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International audience; Users of full-field measurement methods like Digital Image Correlation (DIC) often aim to perform measurements with the best trade-off between spatial resolution, bias and measurement resolution. Whenever two full-field methods are compared, it is essential that these criteria are taken into consideration. Recently a metrological efficiency indicator for full-field measurements has been proposed and discussed. This indicator combines measurement resolution and spatial resolution. It has been shown to be invariant to the subset size in the case of Local DIC. The goal of this article is to discuss a method, which determines both the spatial and the measurement resolutions for a given bias for two different DIC methods, in order to obtain the metrological efficiency indicator for each of these methods. The benefit of this indicator is that it does not depend on setting parameters such as the subset size, which are chosen by the user. As such, it can be considered as intrinsic to each technique, thus enabling fair comparison. Local DIC and triangular finite element based Global DIC will be the subject of this investigation. With this setting, their respective subset and triangular element sizes will be related to the spatial resolution of both methods for a given acceptable bias. By using the metrological efficiency indicator, the performance of the two methods will be compared and discussed to a new level of detail. Generally speaking, the indicator shows that the metrological performance of both methods is similar, confirming their popularity. However, it will be shown that, depending on the choice of what an acceptable bias is, one of the method may be preferred to another. The results show that for the specific DIC versions used in the study, for cases for which a significant bias is acceptable, Local DIC outperforms Global DIC, while the opposite is true in the case for which the bias requirements are more stringent. Finally, the quadratic versions of both DIC versions are shown to significantly outperform their respective linear versions.

International audience

International audience

International audience; This paper introduces and validates a new prediction of sensor noise propagation from images of randomly marked surfaces to displacement maps obtained by Digital Image Correlation (DIC). Images are indeed often affected by sensor noise, which propagates to DIC output. We consider here the 2D global DIC (G-DIC), for which this output is the in-plane displacement calculated at a set of nodes. Predictive formula for the resolution of the displacement at these nodes is already available in the literature. The contribution of the present paper is to revisit this formula to take into account the interpolation required by sub-pixel displacement. A generalization is also proposed to predict the displacement resolution throughout the field of view. It is then extended to several kinds of DIC. The correlation procedure is thoroughly described in order to emphasize the role of the interpolation. A numerical assessment on synthetic data validates the new prediction and shows the improvement brought about by the proposed formula.

International audience; The grid method is a technique suitable for the measurement of in-plane displacement and strain components on specimens undergoing a small deformation. It relies on a regular marking of the surfaces under investigation. Various techniques are proposed in the literature to retrieve these sought quantities from images of regular markings, but recent advances show that techniques developed initially to process fringe patterns lead to the best results. The grid method features a good compromise between measurement resolution and spatial resolution, thus making it an efficient tool to characterise strain gradients. Another advantage of this technique is the ability to establish closed-form expressions between its main metrological characteristics, thus enabling to predict them within certain limits. In this context, the objective of this paper is to give the state of the art in the grid method, the information being currently spread out in the literature. We propose first to recall various techniques that were used in the past to process grid images, to focus progressively on the one that is the most used in recent examples: the windowed Fourier transform. From a practical point of view, surfaces under investigation must be marked with grids, so the techniques available to mark specimens with grids are presented. Then we gather the information available in the recent literature to synthesise the connection between three important characteristics of full-field measurement techniques: the spatial resolution, the measurement resolution and the measurement bias. Some practical information is then offered to help the readers who discover this technique to start using it. In particular, programmes used here to process the grid images are offered to the readers on a dedicated website. We finally present some recent examples available in the literature to highlight the effectiveness of the grid method for in-plane displacement and strain measurement in real situations.

International audience; The predictive formula of resolution displacement maps obtained using Digital Image Correlation tools are revisited and validated in this paper. Indeed, the usual formula available in the literature does not take into account the DIC required sub-pixel interpolation. Moreover, a noiseless DIC tangent operator is defined to also minimise noise propagation from images to displacement maps. These points are now taken into account in the proposed update of the predictive formula. The experimental validation is then done, it is shown that specific image pre-processing tools are required. Usual raw images are indeed corrupted by noise and consequently do not satisfy assumptions needed for predictive formula elaboration. Thanks to the pre-process presented here, image noise properties are corrected, thus assumptions of the predictive formula are satisfied and the prediction is validated.

International audience; L'objectif est ici d'introduire un nouvel outil de caractérisation de lois cohésives, modèle le plus couramment utilisé pour décrire et simuler le phénomène de délaminage. Cet outil est basé sur une méthode de corrélation d'image globale, où la définition de l'espace de recherche cinématique est adéquatement choisie. Après avoir présenté le principe de cette méthode, on s'intéresse à sa validation, ainsi qu'à son comportement au bruit. Cette validation est réalisée numériquement, (i) en construisant un ensemble d'images à l'aide d'un outil de simulation évolué du délaminage et (ii) en identifiant à partir de ces images les caractéristiques du comportement de l'interface.

This work presents a general method providing good control on the discretization error on a local quantity of a finite element solution. Formulated using a general formulation, this method is illustrated in a 2D case of elastoplasticity. Nonlinear part of this implementation has raised issues of a new type in the subject of verification. Mirror and the central problems are defined, and we offer solutions for both.The implementation of this tool is introduced in detail. Thus, the guarantees bounds of the local error on a component of plastic deformation are calculated. An initial study on academic case is presented before focusing on a more complex one. Finally an improved method is introduced, allowing the calculation of more relevant bounds.; Ce travail présente une méthode générale fournissant des bornes garanties de l'erreur de discrétisation sur une quantité locale issue d'un calcul éléments finis. Formulée dans un cadre général, la méthode est illustrée sur un cas 2D d'élastoplasticité. Le cadre non-linéaire de cette implémentation a soulevé des problèmes d'un type nouveau au sein de la thématique de vérification. Après avoir défini les problèmes miroir et central, nous proposons des solutions pour les résoudre.La mise en place de l'outil introduit est détaillée. Ainsi, des bornes garanties de l'erreur locale sur une composante de la déformation plastique sont calculées. Une première étude sur des cas académiques est présentée avant de s'intéresser à un cas plus complexe. Enfin une amélioration de la méthode est introduite, permettant l'obtention de bornes plus pertinentes.

This work presents a general method providing good control on the discretization error on a local quantity of a finite element solution. Formulated using a general formulation, this method is illustrated in a 2D case of elastoplasticity. Nonlinear part of this implementation has raised issues of a new type in the subject of verification. Mirror and the central problems are defined, and we offer solutions for both.The implementation of this tool is introduced in detail. Thus, the guarantees bounds of the local error on a component of plastic deformation are calculated. An initial study on academic case is presented before focusing on a more complex one. Finally an improved method is introduced, allowing the calculation of more relevant bounds.; Ce travail présente une méthode générale fournissant des bornes garanties de l'erreur de discrétisation sur une quantité locale issue d'un calcul éléments finis. Formulée dans un cadre général, la méthode est illustrée sur un cas 2D d'élastoplasticité. Le cadre non-linéaire de cette implémentation a soulevé des problèmes d'un type nouveau au sein de la thématique de vérification. Après avoir défini les problèmes miroir et central, nous proposons des solutions pour les résoudre.La mise en place de l'outil introduit est détaillée. Ainsi, des bornes garanties de l'erreur locale sur une composante de la déformation plastique sont calculées. Une première étude sur des cas académiques est présentée avant de s'intéresser à un cas plus complexe. Enfin une amélioration de la méthode est introduite, permettant l'obtention de bornes plus pertinentes.

National audience; See http://hal.archives-ouvertes.fr/docs/00/59/28/99/ANNEX/r_7KV1JDJU.pdf