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We report a comprehensive data set characterizing and quantifying the geometrically necessary dislocation (GND) density in the crystallographic frame (ραc) and disclination density (ρθ) in fine-grained polycrystalline olivine deformed in uniaxial compression or torsion, at 1000 and 1200 ∘C, under a confining pressure of 300 MPa. Finite strains range from 0.11 up to 8.6 %, and stresses reach up to 1073 MPa. The data set is a selection of 19 electron backscatter diffraction maps acquired with conventional angular resolution (0.5∘) but at high spatial resolution (step size ranging between 0.05 and 0.1 µm). Thanks to analytical improvement for data acquisition and treatment, notably with the use of ATEX (Analysis Tools for Electron and X-ray diffraction) software, we report the spatial distribution of both GND and disclination densities. Areas with the highest GND densities define sub-grain boundaries. The type of GND densities involved also indicates that most olivine sub-grain boundaries have a mixed character. Moreover, the strategy for visualization also permits identifying minor GND that is not well organized as sub-grain boundaries yet. A low-temperature and high-stress sample displays a higher but less organized GND density than in a sample deformed at high temperature for a similar finite strain, grain size, and identical strain rate, confirming the action of dislocation creep in these samples, even for micrometric grains (2 µm). Furthermore, disclination dipoles along grain boundaries are identified in every undeformed and deformed electron backscatter diffraction (EBSD) map, mostly at the junction of a grain boundary with a sub-grain but also along sub-grain boundaries and at sub-grain boundary tips. Nevertheless, for the range of experimental parameters investigated, there is no notable correlation of the disclination density with stress, strain, or temperature. However, a broad positive correlation between average disclination density and average GND density per grain is found, confirming their similar role as defects producing intragranular misorientation. Furthermore, a broad negative correlation between the disclination density and the grain size or perimeter is found, providing a first rule of thumb on the distribution of disclinations. Field dislocation and disclination mechanics (FDDM) of the elastic fields due to experimentally measured dislocations and disclinations (e.g., strains/rotations and stresses) provides further evidence of the interplay between both types of defects. At last, our results also support that disclinations act as a plastic deformation mechanism, by allowing rotation of a very small crystal volume.

Based on Taylor Meshless Method (TMM), the aim of this thesis is to develop a simple, robust, efficient and accurate numerical method which is capable of solving large scale engineering problems and to provide a new idea for the follow-up study on meshless methods. To this end, the influence of the key factors in TMM has been studied by solving three-dimensional and non-linear Partial Differential Equations (PDEs). The main idea of TMM is to use high order polynomials as shape functions which are approximated solutions of the PDE and the discretization concerns only the boundary. To solve the unknown coefficients, boundary conditions are accounted by collocation procedures associated with least-square method. TMM that needs only boundary collocation without integration process, is a true meshless method. The main contributions of this thesis are as following: 1) Based on TMM, a general and efficient algorithm has been developed for solving three-dimensional PDEs; 2) Three coupling techniques in piecewise resolutions have been discussed and tested in cases of large-scale problems, including least-square collocation method and two coupling methods based on Lagrange multipliers; 3) A general numerical method for solving non-linear PDEs has been proposed by combining Newton Method, TMM and Automatic Differentiation technique; 4) To apply TMM for solving problems with singularities, the singular solutions satisfying the control equation are introduced as complementary shape functions, which provides a theoretical basis for solving singular problems; Le but de cette thèse est de développer une méthode numérique simple, robuste, efficace et précise pour résoudre des problèmes d'ingénierie de grande taille à partir de la méthode Taylor Meshless (TMM) et fournir de nouvelles idées principales de TMM est d'utiliser comme fonctions de forme des polynômes d'ordre élevé qui sont des solutions approchées de l'EDP. Ainsi la discrétisation ne concerne que la frontière. Les coefficients de ces fonctions de forme sont obtenus en discrétisant les conditions aux limites par des procédures de collocation associées à la méthode des moindres carrés. TMM est alors une véritable méthode sans maillage sans processus d'intégration, les conditions aux limites étant obtenues par collocation. Les principales contributions de cette thèse sont les suivantes: 1) Basé sur TMM, un algorithme général et efficace a été développé pour résoudre des EDP elliptiques tridimensionnelles; 2) Trois techniques de couplage pour des résolutions par morceaux ont été discutées dans des cas de problèmes à grande échelle: la méthode de collocation par les moindres carrés et deux méthodes de couplage basées sur les multiplicateurs de Lagrange; 3) Une méthode numérique générale pour résoudre les EDP non-linéaires a été proposée en combinant la méthode de Newton, la TMM et la technique de différentiation automatique. 4) Pour résoudre des problèmes avec un bord non régulier, des solutions singulières satisfaisant l'équation de contrôle sont introduites comme des fonctions de forme complémentaires, ce qui fournit une base théorique pour la résolution de problèmes singuliers

The main aim of this thesis is to develop advanced and efficient multi-scale modeling and simulation techniques to study instability phenomena in three common engineering structures, i.e., membrane, film/substrate and sandwich structures, by combining the Technique of Slowly Variable Fourier Coefficients (TSVFC) and the Asymptotic Numerical Method (ANM). Towards this end, based on the Von Karman plate equations, the TSVFC has been firstly used to develop a two-dimensional (2D) Fourier double-scale model for membrane, which has also been implemented into ABAQUS via its subroutine UEL. Then a 2D Fourier model is constructed for film/substrate. Further, making use of deformation features of the film/substrate, a 1D Fourier model is developed by using both the TSVFC and the Carrera’s Unified Formulation (CUF). Subsequently, based on high-order kinematics belonging to Zig-Zag theory, a 2D Fourier model is deduced for sandwich plate. The governing equations for the above models are discretized by the Finite Element Method, and the resulting nonlinear systems are solved by the efficient and robust nonlinear solver ANM. These models are then adopted to study instabilities in these structures. Results show that the established models could accurately and efficiently simulate various instability phenomena. Besides, it’s found that the membrane instability is very sensitive to boundary conditions, and there exists a dimensionless parameter that is almost constant near bifurcation point for various loading cases and geometric parameters, which may be helpful for fast predicting the occurrence of wrinkles; L'objectif de cette thèse est de développer des techniques de modélisation et de simulation multi-échelle avancées et efficaces pour étudier les phénomènes d'instabilité dans trois structures d'ingénierie courantes: membrane, film/substrat et structures sandwich, en combinant la technique des coefficients de Fourier lentement variables (TSVFC) et la méthode numérique asymptotique (ANM). À cette fin, basée sur les équations de la plaque de Von Karman, la TSVFC été utilisée pour développer un modèle de Fourier à bidimensionnel (2D) qui a également été implémenté dans ABAQUS via sa sous-routine UEL. Ensuite, un 2D modèle de Fourier est construit pour le film/substrat. En outre, en utilisant leurs caractéristiques de déformation, un 1D modèle de Fourier est développé en utilisant à la fois le TSVFC et le CUF. Par la suite, sur la base d'une cinématique Zig-Zag d'ordre supérieur, un 2D modèle de Fourier est déduit pour une plaque sandwich. Les équations directrices pour les modèles ci-dessus sont discrétisées par la méthode des éléments finis, et les systèmes non linéaires résultants sont résolus par le solveur non linéaire efficace et robuste ANM. Ces modèles sont ensuite adoptés pour étudier les instabilités dans ces structures. Les résultats montrent que les modèles établis peuvent simuler avec précision et efficacité divers phénomènes d'instabilité. En outre, on constate que l'instabilité membranaire est sensible aux conditions aux limites et qu'il existe un paramètre sans dimension presque constant près du point de bifurcation pour différents cas de charge et paramètres géométriques, ce qui peut être utile pour prédire rapidement l'apparition des rides

International audience; The aim of this paper is to study, through a multiscale analysis, the viscoelastic behavior of glass reinforced sheet molding compound (SMC) composites and SMC-hybrid composites mixing two types of bundle reinforcement: glass and carbon fibers. SMC exhibit more than two distinct characteristic length scales, so that a sequence of scale transitions is required to obtain the overall behavior of the composite. An analytical procedure is used consisting of properly selected well-established micromechanical methods like the Mori-Tanaka (MTM) and the composite cylinders (CCM) accounting for each scale transition. After selecting a representative volume element (RVE) for each scale, the material response of any given length scale is described on the basis of the homogenized behavior of the next finer one. This hierarchical approach is appropriately extended to the viscoelastic domain to account for the time dependent overall response of the SMC composite material. The anisotropic damage has been introduced through a micromechanical model considering matrix penny-shape microcrack density inside bundles. The capabilities of the hierarchical modeling are illustrated with various parametric studies and simulation of experimental data for glass-based SMC composites.

As demonstrated by the loading–unloading tests and the modeling of the grain size effect and the composite effect, mainly owing to the back stress induced by CNTs, carbon nanotube/aluminum (CNT/Al) composites exhibit higher strain hardening capability than the unreinforced ultrafine-grained Al matrix. The back stress induced by CNTs should arise from the interfacial image force and the long-range interaction between statically stored dislocations and geometrically necessary dislocations around the CNT/Al interface. Therefore, this CNT-induced interfacial back stress strengthening mechanism is supposed to provide a novel route to enhancing the strain hardening capability and ductility in CNT/Al composites. IMPACT STATEMENT The present work investigates the roles and origins of back stress in the strain hardening of the carbon nanotubes/aluminum composites for the first time.

International audience

International audience; There are many recent studies concerning reduced-order computational methods, especially reductions by projection on a small-sized basis. But it is difficult to control the quality of the solutions if the basis is fixed once and for all. This is why we attempt to define efficient and low-cost strategies for correction and updating of the basis. These correction steps re-use previously computed quantities such as: vectors and triangulated matrices. The proposed algorithms use alternately full and reduced-size steps, allowing a strong reduction in the number of full-size tangent matrices. Two classes of applications are discussed. First, we consider an algorithm for determining Hopf bifurcation points in 2D Navier-Stokes equations, but which requires time-consuming preliminary frequency-dependent calculations. New reduction-correction procedures are applied to reduce these preliminary computations. The second application concerns the response curves of viscoelastic structures. A key point is the definition of the reduced basis. Vector Taylor series are computed within the asymptotic numerical method and the relevance of this set of vectors is analysed.

The initial problematic arises from the fact that implant failure is often caused by a mismatch between the elastic properties of the bone and those of the implant. Nowadays, an increasing interest is given to this mechanical biocompatibility and led to the development of β-metastable titanium alloys that possess low Young’s modulus, about half that of the conventionally used Ti-6Al-4V alloy. Moreover, lattice structures are currently being the subject of many investigations with the aim of achieving low Young’s modulus and high strength. Their fabrication, with accurate control over the architecture, is made possible thanks to additive manufacturing processes and the several possibilities they offer: design freedom, reduced material usage rate, complex shapes, mass customisation... The present work focuses on the implementation of low modulus titanium alloy Ti-26Nb(at.%) by the means of selective laser melting. An in situ elaboration strategy, based on a mixture of elemental powders, is explored in order to allow potential composition adjustments and to overcome the unavailability of titanium alloy powders. The approach is carried out using two distinct powder morphologies, spherical and irregular. The effects of the numerous parameters of the process (laser power, speed, scanning strategy...) on homogeneity and porosity of the manufactured parts is quantified. A homogeneous alloy can be obtained subject to the use of suitable energy density levels and powder size distributions that take into account the respective fusion temperatures of both elements. Microstructure characterisation highlights a pronounced texture resulting from the scanning strategy. The elaborated samples display a low Young’s modulus associated with a high strength, and hence a favourable strength to elastic modulus ratio compared to the reference cast alloy. Furthermore, an optimization algorithm is developed and allows controlling the mechanical properties of a lattice structure with its geometrical parameters (radius, length and orientation of struts). The combined use of this low Young’s modulus titanium alloy with a lattice structure developed through this algorithm was applied to the design of a total hip prosthesis that was subjected to finite element simulations. Stress-shielding evaluation shows that, compared to a solid design, this kind of prosthesis permits to reduce stress-shielding significantly. By getting closer to a physiological model, this prosthesis can be qualified as “biomimetic” in terms of mechanical behaviour; La problématique initiale part du constat que les échecs d’implants sont souvent causés par une inadéquation entre les propriétés élastiques de l’os et celles de l’implant. Aujourd’hui, ce problème de biocompatibilité mécanique suscite un intérêt croissant et a conduit au développement d’alliages de titane β-métastables qui possèdent un module d’élasticité faible, moitié moindre que celui de l’alliage Ti-6Al-4V classiquement utilisé dans les applications d’implantologie. De plus, les structures architecturées ou treillis font, elles aussi, l’objet d’intenses recherches dans le but de réduire le module d’élasticité et de maximiser la résistance. Leur mise en forme, avec une maîtrise précise de l’architecture, est possible grâce à la fabrication additive et les nombreuses possibilités qu’elle offre : liberté de design, gain matière, pièces complexes, customisation de masse... Ce travail de thèse porte sur la mise en œuvre de l’alliage de titane à bas module d’élasticité Ti-26Nb(%at.) par la technologie de fusion laser sur lit de poudres. Une stratégie d’élaboration in situ de ces alliages à partir de poudres élémentaires de Ti et de Nb est explorée, à la fois pour permettre d’éventuels ajustements de composition, et pour pallier au manque de disponibilité des alliages de titane sous forme de poudres. La démarche est réalisée avec deux morphologies de poudre, irrégulière et sphérique. Les effets des nombreux paramètres de ce procédé (puissance du laser, vitesse et stratégie de balayage...) sur l’homogénéité et la porosité des pièces élaborées sont quantifiés. Un alliage homogène peut être obtenu sous réserve de l’utilisation d’une densité d’énergie adaptée et d’une granulométrie de poudre tenant compte des températures de fusion respectives des éléments. La caractérisation de la microstructure met en évidence une texture marquée, dépendante de la stratégie de balayage. Les pièces élaborées présentent un bas module d’élasticité associé à une résistance mécanique élevée, avec une déformation élastique favorable par rapport à un alliage de référence coulé. Par ailleurs, un algorithme d’optimisation est développé et permet de contrôler les propriétés mécaniques d’une structure architecturée à partir de ses paramètres géométriques (rayon, longueur et orientation des poutres). La combinaison de cet alliage de titane à bas module d’élasticité et d’une structure architecturée développée à partir ce cet algorithme a été appliqué à une prothèse totale de hanche, qui a fait l’objet de simulations par éléments finis. L’évaluation du phénomène de stress-shielding montre que, comparativement à un modèle massif plus rigide, ce type de prothèse permet de réduire de façon significative la déviation des contraintes. En se rapprochant du modèle dit physiologique, cette prothèse peut être qualifiée de « biomimétique » sur le plan du comportement mécanique

Uranium dioxide (UO2) is used as a fuel, in pressurized water nuclear reactors, in the form of pellets produced by powder metallurgy. During power transients, the center part of pellets undergoes visco-plastic deformation by creep mechanisms. These mechanisms can be partially reproduced, out of irradiation, by uniaxial compression tests at high temperature (typically 1500°C). Testing conditions and initial microstructure of the UO2 pellets influence their macroscopic mechanical behavior. At the grain scale, sub-structuring mechanisms are involved, but, up to now, the sub-structure is not quantified and the role of pores on these mechanisms is unknown. In order to provide answers to these points, two batches of pellets (L1 and L2), characterized by a similar grain size, a same volume fraction of pores, but different pores distribution (2.5 times more intra-granular pores in L1 than in L2), were elaborated. They were submitted to mechanical tests under different conditions. The result shows that L1 has as a lower creep rate than L2. Electron Backscatter Diffraction (EBSD) and Electron Channeling Contrast Imaging (ECCI) techniques were used and optimized for porous materials to analyze the evolution of the microstructure after deformation. An original EBSD methodology was implemented to detect Sub-Grain Boundaries (S-GB) with very low disorientation angles (down to 0.1°), study statistically the grain fragmentation into sub-grains and evaluate the average density of the geometrically necessary dislocations. Thanks to ECCI, the arrangement of dislocations in some S-GB was evidenced and analyzed. EBSD and ECCI complementarity allowed relating the distribution of pores within the grains and the S-GB location. The results obtained on the two batches show that the number and the linear fraction of S-GB increases with the deformation level and rate. At high deformation rates, new grains appear by a mechanism of dynamic recovery/recrystallization by rotation of sub-grains. For identical loading conditions and strain rates, the samples of batch L1 have a number and a linear fraction of S-GB that are significantly higher than those of batch L2. Furthermore, in batch L1, S-GB are located essentially in the vicinity of the grain boundaries while they are distributed throughout the grain for batch L2. These microstructural differences seem to be related to a dislocation's mean free path reduction due to the presence of intra-granular pores; Le dioxyde d’uranium (UO2) est utilisé en tant que combustible, sous forme de pastilles élaborées par métallurgie des poudres, dans les réacteurs nucléaires à eau pressurisée. Lors de transitoires de puissance, le centre des pastilles est le siège de mécanismes de déformation visco-plastique qui peuvent être partiellement reproduits, hors irradiation, par des essais de compression uniaxiale à haute température (typiquement 1500°C). Les conditions de sollicitation et la microstructure initiale des pastilles d’UO2 ont une influence sur leur comportement mécanique macroscopique. A l’échelle des grains, des mécanismes de sous-structuration interviennent mais, à ce jour, la sous-structure n’est pas quantifiée et le rôle des pores sur ces mécanismes n’est pas connu. Afin d’apporter des réponses sur ces points, deux lots de pastilles (L1 et L2) de taille de grains similaires, de même fraction volumique de pores, mais ceux-ci étant distribués différemment (2,5 fois plus de pores intra-granulaires dans L1 que dans L2), ont été fabriqués. Ils ont ensuite été soumis à des essais mécaniques dans différentes conditions. Le résultat montre que le lot L2 présente une vitesse de fluage plus élevée que le lot L1. Les techniques Electron BackScatter Diffraction (EBSD) et Electron Channeling Contrast Imaging (ECCI) ont été mises en œuvre et optimisées pour suivre l’évolution de la microstructure après déformation. En EBSD, le développement d’une procédure adaptée aux matériaux poreux a permis de détecter des sous-joints de grains (S-JG) de très faible désorientation (jusqu’à 0,1°), de mener une étude statistique de l'évolution de la sous-structuration des grains et d'évaluer la densité de dislocations géométriquement nécessaires générées. Différents types d’arrangements de dislocations formant les S-JG ont été révélés et analysés par ECCI. Grâce à la complémentarité de l’EBSD et de l’ECCI, la répartition des pores dans les grains et la localisation des S-JG ont pu être mises en regard. Les résultats montrent que le nombre ainsi que la fraction linéaire des S-JG et leur désorientation augmente avec le taux et la vitesse de déformation. Aux forts taux de déformation, cela conduit à la formation de nouveaux grains par un mécanisme de restauration/recristallisation dynamique par rotation de sous-grains. Pour des conditions de sollicitation identiques, les échantillons du lot L1 présentent un nombre et une fraction linéaire de S-JG nettement supérieurs à ceux du lot L2. De plus, dans le lot L1, les S-JG se localisent essentiellement à proximité des joints de grains alors qu’ils sont répartis dans l’ensemble du grain pour le lot L2. Ces différences seraient liées à une réduction du libre parcours moyen des dislocations du fait de la présence des pores intra-granulaires

International audience

International audience; Granular materials are the most recurrent form of solid-state matter on Earth. They challenge researchers and engineers in various fields not only because they occur with a broad variety of grain sizes, shapes and interactions in nature and industry, but also because they show a rich panoply of mechanical states. Despite this polymorphism, all these different types of soils, powders, granules, ores, pharmaceutical products,. .. are instances of the granular matter with the same least common denominator of being sandlike (psammoid in Greek), i.e. solid grains interacting via frictional contacts. This review describes milestone contributions to the field of granular materials since the early elastic-plastic models developed for soils in the 1950s. The research on granular materials has grown into a vast multidisciplinary field in the 1980s with increasing focus on the microstructure and owing to new experimental tools and discrete simulation methods. It turns out that the granular texture, particle-scale kinematics and force transmission are far more complex than presumed in early micromechanical models of granular materials. Hence, constitutive relations cannot easily be derived from the particle-scale behavior although advanced continuum models have been developed to account for anisotropy, intermediate stress and complex loading paths. The subtle elastic properties and origins of bulk friction will be discussed, as well as the effects of particle shape 19and size distribution. The review covers also recent developments in macroscopic modeling such as the thermomechanical approach, anisotropic critical state theory, nonlocal modeling approach, inertial flows and material instabilities. Finally, a brief account is given of open issues and some new frontiers and challenges in the field.

International audience; In the present work, ultrafine-grained Al-Zn-Mg alloy produced by high energy ball milling and Spark PlasmaSintering was synthesized. The Hall-Petch strengthening due to grain refinement (about 150 nm) is preservedand the Orowan's effect attributed to the presence of the hardening phase η’ is obtained in a single-step duringthe sintering. Maximum compression stresses as high as 736 MPa and 830 MPa were reached in quasistatic anddynamic conditions respectively. Hardness was increased from 124 HV5 for a consolidated coarse grainedmaterial to 196 HV for a consolidated ultrafine-grained (UFG) material. The mechanical properties obtained inthis study were all superior to the ones of the reference AA7020-T651.

International audience; Depending on the rate of external loading, the permeability, and the boundary conditions, different drainage conditions are expected during the loading of sands. These drainage conditions are thus neither fully drained nor fully undrained as usually assumed for saturated soils, which implies simultaneous changes in pore volume and in pore water pressure. For a given material, the transition between contractive behavior (increase of the pore water pressure) and dilative behavior (decrease of the pore water pressure) defines the phase transformation state. It is found that the position of this phase transformation state, which is known to be dependent on the relative density, depends on the rate of dilatancy (ε˙v/ε˙1) imposed on the sample. Constraining a loose sample to dilate leads to an unstable state earlier than observed during undrained tests. Even dense samples, which do not exhibit a nonlocalized unstable state during undrained tests, are prone to collapse if the rate to dilatancy is high enough.

This paper proposes an intuitive computational multi-scale homogenisation procedure and tool for the estimation of the effective static and mechanical properties of complex viscoelastic composite material and structures. The proposed solution consists in computing numerically the complex effective properties (storage and loss moduli) as function of frequency. The developed numerical tool is coupled with ABAQUS FEA in order to ease the uptake of the technology code and allows for an accurate and automatic simulation of composite engineering structures with substantially less human intervention and a rational control of the error. Details concerning the numerical implementation and a selection of representative numerical examples are provided.

International audience; This paper deals with the vibration analysis of corrugated beams under large amplitude displacements and temperature variations. The dynamic response depends on the shape, the period and the amplitude of the corrugations as well as on the temperature variations. These different parameters were taken into account using a homogenization process. We coupled the harmonic balance method and the Galerkin technique to derive a frequency amplitude equation, which includes the corrugation shape and temperature. Finally, the influence of different shapes of corrugation (sinusoidal, triangular and square) on the nonlinear vibration response of a corrugated beam was compared to results obtained for a planar beam.

International audience; The aim of this article is to develop a reduction method to determine the forced harmonic response of viscoelastic sandwich structures at a reasonable computational cost. The numerical resolution is based on the asymptotic numerical method. This type of problem is complex, and its number of degrees of freedom is double the number of the undamped structure, leading to a high computational time. To address the problem, three reduction methods are evaluated, which differ from the projection operator. Numerical tests have been performed in the case of cantilever sandwich beams. The comparison of the results obtained by the reduction order resolution with those given by the full order resolution shows both a good agreement and a significant reduction in computational cost.

International audience; This study aims to characterize some safety aspects by examining the geometries of the infrastructure, currently used by societies, against the accumulation of hazardous hydrogen clouds during an accidental leak in areas with limited ventilation. Using ANSYS FLUENT as a modeling tool, the influence of garage roof shape; pyramidal and domed roof compared with the basic model (flat roof), for different leak times, on dispersion and stratification of hydrogen layers, is analyzed. As a result, the domed roof promotes to have a lower hydrogen concentration and presents two remarkable peaks of the Richardson number (Ri) with the highest value more than 2 × 105, which is three times higher than the flat roof. Besides, the influence of the leak time on the dynamic of the flow, concentration, and stratification process are observed: the mole fraction of hydrogen is more than 0.25 after 1 h of leak, whereas it is lower than 0.05 after 100 s. The volume flow and therefore the flammable volume increase. This study highlights the importance of geometrical and sizing parameters on the characteristics of hydrogen leaks and subsequently gives insights to establish performance standards for the availability and reliability of safety critical systems.

Sheet metal forming processes generally involve large deformations together with complex loading sequences. In order to improve numerical simulation predictions of sheet parts forming, physically-based constitutive models are often required. The main objective of this paper is to analyze the strain localization phenomenon during the plastic deformation of sheet metals in the context of such advanced constitutive models. Most often, an accurate prediction of localization requires damage to be considered in the finite element simulation. For this purpose,an advanced, anisotropic elastic-plastic model, formulated within the large strain framework and taking strain-path changes into account, has been coupled with an isotropic damage model. This coupling is carried out within the framework of continuum damage mechanics.In order to detect the strain localization during sheet metal forming, Rice’s localizationcriterion has been considered, thus predicting the limit strains at the occurrence of shear bands as well as their orientation. The coupled elastic-plastic-damage model has been implemented in Abaqus/Implicit. The application of the model to the prediction of Forming Limit Diagrams (FLDs) provided results that are consistent with the literature and emphasized the impact of the hardening model on the strain-path dependency of the FLD. The fully threedimensional formulation adopted in the numerical development allowed for some new results – e.g. the out-of-plane orientation of the normal to the localization band, as well as more realistic values for its in-plane orientation.; International audience; Sheet metal forming processes generally involve large deformations together with complex loading sequences. In order to improve numerical simulation predictions of sheet parts forming, physically-based constitutive models are often required. The main objective of this paper is to analyze the strain localization phenomenon during the plastic deformation of sheet metals in the context of such advanced constitutive models. Most often, an accurate prediction of localization requires damage to be considered in the finite element simulation. For this purpose,an advanced, anisotropic elastic-plastic model, formulated within the large strain framework and taking strain-path changes into account, has been coupled with an isotropic damage model. This coupling is carried out within the framework of continuum damage mechanics.In order to detect the strain localization during sheet metal forming, Rice’s localizationcriterion has been considered, thus predicting the limit strains at the occurrence of shear bands as well as their orientation. The coupled elastic-plastic-damage model has been implemented in Abaqus/Implicit. The application of the model to the prediction of Forming Limit Diagrams (FLDs) provided results that are consistent with the literature and emphasized the impact of the hardening model on the strain-path dependency of the FLD. The fully threedimensional formulation adopted in the numerical development allowed for some new results – e.g. the out-of-plane orientation of the normal to the localization band, as well as more realistic values for its in-plane orientation.

International audience; The dislocation configurations of a Ti2AlC-MAX phase deformed under severe plastic deformation by surface mechanical attrition treatment have been analyzed by transmission electron microscopy. Results show that the microstructure of the deformed Ti2AlC sample is composed of numerous 〈a〉-dislocations, which interact with each other notably with dipolar configurations. In addition, we report here 〈a〉-dislocation dissociations in the basal plane with a dissociation distance of approximately 20 nm, following the reaction 1/3 〈21 ̅1 ̅0〉⇔1/3 〈11 ̅00〉+1/3 〈101 ̅0〉. Finally, strong evidence of zonal dislocations is reported. These original results are discussed in the context of the fundamental deformation mechanisms of nanolayered ternary alloys.

International audience; Since the success of bone regenerative medicine depends on scaffold morphological and mechanical properties, numerous scaffolds designs have been proposed in the last decade, including graded structures that are suited to enhance tissue ingrowth. Most of these structures are based either on foams with a random pore definition, or on the periodic repetition of a unit cell (UC). These approaches are limited by the range of target porosities and obtained effective mechanical properties, and do not permit to easily generate a pore size gradient from the core to the periphery of the scaffold. In opposition, the objective of the present contribution is to propose a flexible design framework to generate various three-dimensional (3D) scaffolds structures including cylindrical graded scaffolds from the definition of a UC by making use of a non-periodic mapping. Conformal mappings are firstly used to generate graded circular cross-sections, while 3D structures are then obtained by stacking the cross-sections with or without a twist between different scaffold layers.. The effective mechanical properties of different scaffold configurations are presented and compared using an energy-based efficient numerical method, pointing out the versatility of the design procedure to separately govern longitudinal and transverse anisotropic scaffold properties. Among these configurations, a helical structure exhibiting couplings between transverse and longitudinal properties is proposed and permits to extend the adaptability of the proposed framework. In order to investigate the capacity of common additive manufacturing techniques to fabricate the proposed structures, a subset of these configurations is elaborated using a standard SLA setup, and subjected to experimental mechanical testing. Despite observed geometric differences between the initial design and the actual obtained structures, the effective properties are satisfyingly predicted by the proposed computational method. Promising perspectives are offered concerning the design of self-fitting scaffolds with on-demand properties depending on the clinical application.

International audience; In this paper, an extended version of Biot's differential equation is considered in order to discuss the quasi-static stability of a response for a solid in the framework of generalized standard materials. The same equation also holds for gradient theories since the gradients of arbitrary order of the state variables and of their rates can be introduced in the expression of the energy and of the dissipation potentials. The stability of a quasi-static response of a system governed by Biot's equations is discussed. Two approaches are considered, by direct estimates and by linearizations. The approach by direct estimates can be applied in visco-plasticity as well as in plasticity. A sufficient condition of stability is proposed and based upon the positivity of the second variation of energy along the considered response. This is an extension of the criterion of second variation, well known in elastic buckling, into the study of the stability of a response. The linearization approach is available only for smooth dissipation potentials, i.e. for the study of visco-elastic solids and leads to a result on asymptotic stability. The paper is illustrated by a simple example.

International audience; Dissimilar welding has been investigated for three different couples of titanium alloys: α/α+β, α/β and α+ββp. Plates of 100 x 60 x 1.6 or 1.8 mm were welded with a Yb: YAG laser. Tensile tests show that the properties of the dissimilar welded specimens were generally controlled by Those of the weakest material except for the α+β/β where the ultimate tensile strength was approximately equal to the average value of both materials. In every case, the welding sample elongation was found to be smaller than that of the base metals. The rupture took place away from the bead and was found to be always located in the alloy having the lowest mechanical properties. Nevertheless, a few large-size pores detected by tomography in the α+β/β couple could explain why rupture for these samples occurred in the weld bead. For each couple, the porosities were situated at the board between the heat affected zone and the molten zone. EBSD map and EDX enabled the observation of different microstructures, which could be correlated to the heterogeneous composition and diffusion into the melted bath.

International audience

International audience; The microstructure of a high-purity Al-6.5Si-1Fe (wt pct) alloy after solidification at various cooling rates was investigated. In most of the cases, the monoclinic beta-Al9Fe2Si2 phase was observed as long and thin lamellae. However, at a very slow cooling rate, Fe-bearing precipitates with Chinese script morphology appeared together with lamellae. Further analysis showed all these Chinese script precipitates correspond also to the monoclinic beta phase. This finding stresses that differentiating second phases according to their shape may be misleading

International audience; The medium Mn steels are a topic of interest from both practical and scientific point of view. Many studies were focused on the microstructure characterization, but only some of them addressed the mechanisms of austenite formation and stabilization. Hence, there are still remaining questions regarding the link between the optimum retained austenite fraction and stability and the austenite formation including both the morphological and kinetics aspects. In this work, different ART annealing treatments were performed on a cold rolled 0.1C – 4.7Mn (wt.%) steel. SEM and TEM observations as well as XRD and magnetic measurements were done to characterize the resulting microstructures. Microstructure evolution was analyzed as a function of soaking time: precipitation and dissolution of cementite; austenite nucleation, growth and stabilization. The experimental observations were compared with the predictions from thermodynamic calculations. Based on the obtained results, a mechanism of austenite formation and stabilization during ART annealing is proposed.

National audience

National audience

The clay soil is composed of solid skeleton and porous and can be studied in the framework of porous media. During the drying process, the initial saturated clay soil deforms mainly due to the changes of water content and capillary pressure. With the further increase in capillary pressure, the soil cracks. In fact, the deformation and the stress in clay soil are tightly related with the water content and capillary pressure. Therefore, it is of great interest to understand the changes in capillary pressure and water content within the clay soil during the drying process. However, it is difficult to obtain these field variables by experiments alone. Therefore, the numerical simulation could be good choice for deeper understanding of the dry process in clay soil. In current research, the combined numerical simulation and laboratory experiments research are conducted; the numerical results are compared with the laboratory observation. This research is the basis for the further study of the cracking process of clay soil due to drying.

In order to predict the lifetime of railtrack components, numerical models are required to better describe the apparition of microstructural defects that lead to material failure. In pearlitic steels, the severe mechanical loading or/and the local temperature increase result in the formation of a hard and brittle phase that is prone to cracking, which is called White Etching Layer (WEL). The current study proposes a phenomenological model, based on the microstructural evolution, to describe the mechanically-induced-WEL kinetics of formation. First, a detailed multiscale microstructural characterization was performed in order to identify suitable microstructural indicators for the WEL formation. The grain size, orientation and morphology, and internal disorientation were characterized for different steps of mechanical WEL formation. This could be achieved thanks to the use of a dedicated test bench that allows to monitor the wheel–rail contact condition (sliding, number of cycles, and contact pressure) independently. The proposed model was validated using tests performed in extreme load and sliding conditions. The numerical and experimental results indicate that the most severe conditions (contact pressure and sliding ratio) catalyze WEL formation, and that the same deformation state can be achieved through different loading paths. This work is part of the multi-disciplinary project MOPHAB, which aims to improve our knowledge and understanding of the mechanisms leading to the formation of the white etching layer in the materials used to construct railways and to develop corresponding numerical models. This project was supported by IRT Railenium and other industrial partners (RATP: Régie Autonome des Transports Parisiens, France, SNCF: Société Nationale des Chemins de Fer Francais, France, SAARSTAHL rail).

International audience; Microstructure evolution of a 3003 sheet cladded with 4004 brazing alloy is investigated during slow heating (1K/min) under secondary vacuum up to isothermal brazing temperature (590°C). Optical and scanning microscopies, EDS chemical analysis and EBSD orientation mapping are used. Experimental results are discussed in the light of thermodynamic calculations using Thermo-Calc. Comparisons show good agreement as long as Mg vaporization does not take place.

International audience; The aim of this paper is to develop a reduction method to determine the modal characteristics of viscoelastic sandwich structures. The method is based on the high order Newton algorithm and reduction techniques. Numerical tests have been performed in the case of sandwich beams and cylindrical shells. The comparison of the results obtained by the reduction method with those given by direct simulation shows both a good agreement and a significant reduction in computational cost.

International audience; In this contribution, an elasto-viscoplastic fast Fourier transform-based (EVPFFT) numerical implementation of the Mesoscale Field Dislocation Mechanics (MFDM) formulation, called MFDM-EVPFFT, is applied to study the reversible plastic behavior of periodic two-phase crystalline composites with an elastoviscoplastic plastic matrix and a purely elastic second phase. Periodic laminate microstructures of this kind with different periods (i.e. sizes) are considered to examine the size dependence of the Bauschinger effect and hardening during cyclic loading. Comparisons with classic composite effects obtained with conventional crystal plasticity are discussed. Specifically, the MFDM-EVPFFT results shed light on the hardening mechanisms due to piling-up/unpiling-up of geometrically-necessary dislocations (GND) during reverse loading.

International audience; For several decades, composite materials have been used in different applications due to their multiple advantages. It has become necessary to understand the dynamic behavior of these materials under critical loads. In-plane (IP) and out-of-plane (OP) dynamic compression tests were investigated on a cubic specimen of composite joints under low to high strain rates using the Split Hopkinson Pressure Bars (SHPB) technique. The specimens consist of two adherents (glass/polyester composite) assembled by an adhesive with different thicknesses (1 and 2 mm). During the experimental tests, the high-speed camera was used to check the damage history and the failure mechanisms on adhesively bonded composite joints. The sample surface temperatures were monitored using an infrared camera (IR). For bonding of two composite substrates, it can be noted that the thickness of the adhesive has a significant effect on the dynamic behavior and the kinetics of the damage. It has been concluded that the damage was localized at the level of the layers in the case of in-plane loading and the level of the adhesive zone in the case of out-of-plane loading. The thermomechanical behavior of cubic specimens of adhesively bonded joints subjected to dynamic compression has been studied. The results underline the sensitivity of the dynamic behavior of the adhesively bonded joints to the strain rate, the loading direction and the appearance of heat dissipation linked to the increase in temperature following the initiation of damage. This rise of temperature can dangerously approach the glass transition temperature of the matrix.

International audience

International audience; Ce travail concerne le calcul des propriétés amortissantes de structures sandwich viscoélastiques. La résolution de ce problème aux valeurs propres non linéaires nécessite des techniques numériques performantes, telles que les méthodes de Newton d'ordre élevé. Cette dernière consiste en l'association d'une technique d'homotopie et d'une méthode de perturbation. La méthode de Newton d'ordre élevé est très efficace [8] mais demande des temps de calculs relativement importants. Une technique de réduction de modèles est alors proposée pour diminuer ces temps de calculs tout en conservant les propriétés de convergence de la méthode de Newton d'ordre élevé. Les applications numériques concernent des plaques sandwich viscoélastiques.

International audience; Spark plasma sintering (SPS) has been used to sinter a milled oxide dispersion-strengthened (ODS) FeAl powder in order to prepare dense nanostructured parts. The effect of processing conditions including sintering temperature, holding time and degassing treatments on the microstructure and mechanical properties of the as-sintered materials was investigated. A hetero-nanostructure that contains nano, ultrafine and micrometric grains was confirmed to be developed whatever the processing window, due to the large temperature difference generated during the SPS process itself. The grain size distribution can be tailored by selecting a combination of sintering temperature and holding time while retaining a full densification, which shows the elaboration flexibility of this SPS processing. Increase of the global hardness by reducing average grain size down to the sub-micrometer scale is straightforward. However, the ductility although improved after pre and in-situ degassing treatments is determined by the sample imperfections such as low inter-particle bonding and high oxygen content.

International audience; This paper discusses the use of an existing adaptive predictive controller to control some Shape Memory Alloy (SMA) linear actuators. The model consists in a truncated linear combination of Laguerre filters identified online. The controller stability is studied in details. It is proven that the tracking error is asymptotically stable under some conditions on the modelling error. Moreover, the tracking error converge toward zero for step references, even if the identified model is inaccurate. Experimentalcresults obtained on two different kind of actuator validate the proposed control. They also show that it is robust with regard to input constraints.

International audience; A simple, first order description of the formation of ε- and α′-martensite in meta-stable austenitic Fe–Mn alloys containing 12–35 wt% Mn was developed. Given the initial volume fractions of ε- and α′-martensite in the non-deformed material, it is possible to calculate the evolution of phase fractions as a function of applied strain. An accurate estimate of the initial amounts of ε- and α′-martensite based solely on the Mn content of the alloy, could not be obtained due to the important role of microstructure features and processing conditions on the martensitic transformations.

International audience; The use of sandwich structures based on composite and/or metallic honeycomb core has grown considerably these last years, especially for aeronautic, aerospace, naval and automotive fields. The proposed work deals with the analysis of milling Nomex® honeycomb core that usually generates poor machined surface quality and problems in assembling parts. The performance of milling process and obtained surface quality are strongly influenced by the honeycomb material properties and cutting parameters. The composite nature of the Nomex® paper forming the honeycomb structure, the thin wall thickness and its alveolar shape make the use of conventional parameters of quantifying the surface quality (as roughness) inappropriate. In this study a new method of measurement of the surface quality has been developed. This innovative approach allows simultaneously the identification and quantification of induced machining defects, as well as the evaluation of the performance of the obtained surface. A new quality criterion was then carried out to obtain the best compromise between high surface quality and optimal cutting conditions. This will improve the use of honeycomb materials in sandwich assemblies for industry.

The formulation of a six-node solid-shell called SHB6, which is a linear, isoparametric element, is discussed. An eigenvalue analysis of the element stiffness matrix is first carried out. Several modifications are introduced into the formulation of the SHB6 element following the assumed strain method adopted by Belytschko and Bindeman. SHB6's coordinates and displacements are related to the nodal coordinates and displacements through the linear shape functions. Applying the simplified form of the Hu-Washizu nonlinear mixed variational principle, in which the assumed stress field is chosen to be orthogonal to the difference between the symmetric part of the displacement gradient and the assumed strain field, the formula is obtained. The newly developed SHB6 element was implemented into the finite element codes INCA and ASTER. It represents some improvement since it converges well and performs much better than the PRI6 six-node three-dimensional element in all of the benchmark problems tested.; International audience; The formulation of a six-node solid-shell called SHB6, which is a linear, isoparametric element, is discussed. An eigenvalue analysis of the element stiffness matrix is first carried out. Several modifications are introduced into the formulation of the SHB6 element following the assumed strain method adopted by Belytschko and Bindeman. SHB6's coordinates and displacements are related to the nodal coordinates and displacements through the linear shape functions. Applying the simplified form of the Hu-Washizu nonlinear mixed variational principle, in which the assumed stress field is chosen to be orthogonal to the difference between the symmetric part of the displacement gradient and the assumed strain field, the formula is obtained. The newly developed SHB6 element was implemented into the finite element codes INCA and ASTER. It represents some improvement since it converges well and performs much better than the PRI6 six-node three-dimensional element in all of the benchmark problems tested.

The formation of a white etching layer (WEL), a very hard and brittle phase on the rail surface, is associated with a progressive transformation of the pearlitic grain to very fragmented grains due to the cumulative passage of trains. Its formation is associated with a complex thermomechanical coupling. To predict the exact conditions of WEL formation, a thermomechanical model previously proposed by the authors needs to be validated. In this study, monotonic and cyclic shear tests using hat-shaped specimens were conducted in the temperature range of 20 C to 400 C to reproduce the WEL formation. The tests showed a strong sensitivity of the material to temperature, which does not necessarily favor WEL formation. For the monotonic tests, no WELs were produced; however, a localization of the plastic deformation was observed for tests performed at 200 C and 300 C. In this temperature range, the material was less ductile than at room temperature, leading to failure before WEL formation. At 400 C, the material exhibited a much more ductile behavior, and nanograins close to WEL stages were visible. For the cyclic tests, a WEL zone was successfully reproduced at room temperature only and confirmed the effect of shear in WEL formation. The same cyclic tests conducted at 200 C and 300 C yielded results consistent with those of the monotonic tests; the deformation was much more localized and did not lead to WEL formation. This work is part of the multi-disciplinary project MOPHAB, which aims to improve our knowledge and understanding of the mechanisms leading to the formation of the white etching layer in the materials used to construct railways and to develop corresponding numerical models. This project was supported by IRT Railenium and other industrial partners (RATP: Régie Autonome des Transports Parisiens, France, SNCF: Société Nationale des Chemins de Fer Francais, France, SAARSTAHL rail).

International audience; At high-temperature, viscoplastic behaviour of polycrystalline uranium dioxide (UO2) is governed by the dislocation flow. At this scale, curvatures in the crystal lattice, associated to the presence of Geometrically Necessary Dislocations (GNDs), have been observed. Here, we develop an approach based on the Field Dislocation Mechanics theory, which models the Geometrically Necessary Dislocations (GNDs) glide, allowing us to consider the size effects associated to GNDs. The proposed model is used to simulate an imposed speed compression test of a polycrystal of UO2. This test shows : accumulation of GNDs at the grain boundaries, glide within the grains and the formation of sub-grains. Results predicted by the model demonstrate, qualitatively, a good agreement with the experimentally observed dislocations substructure in deformed UO2.; À haute température, les déformations viscoplastiques de l’UO2 polycristallin sont gouvernées par le mouvement des dislocations. À cette échelle, des courbures dans le réseau cristallin ont été observées, et sont associées à la présence de Dislocations Géométriquement Nécessaires (GNDs). Une approche basée sur la théorie de la Mécanique des Champs de Dislocations, qui modélise le glissement des GNDs, a été développée pour rendre compte des effets d’échelle associés à la présence de ces GNDs. Le modèle proposé est utilisé pour simuler un essai de compression à vitesse imposée d’un polycristal d’UO2, au cours duquel on observe l’accumulation de GNDs sur les joints de grains, leur glissement dans les grains et la formation de sous grains. Les résultats du modèle montrent, qualitativement, un bon accord avec la sous-structuration de dislocations observée expérimentalement.

Within the framework of sheet metal forming, the importance of hardening models for springback predictions has been often emphasized. While some specific applications require very accurate models, in many common situations simpler (isotropic hardening) models may be sufficient. In these conditions, investigation of the impact of hardening models requires well defined test configurations and accurate measurements to generate the reference data. Specific draw-bend tests have been especially conceived for this purpose. In this work, such a draw-bending experimental device has been designed, for use on a biaxial tension machine. Three different steel sheets have been tested (one mild steel sheet and two HSS sheets) with thicknesses between 0.8 and 2 mm. Up to three different back-force levels were used for the tests. Wall curvatures and springback angles were measured. Finite element simulations of the tests were performed. A parameter sensitivity analysis has been carried out in order to determine the numerical parameters ensuring accurate springback results. The tests were simulated using an isotropic hardening model and a combined isotropic-kinematic hardening model. The impact of the hardening model is explored for the various test configurations and conclusions are drawn concerning their relative importance.; International audience; Within the framework of sheet metal forming, the importance of hardening models for springback predictions has been often emphasized. While some specific applications require very accurate models, in many common situations simpler (isotropic hardening) models may be sufficient. In these conditions, investigation of the impact of hardening models requires well defined test configurations and accurate measurements to generate the reference data. Specific draw-bend tests have been especially conceived for this purpose. In this work, such a draw-bending experimental device has been designed, for use on a biaxial tension machine. Three different steel sheets have been tested (one mild steel sheet and two HSS sheets) with thicknesses between 0.8 and 2 mm. Up to three different back-force levels were used for the tests. Wall curvatures and springback angles were measured. Finite element simulations of the tests were performed. A parameter sensitivity analysis has been carried out in order to determine the numerical parameters ensuring accurate springback results. The tests were simulated using an isotropic hardening model and a combined isotropic-kinematic hardening model. The impact of the hardening model is explored for the various test configurations and conclusions are drawn concerning their relative importance.

With a view to environmental, economic and safety concerns, car manufacturers need to design lighter and safer vehicles in ever shorter development times. In recent years, High Strength Steels (HSS) like Interstitial Free (IF) steels which have higher ratios of yield strength to elastic modulus, are increasingly used for sheet metal parts in automotive industry to meet the demands. Moreover, the application of sheet metal forming simulations has proven to be beneficial to reduce tool costs in the design stage and to optimize current processes. The Finite Element Method (FEM) is quite successful to simulate metal forming processes but accuracy largely depends on the quality of the material properties provided as input to the material model. Common phenomenological models roughly consist in the fitting of functions on experimental results and do not provide any predictive character for different metals from the same grade. Therefore, the use of accurate plasticity models based on physics would increase predictive capability, reduce parameter identification cost and allow for robust and time-effective finite element simulations. For this purpose, a 3D physically based model at large strain with dislocation density evolution approach was presented in IDDRG2009 by the authors [1]. This model allows the description of work-hardening's behavior for different loading paths (i.e. uni-axial tensile, simple shear and Bauschinger tests) taking into account several data from microstructure (i.e. grain size, texture, etc...). The originality of this model consists in the introduction of microstructure data in a classical phenomenological model in order to achieve work-hardening's predictive character for different metals from the same grade. Indeed, thanks to a microstructure parameter set for an Interstitial Free steel, it is possible to describe work-hardening behavior for different loading paths of other IF steels by only changing the mean grain size and the chemical composition. During sheet metal forming processes local material points may experience multi-axial and multi-path loadings. Before simulating actual industrial parts, automotive manufacturers use validation tools - e.g. the Cross-Die stamping test. Such typical stamping tests enable the evaluation of a complex distribution of strains. The work described is an implementation [2] of a 3D dislocation based model in ABAQUS/Explicit and its validation on a Finite Element (FE) Cross-Die model. In order to assess the performance and relevance of the 3D dislocation based model in the simulation of industrial forming applications, the results of thinning profiles predicted along several directions and the strain distribution were obtained and compared with experimental results for IF steels with grain sizes varying in the 8-22 μm value range.; International audience; With a view to environmental, economic and safety concerns, car manufacturers need to design lighter and safer vehicles in ever shorter development times. In recent years, High Strength Steels (HSS) like Interstitial Free (IF) steels which have higher ratios of yield strength to elastic modulus, are increasingly used for sheet metal parts in automotive industry to meet the demands. Moreover, the application of sheet metal forming simulations has proven to be beneficial to reduce tool costs in the design stage and to optimize current processes. The Finite Element Method (FEM) is quite successful to simulate metal forming processes but accuracy largely depends on the quality of the material properties provided as input to the material model. Common phenomenological models roughly consist in the fitting of functions on experimental results and do not provide any predictive character for different metals from the same grade. Therefore, the use of accurate plasticity models based on physics would increase predictive capability, reduce parameter identification cost and allow for robust and time-effective finite element simulations. For this purpose, a 3D physically based model at large strain with dislocation density evolution approach was presented in IDDRG2009 by the authors [1]. This model allows the description of work-hardening's behavior for different loading paths (i.e. uni-axial tensile, simple shear and Bauschinger tests) taking into account several data from microstructure (i.e. grain size, texture, etc...). The originality of this model consists in the introduction of microstructure data in a classical phenomenological model in order to achieve work-hardening's predictive character for different metals from the same grade. Indeed, thanks to a microstructure parameter set for an Interstitial Free steel, it is possible to describe work-hardening behavior for different loading paths of other IF steels by only changing the mean grain size and the chemical composition. During sheet metal forming processes local material points may experience multi-axial and multi-path loadings. Before simulating actual industrial parts, automotive manufacturers use validation tools - e.g. the Cross-Die stamping test. Such typical stamping tests enable the evaluation of a complex distribution of strains. The work described is an implementation [2] of a 3D dislocation based model in ABAQUS/Explicit and its validation on a Finite Element (FE) Cross-Die model. In order to assess the performance and relevance of the 3D dislocation based model in the simulation of industrial forming applications, the results of thinning profiles predicted along several directions and the strain distribution were obtained and compared with experimental results for IF steels with grain sizes varying in the 8-22 μm value range.

International audience; The use of shape memory alloys for micro-actuators constitutes a field of application in which copper–aluminum-based alloys find their usefulness because they can reach higher activation temperatures and are easier to produce than titanium-based alloys, particularly by the method proposed in this work. SMA tapes are a two-dimensional structure that offers many design options such as stamping, punching, and deep drawing, but they are also suitable for laser cutting, engraving, stamping, and EDM machining. This work has been made to study the manufacture of copper-based shape memory alloys (SMAs) using the cold co-rolling process also called the cold-roll bonding (CRB) process. In this process, a thin metal sandwich can be produced with a rolling machine. This sandwich consists of layers of CuNiBe master alloy and Al. During the rolling phase, the sandwich has no shape memory effect (SME) or superelastic effect (SE), so thin strips can be easily produced. After the rolling phase, the sandwich is subjected to a complex heat treatment to gain the SME. To validate this process to produce Cu-based SMAs, several alloys with different CuAlNiBe compositions have been tested. The SMAs obtained were characterized by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The martensitic transformation was studied by Differential Scanning Calorimetry (DSC) and SME and SE were studied by three-point bending tests. This work shows that the CRB is a good process for making a wide variety of Cu-based SMA ribbons.

International audience; In this paper, a mathematical model dealing with a coupled heat, air, and moisture transfer in a building envelope was developed. Based on the three‐following driving potential: vapor pressure, dry air pressure, and temperature, an application on a hygrothermal beha-vior of a real wall was carried out for different climatic conditions. For this purpose, a characterization of the heat and moisture properties of the materials con-stituting the wall made with red brick and cement mortar was carried out in the laboratory. This was used to evaluate experimentally the input parameters of the model as a function of relative humidity. To validate the numerical model, an experimental platform was improved. The wall was set up in a double‐climatic chamber with different boundary conditions, and then the temperature and humidity evolutions were re-corded using several sensors within the wall thickness. The results have highlighted a good agreement be-tween numerical simulation results and experi-mental ones.

International audience; The analysis of multiple necking phenomenon during dynamic stretching of a metallic plate is investigated. For that purpose, a new 1D linear stability analyses is proposed to capture the perturbation evolution. The multidimensional aspect of the stress field within the neck region is taken into account with a Bridgman correction factor. The novelty of the 1D approach is to let the time evolution of the perturbation determined by the linearized field equations. So, no predefined time dependency is assumed as in classical 1D linear stability analysis of the literature, see Zhou et al., An elastic-visco-plastic analysis of ductile expanding ring, Int. J. Impact Eng., 2006. The proposed model, named hereafter 1D-XLSA (standing for 1D-eXtended Linear Stability Analysis), can also be viewed as the restriction to 1D of the 2D-XLSA model developed in Xavier et al., Extension of linear stability analysis for the dynamic stretching of plates: Spatio-temporal evolution of the perturbation, European Journal of Mechanics-A/Solids, 2020. A comparison for a thermo-viscoplastic material with strain hardening is proposed for three possible routes : a 1D model based on the frozen coefficient theory named 1D-CLSA (standing for 1D-Classical Linear Stability Analysis), the proposed 1D-XLSA and 2D-XLSA models. It is shown that while growth rates in late deformation stage are similar, a strong difference in amplitude exists due to large discrepancies in the early deformation stage. The comparison also illustrates the importance of the modeling of multiaxiality of the stress field within the neck region and of initial defects. While differences between models are limited for perturbations with small wavenumbers, large discrepancies are observed for perturbations with large wavenumbers. Our model is also derived for the case of the extension of a cylindrical bar which is representative of the ring expansion during dynamic loading.