Identification de lois de comportement élastique et viscoélastique de panneaux structuraux à base de bois

La détermination rigoureuse des caractéristiques mécaniques est nécessaire pour réduire les coefficients de sécurité utilisés pour les applications techniques de panneaux. Les démarches usuelles pour identifier le comportement élastique ou viscoélastique des matériaux sont basées sur des essais statiquement déterminés comme la flexion, la traction, la compression ou le cisaillement. Dans ces essais, l'état de contraintes est connu aux points de mesure et déterminé uniquement par les conditions limites. Le prix de cette simplicité est qu'on ne peut déterminer qu'une composante viscoélastique de la matrice des rigidités à la fois. De nouvelles méthodes, basées sur la résolution de problèmes inverses, permettent la détermination de plusieurs composantes en même temps. Dans ces essais, le champ des contraintes dépend non seulement des conditions limites, mais aussi de la loi de comportement. Par cette dépendance, il est possible d'augmenter le nombre des mesures et d'accéder à ces composantes. Cependant il convient d'optimiser l'essai pour activer toutes les rigidités qu'on souhaite identifier. Notre méthode contient trois phases : - La recherche de l'essai optimal pour un panneau donné, - L'essai, - L'identification des caractéristiques mécaniques. Nous développerons l'approche pour concevoir les essais : l'optimisation par algorithme génétique. Nous décrirons le dispositif expérimental développé pour mesurer le champ des déplacements via une métrologie optique. Ce dernier et les conditions limites étant connus, le choix des fonctions viscoélastiques pour identifier les paramètres mécaniques a une grande influence sur les temps de calcul (éléments finis). Afin de vérifier la pertinence de cette approche, des résultats préliminaires ont été obtenus par simulation numérique. On montre que la qualité des résultats dépend de l'optimisation de l'essai. Finalement, plusieurs essais d'identifications élastiques et viscoélastiques ont été menés sur des panneaux de contre-plaqués pour valider l'ensemble de cette approche.Rigorous determination of elastic and viscoelastic mechanical characteristics is necessary for reducing the safety factors used for technical applications of panels. Usual investigations to identify elastic and viscoelastic materials rely on statically determined tests as bending, tension, compression or shear. In these tests, the stress is known at the location of measurement points, and determined only by the boundary conditions. Price of simplicity is that one can only find one component of the viscoelastic stiffness matrix. New methods, based on inverse problem solving, allow the determination of several components by one test only. In these tests, the stresses field depends on the boundary conditions, but also on the constitutive equations. Through this dependance, it is possible to increase the number of measurements and access these components. It is necessary to optimise the tests in order to activate these components. Our method contains three phases : - The design of an optimal test for a given panel, - The experiment, - The identification of mechanical characteristics. After a brief description of the inverse problem theory, we will focus on a new approach used to design these tests, i.e. genetic optimization. Once the test is designed, one needs optical measurement methods to access the whole field of deformation. These methods will be described. From the tests, the displacements field and the applied forces are known. To identify the mechanical parameters, the choice of the viscoelastic functions, which have a very strong influence on the computing time of viscoelastic finite elements programs, have to be made. In order to verify the pertinence of this approach, preliminary results were obtained by simulated elastic identifications. The results quality depends on the optimization of tests. Finally, several tests are made, using plywood panels, to validate this new approach

On the use of simulated experiments in designing tests for material characterization from full-field measurements

The present paper deals with the use of simulated experiments to improve the design of an actual mechanical test. The analysis focused on the identification of the orthotropic properties of composites using the unnotched Iosipescu test and a full-field optical technique, the grid method. The experimental test was reproduced numerically by finite element analysis and the recording of deformed grey level images by a CCD camera was simulated trying to take into account the most significant parameters that can play a role during an actual test, e.g. the noise, the failure of the specimen, the size of the grid printed on the surface, etc. The grid method then was applied to the generated synthetic images in order to extract the displacement and strain fields and the Virtual Fields Method was finally used to identify the material properties and a cost function was devised to evaluate the error in the identification. The developed procedure was used to study different features of the test such as the aspect ratio and the fibre orientation of the specimen, the use of smoothing functions in the strain reconstruction from noisy data, the influence of missing data on the identification. Four different composite materials were considered and, for each of them, a set of optimized design variables was found by minimization of the cost function

Identification de lois de comportement élastique et viscoélastique de panneaux structuraux à base de bois

BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

General Anisotropy Identification of Paperboard with Virtual Fields Method

This work extends previous efforts in plate bending of Virtual Fields Method (VFM) parameter identification to include a general 2-D anisotropic material. Such an extension was needed for instances in which material principal directions are unknown or when specimen orientation is not aligned with material principal directions. A new fixture with a multi-axial force configuration is introduced to provide full-field strain data for identification of the six anisotropic stiffnesses. Two paper materials were tested and their Q ij compared favorably with those determined by ultrasonic and tensile tests. Accuracy of VFM identification was also quantified by variance of stiffnesses. The load fixture and VFM provide an alternative stiffness identification tool for a wide variety of thin materials to more accurately determine Q 12 and Q 66