Determination of the size of the representative volume element for random quasi-brittle composites

International audienceA representative volume element (RVE) is related to the domain size of a microstructure providing a ''good" statistical representation of typical material properties. The size of an RVE for the class of quasi-brittle random heterogeneous materials under dynamic loading is one of the major questions to be answered in this paper. A new statistical strategy is thus proposed for the RVE size determination. The microstructure illustrating the methodology of the RVE size determination is a metal matrix composite with randomly distributed aligned brittle inclusions: the hydrided Zircaloy constituting nuclear clad-dings. For a given volume fraction of inclusions, the periodic RVE size is found in the case of overall elastic properties and of overall fracture energy. In the latter case, the term ''representative" is discussed since the fracture tends to localize. A correlation factor between the ''elastic" RVE and the ''fracture" RVE is discussed

Étude statistique de la réponse mécanique lors d'essais de traction directe à l'échelle locale

National audienceLa modélisation de la fissuration du béton nécessite de considérer la liaison entre la pâte de ciment et les granulats du béton, communément appelée interface, auréole de transition. Ce constat a motivé la mise en place d'essais mécaniques spécifiques en laboratoire à l'échelle dite locale afin de caractériser et d'identifier le comportement mécanique d'une pâte de ciment pure et de l'interface pâte de ciment/granulat à partir d'un échantillon composite. Des essais de traction directe sur des éprouvettes de ciment ont été mis en place dans le but d'identifier le comportement mécanique à la rupture de la pâte de ciment pure à l'échelle locale. Pour un type de ciment donné, la dispersion de la réponse mécanique des essais dépend, d'une part, de la dispersion des caractéristiques mécaniques de l'échantillon et, d'autre part, du dispositif expérimental. Cet article s'intéresse à la dispersion de la réponse mécanique sur plusieurs essais qui est grandement influencée par la préparation des échantillons. Une étude statistique numérique met en exergue la forte dépendance de la statistique des réponses mécaniques de l'essai de traction à la position des fixations en acier servant à la traction de l'échantillon. Les résultats de cette étude sont comparés à des observations expérimentales

Étude statistique de la réponse mécanique lors d'essais de traction directe à l'échelle locale

National audienceLa dispersion de la réponse mécanique d'un essai de traction directe sur une pâte de ciment à l'échelle locale, ou échelle de l'interface pâte de ciment/granulat, dépend, d'une part, de la dispersion des caractéristiques mécaniques de la pâte de ciment et, d'autre part, du dispositif expérimental. Une étude statistique numérique quantifie la dépendance de la dispersion des réponses mécaniques d'un essai de traction directe à la position de fixations en acier servant à la traction de l'échantillon. Les résultats de cette étude sont comparés à des observations expérimentales

Étude statistique de la réponse mécanique lors d’essais de traction directe à l’échelle locale

National audienc

Tensile and shear bond strength between cement paste and aggregate subjected to high temperature

International audienceIn the framework of studying the leakage rate of concrete containment of nuclear power plants during an accident, the mechanical characterization of interface between cement paste and limestone aggregate was performed with mechanical tests at the local scale, inside the laboratory of micromechanics and integrity of structures (MISt). The evolution of tensile and shear bond strength between cement paste and aggregate after heating was determined. It was revealed that the tensile bond strength was strongly affected by the temperature rise, with a decrease of about 71% at 60 °C. Moreover, the shear bond strength increased almost linearly with the increase of normal stress. According to the Coulomb criterion, the cohesion and the internal friction angle were determined. These parameters decreased after heating, which indicates occurrence of thermic damage between cement paste and limestone aggregate. Finally, it appears that the tensile bond strength is more affected by the thermal damage than the shear bond strength

Identification of heterogeneous elastoplastic materials by Constitutive Equation Gap Method

International audienceA strategy for identifying the shape and the parameters of elastoplastic laws in heterogeneous material is proposed here. It allows the local characterization of both mechanical properties and stress fields. This method relies on the constitutive equation gap principles and on the measurement of overall loads and strain fields. These fields are obtained by the numerical differentiation of displacement fields measured by Digital Image Correlation. The identification method is based on the iterative minimization of an energy norm involving the secant elastoplastic tensor. We propose here to characterize the robustness of the method and its stability (to noise, initial parameters and mesh) on numerical data sets obtained on numerically-deformed images. Finally, we illustrate the ability of the method to identify the local material properties of a heterogeneous material on real experimental data

Identification of heterogeneous elastoplastic materials by Constitutive Equation Gap Method

International audienceA strategy for identifying the shape and the parameters of elastoplastic laws in heterogeneous material is proposed here. It allows the local characterization of both mechanical properties and stress fields. This method relies on the constitutive equation gap principles and on the measurement of overall loads and strain fields. These fields are obtained by the numerical differentiation of displacement fields measured by Digital Image Correlation. The identification method is based on the iterative minimization of an energy norm involving the secant elastoplastic tensor. We propose here to characterize the robustness of the method and its stability (to noise, initial parameters and mesh) on numerical data sets obtained on numerically-deformed images. Finally, we illustrate the ability of the method to identify the local material properties of a heterogeneous material on real experimental data

Identification of heterogeneous elasto-plastic behaviors using the Constitutive Equation Gap Method

International audienceRecent developments in imaging techniques now facilitate local field measurements (e.g. strain, temperature, etc.). In this study, we identify the spatial distribution of material properties and local stress fields using an inverse identification method based on the constitutive equation gap (CEG). The CEG concept is based both on minimization of a cost function equal to the sum of the potential and complementary energies, and on the deviation between the measured and computed strain fields. We propose a new approach for identifying heterogeneous property fields (mechanical parameters and stress) using a secant elastoplastic tensor and a measured strain field obtained by full-field measurement. The reliability of the method is checked using finite element simulation data as reference full-field measurements. The method is then applied on noisy displacement fields to assess its robustness. Finally, the developed inverse method is tested on real measured data

Identification inverse de paramètres élastoplastiques à partir de mesures de champs

National audienceThe present study focuses on the identification of heterogeneous elastoplastic properties of a material from full-field measurements. An inverse method is used to identify on the one hand the material parameters and on the other hand the stress field. An application to a composite structure submitted to tension is presented to show the feasibility of the method. Finally, the robustness of the method is tested towards initial set of parameters, mesh and noise measurements where Gaussian perturbations were added to displacements values to simulate an experimental errors.La présente étude porte sur l'identification des propriétés élastoplastiques hétérogènes d'un matériau à partir de mesures de champs cinématique. Une méthode inverse est mise en oeuvre afin d'identifier d'une part les paramètres matériaux (3 élastiques et 2 plastiques) et d'autre part le champ de contrainte. Un exemple sur une structure en matériaux composite est présenté en détails afin de montrer la faisabilité de l'approche. La robustesse vis-à-vis des données initiales, maillages et du bruit est étudiée

Méthode d’Eigenerosion : mise en œuvre et extension aux matériaux hétérogènes. Application aux matériaux cimentaires.

National audienceSur la base de l'approche variationnelle de la rupture de Francfort et Marigo [2], Pandolfi et Ortiz [6] ont proposé une méthode — d'Eigenerosion — s'apparentant à une méthode de «killing element » et respectant le critère de Griffith. Les travaux proposés ici portent sur la mise en œuvre de la méthode et sur son extension à la dynamique et aux matériaux hétérogènes