Évaluation métrologique d'algorithmes de corrélation d'images numériques

Cet article rend compte d'un travail collectif mene au sein du GT1 du GDR CNRS 2519, Mesure de champs et identification en mécanique des solidesOptical full field measurement techniques based on digital image correlation get more and more popular. We propose a methodology for the assessment of the metrological performances of the image processing algorithms that constitute their central component, the knowledge of which is required for a global assessment of the whole measurement system. It is based on the generation of synthetic images representative of real speckle patterns, deformed according to a sinusoidal displacement field with varying amplitude and period, and the statistical analysis of the discrepancies between imposed and measured values. It has been applied to various softwares used in the french community. Results show general tendencies rather independent from the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the accuracy with the parameters of the algorithms, such as correlation window size or shape functions can be precised

A minimal model for spontaneous cell polarization and edge activity in oscillating, rotating and migrating cells

How the cells break symmetry and organize their edge activity to move directionally is a fun- damental question in cell biology. Physical models of cell motility commonly rely on gradients of regulatory factors and/or feedback from the motion itself to describe polarization of edge activity. Theses approaches, however, fail to explain cell behavior prior to the onset of polarization. Our analysis using the model system of polarizing and moving fish epidermal keratocytes suggests a novel and simple principle of self-organization of cell activity in which local cell-edge dynamics depends on the distance from the cell center, but not on the orientation with respect to the front-back axis. We validate this principle with a stochastic model that faithfully reproduces a range of cell-migration behaviors. Our findings indicate that spontaneous polarization, persistent motion, and cell shape are emergent properties of the local cell-edge dynamics controlled by the distance from the cell center.Comment: 8 pages, 5 figure

Micromechanical modeling of the elastic behavior of unidirectional CVI SiC/SiC composites

International audienceThe elastic behavior of SiC/SiC composite is investigated at the scale of the tow through a micromechanical modeling taking into account the heterogeneous nature of the microstructure. The paper focuses on the sensitivity of transverse properties to the residual porosity resulting from the matrix infiltration process. The full analysis is presented stepwise, starting from the microstructural characterization to the study of the impact of pore shape and volume fraction. Various Volume Elements (VEs) of a virtual microstructure are randomly generated. Their microstructural properties are validated with respect to an experimental characterization based on high definition SEM observations of real materials, using various statistical descriptors. The linear elastic homogenization is performed using finite elements calculations for several VE sizes and boundary conditions. Important fluctuations of the apparent behavior, even for large VEs, reveal that scales are not separated. Nevertheless, a homogeneous equivalent behavior is estimated by averaging apparent behaviors of several VEs smaller than the Representative Volume Element (RVE). Therefore, the impact of the irregular shape of the pores on the overall properties is highlighted by comparison to a simpler cylindrical porous microstructure. Finally, different matrix infiltration qualities are simulated by several matrix thicknesses. A small increase in porosity volume fraction is shown to potentially lead to an important fall of transverse elastic moduli together with high stress concentrations

Mesure 3d de champs cinématiques dans le cas d'un contraste non uniformément réparti.

Les techniques de corrélation d'image numérique ont récemment été étendues afin de pouvoir analyser des séquences d'images tridimensionnelles, obtenues par exemple par microtomographie X. La résolution et la résolution spatiale des champs de déformation mesurés dépendent directement de la répartition, de la taille et du contraste local des marqueurs utilisés pour réaliser l'appariement des images. Lorsqu'il est présent, le contraste naturel issu des hétérogénéités intrinsèques au matériau peut directement fournir ces marqueurs [1]. Lorsque celui-ci fait défaut, des marqueurs supplémentaires peuvent être ajoutés, mais leur répartition spatiale est difficile à maîtriser [2]. Dans les deux cas, le contraste local peut être fortement non-uniforme, contrairement à ce qui peut être obtenu avec un mouchetis de peinture en corrélation d'image de surface. Habituellement le calcul du critère de ressemblance est réalisé sur des sous-domaines espacés régulièrement. Cette discrétisation spatiale homogène facilite le post-traitement des données mais ne permet pas toujours une exploitation optimale du signal. En effet, les domaines de corrélation ne sont a priori pas centrés sur les marqueurs présentant le meilleur contraste, ce qui peut conduire à des erreurs de mesure. Il est proposé ici de déterminer la position des points de mesure (centre des domaines de corrélation) en fonction de plusieurs critères quantifiant la qualité du signal présent dans l'image. Les premières applications seront réalisées sur le matériau présenté dans [2] : des essais de traction sur une éprouvette entaillée en aluminium marqué par des inclusions de cuivre, puis sur le même matériau renforcé avec de larges particules sphériques de Zircone-Silice. Enfin, la déformation d'un alliage d'Aluminium-Cuivre en cours de solidification [3] sera analysée malgré un contraste local peu abondant dans la structure. [1] Bay ,Smith ,Fyhrie ,Saad, Digital volume correlation : three-dimensional strain mapping using X-ray tomography, Experimental Mechanics, 1999. [2] Bornert ,Chaix ,Doumalin ,Dupré ,Fournel, Jeulin, Maire, Moreaud, Moulinec, Mesure tridimensionnelle de champs cinématiques par imagerie volumique pour l'analyse des matériaux et des structures. Instrumentation, Mesure, Métrologie, 2004. [3] Limodin, Salvo, Suery, Dimichiel, In-situ investigation by X-Ray tomography of the overall and local microstructural changes occurring during partial remelting of an Al-15.8wt%Cu alloy, Acta Materialia, 2007

Caractérisation et modélisation du comportement mécanique de matériaux composites SiC/SiC

National audienceLes composites SiC/SiC sont envisagés comme matériaux de coeur des réacteurs nucléaires de 4e génération. Leur comportement mécanique à été étudié par le biais d'essais biaxés, ayant permis la construction et l'identification d'un modèle d'endommagement ainsi que son implantation dans un code éléments finis

MECHANICAL BEHAVIOUR AND RUPTURE IN CLAYEY ROCKS STUDIED BY X-RAY MICRO-TOMOGRAPHY AND 3D-DIGITAL IMAGE CORRELATION

International audienceThe mechanical behaviour and the rupture of clayey rocks have been experimentally studied by performing in situ triaxial tests on a synchrotron beamline i.e. performing X-ray micro tomography scans under mechanical loading. The 3D images obtained at different steps of the test were then analysed by 3D-Digital Image Correlation method in order to measure incremental strain fields. The results allow to clearly detect the onset of shear strain localization and to characterize its development in a 3D complex pattern

A phase field method to simulate crack nucleation and propagation in strongly heterogeneous materials from direct imaging of their microstructure

International audienceIn this work, crack initiation and propagation in 2D and 3D highly heterogeneous materials models, such as those obtained by micro-CT imagery of cementitious materials, is investigated for the first time by means of the phase field method. A shifted strain split operator algorithm is proposed to handle unilateral contact within cracks in a very efficient manner. The various advantages of the phase field method for voxel-based models are discussed. More specifically, we show that the resolution related to the initial image and thus to meshes for discretizing the same microstructure does not significantly affect the simulated crack path

Demodulation of Spatial Carrier Images: Performance Analysis of Several Algorithms Using a Single Image

http://link.springer.com/article/10.1007%2Fs11340-013-9741-6#Optical full-field techniques have a great importance in modern experimental mechanics. Even if they are reasonably spread among the university laboratories, their diffusion in industrial companies remains very narrow for several reasons, especially a lack of metrological performance assessment. A full-field measurement can be characterized by its resolution, bias, measuring range, and by a specific quantity, the spatial resolution. The present paper proposes an original procedure to estimate in one single step the resolution, bias and spatial resolution for a given operator (decoding algorithms such as image correlation, low-pass filters, derivation tools ...). This procedure is based on the construction of a particular multi-frequential field, and a Bode diagram representation of the results. This analysis is applied to various phase demodulating algorithms suited to estimate in-plane displacements.GDR CNRS 2519 “Mesures de Champs et Identification en Mécanique des Solide

Hardening description for FCC materials under complex loading paths

International audienceThe present work aims at exploring self and latent hardening for FCC polycrystals under complex loading paths at room temperature. Combinations of simple loading paths sequences, such as tension and simple shear, with different orientations with regard to rolling direction, are considered. Experimental results are compared to finite element computations of polycrystalline aggregates taking into account the material microstructure, and to simulations based on mean field models

Comparison of experimental results and finite element simulation of strain localization scheme under cyclic loading

International audienceThe plastic behaviour of FCC materials is studied under cyclic tensile–compression loading at room temperature. The material is an oxygen-free high conductivity copper. The purpose of the work is to model the onset of plasticity, then the cycle by cycle evolution of the localized strain, at grain scale and at mesoscopic scale. A polycrystalline aggregate taking into account the material microstructure is developped to perform finite element simulations corresponding to the experiments. Finite element calculations are carried out on this mesh, using a constitutive law which takes into account the crystallographic orientation of each grain. An analysis of the localisation scheme is performed at different steps of the cyclic loading