Even though the simulations used to predict failure are becoming increasingly predictive, complex multiaxial loading tests are still required to validate the design of structural components in a wide range of industries. Large specimen testing often requires two different scales. A global Far Field to obtain boundary conditions and a local Near Field to evaluate strain gradients around discontinuities such as bolts, notches & mldr; The main goal of this study is to provide a continuous displacement over the whole specimen surface integrating data from multiple cameras. In this paper, we propose a new methodology that generates 3D displacements determined by finite-element stereo digital image correlation in the Near Field and in the Far Field using a unique fractal speckle pattern and an off-line determined texture. The displacements are obtained in the same coordinate system and on the same mesh. Satisfactory data fusion from both Near Field and Far Field images of a biaxial test on a notched laminate composite was obtained with a refined mesh at the notch tip. This methodology can be applied to any tests requiring multiple camera systems and will support the use of the finite-element digital image correlation framework as an experimental-numerical efficient technique.

Even though the simulations used to describe the failure of laminates are becoming more and more predictive, complex testing under multiaxial loadings is still required to validate the design of structural parts in a wide range of industrial domains. It is thus essential to assess the actual boundary conditions to allow for an objective comparison between testing and calculations, in particular since the structural tests are complex and often leads to buckling. Therefore, accurate estimation of force and moment fluxes applied to the specimen is critical. In this context, stereo digital image correlation (SDIC) has proven to be an important measurement tool and provides very well-resolved surface displacement fields, but the exploitation of such measurements to calculate fluxes remains problematic when testing composites. The first objective of this study is both to reduce the uncertainty associated with fluxes determination on a complex test and to simplify the extraction process with respect to existing procedures. The second objective is to make this methodology robust to geometrically non-linear deformations. In this paper, we propose a new methodology that extracts minimal boundary conditions in the form of 3D mechanically admissible displacements fields. The approach developed uses a finite element SDIC (FE-SDIC) method regularized by means of mechanical behaviour admissibility equations. Results show that the new methodology outputs much more accurate fluxes than classical data generated from multiple differentiations of the displacement fields. Excellent noise robustness is obtained and quantified. Numerical predictions have been satisfactorily compared with experimental data from one structural-scale composite specimen under complex testing.

In order to characterize the mechanical behavior of a reinforced concrete beam–column joint, an experiment is designed whereby different histories of increasing cyclic loads are applied, and the structure deformation (as well as the morphology of apparent surface cracks) is studied using multi-view correlation. The complex three-dimensional geometry and the slender nature of the beam and column call for numerous digital cameras were positioned around the specimen. Assessing the feasibility of image acquisition and estimating a priori uncertainties on multi-view correlation became a true challenge. It is shown that the recourse to photo-realistic rendering software provides the needed tools to perform this experiment design optimization.

This paper focuses on the development of fully-coupled 3D thermomechanical field measurement techniques applied for monitoring thermal fatigue tests. First, an original hybrid and multiview system composed of one infrared (IR) camera and two visible light cameras is introduced. The spatial registration of multimodal imaging devices is solved successfully using global Hybrid Multiview Correlation (HMC) based on the NURBS representation of the 3D calibration target and the surface of interest. The measurement uncertainties are estimated with an initial heating up phase prior to the fatigue test. Then, HMC is performed to measure the 3D surface displacement and temperature fields during laser shocks onto an austenitic stainless steel plate. Last, the HMC measurements are validated in comparison with finite element simulations of the test.

A novel approach which combines isogeometric collocation and an equilibrium-based stress recovery technique is applied to analyze laminated composite plates. Isogeometric collocation is an appealing strong form alternative to standard Galerkin approaches, able to achieve high order convergence rates coupled with a significantly reduced computational cost. Laminated composite plates are herein conveniently modeled considering only one element through the thickness with homogenized material properties. This guarantees accurate results in terms of displacements and in-plane stress components. To recover an accurate out-of-plane stress state, equilibrium is imposed in strong form as a post-processing correction step, which requires the shape functions to be highly continuous. This continuity demand is fully granted by isogeometric analysis properties, and excellent results are obtained using a minimal number of collocation points per direction, particularly for increasing values of length-to-thickness plate ratio and number of layers.

This paper introduces a cost-effective strategy to simulate the behavior of laminated plates by means of isogeometric 3D solid elements. Exploiting the high continuity of spline functions and their properties, a proper out-of-plane stress state is recovered from a coarse displacement solution using a post-processing step based on the enforcement of equilibrium in strong form. Appealing results are obtained and the method is shown to be particularly effective on slender composite stacks with a large number of layers. These are indeed the cases where traditional (e.g., "layerwise") approaches are more computationally heavy and where researchers are more inclined to look for alternatives, making the proposed method a very attractive solution.

Wrinkling is to be avoided in the restoration of works on canvas. In the worst cases, it may lead to paint cracking. Visual rendering of the paint may also change because of such mechanism. It is proposed to measure wrinkling under various conditions of contrast and loading via isogeometric stereocorrelation. This method allows low contrasted paints to be analyzed in addition to very large deformations that occur in punch tests on canvas.

Three dimensional displacement fields are measured thanks to an original stereo system made of InfraRed (IR) and visible light cameras. This configuration provides the thermal fields that drive the thermomechanical loading and the 3D surface displacement fields, i.e. the response of the studied material. The calibration of the stereo rig is performed by using a 3D target of known geometry and the large gray level variations that occur on the IR images are accounted for by applying a low-pass filter. Because of a slight disorientation of the target, a further calibration correction step revealed necessary. After correction of the target orientation, the measured displacement fields display a good qualitative agreement (no quantitative comparison has been performed yet) with the expected fields.

The aim of the present study is to measure complex shapes of tested objects by using a priori information given by their CAD model via stereocorrelation. To follow a 3D object during its deformation and to determine 3D surface displacement fields, a first measurement of the object shape is necessary. It is achieved by updating the CAD reference via a global approach to stereocorrelation. Once the 3D shape has been determined, the next step is to measure 3D displacement fields during loading. The kinematics of the deformed shape is assumed to be written within the same isogeometric framework. Isogeometric stereocorrelation is applied to analyze a compression test on a ribbed cylinder in two different configurations of the stereo rig.

A mesh-based framework is developed by extending global stereocorrelation techniques to faceted surfaces with three-noded elements. A two-step self-calibration procedure is followed to determine the projection matrices of the stereo-rig and to update the nominal surface model to match the surface of interest. To prove the feasibility of mesh-based stereocorrelation, two different test parts are analyzed with the present techniques and compared to already validated optical procedures.

DIC can be coupled with computational tools in order to characterize materials by using identification techniques such as finite element model updating or integrated approaches. In this study a framework using CAD-based stereo-DIC coupled with Isogeometric Analyses is followed to implement such identification procedures. Using both techniques allows us to be consistent with the designed geometry and its kinematics as the NURBS formalism is kept during the whole process and fewer degrees of freedom are needed (for the displacement field and the geometric representation of the surfaces) than in classical (finite element) approaches. This technique can be adapted to be written within an integrated framework (whose sensitivity fields are given by an isogeometric code).

Thermal shocks are applied to a 304L austenitic stainless steel plate with a pulsed laser. A stroboscopic reconstruction is used for infrared (IR) and visible light camera measurements. The displacement fields are measured with a digital image correlation (DIC) technique. Different IR devices are used to measure the temperature variations (i.e. medium wave camera and short wave pyrometry). Several ways of determining the emissivity or absorptivity are discussed. The complete 3D thermal loading is numerically determined by minimising the difference between experimental measurements and finite element analyses of thermal fields. An elastoplastic model is then used to compute mechanical fields that are compared with DIC measurements.

The acquisition of images with different modalities may involve different alterations with respect to an ideal model. Inhomogeneous brightness and contrast, blur due to non-ideal focusing, distortions are common. It is proposed herein to account for such effects for instance by registering a calibration target image with an actual optical image to measure lens distortions. An Integrated Digital Image Correlation (I-DIC) algorithm is proposed to account for the above artifacts and the algorithm is detailed. The resolution and uncertainty of the technique are first investigated on synthetic images, and then applied to the measurement of distortions for infrared (IR) images. The procedure is shown to reduce drastically the residual level assessing the validity of the image formation model, but more importantly allowing for a much improved registration of images.

3D kinematic fields are measured using an original stereovision system composed of one infrared (IR) and one visible light camera. Global stereocorrelation (SC) is proposed to register pictures shot by both imaging systems. The stereo rig is calibrated by using a NURBS representation of the 3D target. The projection matrices are determined by an integrated approach. The effect of gray level and distortion corrections is assessed on the projection matrices. SC is performed once the matrices are calibrated to measure 3D displacements. Amplitudes varying from 0 to 800 μm are well captured for in-plane and out-of-plane motions. It is shown that when known rigid body translations are applied to the target, the calibration can be improved when its actual metrology is approximate. Applications are shown for two different setups for which the resolution of the IR camera has been modified.

A new displacement measurement technique is proposed in a stereovision setup, which uses the object of interest as the support of the correlation process. This procedure leads to a global approach to stereocorrelation. The method is presented in its general formulation and is then particularized to the case of non uniform rational B-splines (NURBS). The displacement field is directly measured as a 3D field expressed in a NURBS basis consistent with the existing geometric model. The kinematic measurements are validated against prescribed displacements of a machined Bézier patch. The feasibility in an industrial context is shown with the analysis of 3D displacement fields of a 2- m2 automotive roof panel during a welding operation.

The aim of this study is to measure displacement fields during an experiment on a structure by using a priori information about its surface representation in a stereoDIC framework. The main goal of the present method is to measure the displacement and strain fields in formalisms completely consistent with the surface description via global stereoDIC. Although the majority of stereoDIC methods provide clouds of 3D displacement vectors associated with clouds of 3D points, which are subsequently interpolated to get continuous fields, the proposed CAD-based stereoDIC allows continuous 3D displacement fields to be measured directly. Therefore, there is a direct link between the measured fields and computational results since common shape and kinematic bases are considered. No additional interpolation or data manipulation is needed with such approaches. One practical example will illustrate the use of this novel technique.

It is proposed to develop a multiview framework to perform stereocorrelation by resorting to isogeometric descriptions of the observed three-dimensional surfaces. Once the three-dimensional surfaces have been reconstructed, the corresponding three-dimensional displacement fields are measured within the same space. These fields are subsequently utilized to update isogeometric analyses to calibrate the elastic properties in a tensile experiment. An alternative route consists of preforming such analyses within an integrated framework.

Ces travaux de these sont dedies a la mesure de formes 3D, de champs cinematiques 3D surfaciques et a l'identification de parametres mecaniques a partir de mesures par correlation d'images. Cette methode optique se base sur l'utilisation de cameras comme support de la mesure. Pour cette raison, une etude des differents modeles de camera classiquement utilises est effectuee, puis le passage d'un formalisme continu pour la description des images a une approche discrete utilisant la notion de pixel est decrit. Une attention particuliere est portee aux distorsions optiques et une approche utilisant la correlation d'images pour les evaluer est developpee. Une nouvelle methode de mesure de forme 3D et de champs de deplacements 3D surfaciques par stereo-correlation d'images est introduite. Elle utilise un modele numerique de l'objet observe comme support pour la correlation. Cette methode mene a une formulation globale de la stereo-correlation. Elle peut etre ecrite dans le cas general et particularisee a des surfaces NURBS (non-uniform rational B-splines). Le champ de deplacement est egalement exprime dans un formalisme NURBS completement coherent avec le modele geometrique utilise pour definir la forme observee. Des mesures sont validees en utilisant des mouvements imposes a un carreau de Bezier usine. La faisabilite de la technique dans plusieurs cas industriels est egalement etudiee avec, par exemple, la mesure des deplacements d'une piece composite de train d'atterrissage chargee mecaniquement. Enfin, a partir de cette formulation isogeometrique de la mesure de champs, une etude de la caracterisation de proprietes elastiques est effectuee. Deux methodes inspirees d'approches existantes sont ainsi proposees, utilisant des mesures de champs et des calculs numeriques dans un formalisme isogeometrique pour identifier des parametres de loi de comportement elastique isotrope sur un cas test numerique et pour un essai de traction uniaxiale.