Field statistics in linear viscoelastic composites and polycrystals.

International audienceThe aim of this study is to estimate the effective response, as well as the statistics of the fields (average and fluctuations per phase) in linear viscoelastic heterogeneous materials. To this effect, a variational method based on a rate variational principle (RVP) has recently been introduced by the authors in which, at each time step, the stress or strain fields are approximated by those of a linear thermoelastic comparison composite. In the present study, a different derivation of this estimate is proposed, based on a simple approximation of the stress field along the time steps. The present study also explores the accuracy of the RVP model by comparing its predictions with reference results, either in closed form (for specific two-phase particulate composites) or obtained by full-field simulations (FFT method) for 2D polycrystals. A differential equation for the second moment of the stress field in the individual phases of two-phase particulate composites is given for the first time. These comparisons show that the RVP model delivers a very accurate estimate of the effective behavior as well as of the statistics of the local fields in two-phase particulate composites. For polycrystalline materials subjected to monotonic loading, the effective behavior and the statistics of the local fields are well predicted. The agreement is less accurate for cycling loadings

Using deconvolution to improve the metrological performance of the grid method

WOSInternational audienceThe use of various deconvolution techniques to enhance strain maps obtained with the grid method is addressed in this study. Since phase derivative maps obtained with the grid method can be approximated by their actual counterparts convolved by the envelope of the kernel used to extract phases and phase derivatives, non-blind restoration techniques can be used to perform deconvolution. Six deconvolution techniques are presented and employed to restore a synthetic phase derivative map, namely direct deconvolution, regularized deconvolution, Richardson-Lucy algorithm and Wiener filtering, the last two with two variants concerning their practical implementations. Obtained results show that the noise that corrupts the grid images must be thoroughly taken into account to limit its effect on the deconvolved strain maps. The difficulty here is that the noise on the grid image yields a spatially correlated noise on the strain maps. In particular, numerical experiments on synthetic data show that direct and regularized deconvolutions are unstable when noisy data are processed. The same remark holds when Wiener filtering is employed without taking into account noise autocorrelation. On the other hand, the Richardson-Lucy algorithm and Wiener filtering with noise autocorrelation provide deconvolved maps where the impact of noise remains controlled within a certain limit. It is also observed that the last technique outperforms the Richardson-Lucy algorithm. Two short examples of actual strain fields restoration are finally shown. They deal with asphalt and shape memory alloy specimens. The benefits and limitations of deconvolution are presented and discussed in these two cases. The main conclusion is that strain maps are correctly deconvolved when the signal-to-noise ratio is high and that actual noise in the actual strain maps must be more specifically characterized than in the current study to address higher noise levels with Wiener filtering

Analysis of the mechanical behaviour of composites and their bonded assemblies under out-of-plane loads using a modified Arcan apparatus

International audienceAdhesively bonded assemblies, unlike riveted or bolted joints, do not require holes, which can lead to large stress concentrations in composites. For this reason adhesive bonding is attracting attention particularly when weight is critical. But failure in adhesively bonded assemblies involving composites is often associated with crack initiation in the adhesive or delamination of the composite plies close to the adhesive joint caused by interlaminar or through-thickness stresses. Such assemblies are often characterized by large stress concentrations, which make the analysis of their behaviour for industrial applications difficult. As the mechanical behaviour of adhesives and of composites are quite complex, a large database of experimental results under various loading conditions is necessary to develop accurate numerical models. Unfortunately, few experimental devices allow out-of-plane loading to be studied, except for specific loadings or by using thick composite specimens which are not always representative of industrial applications. This paper presents the possibilities of an experimental device, using a modified Arcan test, and optimized hybrid bonded assemblies, which limit the influence of edge effects. Some test results, showing the potential of the approach, are presented in the form of failure envelope curves for proportional monotonic out-of-plane loadings. Moreover, some aspects of the influence of low temperature on the mechanical behaviour of hybrid bonded assemblies with composites are analyzed

Etude de la transition de phase d'un monocristal d'AMF en Cu-Al-Be par techniques de mesures de champs

Le but de l'étude est d'étudier les phénomènes de transformation de phase se produisant dans un monocristal d'alliage à mémoire de forme (AMF). L'échantillon initialement austénitique devient progressivement martensitique lors d'un essai de traction uniaxial. La méthode de la grille est utilisée sur une face de l'échantillon afin de mesurer les déformations. Sur l'autre face, un calcul de source de chaleur est réalisé à partir de mesures thermiques par caméra infrarouge. Ces mesures complémentaires permettent de suivre et d'analyser les changements de phase

Calcul précis des déformations planes par la méthode de la grille. Application à l'étude d'un multicristal d'aluminium

This work deals with the improvement of the grid method to measure the strain field on the surface of specimens subjected to mechanical tests. For this purpose, a procedure has been proposed to obtain directly the strains from the grid images. The influence of the grid defects has also been characterized and a suitable procedure has been developed to decrease their effects on the strain maps. The proposed tools have been applied in the case of unidirectional and cross grids. The performance of the method has been estimated with appropriate tests. Finally, the method has been applied on a multicristal aluminum in order to illustrate its implementation in a complex case of material behaviour of characterization.Ce travail porte sur l'amélioration de la méthode de la grille afin de mesurer avec de meilleures performances métrologiques des champs de déformation en surface d'éprouvettes soumises à des sollicitations mécaniques. Une procédure d'obtention directe des déformations à partir d'images de grilles a été proposée dans ce but. L'influence des défauts de la grille a également été caractérisée et une procédure adaptée a été développée pour l'éliminer. Les outils proposés ont été appliqués dans le cas de grilles unidirectionnelles et croisées. Les performances métrologiques ont été estimées avec des essais adaptés. Enfin, la méthode a été appliquée à un essai sur un multicristal d'aluminium, montrant ainsi sa mise en oeuvre dans un cas complexe de caractérisation de comportement de matéria

Calcul précis des déformations planes par la méthode de la grille (Application à l'étude d'un multicristal d'aluminium)

Ce travail porte sur l'amélioration de la méthode de la grille afin de mesurer avec de meilleures performances métrologiques des champs de déformation en surface d'éprouvettes soumises à des sollicitations mécaniques. Une procédure d'obtention directe des déformations à partir d'images de grilles a été proposée dans ce but. L'influence des défauts de la grille a également été caractérisée et une procédure adaptée a été développée pour l'éliminer. Les outils proposés ont été appliqués dans le cas de grilles unidirectionnelles et croisées. Les performances métrologiques ont été estimées avec des essais adaptés. Enfin, la méthode a été appliquée à un essai sur un multicristal d'aluminium, montrant ainsi sa mise en oeuvre dans un cas complexe de caractérisation de comportement de matériauCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Modélisation du comportement des assemblages collés : une stratégie de caractérisation

National audienceLes matériaux composites sont de plus en plus utilisés dans le domaine des énergies marines renouvelables en raison de leurs propriétés mécaniques intéressantes : faible masse volumique et propriétés mécaniques élevées. Souvent, ces structures complexes sont obtenues en assemblant plusieurs éléments de natures différentes (matériaux composites et métaux par exemple). Les techniques d'assemblage classiques comme le rivetage, le vissage ou le boulonnage imposent une étape de perçage des matériaux à assembler entrainant à la fois une concentration de contrainte mais favorisant également l'apparition des microfissures. Ces microfissures sont des défauts qui, peuvent potentiellement réduire drastiquement la tenue mécanique de l'assemblage. Le collage structural est pour cette raison considéré de plus en plus comme une solution fiable pour assembler des matériaux de natures différentes. Une part importante des adhésifs utilisés pour le collage structural est constituée de polymères caractérisés par un comportement mécanique fortement non-linéaire, souvent associé à des comportements visqueux et par conséquent, dépendant de la vitesse de sollicitation, de la température ou de la quantité d'eau diffusée dans leur structure. Le choix de l'essai utilisé pour la caractérisation est cruciale à ce stade. Un essai recommandé pour caractériser le comportement mécanique d'assemblage collé jusqu'à rupture, sans être perturbé par les effets de bord, est l'essai Arcan modifié . Cependant, cet essai qui permet de solliciter le joint de colle de manière bi-axiale est difficilement industrialisable. Une alternative possible à l'essai Arcan modifié demeure l'essai Scarf modifié récemment développé au sein de l'IRDL. Cette présentation possède un double objectif : tout d'abord dresser un état des lieux récent sur les dernières avancées en matière de techniques et de moyens de caractérisation des assemblages collés, de modélisations destinées à prédire le comportement macroscopique des assemblages sur le long terme; puis, proposer une stratégie de caractérisation « rapide », destinée à étudier l'effet du vieillissement hydrique

Use of the modified Arcan fixture to study the strength of bonded assemblies for automotive applications

International audienceAdhesive joints are being increasingly used as a good solution to assemble primary structures of metallic and composite materials. Hence, the analysis of the behaviour of adhesively-bonded assemblies under mixed loadings is a critical issue for industry. This article discusses the use of the modified Arcan device to identify the strength of adhesively-bonded metal/metal and metal/composite assemblies. Different assemblies have been tested and their failure envelopes have been determined. Moreover, analyses have also been made of the influence of some crucial parameters on the behaviour of the assemblies in the study such as the composite surface state, the fabric orientation, the humidity and the presence of grease

Numerical simulation of friction stir welding of pure copper plates

International audienceFriction Stir Welding (FSW) is a joining process which is performed at low temperatures, lower than the melting temperature of the base material, thus it is considered a solid state welding process. This feature makes it suitable for copper welding, material whose thermal diffusivity is higher than that of most steel alloys. Large heat losses identified at copper welding by fusion welding processes are thus reduced using FSW process. Because of the shown efficiency and the innovative character of this process, many actions have been initiated in order to optimize it. The aim of this paper is to develop a three-dimensional coupled thermo-mechanical finite element (FE) model of FSW process for pure copper plates using the Coupled Eulerian-Lagrangian (CEL) formulation given in the FE code ABAQUS V6.13. The CEL formulation is one of the few formulations that are capable of handling with such large deformations. The developed numerical model was validated by comparing its results related to the temperatures calculated in the process time with those measured in performed experiments using the same process parameters. This model was capable of simulating the FSW of copper plates and of anticipating the temperature distribution and burrs formation in the weld bead. © Published under licence by IOP Publishing Ltd