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Numerous barriers remain to develop biobased composites in mechanical parts with high requirements (aeronautics, automotive, etc.). The prime objective of this thesis concerns the multi-scale study of the composite and its flax fiber reinforcement and to study its strain rate sensitivity at various scales. The specific application targeted is a long range drone. These researches are motivated by a lack of bibliographical data on the strain rate sensitivity of this material and non-exhaustive existing behavior models. The flax fiber alone already has a composite microstructure with a complex behavior, different from a synthetic fiber and which deserves a particular characterization. This work allows to understand and characterize the influence of this microstructure on the behavior of the composite at the macroscopic scale and to propose adequate homogenization models. Moreover, a constitutive model taking into account the strain rate sensitivity at the composite scale is proposed over a broad range of speeds from quasi-static to dynamic.; De nombreux verrous persistent pour développer des composites biosourcés dans des pièces mécaniques à hauts niveaux d’exigences (aéronautique, automobile, etc.). L’objectif principal de cette thèse concerne ainsi l’étude multi-échelle du composite et de son renfort en fibres de lin et d’étudier sa sensibilité à la vitesse de déformation à différentes échelles. L’application spécifique visée est un drone longue endurance. Ces recherches sont motivées par un manque de données bibliographiques sur la sensibilité à la vitesse de ce matériau et des modèles de comportement existants non-exhaustifs. La fibre de lin seule a déjà une microstructure composite avec un comportement complexe, différent d’une fibre synthétique et qui mérite ainsi une caractérisation particulière. Ces travaux permettent ainsi de comprendre et de caractériser l’influence de cette microstructure sur le comportement du composite à l’échelle macroscopique et de proposer des modèles d’homogénéisation adéquats. De plus, un modèle constitutif prenant en compte la sensibilité à la vitesse de déformation à l’échelle du composite est proposé sur une large plage de vitesses du quasi-statique au dynamique.

Vegetable fibers are increasingly used to substitute glass fibers for polymer strengthening to make eco-composites. Indeed, in addition to the fact that vegetal fibers and glass fibers present equivalent specific properties, the use of the former allows to valorize bio-based and local resources while lightening the overall weight and reducing the cost of composites Therefore, all these advantages provide them to be more and more used in the transport industry (aeronautics, automotive, etc.), and this is expected to increase in light of current environmental issues and the emerging context of the bio-economy aimed at continuing economic growth while preserving the environment and earth resources.However, one of the main difficulties arising when using these natural compounds as polymer matrix reinforcement is their incompatibility with a large part of the latter. Indeed, the hydrophilic nature of vegetal fibers makes them sensitive to moisture absorption and difficult to wet by hydrophobic resins (which represent the majority of polymer matrices used). Thus, to obtain the optimal mechanical performance of these composites, it is necessary to compatibilize these fibers with the polymer matrix to avoid the formation of cavities during the manufacturing process, which would greatly weaken the composite. Nowadays, several chemical and/or physical methods were developed with the aim to achieve this compatibilization (acetylation, alkaline treatment, treatment using peroxides, etc.). However, these techniques are generally harmful to the environment through the use of toxic products and solvents, and can even degrade the initial material (and consequently the mechanical properties).It is in this context that fluorination treatment takes place. Indeed, a treatment under molecular fluorine (F2) of wood or vegetal fibers allows to covalently graft fluorine atoms on the outmost surface of lignocellulosic material surface in substitution of hydroxyl groups responsible for hydrophilicity, and this in a fast and controlled reaction. This grafting, (proved, inter alia, by FT-IR spectroscopy and 19F NMR) allowed to significantly increase the hydrophobicity of fibers without modification of their bulk’s mechanical performance as it is only located on the material’s surface. This treatment thus reduces the gap between the surface energies of the fibers and the matrices; in other words, the wetting of the fiber by the polymer is improved. Thereby, porosity of the composite thus formed is significantly reduce, increasing its mechanical performance, without any chemical coupling agent harmful to humans and the environment.; Les fibres végétales sont de plus en plus utilisées comme bio renfort de matériaux composites, car elles permettent non seulement de valoriser des ressources naturelles, locales et renouvelables, mais aussi de diminuer le poids global de ces matériaux ainsi que leur coût, pour des propriétés mécaniques spécifiques équivalentes aux fibres de verre. En outre, l’utilisation de ces matériaux étant plus respectueuse de l’environnement que l’usage de fibres synthétiques, elle s’inscrit parfaitement dans la problématique environnementale du XXIème, avec l’idée de fabriquer des « éco-composites », présentant une faible empreinte écologique.Toutefois, l’une des principales difficultés rencontrées lors de l’utilisation de ces composés naturels comme renfort de matrice polymère est leurs incompatibilités avec une grande partie ces dernières. En effet le caractère hydrophile des fibres végétales les rend difficiles à mouiller par les résines hydrophobes (qui représentent la majorité des matrices polymères utilisées) induisant une porosité micrométrique qui fragilise le matériau. Ainsi, pour obtenir les performances mécaniques optimales de ces composites, une comptabilisation de ces fibres avec la matrice est nécessaire. À l’heure actuelle, plusieurs méthodes chimiques et/ou physiques sont employées (acétylation, traitement alcalin, peroxydes, etc.). Cependant, elles s’avèrent généralement nocives pour l’environnement de par l’emploi de produits et solvants toxiques, ce qui va à l'encontre de l'idée même de réaliser des éco-composites, censés être plus respectueux de l'environnement.C’est dans ce contexte que le traitement de fluoration intervient. En effet, un traitement sous fluor moléculaire F2 de matériaux ligno-cellulosiques naturels tels que les fibres de lin a permis de greffer de manière covalente des atomes de fluor en substitution de groupements hydroxyles, responsables de l’hydrophilie, de manière rapide et contrôlée. Ce greffage, prouvé, entre autres, par spectroscopie infra-rouge, RMN du 19F et XPS, permet de réduire significativement l’hydrophilie des fibres, sans pour autant dégrader leurs performances mécaniques, car étant uniquement localisé en surface. Ce traitement permet ainsi de réduire l’écart entre les énergies de surface des fibres et des matrices ; en d’autres termes, le mouillage de la fibre par le polymère est amélioré, diminuant significativement la porosité du composites formés à partir de ces renforts fluorés et in fine, augmentant ses performances mécaniques, sans agent de couplage chimique nocif pour les Hommes et l’environnement.

CICE 2021, 10th International Conference on FRP Composites in Civil Engineering, Istanbul, TURQUIE, 08-/12/2021 - 10/12/2021; Flax fiber reinforced polymer (FFRP) composites are demonstrating promising outcomes which makes them potential candidates to replace synthetic composites in various industrial applications. However, there is limited information regarding their long-term performance, and it is usually acknowledged that natural fibers are less resistant than their synthetic counterparts. In this context, it is crucial to study their durability before considering their use for structural rehabilitation and strengthening in construction. This study aims to investigate and predict the performance of FFRP composites with a bio-based epoxy matrix.The test program consisted in exposing FFRP laminates and FFRP strengthened concrete slabs to different accelerated ageing conditions over a total period of 2 years, and with various combinations of temperature and relative humidity in the ranges 20-60 °C and 50-100% RH, respectively. Series of tensile, short beam and pull-off tests were periodically performed on ageing samples in order to evaluate their property evolutions over exposure time in the various environments. Finally, collected experimental data were analyzed using statistical tools, in view of developing a degradation model and evaluating the service lifetime performance of this new bio-based composite.

International audience; Flax fiber appears as a suitable feedstock in the endeavor of deploying a sustainable biobased economy. Its environmental performance as reinforcement in composite materials has been studied in previous Life Cycle Assessments (LCAs). However, these studies only present a coarse Life cycle Inventory (LCI) and often fail to detail all processes of the supply chain or to represent the co-products. This paper aims to bridge this gap and provide data for future LCAs on flax fiber production and transformation.The study focuses on the impacts of producing a bio-based reinforcement material (a fabric product for non-aesthetic purposes) with a system expansion perspective. The functional unit is defined as the production of 2400 m2 flax-based technical textile per year, this corresponds to 1 ha of cultivated land. The geographical scope considers that the production occurs in France and that some manufacturing process are outsourced in China. A Sensitivity Analysis was carried out to assess the influence of the electricity mix in the various countries involved in the manufacturing cycle.A detailed life cycle inventory for flax fiber production and transformation was built and the environmental performance of a flax technical textile was assessed as a cradle-to-gate LCA. The fate of co-products was documented and was shown to contribute to the reduction of the generated environmental impacts.Through a cradle-to-gate LCA, a broader understanding of the environmental performance of a flax-based technical textile was presented by including the valorization of co-products and a wider set of analyzed impact categories, going therefore beyond the existing state-of-the-art. Results show agricultural activities and electricity production to be the biggest contributors to the environmental impacts of flax technical textile; contributions due to land use changes were minor in comparison. Very specifically for this case study, a sensibility analysis showed the influence of an all-French production to be more efficient from an environmental point of view.

Increasing environmental concern has put forward the use of flax fibers instead of glass fibers in composite materials. However, durability performances of these bio-fibers remain one of their main issues. This study focuses on the hydrothermal aging of flax fibers. Flax tows were immersed in distilled water at a temperature of 80 °C for different durations. The effect of the hydrothermal aging on mechanical properties of flax tows was evaluated. Results showed a strong decrease in the maximal strength and the stiffness by 31% and 49%, respectively, until one week of aging. Multi-scale analyses were realized to explain these evolutions. Morphological characterization highlighted a washing of fiber surfaces during the hydrothermal aging, extracting amorphous components as pectins, lignins and amorphous hemicelluloses from cortical tissues. We showed that this morphological evolution impacted the fiber crystallinity. Based on a mechanical analysis, we showed that amorphous components extraction may be at the origin of the material softening. Moreover, the amorphous phase in particular the natural binder pectin would play a major role in the fiber stiffness but does not modify the flaws at the origin of failure.

International audience; Normandy is the world's leading producer of flax fiber (Linum usitatissimu L.) which is mainly exported to China. Flax is a spring cultivated plant which thrives in an oceanic climate, regularly watered and with limited thermal excesses. While the reduction in cycle length due to global warming would help counter the exposure to water scarcity in early summer, thermal conditions could be unfavourable to this crop production, particularly due to the greater occurrence of hot days.; La Normandie est la première région au monde productrice de fibre de lin (Linum usitatissimu L.), majoritairement exportée à destination de la Chine. Le lin est une plante cultivée au printemps qui affectionne un climat océanique, régulièrement arrosé et aux excès thermiques limités. Si la réduction de la durée du cycle liée au réchauffement climatique permettrait de contrer l’exposition au manque d’eau du début de l’été, les conditions thermiques pourraient être défavorables à cette production végétale, notamment à cause de l’occurrence plus importante des jours de chaleur.

International audience; This article reports on the results of a collaborative work aiming at improving the mechanical performances of composites based on flax fabrics and epoxy thermosets or silicone materials. Radiation processing was assessed as a simple pretreatment to modify the surface composition of commercially available fabrics. Two approaches were explored after analysis of the chemical features of the two types of composites.A first method designed for epoxy thermosets is based on the immobilization of bio-based adhesion promoters, partially epoxidized fatty oils exhibiting carbon-carbon unsaturations and epoxy groups expected to act as coupling agents after simultaneous EB radiation-grafting process.The second method was based on the peroxidation by EB-irradiation in air of the flax fabrics selected for the reinforcement of vinylsiloxane-based silicone materials. The thermal decomposition of peroxyl groups during the curing of a silicone matrix including vinylsiloxane units was indeed expected to generate covalent links between the fibers and reactive silicone formulations.The advantage and limitations of the two methods are discussed based on mechanical testing of composite specimens fabricated with the original fabrics, or after implementation of the radiation-induced pre-treatments. The efficiency of the proposed grafting processes is supported by XPS at the different stages of the modification process, as well as by SEM observation of fractured samples. The spectroscopic signature of adhesion promoters for the epoxy composites or of models of the silicone matrix was observed after radiation-promoted immobilization and extensive washing of fabric samples.

International audience; The purpose of the present work is to investigate experimentally the mechanical fatigue behavior of unidirectional flax and carbon fiber reinforced composites. Static and fatigue bending tests were realized on laminates made of flax and carbon fibers impregnated with an epoxy resin. Displacement-controlled bending fatigue tests were conducted on standard specimens with a mean displacement of 50% of the ultimate failure displacement. Specimens were subjected to an applied displacement level of 70% until 104 cycles. A comparison of the fatigue properties of the flax and carbon fiber composites was made. The stiffness degradation curves show that carbon composites have higher resistance than flax composites. Hysteresis cycles, dissipated energy and loss factor evolution were measured and discussed. The results obtained show that the flax fiber composites have a damping property higher than the carbon fiber reinforced composites under fatigue bending tests.

International audience; Ever growing applications of flax fiber-reinforced composites (FFRCs) and their suitability for structural uses involve implementing the design and failure criteria for these composites. Translaminar fracture is one of the primary failure modes of unidirectional (UD) fiber-reinforced composites, and measuring it is essential for design purposes and in many material failure studies. However, the translaminar fracture parameters have not been evaluated for UD-FFRCs; thus, there is no data available in the literature. Moreover, conventional test methods for this failure mode are complex, and there is no standard method for measuring this value in compression. In this study, the translaminar fracture behavior of a UD flax/epoxy composite has been examined, and its fracture toughness in tension and compression in the fiber direction are determined following existing standard meth- ods as well as using a methodology developed using infrared thermography. Compact tension specimens were adapted and used for this purpose. A fractographic study is conducted to examine the fracture surfaces and better understand the failure mechanisms. For tensile tests, the results of infrared thermography are in good agreement with those of ASTM E1922 and lie in the range of values obtained for similar composites in the literature.

The search for alternative solutions to the use of composites wholly or partly derived from renewable resources represents a major environmental challenge and a lever for economic development for future generations. The present study considers the choice of a bio-based composite made from flax fibers associated with a thermoplastic resin. The objective of the research is to analyze the overall mechanical behavior, to identify the main observed physical phenomena, then to analyze their associated mechanisms. An experimental analysis of the mechanical behavior in static and fatigue of unidirectional and cross laminate composites is presented. The techniques of the acoustic emission and micro-meso-macroscopic observations are the main means used for the characterization of the damage phenomena. Then, an experimental study of the vibratory behavior of the composite beams is realized. At first, the dynamic properties of stratified, healthy and damaged composites were determined. Namely, the flexural modulus and the loss factor are evaluated from the frequency responses obtained in linear mode. In a second step, the nonlinear vibration behavior of healthy and damaged composites is explored. Finally, an analytical model that takes into account the components specific to elasticity, plasticity, damage and coupling parameters is identified and used to simulate the overall mechanical behavior of the composite material. Predictive numerical simulations are proposed to evaluate the effect of impact stresses at low speeds / energies on the mechanical behavior of the studied composites.; La recherche de solutions alternatives à l’utilisation de composites issus totalement ou partiellement de ressources renouvelables représente un enjeu environnemental majeur et un levier de développement économique pour les générations futures. La présente étude considère le choix d’un composite biosourcé élaboré à partir de fibres de lin associées à une résine thermoplastique. L’objectif des travaux de recherche consiste à analyser le comportement mécanique global, à identifier les principaux phénomènes physiques constatés, puis à analyser leurs mécanismes associés. Une analyse expérimentale du comportement mécanique en statique et en fatigue des composites stratifies unidirectionnels et croisés est présentée. Les techniques de l’émission acoustique et d’observations micro-méso-macroscopiques sont les principaux moyens utilisés pour la caractérisation des phénomènes de l’endommagement. Ensuite, une étude expérimentale du comportement vibratoire des poutres composites est réalisée. Dans un premier temps, les propriétés dynamiques des composites stratifiés, sains et endommagés ont été déterminées. À savoir le module de flexion et le facteur de perte sont évalués à partir des réponses en fréquences obtenues en mode linéaire. Dans un second temps, le comportement vibratoire non linéaire des composites sains et endommagés est exploré. Finalement, un modèle analytique qui tient compte des composantes propres à l’élasticité, à la plasticité, à l’endommagement et des paramètres de couplage est identifié et utilisé pour simuler le comportement mécanique global du matériau composite.

We have investigated the mechanisms of accelerated aging (immersion in water at 23°C or 70°C) and natural aging (exposure to weather for 24 months) of flax fiber rovings, unsaturated polyester and the resulting composite made by infusion. The correlation of these results is used to check if the usual accelerated aging tests developed for synthetic fiber-reinforced polymers are suitable for plant fiber-reinforced composites in the long term. The role of bark residues appears crucial for the durability of plant fiber biocomposites., Nous étudions les mécanismes de vieillissement accéléré (immersion dans l’eau portée à 23°C ou 70°C) et de vieillissement naturel (exposition aux intempéries pendant 24 mois) de mèches de fibres de lin, d’un polyester insaturé et du composite résultant fabriqué par infusion. La corrélation de ces résultats est utilisée pour évaluer si les conditions hygrothermiques usuelles de vieillissement des composites à fibres synthétiques sont encore adaptées pour prévoir le comportement de ces biocomposites sur le long terme. Le rôle des résidus d’écorces apparaît crucial pour la durabilité des biocomposites à fibres végétales.

International audience; Drawing is the industrial process of refining a roving of fibers so as to obtain a predetermined smaller and more homogeneous cross-section. The ensuing morphological changes are correlated with reductions in the mechanical properties. In this work, we investigate the influence of successive drawings on the morphology of flax fiber rovings. The kinetics of shortening and cross-section reduction of fiber bundles in the rovings are determined by analyzing population changes (length or cross-section) before and after each drawing. It appears that the reduction in cross-section of fiber bundles is more pronounced than is their shortening. Furthermore, drawing greatly reduces the content of the rovings in residues not eliminated by combing, which contributes to improve their impregnation with polymers.; L'étirage est le processus industriel qui consiste à affiner un ruban de fibres, c'est à dire obtenir une section prédéterminée plus faible et la plus homogène possible. Les changements morphologiques qui s'ensuivent sont corrélés à des réductions des propriétés mécaniques du ruban. Dans ce travail, nous étudions l'influence des étirages successifs sur la morphologie de rubans de fibres de lin issus du peignage. Les cinétiques de raccourcissement et de réduction de section des faisceaux de fibres dans les rubans sont déterminées en analysant les variations de population (en longueur ou en section) avant et après chaque étirage. Il ressort que la réduction de section des faisceaux de fibres est plus prononcée que l'est leur raccourcissement. Par ailleurs, ce processus réduit fortement la teneur des rubans en résidus corticaux restant après le peignage, ce qui contribue à améliorer leur imprégnation par les polymères.

Carbon fiber reinforced polymer CFRP has been attracting to repair concrete structures in the last 40 years due to its high mechanical performances and low weight. However using CFRP is still an expensive option and does not match with sustainable development. This paper aims to compare both effects of CFRP and flax fiber reinforced polymer FFRP on repairing by confining mechanically damaged cylindrical concrete specimens under compression. External use of flax fiber is investigated experimentally and analytically in order to check how far it can replace CFRP to repair concrete structures. For this purpose, two compressive damage levels are applied to cylindrical concrete specimens before confining them with three different types of composites. Results showed that using two layers of FFRP composites for repairing damaged concrete lead to slightly lower performances than those when using CFRP ones. FFRP composites allowed not only enhancing properties of damaged concrete strength by 150%, but also giving a great ductility to confined concrete specimens.

International audience

International audience; The aim of this study is to show the interest of the mechanical and dynamical properties of glass-flax hybrid composites. Therefore, various staking sequences of glass-flax hybrid composites were manufactured and tested in free vibrations. The damping coefficients were identified by fitting the experimental responses of free-free bending vibrations. The obtained results show that the staking sequences and the position of flax fiber layers in the hybrid composites changed the properties, so a classification of different stacking sequences was established. In fact, the hybrid laminate made of two glass external layers placed on both sides of four flax layers is very interesting in term of its mechanical and damping properties. Indeed, it showed better specific bending modulus and loss factor than glass composites with proportions of 31 and 39%, respectively. A study of a structure of this composite has been made to validate the obtained results.

This work presents the experimental results of the effect of water ageing in flax fibers reinforced greenpoxy resin matrix composite materials. The experimental study was carried out on two cross-ply laminates ([0/90]s and [+45/−45]s). First, the water uptake of the two composite materials was studied. Second, the mechanical properties were identified as a function of the immersion time. The results obtained show that the Young's modulus of composites decreases progressively to 26% for [0/90]s specimens and 46.36% for [+45/−45]s specimens after 30 days of ageing. Finally, the effects of interactions between water ageing and cyclic fatigue behavior were studied. We consider stiffness loss, hysteresis curves, energy dissipation and loss factor versus number of cycles as a function of different immersions time. A significant decrease in the mechanical characteristics of the composites as a function of the immersion time is observed. [+45/−45]s specimens are more sensitive to both fatigue and water ageing phenomena.

International audience; The purpose of the present paper is to study the effect of hybridation, stacking sequences and fiber orientation on the damping properties of unidirectional carbon/flax fiber reinforced epoxy composites. Non-hybrid and hybrid laminates with different stacking sequences were produced by molding vacuum process. Free vibration tests with an impulse technique were performed on test specimens to investigate the dynamic behavior. Finite element analysis was used to model damping to evaluate the different energies dissipated in the material layer directions of the carbon/flax composites. Close agreement was found between the experimentally measured values and those derived from the numerical simulation for the damping coefficients. The results obtained show that flax layers had a significant effect on damping properties.

International audience; Bio materials have seen an increase of interest from the scientific community and the industry as a possible future generation of mass produced materials, some of the main arguments being their renewability, low production costs and recyclability.The current work is focused on the experimental data required for the viscoelastic characterization of a composite material. Similar work has been conducted on different types of composite materials by Tuttle and Brinson [1] who verified for a carbon epoxy laminate the possibility of long term predicament of creep. Nordin et al [2] studied paper impregnated with phenol-formaldehyde under compression. Muliana [3] conducted experiments on E-glass/vinyl ester materials. Behavior characterization was based on a model presented by Schapery [4]. The main objective of this work is to understand the mechanical behaviors of bio-laminates structures subjected to long and severe operating conditions.The studied material is a bio composite laminate consisting in long flax fibers embedded in an epoxy resin system. The laminates were obtained from pre-impregnated unidirectional fibers, which were cured though a thermo-compression cycle followed by a post curing cycle. Test specimens were cut down to sizes, with the help of an electric saw. The concerned fiber direction was 0° with sample dimensions of 250x25x2 mm.First, testing consisted in quasi static mechanical tests. Second, to characterize linear viscoelastic behavior of the bio-laminates, creep – recovery tests with multiple load levels have been performed for the chosen fiber direction.

The distribution of lignin, 8-5′ and 8-8′ linked lignin substructure, and noncellulosic polysaccharides in hemp (Cannabis sativa L.) phloem fibers were explored based on histochemical and immunological methods. Ultraviolet absorption and potassium permanganate staining were observed mainly in the compound middle lamella (CML) and S1 layers, and rarely in the G-layer of phloem fibers, suggesting that lignin concentration is high at the CML and S1 layers, and very low at the G-layer of hemp fibers. Acriflavine staining, uniform KM1 labeling (8-5′ linked lignin substructure), and no KM2 labeling (8-8′ linked structure) were observed in the G-layer, suggesting that there is a small amount of lignin-like compound with 8-5′ linked structure in the G-layer. In addition, some fiber cells showed a multilayered structure. Uniform arabinogalactan protein (AGP) labeling was observed on the S1 layers and G-layers using JIM14, but little appeared in the CML of hemp fibers, indicating that these layers of the phloem fibers contain AGP. Immunogold labeling of xylan (LM11) and glucomannan (LM21) showed that xylan and glucomannan were mainly present in the S1 layers and the G-layers, respectively. In some phloem fibers, LM21 immunofluorescence labeling showed multilayered structure, suggesting the heterogeneous distribution of glucomannan.

International audience; Hygrothermal ageing and durability are major concerns for the use of plant fiber composites. This paper aims at investigatingthe effectsof hygrothermal ageing on the physical and mechanical properties of unidirectional flaxfiber composites (UD-FFC). Water absorption behavior and the induced swelling were investigatedby immersing UD-FFC in distilled water at 23°Cor70°C. The results show a drasticinfluence of the fibers on the water sorption and induced swelling.The increase involumeis governed by the swelling in the plane transverse to the direction of the fibers. At 70°C, the increases of water uptakeand swelling vs. time are followed by marked falls due tothe degradation of pectins, hemicellulosesand primary wallof the fiber. Damage generated after immersion at 70°C is accompanied by significant reductions of the mechanical properties(-20%), which is not the case after ageing at 23°C.; L’étude du vieillissement hygrothermique et de la durabilité est unepréoccupation majeure pour l’utilisation fiable des composites renforcés par des fibres naturelles. Cet article abordel’influence du vieillissement hygrothermique sur les propriétés physiques et mécaniques d’un polyester insaturé renforcé par des fibres unidirectionnelles de lin. Une étude de l’absorption d’eau et du gonflement induit a préalablement été menée en immergeant les matériaux dans de l’eau distillée à 23°C ou 70°C. Les résultats de l’absorption d’eau indiquent que la présence des fibres augmente considérablement la quantité d’eau absorbée etle gonflementest exacerbé. Le gonflement volumique total du composite est régipar le comportement hydrique des fibres dans le plan transverse. Suite à l’immersion dans l’eau à70°C,le gonflement qui s’ensuit augmente dans un premier temps puis diminue essentiellement du fait de ladégradationdes fibres: dégradation des pectines, des hémicelluloseset de la paroi primaire. Cela se traduit par des abattements des propriétés mécaniques(-20%)qui ne sont pas observés après unvieillissement enimmersion à 23°C.

International audience; This work analyzes the variability of the mechanical properties of flax fiber bundles and strands (a strand is the set of the bundles issuing from the same stem). A first remarkable result is that to obtain average values with good precisions about twenty randomly selected strands are sufficient. Moreover a comparative study of the fracture properties of bundles issued from different strands leads to the assumption that each stem is particular with special properties. A more detailed study shows that correlations exist between the sections of the strands and their mechanical properties. These trends are similar to those reported for unit fibers. (c) 2017 The Authors. Published by Elsevier Ltd.

International audience; L’effet de massification automobile qui accompagne les besoins croissants de mobilité terrestre et l’offre de nouvelles infrastructures routières qui soutient de nouvelles demandes rendent les modèles de mobilité actuels de plus en plus difficilement acceptables. En effet, les personnes souffrent de l’inconfort des villes engorgées (nuisances), de l’insécurité lors de transports du fait de défaillances techniques ou humaines, de la très grande difficulté d’optimiser ses itinéraires en fonction des horaires ou circonstances exceptionnelles (accident par exemple). Par ailleurs, la mobilité se heurte toujours aux contraintes liées aux rigidités des infrastructures composées de nœuds, et de contraintes permanentes qui étouffent nos villes.Pour répondre à ces enjeux, la transition vers la mobilité autonome est en marche et l’écosystème véhicules – infrastructures doit évoluer pour désengorger nos villes. Ces nouveaux systèmes de transport sous-tendent une révolution technologique dans le domaine des transports et devront répondre à des préoccupations socio-économiques capitales en étant tout à la fois adaptables, résilients, et évolutifs, coopératifs et acceptables des points de vue économiques, juridiques et sociaux.

International audience; This work aims to improve the formulation of short flax fiber-reinforced mortar through coating the raw fibers with hydraulic binders like cement or lime, or cleaning them in boiling water prior to the coating. These treatments are chosen for their ability to reduce the water intake by the fibers and to minimize the migration of their extractible compounds toward the cement matrix. Formulations incorporating raw or treated fibers were characterized in the fresh state. The results showed a drastic reduction of water intake, which correlates with an improvement of the properties of the fiber-mortar mixtures in the fresh state, particularly in terms of the heat of hydration, initial setting time and consistency. Mechanical behavior of the different formulations is also evaluated by flexural tests.

This thesis has been carried out as part of the project Fiabilin, which includes 15 different academic and industrial partners, with an aim to develop industrial production of polyamide-11/flax biocomposite. The purpose of this work is to determine multi-scale performances of 100% biosourced composite, in order to substitute composite materials containing glass fibers and/or matrix derived from petroleum. First, we highlighted the flax fiber sensibility toward processing cycles (time and temperature), from mechanical and biochemical structure aspects. Then, we revealed the capacity of PA11-flax association to produce competitive mechanical properties compared to others usual composites. Fiber-matrix interface of the biocomposite was studied at micro and macro scales, showing a higher compatibility than some flax-thermoset resin systems. The end-of-life of the biocomposite was considered by recycling with successive grinding and injections. Then stiffness and strength at break of short fiber biocomposites thus obtained are similar to PPgMA-flax composites, whereas a strong increase of the strain at break according to the number of injection cycles was observed. A life cycle analysis of some composites production steps shows lower environmental impacts of PA11-flax when sizing was made through equivalent material stiffness., Cette thèse de doctorat a été réalisée dans le cadre du projet Fiabilin, qui regroupe 15 partenaires industriels et académiques, et vise à structurer une filière industrielle de production de biocomposites polyamide-11/fibres de lin. Ces travaux ont pour objectif de déterminer des performances multi-échelles de ce composite 100% biosourcé, afin d’envisager son usage en substitution de composites pétrosourcés. Nous avons tout d’abord mis en évidence la sensibilité des fibres de lin aux cycles temps-température des procédés de mise en œuvre, tant du point de vue de leurs propriétés mécaniques que de leur structure biochimique. Ensuite, nous avons montré les capacités du système PA11-lin à produire des performances mécaniques en traction compétitives vis-à-vis d’autres composites pétrosourcés. La qualité de l’interface fibre/matrice du biocomposite a également été étudiée à différentes échelles, montrant une compatibilité supérieure à celles de systèmes lin-résines thermodurcissables. La fin de vie du composite PA11-lin a été envisagée à travers le recyclage par broyages et injections successives. Les propriétés mécaniques du biocomposite à fibres courtes ainsi obtenu sont semblables au composite PPgMA-lin, avec une déformation à rupture accrue et qui augmente significativement avec le nombre de recyclages. Une analyse des cycles de production de plusieurs composites révèle les plus faibles impacts environnementaux du PA11-lin lors d’un dimensionnement des pièces en rigidité équivalentes.

One of the challenges in the widespread use of biocomposites for engineering applications is the influence of environmental conditions on their mechanical properties, particularly for a combination of aging factors such as temperature, moisture, and mechanical stresses. Thus, the purpose of this paper is to study the influence of coupled aging factors by focusing on a 100% bio-based and biodegradable composites made of flax/poly(lactic acid) with several fiber contents. The development of a specific testing setup enabled continuous in-situ measurements and allowed comparing the effects of combined aging factors to those of uncombined aging factors. It was confirmed that the aging temperature in wet conditions led to a loss of elastic properties, especially for higher fiber fractions. While creep tests in dry conditions resulted in little decrease of elastic properties, it was observed that mechanical loading of the materials combined with water immersion resulted in a strong synergistic effect on the loss of stiffness. Finally, the presence of fibers reduced environmental stress cracking mechanisms and increased the time to failure.

International audience; The biosourced Polyamide 11 (PA11) is envisaged as a matrix in structural ecocomposites in combination with long flax fibers. Herein we compare the thermal, rheological, physical, mechanical and hydrothermal behavior of PA11 available as film or powder. These data are used for choosing between the two forms of the PA11 and to determine the temperature, holding time and pressure ranges suited for the elaboration of biocomposites by induction heating. © Lavoisier 2016.

International audience; The association of plant fibers with cement paste raises a number of problems in both fresh and hardened states. Different treatments are applied to flax fibers with various methods within the aim to enhance the rheological properties of the cement mortar-fiber mixtures. Thus, the effects of these treatments on the properties of the fibers are evaluated. The rheological characteristics of the cement mixtures and the mechanical properties of the composites are assessed. Results show an improvement in the properties of the fibers, better rheological behavior of the cement mixtures and an increase of the mechanical strength of the composites. © 2015 Elsevier Ltd. All rights reserved.

This work investigates the effect of hydrothermal aging on the properties of polypropylene/alfa fiber composites. Hydrothermal aging was induced in an environmental testing chamber at 65°C and 75% relative humidity (RH) over a 1000 h period. At the beginning (t=0 h), the results showed that Young’s moduli of the untreated alfa fibers and the acetylation-treated fibers increased by 21% and 36%, respectively, compared with the virgin polypropylene (PP). Additionally, Young’s moduli decreased by 7% for the compatibilized composites composed of maleic anhydride grafted polypropylene (MAPP). After 1000 h of aging, Young’s moduli decreased by 36% for untreated alfa fibers and 29% for the acetylation-treated alfa fibers and the compatibilized composites. Significant degradation was observed in the untreated alfa fiber samples. The Fourier transformed infrared (FTIR) allows us to distinguish the characteristic absorption bands of the main chemical functions present in the composite material before and after aging. The thermal properties showed that the thermal stability and the degree of crystallinity of the composites decreased after hydrothermal aging; this result was corroborated by the dynamical mechanical analysis (DMA) results.

Because of their hydrophilic nature, flax fibers absorb water during the mixing stage and release it gradually during the curing step. Thus, the rheological properties and the setting of the cement paste are deeply disturbed. In addition, the diametrical shrinkage of the fiber in the desorption stage can lead to a weakening of the fiber–matrix bond. It is therefore necessary to carry out treatment of the fiber and/or cement mixture to prevent the fiber/matrix debonding. The results obtained from these formulations show an improvement in the rheology and the mechanical performances of the composites.

International audience; An elementary plant fiber could be assimilated to a laminate, mainly constituted of the secondary wall S2 layer, made of a few non-crystalline polysaccharides reinforced by cellulose fibrils organized in a helix, with a microfibrillar angle (MFA) around 10o relative to the longitudinal fiber axis. This paper investigates the relationships between the MFA, the mechanical properties and the biochemical composition of different varieties of flax. To conduct this study, tensile tests on elementary flax fibers, X-ray diffraction, and solvent extractions have been carried out. Within the different varieties of flax, Young's modulus was found to be negatively correlated with the MFA. The results showed that the ratio between hemicelluloses (matter extracted with alkali) and pectins (hydrolyzed with acids) is highly correlated with the tensile properties; concurrently, we showed the great influence of pectic acids on the fiber' Young's modulus, and on the orientation of the microfibrils.

In the eco-design context, innovation efforts come with new bio-sourced materials that are meant to reduce products’ environmental impact. Insofar as these products are made with new materials, the innovation is also concerned about their perceived quality by consumers. More precisely, the challenge is to reach a compromise between environmental and marketing issues. Indeed, to be satisfying, the perceived quality of the environmental-friendly products has to be similar to the one of products made with classical synthetic materials. Sensorial design, through sensorial evaluation, is the academic discipline that takes into account the sensorial consumers’ perception. However, very few previous studies were led concerning the application of sensorial evaluation methods to bio-sourced materials. The present work proposes to make up for this lack of knowledge. More precisely, we evaluate both a flax fiber bio-sourced material and a glass fiber synthetic material using the same sensorial analysis methods. This evaluation combines a subjective sensorial tactile evaluation to an objective instrumental evaluation of both these materials’ surfaces. The results validated the application of tactile sensorial evaluation methods to a bio-sourced material. Moreover, tactile sensorial profiles were defined for flax fiber bio-sourced materials. Results also enabled to correlate subjective tactile measures with tactile objective instrumental measures. The present gathered knowledge offer, to designers concerned with eco-design and more generally by product design process, a new valuable bio-sourced material decision making aid., Dans un contexte d’éco-conception, les efforts d’innovation amènent de nouveaux matériaux dits bio-sourcés qui participent à réduire l’impact des produits sur l’environnement. S’agissant de nouveaux matériaux, l’innovation porte également sur l’amélioration de la qualité perçue par le consommateur. Le challenge est donc de concevoir des produits respectueux de l’environnement à l’aide de nouveaux matériaux écologiques tout en garantissant une qualité perçue au moins identique à celle des matériaux synthétiques classiques. Le design sensoriel, par l’utilisation de l’évaluation sensorielle, est l’une des disciplines qui permet de prendre en compte le ressenti sensoriel des consommateurs. Cependant, dans la littérature, peu de travaux portent sur l’application des méthodes d’évaluation sensorielle à des matériaux bio-sourcés. Face à ce constat, nos travaux se proposent d’étudier la possibilité d’évaluer, avec des méthodes d’analyse sensorielle, un matériau bio-sourcé à base de fibres de lin de la même manière qu’un synthétique à base de fibres de verre. Plus précisément, nos travaux proposent une approche basée sur une évaluation sensorielle tactile et une évaluation instrumentale objective de la surface de ces deux matériaux. Nos résultats ont permis de valider l’application des méthodes d’évaluation sensorielle tactile sur un matériau bio-sourcé. Nos travaux ont permis la définition de profils sensoriels pour le toucher concernant le matériau à base de fibres de lin. Les résultats obtenus ont aussi permis de valider la possibilité de corréler certains indicateurs tactiles subjectifs avec des mesures instrumentales objectives. L’ensemble des connaissances développées dans le cadre de ces travaux offre aux concepteurs une aide précieuse dans le choix des matériaux bio-sourcés en éco-conception et plus généralement en conception de produits.

International audience; Flax NCF studied is more difficult to impregnate than glass NCF in a longitudinal way. Transverse impregnation is similar in glass and flax fabrics. Infusion processes should then be preferred to RTM injection for this flax NCF. Flax composites show lower tensile properties than glass ones. However flax composite tensile properties are sufficient for various structural applications. In particular, flax-polyurethane composites are mechanically competitive with high agro-resourced content.

International audience; Many works deal with the mechanical properties of flaxfibers cultivated for textile applications and today used for the reinforcement of polymers. Nevertheless, quantities of oleaginousflaxfiber are obtained each year and not promoted. The aim of this work is to study the mechanical properties of single linseed flaxfiber as a function of variety, culture year, dew-retting degree and agronomic factors. Five varieties of oleaginousflax have been characterized by tensile tests on elementary fibers and compared to four varieties of textile flax. These tensile experiments have been carried out on with the same equipment, experimental protocol and environmental conditions. The results show that interesting mechanical properties were obtained with the oleaginous variety and are close of those of textile varieties, such as Agatha or Electra. Considering the diameters and specific properties of these oleaginousfibers, we evidenced that they are good candidates for the substitution of glass fibers in composite materials. To increase the development of flaxfibers, it is important to have a better control of the spread of their mechanical properties. This point could be observed with the Everest variety cultivated for 4 years and no conclusion could be made. We have evidenced that the retting degree has no influence on the diameters and mechanical properties of the fibers; the same conclusion is obtained with agronomic factors such as seeding rate and plant height.

The mechanical properties of flax fibres are analysed as a function of their biochemical and morphological characteristics. The fibres, from the Agatha variety, have been selected from either the top, the middle or the bottom of the stems. The results of each analysis are discussed according to the position of the fibre in the stem and compared among themselves. Considering a flax fibre as a natural composite, this study underlines the complexity of its structure and shows that many parameters intervene in its deformation behaviour.