Contribution à l'étude de composites unidirectionnels renforcés par des fibres de lin : relation entre la microstructure de la fibre et ses propriétés mécaniques

This doctoral dissertation deals with morphological and mechanical characteristics of flax fibres. Indeed, due to their lightness and their good mechanical properties, these fibres are considered as good alternatives for the glass fibres commonly used as reinforcement in polymer composites. At first, sections and longitudinal profiles of flax fibres were observed in order to determine their mean sizes and their scattering and to draw up an internal organisation scheme. Then, tensile tests were then performed on elementary fibres. Their mechanical behaviour displays a typical elasto-visco-plasticity as well as an initial non-linearity which has been attributed to an internal rearrangement of the cell wall constituents. The comparison with other natural fibres and the observation of fibre rupture surfaces allowed to corroborate this hypothesis. The study of fibres issued from two varieties and from different parts of the stems led to the conclusion that the median fibres exhibited optimal dimensions as well as higher mechanical properties than the apical or the basal ones. Finally, unidirectional composites made of flax fibres and polyester were processed by hand lay-up or by compression moulding, with several fibre volume fractions. They were tensile and flexural tested in order to determine their mechanical properties and their damage modes. The results show that flax fibres reinforced composites, in addition to their ecological benefit, are mechanically competitive compared to glass fibres reinforced composites.Cette thèse porte sur la caractérisation morphologique et mécanique des fibres de lin. En effet, leur faible densité et leurs bonnes propriétés mécaniques permettent d’envisager leur utilisation comme renfort de polymères en remplacement des fibres de verre. Des fibres de lin ont d’abord été étudiées à partir d'observations de leurs sections et de leur profil longitudinal, ce qui a permis de déterminer leurs tailles moyennes et leur dispersion et de formuler un schéma d'organisation interne de la fibre de lin. Des essais de traction ont ensuite été réalisés sur des fibres unitaires. Leur comportement élasto-visco-plastique présente une non-linéarité initiale qui a été interprétée comme un réarrangement interne des constituants des parois. La comparaison avec d'autres fibres végétales ainsi que des observations de faciès de rupture de fibres ont permis de valider cette hypothèse de réorganisation. A partir de l’étude de fibres issues de différentes variétés de lin et prélevés en différents endroits le long des tiges, il est apparu que les fibres présentes dans la zone médiane des tiges présentaient des dimensions optimales et des propriétés mécaniques supérieures aux fibres apicales ou basales. Enfin, des composites unidirectionnels lin / polyester ont été fabriqués par contact et par compression, avec différents taux volumiques de fibres. Ceux-ci ont été testés en traction et en flexion par flambement afin de déterminer leurs propriétés mécaniques et de préciser leur mode d'endommagement. Les résultats montrent que, en plus de leur avantage écologique, les composites à base de fibres de lin sont mécaniquement compétitifs par rapport à ceux à base de fibres de verre

Natural fibres as composite reinforcement materials: description and new sources

International audienceThe use of natural fibres to reinforce polymers is currently giving rise to a relatively strong and new interest due to several advantages they offer, such as lightness, stiffness, strength, renewability and worldwide availability. However, certain drawbacks, such as the scattering of their properties and their low hygrothermal stability, common to almost all natural fibres, still hinder their progress in this field. Progressively, research is opening up to investigation into new sources of fibres for such composites with the aim of promoting their development locally. This chapter deals with a survey of these new fibres, aiming at proving that many plants, some of which are still considered as waste, can find an application in the reinforcement of composites

Direct Insights on Flax Fiber Structure by Focused Ion Beam Microscopy

International audienceIn this article, it is shown that focused ion beam (FIB) systems can be used to study the inner structure of flax fibers, the use of which as a reinforcing material in polymer composites still draws much interest from multiple disciplines. This technique requires none of the specific preparations necessary for scanning electron microscopy or transmission electron microscopy studies. Irradiation experiments performed on FIB prepared cross sections with very low Ga + ion beam currents revealed the softer material components of fibers. Thus, it confirmed the presence of pectin-rich layers at the interfaces between the fibers of a bundle, but also allowed the precise localization of such layers within the secondary cell wall. Furthermore, it suggested new insights on the transition modes between the sublayers of the secondary cell wall

Caractérisation mécanique et modélisation de la lamelle mitoyenne au sein d'un faisceau de lin

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Interfaces within flax fibre bundle: Experimental characterization and numerical modelling

International audienceIn natural fibre based-composites, the interfaces within the fibre bundles are not as well described as those between fibres and classical polymer matrices. To compensate for this lack of information, which is however of great importance when trying to model the composite behaviour, morphological and mechanical characterization of the middle lamella present between flax fibres has been carried out. The interfacial strength of the pectic cement was found to be very low compared with that of the fibre/matrix interface. Assuming this fibre/fibre interface to be a cohesive zone material and using the experimental results obtained on pairs of fibres, a model has been developed at the scale of the fibre and applied to study numerically the tensile properties of bundles. The results of the simulations were in agreement with the experimental data obtained on bundles, both leading to a mean bundle strength of 500 MPa

Le virus du Rattle du tabac : indétectable par la technique Elisa

National audienceCertains isolats du phytovirus du Rattle du Tabac (TRV) produisent des particules encapsidées détectables par une réaction sérologique. C'est le cas chez les plantes à bulbes. Dans le cas de la pomme de terre, les entités infectieuses du TRV sont souvent non-encapsidées et par conséquent totalement incapable de se lier à des anticorps. Un test Elisa négatif n'a donc aucune valeur. Le diagnostic basé sur un seul test moléculaire n'est pas non plus une garantie absolue. En France, pour remédier aux insuffisances de tous ces tests de détection, le diagnostic du TRV implique aujourd'hui plusieurs étapes, décrites dans cet articl

Mechanical properties of interfaces within a flax bundle – Part I: Experimental analysis

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