The effect of hygrothermal ageing on the mechanical behaviour of fibre reinforced hybrid eco-composite

L’objectif de ce travail est d’étudier l’effet de différents types de vieillissement sur le comportement mécanique en traction de matériaux composites à fibres de lin et de verre et hybrides lin-verre. L’effet de plusieurs types de vieillissement a été d’abord analysé dans le cas de matériaux composites stratifiés à fibres de lin. Cette étude a permis de suivre l’absorption d’eau dans ces matériaux, et de déterminer leurs paramètres de diffusion en utilisant les modèles de Fick 1D et 3D. Ces paramètres ont été estimés en utilisant une procédure itérative développée à l’aide du logiciel Matlab. Ensuite, une caractérisation des propriétés élastiques et ultimes, à partir des essais de traction des matériaux composites non vieillis et vieillis a été réalisée. Dans le but de réduire l’absorption d’eau et d’améliorer les propriétés mécaniques des composites à fibres de lin, l'hybridation des renforts sergé lin-verre a été choisie. Après l’analyse du comportement mécanique des différents matériaux, l’identification des mécanismes d’endommagement est ensuite menée en utilisant une méthodologie associant la technique d’émission acoustique (EA) et des observations microscopiques. Les signaux d’EA ont été analysés, en considérant une analyse multivariable avec le Logiciel Noesis. La dernière partie aborde une 1ère étape de la modélisation des propriétés élastiques des composites hybrides lin-verre. A partir d’une décomposition de la cellule de base, l’application de la théorie des stratifiés a permis de déterminer les propriétés élastiques du composite en fonction de ses constituants (fibres et matrice) en tenant compte de la géométrie de l’armure.The main objective of this work is to study the effect of several ageing processes on the tensile mechanical behaviour of flax-glass fibres non-hybrid and hybrid composite materials. First, the effect of several types of ageing was analysed in the case of flax fibre reinforced composites. This study enabled to determine the diffusion parameters of these materials by using 1D and 3D Fick’s model. For this purpose, an analytical modelling was applied, using optimisation toolbox of Matlab. The second part aims at analysing the tensile mechanical properties of the unaged and aged composite materials. With the aim to improve the mechanical properties as well as the moisture resistance behaviour of flax fibre reinforced composites, hybridation of twill flax-glass fibres was proposed. Next, the Acoustic Emission (AE) technique combined with scanning electron microscopy observations was used to identify the damage events leading to overall failure of the studied composites. The AE signals obtained during mechanical tensile tests were analysed and classified using a non-supervised method with Noesis Software. The last part is devoted to the first step in determining the elastic behaviour of flax-glass hybrid composites. By discretizing the unit-cell of the composite, the use of the classical thin laminate theory made it possible to determine the elastic properties of the composite, considering constituents (resin and fibres) and microstructure geometry

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

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