The development of composites based on vegetal fibers requires a good control of manufacturing process. The aim of this work is to determine the key parameters to produce high grade flax / epoxy unidirectional laminated composite by thermocompression. So, many processing parameters have been tested and ranked according to their influence on mechanical properties. Since variability can be high for this kind of materials, statistical analyses have been used to determine if properties variations were significant or not. Among all studied parameters, the three which have been identified as first rank influence on mechanical properties are: fibers conditioning, curing pressure and exit plate temperature.

The use of vegetal fibers to reinforce polymeric matrix composites is challenging and goes with questions of durability issues, especially when exposed to damp conditions. The aim of this study is to quantify, up to 1 year, the impact of humidification/drying cycles, i.e. 3.5 days at 90% HR and 3.5 days at 40% HR, both at 55°C, on the longitudinal mechanical properties of a unidirectional flax/epoxy composite. Then, by a multi-scale analysis, the objective is to identify the causes of mechanical properties evolutions. According to the results of this study, a cyclic hygrothermal ageing induces irreversible degradations of fibers (matrix-embedded or not) and interfaces within the composite. Indeed fiber/fiber and fiber/matrix debondings have been observed by morphological analyses and a loss of fibers hydrophilic components has been highlighted by chemical and microstructural characterizations. A plasticization of flax fibers has also been shown during this kind of ageing. All these modifications induce a drop of composite's moduli after the first cycles of exposure and a decrease of ultimate tensile stress and strain after long time exposure. But these mechanical properties evolutions are much lower than the values found in the

The development of composites based on plant fibres has considerably grown in last decades; however, their high sensitivity to moisture limits their use in structural applications. The objective of this study is to determine and understand the impact of moisture variations during one-year cyclic hygrothermal ageing i.e. 52 cycles on the transverse properties of a unidirectional epoxy flax composite. A cycle consists of 3.5 days at 90% RH followed by 3.5 days at 40% RH. Compared to the studies usually carried out to determine the impact of ageing on the longitudinal properties of unidirectional composites, the determination of the transverse mechanical properties during this type of ageing allowed to focus on the contribution of the matrix and the interface on the evolution of the composite properties. The multi-scale analyzes provided identification of the phases responsible for the evolution of transverse mechanical properties induced by ageing. The transverse ultimate tensile strength drops by about 20% after the first week of ageing and slightly decreases up to 2 months. This evolution seems to be induced by the creation of flaws, in particular the phenomena of fibre-fibre and matrix-fibre debonding and the creation of cracks in the fibres initiated at the lumen level, revealed by SEM. The transverse modulus also strongly drops after

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