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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.

In this paper, we use isogeometric Kirchhoff plates to approximate composite laminates adopting the classical laminate plate theory. Both isogeometric Galerkin and collocation formulations are considered. Within this framework, interlaminar stresses are recovered through an effective post-processing technique based on the direct imposition of equilibrium in strong form, relying on the accuracy and the higher continuity typically granted by isogeometric discretizations. The effectiveness of the proposed approach is proven by extensive numerical tests., 28 pages, 11 figures, 13 tables

International audience; 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.

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.

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.

International audience; 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 elds 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 dierent congurations of the stereo rig.

International audience; 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.

International audience; 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.

International audience; 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.

International audience; 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. 1. Introduction Thermal fatigue may occur in pipes of nuclear power plants due, for instance, to the turbulent mixing of two fluids that have different temperatures. To study the material and structure response to constrained temperature variations, several experimental setups have been designed in different laboratories. However the temperature measurements have only been punctual via thermocouples and out of the zone of interest to prevent crack initiation on the connexion with the sensor.[1–6] Based on these measurements , the mechanical equivalent strain variation in the crack initiation region is evaluated thanks to numerical thermomechanical simulations. The number of cycles to crack initiation under such an equivalent strain is then compared with the number of cycles to failure in classical isothermal uniaxial fatigue tests. In many cases, it appears that this number of cycles for crack initiation is lower in thermal fatigue, namely, that crack initiation predictions in thermal fatigue based on classical isothermal fatigue tests would be non-conservative.[5] Some mechanical hypotheses can be proposed to explain such discrepancies, but they will not be discussed herein. In this paper, an effort is made to better estimate the experimental variations of temperature and strain fields. A new testing setup is proposed where thermal shocks are applied with a pulsed laser beam while the thermal and kinematic fields on the specimen surface are respectively measured with infrared (IR) and visible light cameras.

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).

International audience; 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.

Proceedings of the 2016 Annual Conference on Experimental and Applied Mechanics; International audience; 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 which 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.

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 Bezier 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.

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.

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.

Optical aberrations are one of the biases affecting images. Their correction can be performed as a transformation of the image. In this paper, it is proposed to use digital image correlation to calculate the distortion fields. A correlation calculation is performed between a known numerical pattern and a picture of the same pattern printed or etched on a plate. This new procedure is applied to determine the distortion fields of a camera lens. First, a parametric description of the distortion field is used to reduce the degrees of freedom required to describe optical distortions. Second, a non-parametric model based upon splines is used. The distortion fields found by both methods are compared. A resolution analysis is performed for two different procedures (removing the lens between pictures or not). The standard displacement resolution is found to be of the order of 10-2 pixel in the first case, and 2.5 × 10 − 3 pixel in the second case.

International audience; A new calibration procedure is proposed for a stereovision setup. It uses the object of interest as the calibration target, provided the observed surface has a known de nition (e.g., its CAD model). In a rst step, the transformation matrices needed for the calibration of the setup are determined assuming that the object conforms to its CAD model. Then the 3D shape of the surface of interest is evaluated by deforming the a priori given freeform surface. These two steps are performed via an integrated approach to stereoDIC. The measured 3D shape of a machined Bézier patch is validated against data obtained by a coordinate measuring machine. The feasibility of the calibration method's application to large surfaces is shown with the analysis of a 2-m2 automotive roof panel.

The aim of this study is to measure the (complex) shape of an object by using a priori information given by its CAD representation in a 3D-DIC framework. In order to follow a 3D object during its deformation and to determine 3D surface displacement fields, a first measurement of the object shape is necessary. The main goal of the present method is to obtain a CAD representation of this measurement by updating the CAD reference via a global approach to 3D-DIC. Although in the majority of the shape measurement methods a cloud of points is obtained and subsequently interpolated to create a continuous description of the surface, CAD-based stereoDIC is devised to directly measure a 3D shape described by NURBS. This approach allows a direct link to be established between the theoretical CAD model and its true geometry thereby yielding the metrology of the measured surface in the CAD language.