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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.

International audience; The present work deals with the use of asymptotic numerical methods (ANM) to manage crack onset and crack growth in the framework of Continuum Damage Mechanics (CDM). More specifically, an application of regularization techniques to a 1D cohesive model is proposed. The standard "triangle" damageable elastic model, often used in finite element codes to describe fracture of brittle materials, was chosen. Results associated with load-unload cycle showed that ANM is convenient to take numerically this specific non regular behaviour into account. Moreover, the present paper also shows that the chosen damageable interface model can be introduced in the generalized standard material formalism which unables us to define a complete energy balance associated with the damage process. In such a framework, the new damage state variable is a displacement. Finally, a 1D finite element application to a simple elastic damageable structure is shown to emphasize the potentialities of such an approach.

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; This work deals with the convergence acceleration of iterative nonlinear methods. Two convergence accelerating techniques are evaluated: the Modified Mininal Polynomial Extrapolation Method (MMPE) and the Padé approximants. The algorithms studied in this work are iterative correctors: Newton’s modified method, a high-order iterative corrector presented in Damil et al. (Commun Numer Methods Eng 15:701–708, 1999) and an original algorithm for vibration of viscoelastic structures. We first describe the iterative algorithms for the considered nonlinear problems. Secondly, the two accelerating techniques are presented. Finally, through several numerical tests from the thin shell theory, Navier–Stokes equations and vibration of viscoelastic shells, the advantages and drawbacks of each accelerating technique is discussed.

International audience; In this work, we propose an iterative linear solver for the linearized equations coming from the Newton-Raphson method. In structural mechanics, the computation of the non-linear solution, with a Newton-Raphson method for example, requires the solution of sparse linear systems of equations: $K^i U^i = F(U^j )$ with $i = 1,...,k$ and $j = 1,...,i − 1$ with $K^i$ designates a $N × N$ symmetric matrix (the tangent matrix), $U^i$ is the unknown displacement vector $(U^i ∈ \mathbb{R}^N)$ and $F(U^j )$ is a load vector which depends on the previous solutions $U^j$ with $j = 1,...,i − 1$.

Automotive industries are more and more subject to restrictive environmental constraints. Weight reduction of structures seems to be an interesting way to satisfy these requirements. This can be achieved either by using new materials, such as high strength steels or by adopting appropriate dimensioning methods to predict the occurrence of strain localization. Forming Limit Diagram (FLD) is a concept widely used to characterize the formability of thin metal sheets. Analytical determination of FLDs is usually based on the use of localization criteria. Some of the existing material instability criteria are for example based on empirical observations, on the maximum load principle [1-3], on the existence of an initial defect in the sheet [4], on a perturbation method [5] or on bifurcation analysis [6]. Although numerous criteria have been developed, they all have advantages but also drawbacks and limitations. Their confrontation on a wide range of materials is still insufficiently developed to compare their respective capability of accurately predicting FLDs for new materials. Adaptations of some criteria to advanced constitutive laws are also made necessary by the use of new high strength materials. The aim of this paper is to give a general formulation of some localization criteria allowing the comparison of the predicted FLDs for a wide range of materials. An implementation of these criteria coupled with different phenomenological constitutive laws is presented and compared for different materials including an aluminium alloy, a brass and a dual phase steel.; International audience; Automotive industries are more and more subject to restrictive environmental constraints. Weight reduction of structures seems to be an interesting way to satisfy these requirements. This can be achieved either by using new materials, such as high strength steels or by adopting appropriate dimensioning methods to predict the occurrence of strain localization. Forming Limit Diagram (FLD) is a concept widely used to characterize the formability of thin metal sheets. Analytical determination of FLDs is usually based on the use of localization criteria. Some of the existing material instability criteria are for example based on empirical observations, on the maximum load principle [1-3], on the existence of an initial defect in the sheet [4], on a perturbation method [5] or on bifurcation analysis [6]. Although numerous criteria have been developed, they all have advantages but also drawbacks and limitations. Their confrontation on a wide range of materials is still insufficiently developed to compare their respective capability of accurately predicting FLDs for new materials. Adaptations of some criteria to advanced constitutive laws are also made necessary by the use of new high strength materials. The aim of this paper is to give a general formulation of some localization criteria allowing the comparison of the predicted FLDs for a wide range of materials. An implementation of these criteria coupled with different phenomenological constitutive laws is presented and compared for different materials including an aluminium alloy, a brass and a dual phase steel.

In this work, an advanced anisotropic elastic-plasticity model is combined with a damage model and a strain localization criterion in the aim to describe accurately the mechanical behavior of sheet metals. Large strain, fully three-dimensional, implicit time integration algorithms are developed for this model and implemented in the finite element code Abaqus. The resulting code is used to predict the strain localization limits as well as the springback after forming of sheet steels. The impact of strain-path dependent hardening models on the limit strains and on the amount of springback is addressed.; International audience; In this work, an advanced anisotropic elastic-plasticity model is combined with a damage model and a strain localization criterion in the aim to describe accurately the mechanical behavior of sheet metals. Large strain, fully three-dimensional, implicit time integration algorithms are developed for this model and implemented in the finite element code Abaqus. The resulting code is used to predict the strain localization limits as well as the springback after forming of sheet steels. The impact of strain-path dependent hardening models on the limit strains and on the amount of springback is addressed.

International audience; A large strain elastic-plastic single crystal constitutive law, based on dislocation annihilation and storage, is implemented in a new self-consistent scheme, leading to a multiscale model which achieves, for each grain, the calculation of plastic slip activity, with help of regularized formulation drawn from visco-plasticity, and dislocation microstructure evolution. This paper focuses on the relationship between the deformation history of a BCC grain and induced microstructure during monotonic and two-stage strain paths.

International audience; Sheet metal forming processes often involve complex loading sequences. To improve the prediction of some undesirable phenomena, such as springback, physical behavior models should be considered. This paper investigates springback behavior predicted by advanced elastoplastic hardening models which combine isotropic and kinematic hardening and take strain-path changes into account. A dislocation-based microstructural hardening model formulated from physical observations and the more classical cyclic model of Chaboche have been considered in this work. Numerical implementation was carried out in the ABAQUS software using a return mapping algorithm with a combined backward Euler and semi-analytical integration scheme of the constitutive equations. The capability of each model to reproduce transient hardening phenomena at abrupt strain-path changes has been shown via simulations of sequential theological tests. A springback analysis of strip drawing tests was performed in order to emphasize the impact of several influential parameters, namely: process, numerical and behavior parameters. The effect of the two hardening models with respect to the process parameters has been specifically highlighted.

International audience; The impact of material models on the numerical simulation of springback is investigated. The study is focused on the strain-path sensitivity of two hardening models. While both models predict the Bauschinger effect, their response in the transient zone after a strain-path change is fairly different. Their respective predictions are compared in terms of sequential test response and of strip-drawing springback. For this purpose, an accurate and general time integration algorithm has been developed and implemented in the Abaqus code. The impact of several numerical parameters is also studied in order to assess the overall accuracy of the finite element prediction. For some test geometries, both material and numerical parameters are shown to clearly influence the springback behavior at a large extent. Moreover, a general trend cannot always be extracted, thus justifying the need for the finite element simulation of the stamping process.

This thesis concerns the study of metamaterials for water surface waves. The study is based on a laboratory experiment which makes it possible to measure the wave field precisely.In the first part, we demonstrate experimentally and numerically that metamaterials can be used to control the wave propagation and resonance properties of a closed cavity, including the cloaking of its eigenmodes. The anisotropic medium is designed using coordinate transformation theory and the homogenization of a three-dimensional linear water wave problem. This medium consists of a set of vertical plates whose spacing is much lower than the wavelength. This structure imposes an anisotropic bathymetry which influences the propagation of the waves differently according to their direction of propagation. Three different cavities manufactured by a 3D printer are tested and compared to the reference case with bathymetry without structuring. Fourier Transform Profilometry, as well as confocal displacement sensors, are used for measurements of water surface deformation resolved in time and space. Experimental data shows a remarkable ability of the metamaterial to influence the anisotropic propagation of waves on the water surface.The second part concerns the metamaterials submerged between two water layers for which a homogenized model is proposed, and the numerical solution by the modal method is provided. The anisotropic properties of such a structure are investigated experimentally using the same technique. An analysis based on the Bloch-Floquet formalism is performed to verify the dispersion relation of this medium predicted by the homogenization method.The main objective of the third part of this thesis is to experimentally study topologically protected edge states in a waveguide with periodic geometry in both linear and non-linear regimes. One of the representations of topological states, provided by the Su-Schrieffer-Heeger (SSH) model, is applied to describe the observed phenomena. A waveguide with periodic width is compared to the regular case of a rectangular reservoir with constant width. Confocal displacement sensors are used to measure the wave field very precisely. The experimental data is compared with the results of the 2D numerical simulations and the prediction of the SSH model. The results obtained show that this very simple configuration presents all the properties of the SSH model with excellent agreement.; La thèse concerne l'étude des métamatériaux dans le contexte des ondes à la surface de l'eau. Cette étude s'appuie sur une expérience en laboratoire qui permet de mesurer précisément le champ des ondes.Dans la première partie, nous démontrons expérimentalement et numériquement que les métamatériaux peuvent être utilisés pour contrôler la propagation des ondes et les propriétés de résonance d'une cavité fermée, y compris le « cloaking » de ses modes propres. Le milieu anisotrope est conçu à partir de la théorie de la transformation des coordonnées et de l'homogénéisation d'un problème d’onde tridimensionnel. Ce milieu est constitué par un ensemble de plaques verticales dont l’espacement est très inférieur à la longueur d’onde. Cette structure impose une bathymétrie anisotrope qui influe sur la propagation des ondes différemment selon leur direction de propagation par rapport à ce milieu structuré. Trois cavités différentes, fabriquées par une imprimante 3D, sont testées et comparées au cas de référence avec bathymétrie sans structuration. La profilométrie par transformée de Fourier, ainsi que des capteurs de déplacement confocaux, sont utilisés pour les mesures de la déformation de la surface de l'eau résolues en temps et en espace. Les données expérimentales montrent une capacité remarquable du métamatériau à influer sur la propagation anisotrope des ondes à la surface de l'eau.La deuxième partie concerne les métamatériaux immergés entre deux eaux pour lesquels un modèle homogénéisé est proposé et la solution numérique par méthode modale est fournie. Les propriétés anisotropes d'une telle structure sont étudiées expérimentalement en utilisant la même technique. Une analyse basée sur le formalisme de Bloch-Floquet est effectuée pour vérifier la relation de dispersion de ce milieu prédite par la méthode d'homogénéisation.L'objectif principal de la troisième partie de cette thèse est d'étudier expérimentalement les états de bord topologiquement protégés dans un guide d'ondes à géométrie périodique en régime linéaire et non linéaire. Une des représentations des états topologiques, fournie par le modèle Su-Schrieffer-Heeger (SSH), est appliquée pour décrire les phénomènes observés. Un guide d'ondes avec une largeur périodique est comparé au cas régulier d’un réservoir rectangulaire avec une largeur constante. Des capteurs de déplacement confocaux sont utilisés pour mesurer le champ d'onde très précisément. Les données expérimentales sont comparées aux résultats des simulations numériques 2D et à la prédiction du modèle SSH. Les résultats obtenus montrent que cette configuration très simple présente toutes les propriétés du modèle SSH avec un excellent accord.

International audience; The thermal wrinkling behavior of thin membranes is investigated in this paper. Wrinkles often occur at multiple length scales where induced compressive stresses are located during thermal loading. In the present study, the method of double scale Fourier series is used to deduce the macroscopic membrane wrinkling equations. The obtained equations account for the global and local wrinkling modes. Numerical examples are conducted to assess the validity of the approach developed. The present membrane's model needs only few degrees of freedom to recover more accurately the post-buckling equilibrium curves and the wrinkling shapes. Different parameters such as membrane's aspect ratio, wave number, and pre-stressed membranes are discussed from a numerical point of view and the properties of the wrinkles (critical load, wavelength, size, and location) are presented.

International audience; This paper revisits the modeling of compressive failure of long fiber composite materials by considering a multiscale finite element approach. It is well known that this failure follows from a fiber microbuckling phenomenon. Fiber microbuckling is governed by both material and geometrical quantities: the elastoplastic shear behavior of the matrix and the fiber misalignment. Although all these parameters are easily accounted by a finite element analysis at the local level, the failure is also influenced by macrostructural quantities. That is why a multilevel finite element model (FE²) is relevant to describe the compressive failure of composite. Furthermore, fiber local buckling leads to a loss of ellipticity of the macroscopic model, which can be a criterion of failure.

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WOS; International audience; The tubular joints, frequently employed in the offshore industry, are submitted to stresses resulting from elementary loadings: tension/compression, in-plane bending and out-of-plane bending. This work concerns the analysis of the recommendations commonly used for the fatigue design of welded joints submitted to combined loadings. Particularly, it deals with the fatigue behaviour of T-joints submitted to deviated-bending: first, a finite element analysis was developed and a post-processing based on the structural stress approach, as proposed by the International Institute of Welding (IIW). Then, fatigue tests were conducted on T-joints submitted to deviated-bending. Comparisons between experimental and numerical results showed that this kind of recommendations is not systematically conservative. Thus, an alternative approach based on structural stresses and taking into account the multi-axial stress state in the weld toe was developed in order to complete the recommendations for the fatigue design of tubular welded joints.

International audience; Understanding the routes of pathogen introduction contributes greatly to efforts to protect against future disease emergence. Here, we investigated the history of the invasion in North America by the fungal pathogen Microbotryum lychnidis-dioicae, which causes the anther smut disease on the white campion Silene latifolia. This system is a well-studied model in evolutionary biology and ecology of infectious disease in natural systems. Analyses based on microsatellite markers show that the introduced American M.lychnidis-dioicae probably came from Scotland, from a single population, and thus suffered from a drastic bottleneck compared with genetic diversity in the native European range. The pattern in M.lychnidis-dioicae contrasts with that found by previous studies in its host plant species S.latifolia, also introduced in North America. In the plant, several European lineages have been introduced from across Europe. The smaller number of introductions for M.lychnidis-dioicae probably relates to its life history traits, as it is an obligate, specialized pathogen that is neither transmitted by the seeds nor persistent in the environment. The results show that even a nonagricultural, biotrophic, and insect-vectored pathogen suffering from a very strong bottleneck can successfully establish populations on its introduced host.

International audience; Biologic invasions can have important ecological, economic and social consequences, particularly when they involve the introduction and spread of plant invasive pathogens, as they can threaten natural ecosystems and jeopardize the production of human food. Examples include the grapevine downy mildew, caused by the oomycete Plasmopara viticola, an invasive species native to North America, introduced into Europe in the 1870s. We investigated the introduction and spread of this invasive pathogen, by analysing its genetic structure and diversity in a large sample from European vineyards. Populations of P.viticola across Europe displayed little genetic diversity, consistent with the occurrence of a bottleneck at the time of introduction. Bayesian coalescent analyses revealed a clear population expansion signal in the genetic data. We detected a weak, but significant, continental-wide population structure, with two geographically and genetically distinct clusters in Western and Eastern European vineyards. Approximate Bayesian computation, analyses of clines of genetic diversity and of isolation-by-distance patterns provided evidence for a wave of colonization moving in an easterly direction across Europe. This is consistent with historical reports, first mentioning the introduction of the disease in Bordeaux vineyards (France) and sub-sequently documenting its rapid spread across Europe. This initial introduction in the west was probably followed by a leap-frog' event into Eastern Europe, leading to the formation of the two genetic clusters we detected. This study shows that recent population genetics methods within the Bayesian and coalescence frameworks are extremely powerful for increasing our understanding of pathogen population dynamics and invasion histories.

International audience; In order to characterise the behaviour of concrete under high pressures and high strain-rates, dynamic 1D-strain compression tests were performed. A cylindrical specimen is embedded in a steel confinement ring and compressed between 2 cylindrical plugs with a SHPB (Split Hopkinson Pressure Bars) device. Moreover, a new method was used to process the experimental data. It is based on numerical simulations of the cell loaded by an internal pressure that allow to build a relation between the pressure applied by the concrete to the inner surface of the cell and the external hoop strain measured by gauges. Stresses and strains in the specimen are computed at any loading time and the evolution of the deviatoric stress versus the pressure (deviatoric behaviour) and of the pressure versus volumetric strain (spherical behaviour) is deduced. The method is validated by several numerical simulations of the test involving different friction coefficients between the cell and the specimen. Three 1D-strain compression tests were performed and processed with the MB50 high-performance concrete. They showed that the deviatoric and spherical behaviours are almost independent of the strain rate in the observed range of strain rates (80-221 s-1). The deviatoric strength is seen to increase regularly with the hydrostatic pressure. The spherical behaviour indicates a fairly constant dynamic modulus of compressibility (around 5 to 6 GPa) up to a pressure of 900 MPa.

International audience; X-ray micro-computed tomography (CT) is used to generate 3D- and 2D-imaging reconstruction of the meso-structure of high-density polyethylene (HDPE)/short hemp fibre composites subjected to in situ tensile testing. These composites exhibit marked fibre/matrix debonding mechanisms starting at low strain level that are carefully analysed. In particular, debonding causes the formation of micro-cracks that first starts at the tip of fibres oriented parallel to the tensile direction and along fibres oriented perpendicular to the tensile direction. With increasing strain level, the thickness of these micro-cracks increases and coalescence mechanisms between adjacent micro-cracks are noted. The coalescence mechanism of some micro-cracks is believed to be at the origin of the material rupture.

Molecular dynamics (MD) simulations of liquid tin between its melting point and 1600 °C have been performed in order to interpret and discuss the ionic structure. The interactions between ions are described by a new accurate pair potential built within the pseudopotential formalism and the linear response theory. The calculated structure factor that reflects the main information on the local atomic order in liquids is compared to diffraction measurements. Having some confidence in the ability of this pair potential to give a good representation of the atomic structure, we then focused our attention on the investigation of the atomic transport properties through the MD computations of the velocity autocorrelation function and stress autocorrelation function. Using the Green-Kubo formula (for the first time to our knowledge for liquid tin) we determine the macroscopic transport properties from the corresponding microscopic time autocorrelation functions. The selfdiffusion coefficient and the shear viscosity as functions of temperature are found to be in good agreement with the experimental data.

International audience; Investigations of precipitation hardening are performed in term of analysis of distributions of geometrically necessary dislocations (GND) surrounding particles. The dislocation microstructures are computed from three dimensional discrete dislocation dynamics (DDD) and strain gradient plasticity (SGP) models. DDD simulations of spherical particle embedded in a single crystal matrix undergoing single slip provide the GND structures and the associated work-hardening. A 3D periodic arrangement of particles with cubic symmetry is considered. It is found that a network of slip and kink deformation bands develops, which is strongly dependent on the crystal lattice orientation of the matrix with respect to the particle array. For some relative orientations, the strain hardening is increased by the distributions of GND which act as additional barrier against slip. Some of these features are also captured with the SGP model in contrast to conventional continuum crystal plasticity.

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National audience; See http://hal.archives-ouvertes.fr/docs/00/59/26/94/ANNEX/r_4V1Q53J6.pdf

International audience; Understanding the mechanisms responsible for divergence and specialization of pathogens on different hosts is of fundamental importance, especially in the context of the emergence of new diseases via host shifts. Temporal isolation has been reported in a few plants and parasites, but is probably one of the least studied speciation processes. We studied whether temporal isolation could be responsible for the maintenance of genetic differentiation among sympatric populations of Ampelomyces, widespread intracellular mycoparasites of powdery mildew fungi, themselves plant pathogens. The timing of transmission of Ampelomyces depends on the life cycles of the powdery mildew species they parasitize. Internal transcribed spacer sequences and microsatellite markers showed that Ampelomyces populations found in apple powdery mildew (Podosphaera leucotricha) were genetically highly differentiated from other Ampelomyces populations sampled from several other powdery mildew species across Europe, infecting plant hosts other than apple. While P. leucotricha starts its life cycle early in spring, and the main apple powdery mildew epidemics occur before summer, the fungal hosts of the other Ampelomyces cause epidemics mainly in summer and autumn. When two powdery mildew species were experimentally exposed to Ampelomyces strains naturally occurring in P. leucotricha in spring, and to strains naturally present in other mycohost species in autumn, cross-infections always occurred. Thus, the host-related genetic differentiation in Ampelomyces cannot be explained by narrow physiological specialization, because Ampelomyces were able to infect powdery mildew species they were unlikely to have encountered in nature, but instead appears to result from temporal isolation

International audience; One of the main consequences of the nonassociative character of plastic strains in geomaterials is the existence of failure states strictly inside the Mohr-Coulomb plastic limit surface. This point is first emphasized by considering proportional strain loading paths as generalizations of the classical undrained triaxial path. It is shown that the sign of the second-order work is a proper criterion for analyzing these particular failure states. Then, experimental results and theoretical curves (obtained from an incrementally nonlinear constitutive relation) are compared for the case of proportional stress paths in axisymmetric conditions. Main features of the second-order work criterion are identified, such as the existence of a bifurcation domain together with a number of instability cones inside the Mohr-Coulomb surface. Furthermore, decomposing the second-order work into its isotropic and deviatoric parts makes it possible to compare each of the respective contributions to material instability. Finally, a heuristic boundary value problem is simulated via finite element modelling. A spectral analysis of the symmetric part of the stiffness matrix is conducted to extract the first vanishing eigenvalue and its associated eigenvector. It is found that the displacement field related to this eigenvector appears to be very close to the displacement computed just before global numerical breakdown signalling an effective failure. A plausible explanation is that, considering a material point, the flow rule at the boundary of the bifurcation domain almost coincides with the one describing the failure mechanism on the Mohr-Coulomb plastic limit surface.

International audience; This paper deals with the thermomechanical modeling of the macroscopic behavior of NiTi shape memory alloys (SMAs). A phenomenological 3D-model, based on thermodynamics of irreversible processes is presented. Three main physical mechanisms are considered: the martensitic transformation, the reorientation of martensite and the inelastic accommodation of twins in self-accommodated martensite. The description of such strain mechanisms allow an accurate analysis of SMA behavior under complex thermomechanical paths, especially when transformation occurs at low stress level. Moreover, some key characteristics such as tension–compression asymmetry and internal loops inside the major hysteresis loop are taken into account. Numerical simulations for various theromechanical loading paths are presented to illustrate the present model capability to capture the complex behavior of SMAs

International audience; Failure in soils and the yielding mechanisms involved are essential to many civil engineering issues such as slope stability. Sudden collapse affecting slopes can be observed after rainfalls, sometimes leading to the devastating flow of a soil-water mixture. Although the notion of failure in soils was long described as a perfect plastic limit, the scientific community now recognizes that failure can occur well before the Mohr-Coulomb criterion is met. This paper considers the collapse of granular soils in relation to a sudden burst of kinetic energy. First, a correlation between the bursts of kinetic energy (leading to sudden avalanches) and the vanishing of the second-order work is clearly obtained from discrete element simulations. Then the link between the vanishing of the second-order work and the increase in kinetic energy is established from a theoretical approach. Finally, these results are confirmed based on laboratory experimental tests where the sudden collapse of soil specimens was obtained.

International audience; A Kossel microdiffraction experimental set up is under development inside a Scanning Electron Microscope (SEM) in order to determine the crystallographic orientation as well as the inter- and intragranular strains and stresses. An area of about one cubic micrometer can be analysed using the microscope probe, which enables to study different kinds of elements such as a grain boundary, a crack, a microelectronic component, etc. The diffraction pattern is recorded by a high resolution Charge-Coupled Device (CCD) camera. The crystallographic orientation, the lattice parameters andthe elastic strain tensor of the probed area are deduced from the pattern indexation using a homemade software. The purpose of this paper is to report some results achieved up to now to estimate the reliability of the Kossel microdiffraction technique.

International audience; This paper presents an experimental investigation on the behaviour of structural adhesive bonding under quasi-static and moderately high loading rates. It addresses the effects of the loading rate on the strength of the adhesively bonded joints under dynamic tensile. A comparison has been achieved between the strength and the damage of specimens' made of aluminium and lamina substrates. High rate tests showed ringing in the force/displacement curves.

International audience

International audience; This paper presents a synthesis of the works performed by various teams from France, Italy and Canada around the question of second‐order work criterion. Because of the non‐associative character of geomaterials plastic strains, it is now recognized that a whole bifurcation domain exists in the stress space with various possible modes of failure. In a first part these failure modes are observed in lab experimental tests and in discrete element modelling. Then a theoretical study of second‐order work allows to establish a link with the kinetic energy, giving a basis to explain the transition from a prefailure (quasi)static regime to a postfailure dynamic regime. Eventually the main features of geomaterials failure are obtained by applying second‐order work criterion to five different constitutive rate‐independent models—three being phenomenological and two micromechanical. As a whole this paper tries to gather together all the elements for a proper understanding and use of second‐order work criterion in geomechanics.

International audience; Local texture and microstructure have been investigated in order to study the deformation mechanisms during equal channel angular extrusion of a high purity nickel single crystal of cube initial orientation. A detailed texture and microstructure analysis by various diffraction techniques reveals the complexity of the deformation patterns in different locations of the billet. A modeling approach, taking into account the slip system activity, is used to interpret the development of this heterogeneous deformation.

International audience; Local approach to brittle fracture for low-alloyed steels is discussed in this paper. A bibliographical introduction intends to highlight general trends and consensual points of the topic and evokes debatable aspects. French RPV steel 16MND5 (equ. ASTM A508 Cl.3), is then used as a model material to study the influence of temperature on brittle fracture. A micromechanical modelling of brittle fracture at the elementary volume scale already used in previous work is then recalled. It involves a multiscale modelling of microstructural plasticity which has been tuned on experimental inter-phase and inter-granular stresses heterogeneities measurements. Fracture probability of the elementary volume can then be computed using a randomly attributed defect size distribution based on realistic carbides repartition. This defect distribution is then deterministically correlated to stress heterogeneities simulated within the microstructure using a weakest-link hypothesis on the elementary volume, which results in a deterministic stress to fracture. Repeating the process allows to compute Weibull parameters on the elementary volume. This tool is then used to investigate the physical mechanisms that could explain the already experimentally observed temperature dependence of Beremin's parameter for 16MND5 steel. It is showed that, assuming that the hypothesis made in this work about cleavage micro-mechanisms are correct, effective equivalent surface energy (i.e. surface energy plus plastically dissipated energy when blunting the crack tip) for propagating a crack has to be temperature dependent to explain Beremin's parameters temperature evolution.

The effect of precipitation on Ni-rich NiTi Shape Memory Alloys on the thermomechanical properties is investigated. Particulary, the relation between the chemical evolution of the alloy and phase transformation characteristics are studied in order to evaluate the impact on the global mechanical response. The evolution of chemical composition of the alloy is computed using a diffusion law, according to the precipitates population characteristics. The mechanical response is simulated using a Representative Unit Cell which takes into account the structural effect of the precipitates as well as the effect of the composition change. The results show an important effect on the global mechanical behavior even for a small volume fraction of precipitates.Copyright © 2010 by ASME

WOS; International audience; This paper deals with geometrically nonlinear vibrations of sandwich beams with viscoelastic materials. For this purpose, a new finite element formulation has been developed, in which a zig-zag model is used to describe the displacement field. The viscoelastic behaviour is handled by using hereditary integrals and their relationships with complex moduli. An efficient solution procedure based on the harmonic balance method is also developed. To demonstrate its abilities, various problems of nonlinear vibrations of sandwich beams are considered. First, the results derived from the proposed approach are compared with those of nonlinear dynamic analyses using direct time integration and to experimental data. Then, the influence of the vibration amplitude on the damping properties of sandwich beams is investigated. The effect of an initial axial strain is also examined.

International audience; The development and the optimization of orthopaedic implants require precise studies involving geometrical, mechanical and material considerations. Moreover, the long term performance of the implant results from a compromise between the kinematic of the movement, the mechanical behavior and the bone tissue interaction. Obviously, the clinical evaluation is the best way to assess the implant performance. Nevertheless, the focus of this paper is to propose a methodology to estimate the immediate post-operative interactions between a proximal interphalangeal (PIP) joint prosthesis and the surrounding bone tissue. A finite element model of two PIP prostheses has been developed: an Ascension PIP pyrocarbon prosthesis and a new prosthesis which is currently under development (prototype prosthesis). Extreme loadings are applied to the bone models, which can be made either of young or old bone tissue: the strain state at the interface between the bone and the implant is compared to bone tissue remodeling curves. An iterative procedure is then adopted in order to improve the design of the prototype stem such that the final strain state is compatible with physiological conditions. This study shows that the strain level is always higher in the pyrocarbon implant than in the prototype prosthesis. Such high strain levels can induce bone necrosis and could explain the clinical observations of radiolucent lines around the implant resulting from a settling to a stable position.

International audience; This paper presents a new meshless method using high degree polynomial shape functions. These shape functions are approximated solutions of the partial differential equation (PDE) and the discretization concerns only the boundary. If the domain is split into several subdomains, one has also to discretize the interfaces. To get a true meshless integration-free method, the boundary and interface conditions are accounted by collocation procedures. It is well known that a pure collocation technique induces numerical instabilities. That is why the collocation will be coupled with the least-squares method. The numerical technique will be applied to various second order PDE's in 2D domains. Because there is no integration and the number of shape functions does not increase very much with the degree, high degree polynomials can be considered without a huge computational cost. As for instance the p-version of finite elements or some well established meshless methods, the present method permits to get very accurate solutions.

In the aerospace industry, carbon-ber reinforced plastic (CFRP) materials are becoming increasingly popular. Due to mechanical fracture and hence safety related issues, CFRP components must be inspected for defects with non-destructive methods. This thesis focuses on non-destructive testing of CFRP materials with optical lockin thermography. The eld of quantitative analysis of thermographic measurements is enhanced. The data of geometrical parameters e.g. depth, size and shape of defects in structures of globally homogeneous and anisotropic CFRP materials is required for fracture mechanics. To evaluate defects in a quantitative way, image processing algorithms are applied to thermographic phase images in order to get panoramic views of extended aircraft parts and to compare measurements before and after a fatigue load in order to determine potential defect growth. Images of lockin and ultrasound excited thermography are combined with data-fusion techniques to get improved information on defects such as impacts. The image formation process can be modeled through a point-spread function, which depends on the depth of the defect and the modulation frequency. A function is computed by using Green's functions and is adapted to anisotropic materials. The quantities depth, size and shape of a defect are determined through inverse numerical lters. Measurements are compared to numerical simulations and a reconstruction algorithm of planar subsurface defects is validated, Les matériaux composites carbone-époxy connaissent un intérêt grandissant dans le domaine de l'aéronautique. Des tests de contrôle non-destructif permettent de détecter des défauts. Ce travail s'attache particulièrement au contrôle non-destructif de matériaux composite carbone-époxy par la thermographie optique lockin. Il élargit le domaine de l'analyse quantitative des mesures thermographiques. Les paramètres géométriques tels la profondeur, la taille et la forme des défauts sont déterminés dans des pièces en matériau composite anisotrope et globalement homogène. Dans un but d'évaluation quantitative des défauts, des techniques de traitement d'image sont appliquées à des images de longues pièces aéronautiques pour former des vues panoramiques. Des images thermiques prises avant et après chargement mécanique sont superposées afin de pouvoir déterminer un endommagement. Différentes images thermale (lockin et excitation ultrasonore) sont fusionnées afin d'obtenir plus d'information sur des défauts du type impacts. La formation d'images est modélisée par une fonction de point qui dépend de la profondeur du défaut et de la fréquence de modulation. Un modèle est calculé en utilisant des fonctions de Green et adapté à des matériaux anisotropes. Les grandeurs : profondeur, taille et forme du défaut sont déterminées par des problèmes inverses. Les mesures sont comparées aux simulations numériques et un algorithme de reconstruction des défauts planaires est validé

In the aerospace industry, carbon-ber reinforced plastic (CFRP) materials are becoming increasingly popular. Due to mechanical fracture and hence safety related issues, CFRP components must be inspected for defects with non-destructive methods. This thesis focuses on non-destructive testing of CFRP materials with optical lockin thermography. The eld of quantitative analysis of thermographic measurements is enhanced. The data of geometrical parameters e.g. depth, size and shape of defects in structures of globally homogeneous and anisotropic CFRP materials is required for fracture mechanics. To evaluate defects in a quantitative way, image processing algorithms are applied to thermographic phase images in order to get panoramic views of extended aircraft parts and to compare measurements before and after a fatigue load in order to determine potential defect growth. Images of lockin and ultrasound excited thermography are combined with data-fusion techniques to get improved information on defects such as impacts. The image formation process can be modeled through a point-spread function, which depends on the depth of the defect and the modulation frequency. A function is computed by using Green's functions and is adapted to anisotropic materials. The quantities depth, size and shape of a defect are determined through inverse numerical lters. Measurements are compared to numerical simulations and a reconstruction algorithm of planar subsurface defects is validated; Les matériaux composites carbone-époxy connaissent un intérêt grandissant dans le domaine de l'aéronautique. Des tests de contrôle non-destructif permettent de détecter des défauts. Ce travail s'attache particulièrement au contrôle non-destructif de matériaux composite carbone-époxy par la thermographie optique lockin. Il élargit le domaine de l'analyse quantitative des mesures thermographiques. Les paramètres géométriques tels la profondeur, la taille et la forme des défauts sont déterminés dans des pièces en matériau composite anisotrope et globalement homogène. Dans un but d'évaluation quantitative des défauts, des techniques de traitement d'image sont appliquées à des images de longues pièces aéronautiques pour former des vues panoramiques. Des images thermiques prises avant et après chargement mécanique sont superposées afin de pouvoir déterminer un endommagement. Différentes images thermale (lockin et excitation ultrasonore) sont fusionnées afin d'obtenir plus d'information sur des défauts du type impacts. La formation d'images est modélisée par une fonction de point qui dépend de la profondeur du défaut et de la fréquence de modulation. Un modèle est calculé en utilisant des fonctions de Green et adapté à des matériaux anisotropes. Les grandeurs : profondeur, taille et forme du défaut sont déterminées par des problèmes inverses. Les mesures sont comparées aux simulations numériques et un algorithme de reconstruction des défauts planaires est validé

AgCdO is one of the most widely used contact materials in the world because of its outstanding performance. Nevertheless, due to environmental considerations, it will soon be completely forbidden by European environmental directives. Therefore, finding a good substitute is of crucial importance. Electrical arc erosion plays a crucial role in the reliability and life of power switching devices. Depending on the contact material's behavior in response to an electrical arc, surface damage can induce severe changes in contact material properties that will impact the power switching device's functioning. Consequently, electrical arc effects and consequences on the contact material surface are of first importance. In this context, we have focused our research activities on the following axes. First of all, in order to better understand AgCdO (Current contact material in aerospace industry) and AgSnO2 (Potential candidate to AgCdO substitution) arc erosion behaviors, arc erosion experiments, where the power switching devices have been subjected to different numbers of arc discharges, have been realized. Further, a general macroscopic electrical contact arc erosion model valid for low and high currents was developed. To compare model results to experimental data, this model describes the complete breaking process of electrical contacts and gives the total amount of material removed after one breaking operation. In parallel, arc erosion experiments on AgCdO power switching devices have been conducted at high currents (0 -> 1000 A) in order to validate the arc erosion model. Next, using the general arc erosion model, the properties having the greatest influence on the electrical arc erosion process have been determined through simulations on silver contact material. At this stage, ab initio calculations were needed to obtain ranges of variation of certain silver contact material properties. Finally, an investigation of the trends of changing local contact material composition of AgSnO2 on these identified material properties was performed. This study was based on ab initio calculations for two different oxide compositions of AgSnO2. These will allow us to give directions to aid the design of a good substitute for AgCdO, and therefore, to complete the main objective of this research work; L'AgCdO est l'un des matériaux de contact parmi les plus répandus dans le monde du fait de ses excellentes propriétés. Cependant, étant donné la présence de Cadmium dans celui-ci, il sera bientôt totalement interdit par des directives européennes d'ordre environnementales. Trouver un matériau ne contenant pas de Cadmium et ayant au moins d'aussi bonnes propriétés est donc la clé du projet. Le phénomène d'érosion du matériau de contact suite à l'impact d'un arc électrique joue un rôle de premier ordre dans la fiabilité et la durée de vie des dispositifs de commutation de puissance. Celui-ci influe en effet sur les propriétés du matériau et affecte ainsi le fonctionnement du dispositif de commutation de puissance. Dans ce contexte, nos axes de recherche ont été les suivants. Avant tout, et ce afin de mieux comprendre le comportement à l'érosion suite à l'impact d'un arc électrique de l'AgCdO (matériau de contact utilisé actuellement dans l'industrie aérospatiale) et celui de l'AgSnO2 (candidat potentiel au remplacement de l'AgCdO), des essais ont été réalisés en soumettant les dispositifs de commutation de puissance à différents nombres de décharges. Ensuite, un modèle macroscopique permettant de prédire l'érosion d'un matériau suite à l'impact d'un arc électrique, valable pour faibles et forts courants, a été développé. Ce modèle décrit le processus complet de coupure et donne la quantité totale de matériau érodé suite à une opération de commutation. En parallèle, des essais ont été réalisés sur des dispositifs de commutation de puissance AgCdO à forts courants (0 -> 1000 A) afin de valider ce modèle d'érosion. Les propriétés matériau les plus influentes sur le processus d'érosion ont ensuite été déterminées à l'aide de ce modèle. Enfin, une étude ab initio portant sur les effets d'un changement de composition locale du matériau sur ses propriétés identifiées comme influentes sur le processus d'érosion a été réalisée. Ainsi, l'ensemble de ces travaux permettront de conclure sur des directives dans l'élaboration du matériau de contact qui remplacera l'AgCdO, objectif principal de ces travaux de recherche

International audience; This paper describes the application of a steady state elastic-viscoplastic finite element model to the prediction of strip flatness and out of bite buckling during cold rolling of thin strips. This model is applied here to the last stand of a tinplate tandem cold mill. Results show that strip buckling reshuffles the non-homogeneous strip velocity profile just at the roll bite exit; strip stresses inside the roll bite are little impacted, so that roll stack deformation and roll force profile are not modified. The paper focuses on 'latent strip flatness', i.e. the profile in the transverse direction of the stress in the rolling direction. The profile predicted by the model far downstream the roll bite is in good agreement with shapemeter measurements only if strip buckling (i.e. manifest flatness) just at the roll bite exit and its interaction with the roll bite are considered, using a strong coupling of bite model and post-bite buckling model.

International audience; Nitrogen jets under high pressure and low temperature have been introduced recently. The process consists in projecting onto a surface a low temperature jet obtained from releasing the liquid nitrogen stored in a high pressure tank (e.g. 3000 bars) through a nozzle. It can be used in a range of industrial applications, including surface treatment or material removal through cutting, drilling, striping and cleaning. The process does not generate waste other than the removed matter, and it only releases neutral gas into the atmosphere. This work is aimed at understanding the mechanisms of the interaction between the jet and the material surface. Depending on the impacted material, the thermo-mechanical shock and blast effect induced by the jet can activate a wide range of damage mechanisms, including cleavage, crack nucleation and spalling, as well as void expansion and localized ductile failure. The test parameters (standoff distance, dwell time, operating pressure) play a role in selecting the dominant damage mechanism, but combinations of these various modes are usually present. Surface treatment through phase transformation or grain fragmentation in a layer below the surface can also be obtained by adequate tuning of the process parameters. In the current study, work is undertaken to map the damage mechanisms in metallic materials as well as the influence of the test parameters on damage, along with measurements of the thermo-mechanical conditions (impact force, temperature) in the impacted area.

AgCdO is one of the most widely used contact materials in the world because of its outstanding performance. Nevertheless, due to environmental considerations, it will soon be completely forbidden by European environmental directives. Therefore, finding a good substitute is of crucial importance. Electrical arc erosion plays a crucial role in the reliability and life of power switching devices. Depending on the contact material's behavior in response to an electrical arc, surface damage can induce severe changes in contact material properties that will impact the power switching device's functioning. Consequently, electrical arc effects and consequences on the contact material surface are of first importance. In this context, we have focused our research activities on the following axes. First of all, in order to better understand AgCdO (Current contact material in aerospace industry) and AgSnO2 (Potential candidate to AgCdO substitution) arc erosion behaviors, arc erosion experiments, where the power switching devices have been subjected to different numbers of arc discharges, have been realized. Further, a general macroscopic electrical contact arc erosion model valid for low and high currents was developed. To compare model results to experimental data, this model describes the complete breaking process of electrical contacts and gives the total amount of material removed after one breaking operation. In parallel, arc erosion experiments on AgCdO power switching devices have been conducted at high currents (0 -> 1000 A) in order to validate the arc erosion model. Next, using the general arc erosion model, the properties having the greatest influence on the electrical arc erosion process have been determined through simulations on silver contact material. At this stage, ab initio calculations were needed to obtain ranges of variation of certain silver contact material properties. Finally, an investigation of the trends of changing local contact material composition of AgSnO2 on these identified material properties was performed. This study was based on ab initio calculations for two different oxide compositions of AgSnO2. These will allow us to give directions to aid the design of a good substitute for AgCdO, and therefore, to complete the main objective of this research work, L'AgCdO est l'un des matériaux de contact parmi les plus répandus dans le monde du fait de ses excellentes propriétés. Cependant, étant donné la présence de Cadmium dans celui-ci, il sera bientôt totalement interdit par des directives européennes d'ordre environnementales. Trouver un matériau ne contenant pas de Cadmium et ayant au moins d'aussi bonnes propriétés est donc la clé du projet. Le phénomène d'érosion du matériau de contact suite à l'impact d'un arc électrique joue un rôle de premier ordre dans la fiabilité et la durée de vie des dispositifs de commutation de puissance. Celui-ci influe en effet sur les propriétés du matériau et affecte ainsi le fonctionnement du dispositif de commutation de puissance. Dans ce contexte, nos axes de recherche ont été les suivants. Avant tout, et ce afin de mieux comprendre le comportement à l'érosion suite à l'impact d'un arc électrique de l'AgCdO (matériau de contact utilisé actuellement dans l'industrie aérospatiale) et celui de l'AgSnO2 (candidat potentiel au remplacement de l'AgCdO), des essais ont été réalisés en soumettant les dispositifs de commutation de puissance à différents nombres de décharges. Ensuite, un modèle macroscopique permettant de prédire l'érosion d'un matériau suite à l'impact d'un arc électrique, valable pour faibles et forts courants, a été développé. Ce modèle décrit le processus complet de coupure et donne la quantité totale de matériau érodé suite à une opération de commutation. En parallèle, des essais ont été réalisés sur des dispositifs de commutation de puissance AgCdO à forts courants (0 -> 1000 A) afin de valider ce modèle d'érosion. Les propriétés matériau les plus influentes sur le processus d'érosion ont ensuite été déterminées à l'aide de ce modèle. Enfin, une étude ab initio portant sur les effets d'un changement de composition locale du matériau sur ses propriétés identifiées comme influentes sur le processus d'érosion a été réalisée. Ainsi, l'ensemble de ces travaux permettront de conclure sur des directives dans l'élaboration du matériau de contact qui remplacera l'AgCdO, objectif principal de ces travaux de recherche

International audience; An experimental method is presented in this paper to measure flash temperatures of sliding surfaces. High sliding velocities are reached by using a ballistic set-up equipped with a high speed camera. The temperature field on the friction surface was recorded during the process. Tests were conducted under dry sliding conditions by using an identical material for the rubbing bodies, which are of middle hard steel (C22). Experiments showed that the temperature distribution generated by frictional heating is made up of small hot spots that correspond to the friction of asperities located on the sliding surface during very short time. Deduced from observations, maximum local surface temperatures can exceed about 1100 °C around an area less than 100 μm in diameter.

International audience; Hard X-ray diffraction experiments have provided evidence of a strongly heterogeneous distribution of dislocation densities along the axis of cylindrical ice single crystals oriented for basal slip in torsion creep. The dislocation arrangements showed a complex scale-invariant character, which was analyzed by means of statistical and multifractal techniques. A trend to decreasing autocorrelation of the dislocation distribution was observed as deformation proceeds. At low strain levels, long-range spatial correlations control the distribution, but short-range correlations in relation with cross-slip progressively prevail when strain increases. This trend was reproduced by a model based on field dislocation dynamics, a theory accounting for both long-range elastic interactions and short-range interactions through transport of dislocation densities.

International audience; Statistics of acoustic emission accompanying plastic deformation and of stress serrations caused by the Portevin-Le Chatelier effect are studied during tension of an Al3%Mg alloy at room temperature. Power-law distributions of acoustic emission reflecting self-organization of dislocations and intermittency of plastic flow are found, irrespective of the strain rate, both before and after the critical strain for the onset of the serrated flow. In contrast, several regimes including both power-law and peaked distributions are observed at the macroscopic scale of stress serrations, depending on the applied strain rate.

Titanium alloys dedicated to biomedical applications may display both clinical and mechanical biocompatibility. Based on nontoxic elements such as Ti, Zr, Nb, Ta, they should combine high mechanical resistance with a low elastic modulus close to the bone elasticity (E=20 GPa) to significantly improve bone remodelling and osseointegration processes. These elastic properties can be reached using both lowering of the intrinsic modulus by specific chemical alloying and superelasticity effects associated with a stress-induced phase transformation from the BCC metastable beta phase to the orthorhombic alpha(″) martensite. It is shown that the stability of the beta phase can be triggered using a chemical formulation strategy based on the electronic design method initially developed by Morinaga. This method is based on the calculation of two electronic parameters respectively called the bond order (B(o)) and the d orbital level (M(d)) for each alloy. By this method, two titanium alloys with various tantalum contents, Ti-29Nb-11Ta-5Zr and Ti-29Nb-6Ta-5Zr (wt%) were prepared. In this paper, the effect of the tantalum content on the elastic modulus/yield strength balance has been investigated and discussed regarding the deformation modes. The martensitic transformation beta-->alpha(″) has been observed on Ti-29Nb-6Ta-5Zr in contrast to Ti-29Nb-11Ta-5Zr highlighting the chemical influence of the Ta element on the initial beta phase stability. A formulation strategy is discussed regarding the as-mentioned electronic parameters. Respective influence of cold rolling and flash thermal treatments (in the isothermal omega phase precipitation domain) on the tensile properties has been investigated.