Study of image characteristics on digital image correlation error assessment

In this paper, errors related to digital image correlation (DIC) technique applied to measurements of displacements are estimated. This work is based on the generation of synthetic images representative of real speckle patterns. With these images, various parameters are treated in order to determine their impact on the measurement error. These parameters are related to the type of deformation imposed on the speckle, the speckle itself (encoding of the image, image saturation) or the software (subset size)

Shear Test on CFRP Full-Field Measurement and Finite Element Analysis

The purpose of this work is to study the Iosipescu shear test and more precisely its ability to characterize the shear modulus of a carbone/epoxy composite material. The parameters influencing this identification are the fibre orientation, the geometry of the notch and the boundary conditions. Initially these parameters were studied through the finite element analysis of the shear test. Then, the measurement of the shear strains was carried out by traditional methods of measurement (strain gauges) but also by optical methods. These optical methods: the digital image correlation and the electronic speckle pattern interferometry (ESPI); allow for various levels of loading, to reach a full-field measurement of the shear strain. This enabled us to study the strain distribution on the section between the two notches. The finite element model enabled us to study the parameters influencing the calculation of the shear modulus in comparison with strain gauges, image correlation and ESPI. This work makes it possible to conclude on optimal parameters for the Iosipescu test

Identification of materials properties using displacement field measurement

The aim of this work is to identify parameters driving constitutive equations of materials with displacement field measurements carried out by image stereo-correlation during an unidirectional tensile test. We evaluate two identification techniques. The first one is the virtual fields method which consists in writing the principle of virtual work with particular virtual fields. It is generally used in the case of linear elasticity and it requires a perfect knowledge of the model in terms of boundary condition since the virtual fields used must be kinematically admissible. This method allows to determine parameters by a direct and fast calculation, without iterations. The second method is the finite element model updating method. It consists in finding constitutive parameters that achieve the best match between finite element analysis quantities and their experimental counterparts. This method is more adaptable than the virtual field method but it needs to spend more calculation time

Développement de méthodes d'intégration des mesures de champs

Les mesures optiques dimensionnelles sont des techniques en plein essor dont la maîtrise et l'exploitation soulèvent encore de nombreuses questions. Pour une meilleure compréhension d'un système de mesure par stéréo-corrélation d'images, des études de caractérisation et d'évaluation de l'erreur de mesure en corrélation à partir d'images synthétiques et en stéréovision à partir d'images réelles ont été réalisées. Les résultats mettent en avant l'influence de plusieurs paramètres : fenêtre de corrélation, déformation et gradient de déformation. La dernière partie de l'étude est consacrée à l'identification de comportements constitutifs à partir des mesures de champs. Deux méthodes sont mises en œuvre : la méthode des champs virtuel pour l'identification des paramètres élastiques d'un matériau et l'identification paramétrique par recalage éléments finis pour le cas des comportements élasto-plastiques. ABSTRACT : The optical dimensional measurements are emergent techniques whose control and exploitation still address many questions. For a better understanding of the stereo-correlation measurement systems, studies are made to characterize and assess the digital image correlation measurement error by the way of synthetic images. The stereovision characterization is made using real images. The results highlight the influence of several parameters : subset size, strain and strain gradient. The last part of this study is devoted to the identification of constitutive behaviour law using full-field measurements. Two identification methods are used : the virtual fields method to determine the elastic parameters of a material and the finite element model updating method to identify the elasto-plastic behaviour law

Digital image stereo-correlation applied to the identification of elastomers

As a complement to classic characterization in the identification of elastomers, full-field measurement methods such as image stereo correlation can undeniably contribute to the validation of elastomeric connection behaviour. Indeed, these measurements enable three-dimensional displacement and strains fields over the totality of the studied surface to be finely reached. Characterization tests in shearing on an elastomer were followed by image stereocorrelation up to high strain levels. The experimental results could thus be confronted with theoretical results using an Alexander hyperelastic behaviour law, which enabled the relevance of this law to be validated locally

Global and local identification on composte material

One of the main challenges in composite design and development is to be able to compute the damage state at any point of a composite structure during a complex loading. In this context, knowledge of the spatial distribution of the material elastic properties provides a quantitative estimation of the damage level and localization. The work presented here intends to access to such distribution through the association of kinematic fields and finite element model updating method. Such iterative technique has many interesting advantages for structural analysis and industrial requirements, including the numerical framework, the ability to explore complex shapes and loads and a treatment based on surface measurement without any assumption on volume distribution

Global behaviour of a composite stiffened panel in buckling. Part 2: Experimental investigation

The present study analyses an aircraft composite fuselage structure manufactured by the Liquid Resin Infusion (LRI) process and subjected to a compressive load. LRI is based on the moulding of high performance composite parts by infusing liquid resin on dry fibres instead of prepreg fabrics or Resin Transfer Moulding (RTM). Actual industrial projects face composite integrated structure issues as a number of structures (stiffeners, …) are more and more integrated onto the skins of aircraft fuselage. A post-buckling test of a composite fuselage representative panel is set up, from numerical results available in previous works. Two stereo Digital Image Correlation (DIC) systems are positioned on each side of the panel, that are aimed at correlating numerical and experimental out-of-plane displacements (corresponding to the skin local buckling displacements of the panel). First, the experimental approach and the test facility are presented. A post-mortem failure analysis is then performed with the help of Non-Destructive Techniques (NDT). X-ray Computed Tomography (CT) measurements and ultrasonic testing (US) techniques are able to explain the failure mechanisms that occured during this post-buckling test. Numerical results are validated by the experimental results

Stéréo-corrélation d'image : application aux vibrations d'une paroi mince en usinage

La réalisation de pièces d'aubage ou de certaines structures aéronautiques, présentant des parois minces, engendre généralement des vibrations d'usinage. Ces phénomènes dégradent l’état de surface obtenu et réduisent la durée de vie d'outil ou de broche. Une démarche expérimentale visant à préciser les vibrations de paroi mince pendant l’usinage est présentée ici. Des mesures de champs par stéréo-corrélation d’image associées à des mesures fréquentielles ponctuelles par accéléromètres, mettent en évidence le comportement de la paroi en fraisage radial

Identification of elastomers by digital image correlation

As a complement to classic characterization in the identification of elastomers, full-field measurement methods such as digital image stereo correlation can undeniably contribute to the validation of elastomeric connection behavior. Indeed, these measurements enable three-dimensional displacement and strains fields over the totality of the studied surface to be finely reached. Characterization tests in shearing on an elastomer were followed by image stereo correlation up to high strain levels. The experimental results could thus be compared with theoretical results using an Alexander hyper-elastic constitutive law, which enabled the relevance of this law to be validated locally

Identification du comportement en fatigue bi-axiale de composite stratifié

La fatigue est un enjeu majeur dans la conception des structures en composites. Elle correspond à l'application d'un chargement répété sur la structure. Il a été montré à de nombreuses reprises dans la littérature, qu'une pièce mécanique pouvait rompre en fatigue alors que le chargement appliqué est inférieur à la charge critique à rupture statique. La durée de vie de la structure est ainsi dépendante de l'intensité du chargement appliqué, ainsi que du nombre de cycles réalisés. La difficulté de prédiction de l'évolution de l'endommagement sous une sollicitation cyclique pousse généralement les concepteurs à sur-dimensionner les structures, en utilisant des coefficients de sécurité importants. De nombreuses études ont donc été menées pour prédire l'endommagement des composites face à de telles sollicitations. Divers modèles et critères de rupture en fatigue ont été élaborés et sont couramment utilisés pour le dimensionnement de structures. Pour étudier le comportement d’un composite plastique renforcé de fibres de carbone sous un chargement bi-axial, un nouveau type d’éprouvette cruciforme a été conçu. Cette éprouvette est soumise à un chargement bi-axial statique et cyclique en fatigue. Les essais seront suivis simultanément en utilisant la stéréo-corrélation d’images numériques (DIC), et la thermographie infrarouge. Une comparaison entre les mesures expérimentales et un modèle éléments finis est réalisée pour valider le design de l’éprouvett