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Les fibres naturelles telles que le lin, le chanvre, le bambou ou la miscanthus sont de plus en plus utilisées pour renforcer les composites industriels afin de réduire le poids, le coût et l’impact environnemental des produits. Elles remplacent les composites conventionnels tels que les composites à base de résine polymère et fibres synthétiques. Les méthodes de parachèvement par usinage de ces produits agrocomposites demeurent un verrou technologique et un défi scientifique. Ceci est dû principalement à la structure complexe des fibres végétales constituée de cellulose et issue des feuilles ou des tiges de plantes cultivées. Ce travail de thèse propose une analyse multiéchelle du comportement à la coupe de ces matériaux renouvelables qui exploite un procédé 2D de coupe orthogonale et un procédé 3D de coupe par fraisage. L’objectif étant de mieux appréhender les mécanismes physiques majeurs activés par le processus d’enlèvement de matière des agrocomposites. Aussi, pour identifier les effets d’échelle observés en usinage, une caractérisation tribo-mécanique des agrocomposites stratifiés par nanoindentation et rayage ainsi que des essais mécaniques spécifiques ont été réalisés. Les fibres végétales se différencient des fibres synthétiques par une flexibilité transversale qui leur confère une grande capacité à se déformer lors du contact avec l’outil de coupe. Ainsi, la rigidité mécanique du contact outil/matière contrôle ici la coupe par cisaillement plastique des fibres végétales et, par conséquence, la qualité de la surface usinée des agrocomposites. Les fibres végétales, associées à une matrice polymère thermoplastique, présentent par ailleurs un comportement mécanique élastoplastique avec un endommagement ductile lorsqu’elles sont sollicitées suivant leur direction transversale. Ceci explique la production de copeaux continus en usinage par opposition aux composites synthétiques conventionnels. Les comportements mécanique et tribologique des fibres végétales en usinage sont fonction de l’échelle de contact. Ceci explique le caractère multiéchelle de la coupe des agrocomposites dont l’usinabilité est intiment liée à la taille du renfort fibreux. / Natural fibers such as flax, hemp, bamboo or miscanthus are increasingly used as fibrous reinforcement in order to reduce the weight, the cost and the environmental impact of products. They replace the conventional composites based on polymer resin and synthetic fibers. The finishing operations by machining of these biocomposite products remain a technological issue and a scientific challenge. This is mainly due to the complex structure of natural fibers composed of cellulose and extracted from plant leaf or plant stem. This research work provides a multiscale analysis of cutting behavior of these renewable materials in 2D orthogonal cutting and 3D milling processes. The primary objective is to better understand the major physical mechanisms activated by the material removal process of biocomposites. Furthermore, to identify the scale effects observed in machining, a tribo-mechanical characterization of stratified biocomposites by nanoindentation and scratch as well as specific mechanical tests were carried out. Natural fibers are distinguished from synthetic fibers by a transverse flexibility, which enable them good ability to deform upon contact with the cutting tool. Thus, the mechanical tool/material contact stiffness controls the cutting by plastic shearing of plant fibers and, consequently, it controls the quality of the biocomposite-machined surfaces. Otherwise, natural fibers, associated with a thermoplastic polymer matrix, have an elastoplastic behavior with a ductile damage when they are stressed in their transverse direction. This explains the production of continuous chips when machining biocomposites, unlike conventional synthetic composites. The mechanical and tribological behaviors of plant fibers in machining are dependent on the contact scale. This explains the multiscale cutting character of biocomposites where the machinability is intimately related to the size of the fibrous reinforcement.

Dans cette thèse, un composite unidirectionnel à renfort lin (composite UD biosourcé) a été développé et élaboré par la technique de presse à chaud. Le comportement en traction des composites à renfort végétal montre en général deux domaines, mais un troisième domaine est identifié dans ce travail. Un modèle phénoménologique développé précédemment pour décrire le comportement en traction d'un composite à renfort en fils torsadés a été testé avec le composite UD biosourcé. Nous montrons que l'ajout d'un phénomène de consolidation au modèle précédent est nécessaire pour simuler correctement le troisième domaine. Un second modèle mécanique a été par ailleurs développé pour identifier expérimentalement les propriétés mécaniques effectives du renfort en lin lorsqu'il est piégé dans la matrice. La distribution statistique de l'orientation locale du renfort a été mesurée pour pouvoir prendre en compte l'orientation des fibres. Pour cela, la technique du tenseur de structure a été appliquée sur des images optiques du pli de lin. Par ailleurs, ce modèle permet d'étudier l'influence des porosités sur les propriétés mécaniques. Les deux modèles permettent d'effectuer des prévisions efficaces du comportement mécanique du composite de fibre de lin unidirectionnel. En complément des modèles de mécanique, le comportement en sorption du composite de lin UD a également été analysé. Le modèle de Langmuir et le modèle de Fick ont été appliqués sur nos composites UD. Les résultats montrent que la configuration unidirectionnelle du renfort de lin favorise la sorption d'eau des composites associés.Résumé en anglais / In this Ph.D work, unidirectional flax fiber composite (UD biobased composite) has been designed and manufactured based on the hot platen press process. Plant fiber composites usually exhibit two regions under tensile load, but three regions have been identified in this work. A phenomenological model, previously developed to describe the tensile mechanical behavior of twisted plant yarn composites, has been tested with the UD biobased composite. We show that the addition of a strengthening phenomenon to the previous model is necessary to simulate correctly the third region. A second mechanical model has also been developed for experimental identification of the effective mechanical properties of flax reinforcement when embeded in matrix. A statistical distribution of local orientation of UD reinforcement was obtained allowing taking the fiber orientation into account. To that end, structure tensor method was applied to optical images of flax ply. Furthermore, this model allows the effect of porosity on mechanical properties to be studied. Both models provide effective forecast of the mechanical behavior of unidirectional flax fiber composite. Besides the mechanic models, sorption behavior of UD flax composite also has been analyzed. Langmuir's model and Fick's model were applied on our UD composite. The results show that the unidirectional configuration of the flax reinforcement promotes the water sorption from the associated composites.

O presente estudo teve por objetivo avaliar características agronômicas e tecnológicas da fibra do algodoeiro mediante a antecipação da abertura das maçãs pelo uso de maturador em épocas distintas. O experimento foi conduzido na área experimental do Departamento de Produção Vegetal da USP/ESALQ em Piracicaba, Estado de São Paulo, no ano agrícola de 2005/06. O delineamento experimental utilizado foi em blocos ao acaso com 4 repetições. Os tratamentos consistiram na aplicação do maturador (ethephon/amads) em épocas distintas de abertura natural dos frutos, correspondendo a 7 tratamentos com a aplicação do produto aos 15, 30, 54, 75, 87, 92 e 100% de abertura natural, respectivamente. Em dois metros lineares de cada parcela foram realizadas imediatamente antes e 15 dias após a aplicação do produto, onde ocorreu a contagem de maçãs, capulhos e folhas, determinando as porcentagens de abertura dos frutos e as desfolhas ocorridas. Na área útil de cada parcela foi determinada a produção de algodão em caroço e, em amostras de 20 capulhos colhidos aleatoriamente por parcela, foram realizadas as análises das características agronômicas (massa de um capulho, massa média de 100 sementes, porcentagem de fibra e germinação) e das características tecnológicas da fibra (comprimento, uniformidade de comprimento, tenacidade e índice micronaire e maturidade). De acordo com os dados experimentais obtidos, pôde-se concluir que a melhor desfolha ocorreu aos 87% capulhos. A aplicação de maturador com 84% de capulhos garante maior produção. Quanto as características tecnológicas da fibra, houve redução significativa para índice micronaire quando a aplicação foi realizada aos 15, 30, 54% de capulhos e a maturidade foi menor quando a aplicação ocorreu aos 15 e 30 % de capulhos. / The present study had as objective, to estimate the agronomic and technological characteristics of the cotton plant by means the anticipation of open bolls using the harvest-aids applied in different time. The experiment was made at experimental area of Agricultural Departament at USP/ESALQ in Piracicaba, São Paulo State during the 2005/06 season. The experimental design was in randomized blocks with four replications. The trataments were made up of in application of harvesting-aids (ethephon/amads) in different time of natural opening and the experiment was composed of seven treatments, using the product at 15, 30, 54, 75, 87, 92, 100 % open bolls, respectively. Two linear meters in each plot were immediately available before and fifteen days after the application of the product, the bolls, the open bolls and the leaves were counted, estimating the parameters of percentage of opening and defoliation. The cotton yield was determinated in the useful area of each plot and samples of 20 open bolls randomized chosen per plot were analyzed for agronomical lab characteristics (average open boll mass, mass of 100 seeds, fiber percentage and germination) and fiber technological characteristics (length, length uniformity, strength, micronaire and maturity). According to the experimental data obtained, the best falling off of the leaves occurred to the 87% open bolls. The application of maturation with 84% open boll guarantees greater production. As the technological characteristics of the fibre, there was significative reduction to the micronaire index as the application was achieved to 15, 30, 54% open bolls and the maturity was smaller when the application occured to 15 and 30% open bolls.

O presente estudo teve por objetivo avaliar características agronômicas e tecnológicas da fibra do algodoeiro mediante a antecipação da abertura das maçãs pelo uso de maturador em épocas distintas. O experimento foi conduzido na área experimental do Departamento de Produção Vegetal da USP/ESALQ em Piracicaba, Estado de São Paulo, no ano agrícola de 2005/06. O delineamento experimental utilizado foi em blocos ao acaso com 4 repetições. Os tratamentos consistiram na aplicação do maturador (ethephon/amads) em épocas distintas de abertura natural dos frutos, correspondendo a 7 tratamentos com a aplicação do produto aos 15, 30, 54, 75, 87, 92 e 100% de abertura natural, respectivamente. Em dois metros lineares de cada parcela foram realizadas imediatamente antes e 15 dias após a aplicação do produto, onde ocorreu a contagem de maçãs, capulhos e folhas, determinando as porcentagens de abertura dos frutos e as desfolhas ocorridas. Na área útil de cada parcela foi determinada a produção de algodão em caroço e, em amostras de 20 capulhos colhidos aleatoriamente por parcela, foram realizadas as análises das características agronômicas (massa de um capulho, massa média de 100 sementes, porcentagem de fibra e germinação) e das características tecnológicas da fibra (comprimento, uniformidade de comprimento, tenacidade e índice micronaire e maturidade). De acordo com os dados experimentais obtidos, pôde-se concluir que a melhor desfolha ocorreu aos 87% capulhos. A aplicação de maturador com 84% de capulhos garante maior produção. Quanto as características tecnológicas da fibra, houve redução significativa para índice micronaire quando a aplicação foi realizada aos 15, 30, 54% de capulhos e a maturidade foi menor quando a aplicação ocorreu aos 15 e 30 % de capulhos. / The present study had as objective, to estimate the agronomic and technological characteristics of the cotton plant by means the anticipation of open bolls using the harvest-aids applied in different time. The experiment was made at experimental area of Agricultural Departament at USP/ESALQ in Piracicaba, São Paulo State during the 2005/06 season. The experimental design was in randomized blocks with four replications. The trataments were made up of in application of harvesting-aids (ethephon/amads) in different time of natural opening and the experiment was composed of seven treatments, using the product at 15, 30, 54, 75, 87, 92, 100 % open bolls, respectively. Two linear meters in each plot were immediately available before and fifteen days after the application of the product, the bolls, the open bolls and the leaves were counted, estimating the parameters of percentage of opening and defoliation. The cotton yield was determinated in the useful area of each plot and samples of 20 open bolls randomized chosen per plot were analyzed for agronomical lab characteristics (average open boll mass, mass of 100 seeds, fiber percentage and germination) and fiber technological characteristics (length, length uniformity, strength, micronaire and maturity). According to the experimental data obtained, the best falling off of the leaves occurred to the 87% open bolls. The application of maturation with 84% open boll guarantees greater production. As the technological characteristics of the fibre, there was significative reduction to the micronaire index as the application was achieved to 15, 30, 54% open bolls and the maturity was smaller when the application occured to 15 and 30% open bolls.

Le plâtre est un matériau de grande disponibilité et très connu pour ses qualités : il est favorable à la protection de l’environnement, assez malléable, de faible densité, aux propriétés fonctionnelles remarquables (coupe-feu, isolant thermique, régulateur de l’hygrométrie des enceintes), décoratif, ... Ce qui justifie l’intérêt accordé à ce matériau pour les constructions. Sa grande fragilité préoccupante est à l’origine des travaux de recherches dans le monde entier en vue de son renforcement. Les fibres de verre et de sisal sont les renforts les plus utilisés à ce jour. Le renforcement par des fibres végétales est de plus en plus recherché. La texture micro structurale poreuse du plâtre favorise son caractère d’isolant thermique. Les textures mises en œuvre jusqu’à présent sont limitées à des porosités comprises entre 30 et 55%. La réduction du coût de ce matériau pour une large utilisation est encore possible et souhaitée. Deux leviers sont exploités dans ce travail, notamment un allègement de la masse de plâtre pour augmenter le taux de porosité et un renforcement de la tenue mécanique par incorporation de fibres végétales produites localement. L’objectif de ce travail est d’évaluer les caractéristiques mécaniques, thermiques et hygrométriques d’un matériau constitué de plâtre pris, à grande porosité, renforcé d’une nouvelle fibre végétale : le Rhecktophyllum Camerunense (RC), une fibre des forêts humides équatoriales. La fibre de sisal, d’utilisation connue pour le renforcement du plâtre, sert de référence à des fins de comparaison. Une série d’expérimentations est menée à cet effet. Une caractérisation physico-chimique des constituants est effectuée, des essais mécaniques de traction et de flexion sont effectués sur les constituants et les matériaux composites plâtre/fibres résultants, la cinétique d’adsorption d’humidité par les constituants et le matériau fibreux est suivie. Le comportement thermique des matériaux plâtre et plâtre/fibres est aussi mesuré. Les fibres utilisées, le sisal et le RC, sont à fort taux de cellulose (entre 49 et 78,8%), la fibre de RC est tubulaire avec 35,5% de porosité. Le plâtre est gâché à l’eau déminéralisée à un rapport massique E/P égal à 1 à partir de la poudre de semihydrate β. Sa microstructure cristalline est constituée de cristaux de gypse sous forme d’aiguilles enchevêtrées avec des vides intercristallins. Sur le plan du comportement mécanique, les résultats obtenus révèlent que le plâtre se montre fragile et présente un module d’élasticité en traction de 1,72 GPa, une résistance à la traction de 0,86 MPa et un allongement à la rupture de 1,16%. En flexion trois points, son module d’élasticité est de 0,64 GPa et sa contrainte à la rupture, de 0,13 MPa. La fibre de sisal est raide et fragile. Son module d’élasticité est compris entre 9 et 21 GPa, elle admet un allongement à rupture de 3 à 7%. Par contre, la fibre de RC est assez ductile avec un module d’Young moyen de 0,7 GPa et un allongement à rupture de 24,2%. L’adhésion du plâtre sur les fibres est faible : il adhère plus sur le sisal que sur le RC. Le sisal renforce mieux le plâtre par une augmentation plus sensible du module d’élasticité de 42,5%, contre 16,3% pour le RC, ce dernier lui apportant plutôt une grande ductilité élastique. Les fibres de RC apportent le maximum de renforcement en traction au plâtre lorsqu’elles sont tissées en unidirectionnel et en flexion lorsqu’elles sont uniformément réparties dans le volume suivant la direction longitudinale de la structure. (...) / The plaster is a material of high availability and very known for its qualities : it is favourable to the protection of the environment, quite malleable, of low density, its functional properties are remarkable (firewall, thermal insulation, regulator of the hygroscopy of enclosures), decorative, ... What justifies the interest attached to this material for constructions. Its great alarming brittleness is at the origin of the research tasks in the whole world for its strengthening. The glass fibers and sisal are the more used reinforcements to this day. The strengthening by plant fibers is more and more researched. The microstructure of the plaster is porous ; that promotes its heat insulation character. The textures implemented so far are limited to porosities ranging between 30 and 55%. The reduction of cost of this material for a wide use is still possible and desired. Two levers are exploited in this work, in particular a lightening of the plaster weight to increase the proportion of air voids and a reinforcement of the mechanical resistance with locally produced fibers. The objective of this work is to evaluate the mechanical, thermal and hygrometrical characteristics of a material made up of harden plaster, with high porosity, strengthened by a new plant fiber : the Rhecktophyllum Camerunense (RC), a fiber of humid equatorial forests. The sisal fiber, of known use for the strengthening of the plaster, serves as a reference for comparison purposes. A serie of experiments is conducted to this effect. A physicochemical characterization of constituents is performed. Mechanical tests of tensile and of bending are performed on the constituents and the resulting plaster/fiber composite materials. The kinetic adsorption of moisture by the constituents is followed. The thermal behaviour of plaster and plaster/fiber is also measured. The fibers used, sisal and RC are with high rates of cellulose (between 49 and 78.8% ), the fiber of RC is tubular with 35.5 % of porosity. The plaster is dissolved in demineralized water to a mass ratio W/P equals to 1 from the powder of semihydrate β. Its crystalline microstructure is composed of gypsum crystals in the form of needles tangled with the empty intercristallins. As far as the mechanical behavior is concerne, the result reveals that the plaster is weak, its Young’s modulus in tensile is 1.72 GPa, its tensile strength is 0.86 MPa and its elongation at break is 1.16 %. In three points bending test, its modulus of elasticity is 0.64 GPa and its constraint at break is 0.13 MPa. The sisal fiber is stiff and fragile. Its Young’s modulus is between 9 and 21 GPa, it admits an elongation at break of 3 to 7 %. On the other side, the fiber of RC is quite ductile : the means of Young’s module is 7 GPa and the elongation at break is 24.2 %. The adhesion of the plaster on the fiber surface is low : it adheres more on the sisal than on the RC. The sisal strengthened better the plaster with a sensitive increase of the Young’s modulus of 42.5 %, against 16.3 % for the RC. But the RC fiber gives rather high elastic ductility. The fibers of RC deliver maximum capacity in tensile to the plaster when they are woven into unidirectional. They offer high capacity in bending when they are uniformly distributed inside the volume according to the longitudinal direction of the structure. (...)