramètres morphologiques (qui s’ajoutent à la fraction volumique en fibres du composite) sont fournies pour prédire le module de cisaillement transverse et le module de compressibilité plan de tout matériau composite UD. Ces deux modules sont liés l’un à l’autre. Des calculs analytiques très simples rendent les études paramétriques possibles moins coûteuses que les caractérisations numériques ou expérimentales.

Modifications alter hygro-mechanical properties of wood in non-trivial ways that depend on modification treatment and wood microstructure. Generic micromechanical models with modifications on the cellular scale of spruce are proposed and studied, such as partial and entire lumen filling with isotropic materials, as well as modification of S2-layer properties. Based on a hierarchical micromechanical model, hygro-mechanical response surfaces of the modified, orthotropic material are predicted. Simulation results are compared to experimental data. The findings can be used for optimizing modification treatments, as well as for calculating the behavior in graded situations, common to treatments with limited modification depth.

International audience; Despite the great similarities in the physical and chemical properties of flax and primary hemp fibres, literature generally reports differences in their average tensile properties. This work proposes a numerical investigation of the relationships between the morphological, biochemical and cell wall features, along with their variability, and the mechanical properties of flax and hemp fibres. A previously developed 3D micromechanical model of plant fibre is used to carry out a sensitivity analysis. Results show that the difference observed in the average tensile properties of flax and hemp fibres is partly due to the differences in scattering of some their morphological and ultrastructural properties. Despite a similar mean value, the reduced variability in the flax fibre features, seems to lead to higher average tensile properties.

This work focuses on the use of a heterogeneous generation method in order to develop a micromechanical model of Hot Mix Asphalt (HMA) that takes into account the effects of characteristics at the ...

This paper aims to highlight the effects of the compatibility between nano-clay platelets and polymer matrix and capability of combined approaches (experimental and modeling) to capture the visco-elastic properties of the studied materials. In this context, a study, as complete as possible, is conducted through microstructural observation, mechanical characterization and micromechanical modeling. Nano-clay reinforced Poly(lactic acid) blends with different modified nano-clay platelets (Cloisite 30B and Cloisite 93A) were prepared. The linear elastic and visco-elastic behaviors of nano-reinforced samples were characterized. And a combination of lenght–scale–sensitive experimental techniques helped to describe the micro-structure through different microstructural parameters. These parameters were used as input into a viscoelastic micromechanical model. Comparison between experimental data (Complex, loss and storage moduli) and model predictions were in good agreement.

International audience; This note deals with the study of stress-sensitive relative permeability experimentally observed in low permeable sandstones. These sandstones are made up of quartz grains surrounded by permeable interfaces between grains and pores. A micromechanical model of relative permeability behaviour under loading highlights the role of the closure of interfaces. Morphological models adapted to permeable sandstones show conversely that the relative permeabilities do not depend on the loading.; Cet article est consacré à la modélisation de la sensibilité de la perméabilité relative au confinement observée expérimentalement pour les grès de faible perméabilité. Ces grès sont décrits comme un assemblage de grains de quartz entourés de joints perméables et de pores. Un modèle micromécanique de comportement de la perméabilité relative sous chargement met enévidenceenévidence le rôle de la fermeture des interfaces. Les modèles morphologiques adaptés aux grès perméables montrent inversement que les perméabilités relatives ne dépendent pas du chargement.

International audience; In this work, we aim to study the elastic behavior of wood-aggregate concrete. The objective is to determine the relevant microstructural and mechanical parameters of the constituents that influence its elastic properties. To reach this objective, we combined experiments and modeling. Experimental results show that the evolution over time of the elastic properties of wood-aggregate concrete is complex: it depends on storage conditions, content of binder and drying time. To well understand this evolution, we developed a micromechanical model based on the self-consistent scheme. This model captures well the experimental data and provides a predictive tool for the behavior of plant-based concrete.

International audience; In this paper, we propose a polycrystalline model to study the elastic–plastic behavior of porous rocks. The proposed model will be applied to sandstone. For this purpose, the microstructure of porous rocks will be represented by an assembly of discrete grains and pores. The plastic deformation of each grain is related to the frictional sliding along a number of weakness planes. A specific plastic model is devised to describe the sliding phenomena to take into account the characteristics of rocks such as the pressure dependency and the volumetric dilatancy. A homogenization approach–self-consistent is developed to account for the interactions between grains and pores. An efficient numerical procedure for model implementation is proposed. Finally, we present a series of numerical investigations and comparisons with experimental data to assess the capability of the proposed model to capture the main features of mechanical behaviors of porous rocks.

International audience; Strain-crystallising rubber exhibits interesting properties: for instance, fatigue lifetime is known to be modified by this microstructural evolution which dissipates energy and creates a strong anisotropic reinforcement. We develop herein a micro-sphere 3D constitutive model for such strain-crystallising rubber. It is based on a simplified 1D micromechanical model that we extend with a micro-sphere approach to a full thermodynamically consistent evolutive anisotropic model. A specific numerical strategy is then proposed. The model is assessed on several significative configurations and reproduces the main experimental features while predicting the evolution of anisotropy as a function of the loading history. We finally show that it can also predict the crystallised zone in front of a mode I crack.

International audience; A micromechanical elastoplastic model is proposed in this study for anisotropic sedimentary rocks. With a two-step homogenization procedure, a macroscopic criterion is established for geomaterials made up of a porous matrix reinforced by rigid inclusions. The effects of porosity, inclusions and the inherent anisotropy are explicitly taken into account. Based on [18], a scalar anisotropy parameter is introduced to describe the anisotropic characteristics of the material. With a non-associated plastic flow rule and a strain hardening law, the proposed model is applied to describe the macroscopic mechanical behavior of Tournemire shale with different initial confining pressures and different loading orientations. Comparing numerical results with experimental data, the efficiency of the proposed micromechanical model is assessed and verified.

The mechanical properties of Hot Mix Asphalt (HMA) mixes are often characterized by its viscoelastic behavior. Viscoelasticity coupled with the thermo-mechanical nature of asphalt offers time-temperature dependence to the mechanical properties of HMA mixes. Also, the wide range of distribution of particles, and several orders of magnitude of differences in the mechanical properties between constituents cause a wide range of stress and strain distribution within the microstructure. In this paper, micromechanical modeling has been performed to study the relationship between individual material properties, their interaction within the microstructure, and the macroscopic properties of HMA mix. The analytical model based on N phase self-consistent scheme allows calculating the complex modulus and phase angle of HMA mixes from the mechanical properties of its constituents and designed mix data. In the mix microstructure, aggregates and sand are classified using simple sieve analysis test. Asphalt and sand mastic film thicknesses around the aggregates are calculated for each class and introduced into the model.

This work aims mainly to derive closed-form expressions for the effective properties of layered composites when linear uncoupled and coupled phenomena, such as conduction, elasticity, thermoelectricity and piezomagnetoelectricity, are concerned. In addition, properties may be graded in the direction normal to the layer plane. The remarkable feature of the obtained results is that they are expressed in a unified and very compact way for all uncoupled and coupled linear mechanical and physical phenomena. Our results include as special ones all the relevant results reported up to now in the literature. The key to obtaining our general compact coordinate-free results resides in extending Hill’s interfacial relations for a perfect elastic interface to the setting of an arbitrary finite number of uncoupled and coupled linear phenomena. The effects of the presence of stiffeners and softeners on the effective properties of layered composites are also investigated. In particular, four limiting cases for piezoelect...

International audience; A new micromechanical modelling approach for brittle damage in initially orthotropic materials is presented. The proposed strain-based energy formulation allows to derive a fully anisotropic multilinear model for microcracked materials with arbitrary oriented defects. The thermodynamics framework provides a standard procedure for the damage evolution law. The new model explicitly accounts for the interaction between primary and induced anisotropies. Moreover, the very challenging issue of opening-closure effects (unilateral behavior) is addressed in this framework.

Following the proposals of the European Commission, to achieve the goals of emission reduction of carbon dioxide (CO2) from cars. To achieve this objective, automakers must reduce the weight of the car. Thus the supplier Faurecia, manufacturer of car seats desires to replace metal structures by structure plastic-metal hybrid (PMH). And they desire also to use a composite material to replace metal in order to reduce weight. Moreover in order to improve the adhesion strength at the interface piece hybrid structure,Atmospheric plasma technique was used.In this context, we studied the effects of plasma treatment on term of adhesion in the hybrid structure in order to provide the best condition of treatment. The objective of this study was to characterize the effect of plasma treatment by determination at the parameters in the process such as the scanning speed, the distance between the substrate and the plasma torch and the number of passes of the torch, then to predict the initiation of delamination under quasi-static loading test in specimen of single lap shear by using the criterion of rupture. The other objective was to study the micromechanical model to assess the reliability of them. And they will be applied to predict the mechanical behavior of Short Glass Fiber reinforced thermoplastic. In order to achieve the objectives presented, the several tests such as tensile test, single lap shear test and ARCAN-Mines test have been executed. In parallel, techniques acoustic emission (AE), infrared radiation (IR) and optical microscopy were used in order to follow the failure mechanisms of the specimen studied. Finally, the finite element method was used to simulate the tests and allow to verify the reliability of the failure criterion.; A la suite des propositions de la commission européenne, visant à concrétiser les objectifs de réduction des émissions de dioxyde de carbone (CO2) des voitures. Pour atteindre cet objectif, les constructeurs automobiles doivent réduire le poids de la voiture. Ainsi l'équipementier FAURECIA, fabricant de sièges de voiture désire remplacer les structures métalliques par des structures hybrides plastique-métal (PMH). Et en plus, il désire également utiliser un matériau composite en remplacement du métal pour diminuer le poids et utiliser la technique du plasma atmosphérique pour améliore le force d'adhérence à l'interface de pièce structure hybride.C'est dans ce contexte que nous avons étudié des effets de plasma traitement sur l'adhérence dans la structure hybride pour proposer la meilleure condition de traitement. L'objectif de ce travail était de caractériser l'effet de plasma traitement par détermination des paramètres dans le processus de traitement telle que la vitesse de balayage, la distance entre le substrat et la torche plasma et le nombre de passages de la torche, puis de prédire l'initiation du délaminage sous chargement quasi-statiques dans l'éprouvette de simple recouvrement par l'utilisation le critère de la rupture. Un autre objectif était d'étudier les modèle micromécanique pour évaluer la fiabilité de leur. Et ils seront appliqués pour prévoir les comportements mécaniques de matériau thermoplastique renforcé par fibre de verre court. Afin d'atteindre les objectifs présenté, les plusieurs essais telles que l'essai de traction, l'essai simple recouvrement et l'essai de l'ARCAN-Mines sont été réalisé. En parallèle, les techniques de l'émission acoustique (EA), du rayonnement infrarouge (RI) et de la microscopie optique ont été utilisées pour suivre les mécanismes de la rupture de l'éprouvette étudiée. En enfin, la méthode des éléments finis a été utilisé pour simuler les essais et pour permettre de vérifier la fiabilité du critère de rupture.

Following the proposals of the European Commission, to achieve the goals of emission reduction of carbon dioxide (CO2) from cars. To achieve this objective, automakers must reduce the weight of the car. Thus the supplier Faurecia, manufacturer of car seats desires to replace metal structures by structure plastic-metal hybrid (PMH). And they desire also to use a composite material to replace metal in order to reduce weight. Moreover in order to improve the adhesion strength at the interface piece hybrid structure,Atmospheric plasma technique was used.In this context, we studied the effects of plasma treatment on term of adhesion in the hybrid structure in order to provide the best condition of treatment. The objective of this study was to characterize the effect of plasma treatment by determination at the parameters in the process such as the scanning speed, the distance between the substrate and the plasma torch and the number of passes of the torch, then to predict the initiation of delamination under quasi-static loading test in specimen of single lap shear by using the criterion of rupture. The other objective was to study the micromechanical model to assess the reliability of them. And they will be applied to predict the mechanical behavior of Short Glass Fiber reinforced thermoplastic. In order to achieve the objectives presented, the several tests such as tensile test, single lap shear test and ARCAN-Mines test have been executed. In parallel, techniques acoustic emission (AE), infrared radiation (IR) and optical microscopy were used in order to follow the failure mechanisms of the specimen studied. Finally, the finite element method was used to simulate the tests and allow to verify the reliability of the failure criterion.; A la suite des propositions de la commission européenne, visant à concrétiser les objectifs de réduction des émissions de dioxyde de carbone (CO2) des voitures. Pour atteindre cet objectif, les constructeurs automobiles doivent réduire le poids de la voiture. Ainsi l'équipementier FAURECIA, fabricant de sièges de voiture désire remplacer les structures métalliques par des structures hybrides plastique-métal (PMH). Et en plus, il désire également utiliser un matériau composite en remplacement du métal pour diminuer le poids et utiliser la technique du plasma atmosphérique pour améliore le force d'adhérence à l'interface de pièce structure hybride.C'est dans ce contexte que nous avons étudié des effets de plasma traitement sur l'adhérence dans la structure hybride pour proposer la meilleure condition de traitement. L'objectif de ce travail était de caractériser l'effet de plasma traitement par détermination des paramètres dans le processus de traitement telle que la vitesse de balayage, la distance entre le substrat et la torche plasma et le nombre de passages de la torche, puis de prédire l'initiation du délaminage sous chargement quasi-statiques dans l'éprouvette de simple recouvrement par l'utilisation le critère de la rupture. Un autre objectif était d'étudier les modèle micromécanique pour évaluer la fiabilité de leur. Et ils seront appliqués pour prévoir les comportements mécaniques de matériau thermoplastique renforcé par fibre de verre court. Afin d'atteindre les objectifs présenté, les plusieurs essais telles que l'essai de traction, l'essai simple recouvrement et l'essai de l'ARCAN-Mines sont été réalisé. En parallèle, les techniques de l'émission acoustique (EA), du rayonnement infrarouge (RI) et de la microscopie optique ont été utilisées pour suivre les mécanismes de la rupture de l'éprouvette étudiée. En enfin, la méthode des éléments finis a été utilisé pour simuler les essais et pour permettre de vérifier la fiabilité du critère de rupture.

International audience; A micromechanical model is proposed to describe the viscoplastic deformation of cohesive frictional geomaterials with a granular texture. Inspired by the widely used polycrystalline theory, the macroscopic viscoplastic deformation is attributed to the subcritical sliding along a number of oriented weakness planes, which are assimilated to the crystallographic planes in single crystals. Main features of geomaterials, such as pressure sensitivity and plastic compressibility/dilatancy, are taken into account by introducing an appropriate yield criterion and a non-associated plastic potential for each weakness plane. A simple interaction law is adopted to relate the local stress and strain fields to the macroscopic ones of the representative elementary volume. Computational aspects about the implementation of constitutive equations of both single crystal and polycrystal are discussed in detail. The performance of proposed model is checked through the comparisons between numerical results and experimental data on both triaxial compression tests and creep tests performed on a typical rock - granite.

International audience; This paper discusses the notion of failure in a granular assembly by examining the key microstructural mechanisms which are most likely to trigger the nucleation and propagation of instabilities within a granular material. For this purpose, the key variable to predict the occurrence of failure, known as second-order work, is expressed from variables on the grain scale. The local behaviour incidents (where contacts may open or slide), compared to the global response of the assembly, are analysed by two approaches. First, numerical computations made by a discrete element model confirm the microscopic definition of the second-order work. Secondly, a micromechanical model, based on a homogenization procedure, relating the macroscopic behaviour to microscopic ingredients, namely contact planes, points to a close link between the occurrence of failure on the macroscopic scale as well as on the contact planes.

International audience; In this article, we introduce a new micromechanical model for alkali-silica reaction. Our idea was to build a model with the following characteristics. First, the model has to be simple enough to be used to compute damage under loading and chemical attack at the level of each element in a structure code. Second, its parameters must be easy to identify on available alkali-silica reaction lab experiments. We have chosen to model the behavior of concrete containing aggregates such that most of the damage occurs in the cement paste. Using micromechanics and an energy criterion, the model remains analytical except for the minimization of the energy. The parameters were identified on Multon's triaxial experiments and good results were obtained for compressive loadings up to 10 MPa.

This paper presents a new micromechanical model for a collection of cohesive zone models embedded between each mesh of a finite element-type discretization. It aims to fully extend the previous linear results of Blal et al. (2012) [11] to the calibration of damageable cohesive parameters (cohesive peak stress, critical opening displacement, cohesive energy, etc). The main idea of the approach consists in replacing the underlying cohesive-volumetric discretization by an equivalent ‘matrix-inclusions’ composite. The overall behavior of this equivalent composite is estimated using homogenization schemes (Hashin–Shtrikman estimate and the modified secant method) and is given in a closed-form as function of both cohesive and bulk properties and the mesh density. In the particular case of a bilinear cohesive law a micromechanical damage model for quasi-brittle materials is derived. The corresponding local-to-global relationships are obtained for any overall triaxiality loading ratio.

International audience; This paper deals with the modeling of Shape Memory Alloys (SMA) with embedded precipitates, and describes the effect of these precipitates on the phase transformation properties. It is based on a micromechanical constitutive model and uses a double scale transition technique. The first scale transition considers elastic inclusions embedded in an SMA matrix. It uses the Mori–Tanaka homogenization technique to determine the equivalent behavior of an SMA single crystal. The second scale transition leads to the effective behavior of an SMA polycrystalline aggregate. It is based on the behavior of each grain and uses a self-consistent homogenization technique. The constitutive non-linear equations of this model are then numerically solved. The results obtained under different loadings are detailed and discussed at the grain and at the sample (macroscopic) scales. Finally the effect of the volume fraction of precipitates on the transformation properties are discussed.

Cemented granular aggregates include a broad class of geomaterials such as sedimentary rocks and some biomaterials such as the wheat endosperm. We present a 3D lattice element method for the simulation of such materials, modeled as a jammed assembly of particles bound together by a matrix partially filling the interstitial space. From extensive simulation data, we analyze the mechanical properties of aggregates subjected to tensile loading as a function of matrix volume fraction and particle-matrix adhesion. We observe a linear elastic behavior followed by a brutal failure along a fracture surface. The effective stiffness before failure increases almost linearly with the matrix volume fraction. We show that the tensile strength of the aggregates increases with both the increasing tensile strength at the particle-matrix interface and decreasing stress concentration as a function of matrix volume fraction. The proportion of broken bonds in the particle phase reveals a range of values of the particle-matrix adhesion and matrix volume fraction for which the cracks bypass the particles and hence no particle damage occurs. This limit is shown to depend on the relative toughness of the particle-matrix interface with respect to the particles.

International audience; 9-12%Cr quenched and tempered martensitic steels are known to soften under cyclic loadings at high temperature. The present article proposes a model based on physical mechanisms described at the scale of slip systems. This model describes explicitly the microstructural recovery (corresponding to a decrease of the dislocation density and subgrain coarsening) observed experimentally. The scale transition is carried out in the framework of self-consistent homogenization schemes. The model assumptions and its physical basis are explicitly discussed. The parameters are identified on a very limited amount of experimental data. The model turns out to give very good predictions and extrapolations for the cyclic softening effect observed in uniaxial tension-compression loadings for strain ranges larger than 0.3%. Stress-relaxation and creep behavior can also be simulated for high stresses. In addition the cyclic softening effect is reproduced for multiaxial tension-torsion loadings.

International audience; The present paper is devoted to a micromechanical model of porous rocks and its application to a sandstone. This original model takes advantage of a recent homogenization-based macroscopic yield function which couples Drucker-Prager type plasticity of the solid matrix and evolving porosity. Its formulation and implementation are described. Application to a Vosges sandstone shows that, except for very low confining pressures for which the mechanical behavior is quasi-brittle, the model predicts well the ductile behavior at moderate or high confining pressures (for which the pore collapse mechanism is expected to play a dominant role).

This paper is devoted to micromechanical modeling of isotropic damage in brittle materials . The damaged materials will be considered as heterogeneous media composed of solid matrix weakened by isotropically distributed microcracks . The original contribution of the present work is to provide a proper micromechanical thermodynamic formulation for damage-friction modeling in brittle materials with the help of Eshelby’s solution to matrix-inclusion problems. The elastic and plastic strain energy involving unilateral effects will be fully determined. The condition of microcrack opening–closure transition will be determined in both strain-based and stress-based forms. The effect of spatial distribution of microcracks will also be taken into account. Further, the damage evolution law is formulated in a sound thermodynamic framework and inherently coupled with frictional sliding . As a first phase of validation, the proposed micromechanical model is finally applied to reproduce basic mechanical responses of ordinary concrete in compression tests.

Solving boundary value problems requires implementation of sufficiently robust constitutive models. Most models try to incorporate a great deal of phenomenological ingredients, but this refining often leads to overcomplicated formulations, requiring a large number of parameters to be identified. A powerful alternative can be found with micromechanical models, where the medium is described as a distribution of elementary sets of grains. The complexity is not related to the constitutive description, but to the multiplicity of contacts oriented along all the directions of the physical space. This paper proposes an advanced micromechanical model that introduces an intermediate scale (mesoscopic scale): elementary hexagonal patterns of adjoining particles. This is advantageous with respect to current micromechanical models that generally describe the material through a single distribution of contacts. This new approach makes it possible to take many constitutive properties into account in a very natural way, such as the occurrence of diffuse failure modes. These preliminary results are presented in order to give clear insights into the capability of such multiscale approaches.

International audience; In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion based on bifurcation theory first proposed by Rice is applied to elastic-plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale transition scheme. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities, softening mechanisms so that the behavior during complex loading paths can be accurately described. Based on this formulation, Forming Limit Diagrams (FLDs) are derived and compared with a reference model for multiphase steels involving linear and complex loading paths. Furthermore, the effect of various physical and microstructural parameters on the ductility limit of a single-phase steel is qualitatively studied with the aim of helping in the design of new materials.

In the work presented in this paper, several strain rate potentials are examined in order to analyze their ability to model the initial stress and strain anisotropy of several orthotropic sheet materials. Classical quadratic and more advanced non-quadratic strain rate potentials are investigated in the case of FCC and BCC polycrystals. Different identifications procedures are proposed, which are taking into account the crystallographic texture and/or a set of mechanical test data in the determination of the material parameters.; International audience; In the work presented in this paper, several strain rate potentials are examined in order to analyze their ability to model the initial stress and strain anisotropy of several orthotropic sheet materials. Classical quadratic and more advanced non-quadratic strain rate potentials are investigated in the case of FCC and BCC polycrystals. Different identifications procedures are proposed, which are taking into account the crystallographic texture and/or a set of mechanical test data in the determination of the material parameters.

International audience; A micromechanical model is proposed for plastic deformation and damage in hard clay called argillite. The argillite is considered as a two phase composite with the clay matrix and mineral inclusions. The clay matrix is described by an elastoplastic damage model while the inclusions are considered as linear elastic media. The macroscopic inelastic behavior of material is determined using Hill's incremental method and a Mori–Tanaka scheme for the linear comparison composite. The validity of the proposed model is verified through comparisons with experimental data for various laboratory tests. Influences of microscopic deformation mechanisms on macroscopic behavior are discussed.

The aim of this PhD thesis is to elaborate a predictive tool for the loss of ductility of polycrystalline multiphase steels during monotonous and sequential loading paths. Another major interest of this tool is to help design new materials thanks to knowledge of the impact of various physical and microstructural parameters on the ductility limit. At first, the elastic-plastic behavior at the local scale, i.e. at the grain level, is modeled using a micromechanical law formulated within the large strain framework. A rate-independent regularization technique, without introducing ratesensivity, is used to determine slip activity. The intragranular heterogeneities are introduced into the model following the original work of Peeters. The internal variables of the model are represented by three types of dislocation densities whose evolution is based on the mechanisms of creation, storage and annihilation. A comparison of the internal variable evolution with micrographs of grains of ferritic steel for monotonous and complex loadings allowed the validation of this modeling and demonstrates the macroscopic effects related to the intragranular microstructure. Secondly, the macroscopic behavior of the polycrystalline material is deduced from the singlecrystal behavior thanks to the self-consistent scheme in the sense of Hill. It is shown that this modeling allows the correct prediction of the behavior of multiphase steels. In a last part, a criterion based on the ellipticity loss of the elastic-plastic tangent modulus, the Rice criterion, is integrated once the macroscopic behavior is known. This criterion is used to plot forming limit diagrams, which have proven to be correct for various multiphase steels. Several attempts were undertaken as well in order to address possible differences with experiments.; Cette thèse a pour objectif d'élaborer un outil de prédiction de la perte de ductilité d'aciers multiphasés polycristallins soumis à des chargements monotones et séquentiels. Cet outil présente également l'intérêt d'aider à l'élaboration de nouveaux matériaux grâce à la connaissance de l'influence de différents paramètres physiques et microstructuraux sur la limite de ductilité. Dans un premier temps, le comportement élastoplastique à l'échelle locale, c'est-à-dire du grain, est modélisé par une loi micromécanique formulée dans le cadre des grandes déformations. Une loi de régularisation, inspirée de la viscoplasticité, mais sans introduire de sensibilité à la vitesse, est utilisée afin de déterminer l'activité de glissement. L'hétérogénéité intragranulaire est introduite dans le modèle en s'inspirant des travaux de Peeters. Les variables internes du modèle sont représentées par trois types de densités de dislocations dont leurs évolutions reposent sur les mécanismes de création, de stockage et d'annihilation. Une comparaison de l'évolution des variables internes avec des micrographies de grains d'acier ferritique issues d'essais monotones et complexes a permis de valider cette modélisation et de montrer les effets macroscopiques liés à la microstructure intragranulaire. Dans un second temps, le comportement macroscopique de l'agrégat polycristallin est déduit à partir du comportement monocristallin à l'aide de l'approche autocohérente au sens de Hill. Il est montré que cette modélisation permet de prédire correctement le comportement d'aciers multiphasés. Dans un dernier temps, un critère basé sur la perte d'ellipticité du module tangent élastoplastique, le critère de Rice, est intégré une fois le comportement macroscopique connu. Ce critère est utilisé pour tracer les courbes limites de formage et il est montré qu'elles s'avèrent correctes pour différents aciers multiphasés. Différentes pistes ont été exploitées afin de répondre aux éventuels écarts avec l'expérience.

The aim of this PhD thesis is to elaborate a predictive tool for the loss of ductility of polycrystalline multiphase steels during monotonous and sequential loading paths. Another major interest of this tool is to help design new materials thanks to knowledge of the impact of various physical and microstructural parameters on the ductility limit. At first, the elastic-plastic behavior at the local scale, i.e. at the grain level, is modeled using a micromechanical law formulated within the large strain framework. A rate-independent regularization technique, without introducing ratesensivity, is used to determine slip activity. The intragranular heterogeneities are introduced into the model following the original work of Peeters. The internal variables of the model are represented by three types of dislocation densities whose evolution is based on the mechanisms of creation, storage and annihilation. A comparison of the internal variable evolution with micrographs of grains of ferritic steel for monotonous and complex loadings allowed the validation of this modeling and demonstrates the macroscopic effects related to the intragranular microstructure. Secondly, the macroscopic behavior of the polycrystalline material is deduced from the singlecrystal behavior thanks to the self-consistent scheme in the sense of Hill. It is shown that this modeling allows the correct prediction of the behavior of multiphase steels. In a last part, a criterion based on the ellipticity loss of the elastic-plastic tangent modulus, the Rice criterion, is integrated once the macroscopic behavior is known. This criterion is used to plot forming limit diagrams, which have proven to be correct for various multiphase steels. Several attempts were undertaken as well in order to address possible differences with experiments., Cette thèse a pour objectif d'élaborer un outil de prédiction de la perte de ductilité d'aciers multiphasés polycristallins soumis à des chargements monotones et séquentiels. Cet outil présente également l'intérêt d'aider à l'élaboration de nouveaux matériaux grâce à la connaissance de l'influence de différents paramètres physiques et microstructuraux sur la limite de ductilité. Dans un premier temps, le comportement élastoplastique à l'échelle locale, c'est-à-dire du grain, est modélisé par une loi micromécanique formulée dans le cadre des grandes déformations. Une loi de régularisation, inspirée de la viscoplasticité, mais sans introduire de sensibilité à la vitesse, est utilisée afin de déterminer l'activité de glissement. L'hétérogénéité intragranulaire est introduite dans le modèle en s'inspirant des travaux de Peeters. Les variables internes du modèle sont représentées par trois types de densités de dislocations dont leurs évolutions reposent sur les mécanismes de création, de stockage et d'annihilation. Une comparaison de l'évolution des variables internes avec des micrographies de grains d'acier ferritique issues d'essais monotones et complexes a permis de valider cette modélisation et de montrer les effets macroscopiques liés à la microstructure intragranulaire. Dans un second temps, le comportement macroscopique de l'agrégat polycristallin est déduit à partir du comportement monocristallin à l'aide de l'approche autocohérente au sens de Hill. Il est montré que cette modélisation permet de prédire correctement le comportement d'aciers multiphasés. Dans un dernier temps, un critère basé sur la perte d'ellipticité du module tangent élastoplastique, le critère de Rice, est intégré une fois le comportement macroscopique connu. Ce critère est utilisé pour tracer les courbes limites de formage et il est montré qu'elles s'avèrent correctes pour différents aciers multiphasés. Différentes pistes ont été exploitées afin de répondre aux éventuels écarts avec l'expérience.

International audience; The present study deals with the formulation of a new micromechanical model for anisotropic damage in brittle rocks and its numerical implementation and application. The basic idea is to integrate Eshelby solution-based homogenization approaches into the standard thermodynamics framework for the description of inelastic deformation contributed by microcracks; the anisotropic damage is coupled with frictional sliding which occurs on closed cracks. The identification of the micromechanical model requires only six parameters, each with a clear physical meaning. Comparisons of the model's predictions with experimental data are performed on both conventional and true triaxial compression paths, respectively, for two granites. The proposed model is implemented into the finite element software Abaqus. Its application to an underground excavation problem shows that the proposed model is able to describe general responses and damaged zone evolution due to excavation.

A simple micromechanical model is presented in order to predict the phase transformation between austenite and a twinned martensite (or reorientation process of martensite platelets) for SMA single crystals. The predictions are in good agreement with the experimental tensile curves obtained in three Cu–Al–Ni single crystals with three different orientations [Continuum Mech. Thermodyn. 8 (1996) 309]. Moreover, the initiation curve of phase transformation (or reorientation) can be predicted in the stress space for proportional loading on Cu–Al–Ni polycrystals.

The present paper is concerned with the development of a micromechanical model of the hardening, rate-sensitivity and thermal softening of bcc crystals. In formulating the model we specifically consider the following unit processes: double-kink formation and thermally activated motion of kinks; the close-range interactions between primary and forest dislocations, leading to the formation of jogs; the percolation motion of dislocations through a random array of forest dislocations introducing short-range obstacles of different strengths; dislocation multiplication due to breeding by double cross-slip; and dislocation pair annihilation. The model is found to capture salient features of the behavior of Ta crystals such as: the dependence of the initial yield point on temperature and strain rate; the presence of a marked stage I of easy glide, specially at low temperatures and high strain rates; the sharp onset of stage II hardening and its tendency to shift towards lower strains, and eventually disappear, as the temperature increases or the strain rate decreases; the parabolic stage II hardening at low strain rates or high temperatures; the stage II softening at high strain rates or low temperatures; the trend towards saturation at high strains; the temperature and strain-rate dependence of the saturation stress; and the orientation dependence of the hardening rate., Comment: 27 pages (LaTeX) and 15 Figures (jpg)

The thermo-mechanical behavior of ceramic-matrix composites is analyzed in the framework of continuum damage mechanics through the introduction of internal variables. The choice of these variables is based upon the study of a micromechanical model describing the degradation mechanisms that have been observed in the material. The model identification is performed by studying two tension tests. The model is then used to predict the result of a Iosipescu shear test. An extension for cyclic and thermal loadings is presented.

A micromechanical model called MESOTEX is presented for prediction of the elastic behaviour of composites reinforced with non-hybrid weave (plain weave, satin weave and twill weave) and hybrid weave (hybrid plain weave and hybrid twill weave) fabrics. By using the classical thin laminate theory applied to each woven structure, this analytical model takes into account the strand undulations in the two directions and also integrates the geometrical and mechanical parameters of each constituent (resin, fill and warp strands). A representative volume is chosen for each woven composite and the fibre architecture is described by several functions. To determine the effectiveness of this analysis, the elastic properties predicted for each woven composite are compared with experimental results and results extracted from the literature. This correlation shows excellent agreement between measured and predicted values, with a very low calculation cost (CPU time of less than 0·01 s).

Nous presentons une quantification des caracteristiques micromecaniques decrivant la rupture ductile, tels les vides, en deformation uniaxiale obtenue lors d'un test d'ecaillage sur du tantale de purete commerciale. Nous decrivons l'evolution des vides jusqu'a la rupture par: (i) la fraction volumique de vide (porosite) et sa distribution en fonction de la distance par rapport au plan d'ecaillage; et (ii) la distribution des diametres des vides. Ces resultats completent la discussion sur les implications de la coalescence des vides pour la modelisation micromecanique de la rupture ductile presentee par D.L. Tonks et al. lors de cette meme conference.

Ceramics can deform non-elastically under compression and shear by sliding and ride up of fragments or grains, with accompnying competition between dilatancy and pore compaction. This paper describes a micromechanical model of such deformation based on multiplane plasticity and an analogy to the dynamics of atomic dislocations.

A micromechanical model describing the martensitic transformation on the grain scale has been developed, using Finite Elements. First results gained from the simulation illustrate how the morphological evolution within the grain is directly controlled by the internal stress state. The reversible and irreversible part of transformation "plasticity" strain and their evolution with the transformation can then be obtained from these calculations.