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In this paper, the evolution of damage when curving a substrate with an alumina layer has been studied. A sample preparation method has been used, so that substrate is free from any stress due to the adhesive insulation during the anodization of one side. The curvature evolution as a function of alumina layers thickness from 50 to 200 μm on aluminum substrates from 0.5 to 1.4 mm thick has been observed. Crack initiations start around film defects and spread transversely and longitudinally to eventually intersect. SEM examination of the anodized layers has shown that the stresses involved by curving create cracks up to 3 μm wide to provide accommodation of substrate-coating assembly. An approach has been adopted to describe the different stages of defects appearance with respect to stresses evolution. Due to internal stress effects, cracks continue to propagate and secondary ones are created in order to release the stored elastic energy in the interface substrate-alumina.

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International audience; Plant-based reinforced polymer often present non linear behavior under tensilesolicitation. It is clearly the case for flax fiber reinforcement for which a yield point separates the composite behavior in two regions. This is visible on experimental curves inarticles that do not deal only with unidirectional reinforcement but also with reinforcement by random mat of flax. In a previous work we proposed a viscolastoplastic model to study the non linear effects

International audience; The experienced prepreg flax/epoxy consists of twisted flax yarns coming from textile industry. It is mainly used in sports and leisure industry. The required added-value is the good capacity of vibration damping. A first study showed that the behavior of the flax/ epoxy is not just elastic and that its resistance is not optimal. We reached at best case 350 MPa where the mixtures law look forward to the double! The objective in the long term of the work consists in the optimization of composite prepreg. We studied the influence of the torsion of the streak by comparing a WOVEN comprising a weft yarn for 19 warp yarn , with an UD, an UD SEWN with 45° and a MAT having a privileged orientation. The best result is obtained for WOVEN. It is believed that the wet spinning optimizes the separation of elementary fibers. We studies the influence of textile treatment by comparing an UNBLEACHED 2/2 twill with the same LEACHED (WASHED), BLEACHED and MERCERIZED fabric. The best result is obtained for MERCERIZED fabric.

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International audience; Flax fibre reinforced polymers are at the heart of current scientific and societal concerns. However their mechanical behaviour is still poorly known despite significant scientific efforts, in particular the non-linear behaviour observed under tensile tests. In this paper, results of hardening, creep and repeated progressive tests under uniaxial loading performed on various quasi-unidirectional flax fibre reinforced composites are analysed in terms of the volume fraction of fibre, the titration of yarn reinforcement and temperature testing. To identify the respective parts of pure elastic, viscoelastic and viscoplastic components in the global mechanical response, a phenomenological model is built based on eight independentparameters. The parameter identification based on experimental data and checked by a sensitivity analysis shows that the viscoelastic effects are non-significant at room temperature. The non-linear behaviour can mainly be attributed to viscoplastic effects. Viscoplastic modelling is based on a combination of two hardenings; the first classical (linear) one describes the translation of the elastic domain ; the second (non-linear) one describes a translation coupled with a contraction of the elastic domain during loading that improves the unloading behaviour during repeated progressive loading tests.

International audience; We evaluate the elastic properties of glass, carbon, and flax fibers that are used to manufacture composite materials. The transverse elastic properties of fibers are more difficult to measure than the longitudinal properties. But the knowledge of the transverse properties is mandatory to predict correctly the mechanical behavior of a composite material. We apply the laser resonant ultrasound spectroscopy (RUS) technique to evaluate the transverse Young modulus and Poisson ratio of fibers. The mechanical vibrations of a cross section are generated by a pulsed subnanosecond laser beam focused and shaped on the fiber as a line parallel to the fiber. The vibrations are detected optically by the probe beam of an interferometer, focused at the zone of excitation. The measured eigenfrequencies range from 5 MHz for flax fibers to 500 MHz for carbon and glass fibers. Using finite element modeling, the eigenmodes of fibers are identified and the mechanical properties of the fiber materials are evaluated. The transverse elastic properties of carbon fibers are significantly different from the longitudinal properties. For flax fibers, the strong damping of modes which is observed could explain the strong damping of flax fiber reinforced polymers.

International audience; L'utilisation de matériaux composites à matrice Polyester renforcés par des fibres de verre soulève de plus en plus des questions tant au niveau sanitaire qu'environnementale. Une alternative consiste à développer des composites à fibres végétales avec des propriétés mécaniques élevées et un impact environnementale faible. Pour un meilleur développement de ces composites, il est nécessaire d'avoir une information supplémentaire sur leur durabilité. Ce travail présente les résultats expérimentaux de l'effet du vieillissement en eau des matériaux composites à fibres de lin. L'étude expérimentale a été effectuée sur deux stratifiés (unidirectionnel et sergé). Dans un premier temps, les propriétés mécaniques ont été identifiées en fonction de la durée du vieillissement. Dans la deuxième étape, les effets des interactions entre le vieillissement et les mécanismes d'endommagement ont été étudiés avec les techniques d'émission acoustique.

The aim of the study is the determination of the mechanical properties of thin films by optical metrology and numerical analysis. The centers of interest are the residual stresses on the one hand and the tensile test on the other hand. The optical methods used are interferometric fringes projection (contouring) and numerical speckle photography.Contouring gives the out-of-plane displacements. Residual stresses in thin films are measured by courbure analysis and/or bulging test. We conclude by three experimental works:1. the validity limits of the Stoney's formula even with free standing bilayers (SiO2/Si);2. the evidence of buckling in compressed bilayer membranes (SiO2/Si);3. a good agreement between bulging tests and nanoindentation tests for film in a state of residual tension (polimiide).Numerical speckle photography gives the in-plane displacements field. We adapted this method to a tensile test for thin film that has been fabricated during the PhD. An original algorithm with subpixel displacement sensivity has been developped and has been fully compared, by simulations and experiments, with classical correlation methods. The rapidity of the algorithm and its high resolution allows the calculation of the displacements fields during the tensile test. The in-plane strains are then calculated ans allows us to obtain the stress/strain relations and to calculate Young's modulus E and Poisson's ratio v. To validate the tensile test, we have tested two kinds of materials: the first one is ductile (electrodeposited copper) and the second one is fragile (monocristal of silicon)., La thèse est consacrée à la caractérisation mécanique des films minces à l'aide de mesures de déplacements par voie optique et traitements numériques. Les domaines d'études sont d'une part les contraintes résiduelles et d'autre part l'essai de traction. Les moyens de mesure employés sont la projection de franges et la photographie numérique de speckle.La projection de franges permet de mesurer la carte des déplacement hors-plan. Elle est adaptée à la mesure des contraintes résiduelles dans les films minces par analyses de courbure et/ou essais de gonflement. On montre suite à trois études expérimentales :1. la précaution à prendre dans l'utilisation de la formule de Stoney même pour des bicouches libre-libre (SiO2/Si) ;2. la présence de flambage dans les membranes bicouches comprimées encastré-encastré (SiO2/Si) ;3. un parfait accord entre essais de gonflement et essais de nano-indentation pour des films tendus (polimiide).La photographie numérique de speckle permet de mesurer la carte des déplacements dans le plan. Elle est adaptée avec succès à un banc d'essais de traction conçu durant la thèse et spécialement dédié aux films minces. Un algorithme original de calcul a été développé pour obtenir une résolution subpixel. Les performances de l'algorithme sont comparées par des simulations et des expériences aux algorithmes classiques de corrélation. Sa rapidité et sa haute résolution permettent de calculer les cartes de déplacements en temps réel. Les déformations dans le plan en sont déduites et permettent le tracé des courbes contrainte/déformation puis le calcul du module d'Young E et du coefficient de Poisson v. Les matériaux choisis pour valider le banc d'essai sont le cuivre électrodéposé, matériaux ductile, puis le silicium monocristallin, matériaux fragile.

The aim of the study is the determination of the mechanical properties of thin films by optical metrology and numerical analysis. The centers of interest are the residual stresses on the one hand and the tensile test on the other hand. The optical methods used are interferometric fringes projection (contouring) and numerical speckle photography.Contouring gives the out-of-plane displacements. Residual stresses in thin films are measured by courbure analysis and/or bulging test. We conclude by three experimental works:1. the validity limits of the Stoney's formula even with free standing bilayers (SiO2/Si);2. the evidence of buckling in compressed bilayer membranes (SiO2/Si);3. a good agreement between bulging tests and nanoindentation tests for film in a state of residual tension (polimiide).Numerical speckle photography gives the in-plane displacements field. We adapted this method to a tensile test for thin film that has been fabricated during the PhD. An original algorithm with subpixel displacement sensivity has been developped and has been fully compared, by simulations and experiments, with classical correlation methods. The rapidity of the algorithm and its high resolution allows the calculation of the displacements fields during the tensile test. The in-plane strains are then calculated ans allows us to obtain the stress/strain relations and to calculate Young's modulus E and Poisson's ratio v. To validate the tensile test, we have tested two kinds of materials: the first one is ductile (electrodeposited copper) and the second one is fragile (monocristal of silicon).; La thèse est consacrée à la caractérisation mécanique des films minces à l'aide de mesures de déplacements par voie optique et traitements numériques. Les domaines d'études sont d'une part les contraintes résiduelles et d'autre part l'essai de traction. Les moyens de mesure employés sont la projection de franges et la photographie numérique de speckle.La projection de franges permet de mesurer la carte des déplacement hors-plan. Elle est adaptée à la mesure des contraintes résiduelles dans les films minces par analyses de courbure et/ou essais de gonflement. On montre suite à trois études expérimentales :1. la précaution à prendre dans l'utilisation de la formule de Stoney même pour des bicouches libre-libre (SiO2/Si) ;2. la présence de flambage dans les membranes bicouches comprimées encastré-encastré (SiO2/Si) ;3. un parfait accord entre essais de gonflement et essais de nano-indentation pour des films tendus (polimiide).La photographie numérique de speckle permet de mesurer la carte des déplacements dans le plan. Elle est adaptée avec succès à un banc d'essais de traction conçu durant la thèse et spécialement dédié aux films minces. Un algorithme original de calcul a été développé pour obtenir une résolution subpixel. Les performances de l'algorithme sont comparées par des simulations et des expériences aux algorithmes classiques de corrélation. Sa rapidité et sa haute résolution permettent de calculer les cartes de déplacements en temps réel. Les déformations dans le plan en sont déduites et permettent le tracé des courbes contrainte/déformation puis le calcul du module d'Young E et du coefficient de Poisson v. Les matériaux choisis pour valider le banc d'essai sont le cuivre électrodéposé, matériaux ductile, puis le silicium monocristallin, matériaux fragile.

International audience; This paper describes a numerical method applied to speckle photography to measure the inplane displacement field on a thin film and specially adapted to microtensile test. Speckle photography is chosen to avoid touching and stressing the specimen. A Spectral Phase Algorithm particularly useful in the case of small displacement or/and strain is compared with the Intercorrelation and Double FFT methods. A final algorithm using first the Intercorrelation method without interpolation and secondly the Spectral Phase Algorithm is then adopted to estimate with good accuracy the in-plane displacement field in quasi-real time during the microtensile tests. A first application of the method to the tensile tests performed on thin films of electroplated copper of 18, 35 and 70 µm of thickness is then presented.