Effet du vieillissement artificiel sur les paramètres d'un critère de rupture interlaminaire en mode mixte (I+II) d'un matériau composite à armure tissée hybride

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Influence of the scattering of flax fibres properties on flax/epoxy woven ply stiffness

International audienceThe natural character of plant fibres was often cited as a disadvantage which results in the scattering of their properties. This work aims at studying the influence of this scattering on the stiffness of an epoxy-flax fibres twill fabric ply. Elastic coefficients were estimated using an analytical model in an elliptical configuration based on the use of the classical laminate theory. Most of flax fibre elastic coefficients were obtained in the literature; in addition, a flax fibre Poisson's ratio of 0.498 was estimated from an experimentally inverse approach. Then, the use of the analytical model evidenced that the Young's moduli of woven composite were strongly correlated to longitudinal and transversal fibre ones; moreover, the fibre transversal modulus had a significant impact on the Poisson's ratios of the composite. As regards to the yam configuration and architecture, we showed that the mean twisting angle significantly affected stiffness and Poisson's ratio of the composite; in the same way, the importance of waviness described by its waviness ratio was underlined whereas the scattering into yam morphology had low influence on composite performances when the waviness ratio was high. (C) 2017 Elsevier Ltd. All rights reserved

Thermo-mechanical behaviour of flax/green epoxy composites: Evaluation of thermal expansion coefficients and application to internal stress calculation

International audienceThis work aims at evaluating the coefficients of linear thermal expansion (CLTE) of flax/green epoxy unidirectional composites and the CLTE of flax fibre. This required using high precision measuring instrument to experimentally evaluate the CLTE of unidirectional composites with various fibre contents. The flax fibre CLTE were first estimated using an inverse approach with two micromechanical models. From the longitudinal and transverse CLTE, the internal stresses of various symmetric and antisymmetric laminates, due to temperature variation, were then predicted by a 2D analytical model based on classical laminate theory. The inverse approach results showed that the transverse CLTE of flax fibre was positive and estimated at 75 ± 5 × 10 − 6 /K whereas the longitudinal CLTE was negative and equal to-1.2 ± 0.1 × 10 − 6 /K, highlighting the high anisotropy of flax fibres. The internal stress analysis in flax fibre laminates showed that the stacking sequence had a significant effect on the internal stresses, whatever the temperature variation. Regarding stacking the layers, choosing symmetric cross-ply laminate was more interesting than antisymmetric one for minimising the internal stresses. The normal stresses reached their maximum absolute values for the cross-ply laminates, whereas the maximum shear stress occurred in the [0/60] s and [0/60/0/60] stacking. This study highlighted the importance of choosing an optimised stacking sequence, such as the 0/30 • oriented laminates, and a relevant curing cycle prior to the manufacturing process, in order to obtain flax fibre laminates with low internal stresses

Effet de l'humidité sur le comportement mécanique des composites à fibres de lin

International audienceL'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

Influence of hygrothermal ageing on the damage mechanisms of flax-fibre reinforced epoxy composite

International audienceThis paper describes the influence of hygrothermal ageing on mechanical properties and damage behaviour of quasi-unidirectional flax-fibre reinforced epoxy (FFRE) composite. The evolution of water absorption for FFRE composite appears to be Fickian and the kinetics parameters are influenced by the temperature variation. Young’s modulus and tensile strength are clearly affected by the hygrothermal ageing because a significant reduction in Young’s modulus is shown while tensile strength decreases much less for water-saturated composites. The decrease of both properties could be explained by a reorientation of flax microfibrils and the plasticiser effect of water on the matrix, which are both stimulated by moisture absorption. Acoustic emissions analysis combined with scanning electron microscopy enabled investigating the effects of hygrothermal ageing on the process of degradation of flax-fibre composite

Mode-I interlaminar fracture toughness of flax, glass and hybrid flax-glass fibre woven composites: Failure mechanism evaluation using acoustic emission analysis

International audienceThe mode-I interlaminar fracture toughness of flax, glass and hybrid flax-glass fibre woven composites was studied by using a DCB test. The acoustic emission signals recorded during the tests and scanning electron microscope images were used to analyse the damage mechanism of each composite. The crack initiation for the flax-fibre laminate needs the highest energy (1079 versus 945 for hybrid flax-glass fibre and 923 J/m2 for glass-fibre laminates). The morphology of the flax fibres, short and bonded together in bundles to manufacture the twill fabric, allows the creation of a larger amount of fibre bridging as the origin of this highest energy. Furthermore, hybridisation of glass fibres with flax fibres in an appropriate combination offers an interesting solution when the toughness of glass fibre composites needs to be increased. More interesting is the considerable advantage of the composite structure weight reduction due to the low flax fibre density

Caractérisation physique et mécanique du bois et des fibres issus d’une palme mûre de palmier dattier

La disponibilité en quantité importante des sous-produits renouvelable des palmiers dattiers laisse envisager leur utilisation comme renfort dans les matériaux composites et les structures d’isolation. Ceci nécessite la caractérisation physico-mécanique des différentes composantes du palmier. Dans ce contexte, cette étude a été entamée pour la caractérisation d’une composante importante du palmier qui est la palme mûre. La palme étant décomposée essentiellement en deux parties, le Pétiole et le Rachis, la caractérisation a été portée alors sur le bois fibreux et les fibres de ces deux parties. La caractérisation physique concernera la masse volumique des fibres et de la matrice végétale (lignine) ainsi que le taux d’humidité dans les différentes parties de la palme. Les essais mécaniques ont permis de déterminer les caractéristiques mécaniques du bois fibreux et des fibres extraites des deux parties de la palme après le séchage. À l’issue de cette étude, les résultats obtenus montrent l’influence de la position de l’échantillon testé sur les propriétés physico-mécaniques. Ils ont permis également de situer ce type de bois fibreux et ces fibres parmi d’autres résultats de la littérature scientifique et d’envisager de les utiliser dans la mise en œuvre des matériaux composites et dans des éléments d’isolation

Are there similarities between quasi-static indentation and low velocity impact tests for flax-fibre composites?

International audienceFlax-fibre composites are increasingly used as a replacement of classical synthetic composite materials. Due to the good energy absorption properties of flax fibres, they represent a promising alternative in structures susceptible to low velocity impact (LVI) damage. However, this type of dynamic loading is complex, expensive to perform and not necessarily easy to fully investigate. A simpler way to tackle this problem consists in investigating quasi-static indentation (QSI) tests, but this alternative remains relatively under-researched for natural fibre composites. Thus, this paper aims at providing a comparison between both types of loading to facilitate the later analysis and modelling of flax fabric laminates submitted to LVI. Six layers of a flax 2/2 twill fabric were used as reinforcement for epoxy laminates made through vacuum infusion. Specimens were then submitted to instrumented LVI and QSI tests at comparable energy levels, with a 1.5 %–3.9 % difference only. Load-displacement curves and visible damage were first analysed and compared between both test types. Then, the internal damage within QSI specimens were investigated using acoustic emission (AE). Our findings showed good analogies between both testing methods in all the stages of damage development. Great similarities were found in load-displacement curves (in shape, stiffness and peak load), in energy absorption capacity (at 5 and 10 J) and in visible damage. Actually, the differences between QSI and LVI remain low, i.e. 2.1 % for linear stiffness, from 0.2 to 5.6 % for peak load and less than 7% for the proportions of absorbed energy. Comparison of the QSI damage analysed from the AE data with LVI results from literature suggested similar mechanisms and onset sequences. These results revealed that QSI monitoring could provide characteristic indications on the damage evolution of flax-fibre woven composites during an LVI test