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 the first week by about 18% to reach about 45% loss after one year of ageing. These evolutions are mainly linked to the physical plasticization of the epoxy matrix demonstrated by the reversible change of its glass transition temperature, but analyses showed that water does not lead to chemical degradation of the matrix.

在过去的几十年中，基于植物纤维的复合材料的开发有了长足的发展。但是，它们对水分的高度敏感性限制了它们在结构应用中的使用。这项研究的目的是确定和了解一年的循环湿热老化过程中水分变化的影响，即52个循环对单向环氧亚麻复合材料的横向性能的影响。一个周期包括在90％相对湿度下3.5天，然后在40％相对湿度下3.5天。与通常进行的确定时效对单向复合材料纵向性能影响的研究相比，在此类时效过程中确定横向机械性能可以使研究重点集中在基体和界面对材料演化的影响上。复合特性。多尺度分析提供了确定由时效引起的横向力学性能演变的相的识别。老化的第一周后，横向极限抗拉强度下降约2​​0％，直至2个月时，横向极限抗拉强度略有下降。扫描电镜显示，这种演变似乎是由缺陷的产生引起的，特别是纤维和基体纤维的脱粘现象以及在管腔水平上引发的纤维中裂缝的产生。在第一周后，横向模量也急剧下降了约18％，在老化了一年后达到了约45％的损耗。这些演变主要与环氧基质的物理增塑有关，这可以通过其玻璃化转变温度的可逆变化来证明，