Investigation of the Effect of Water and Moisture on the Performance of Flax Fibre Based Bio-Composites

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Flax fibre composite is a bio-material that can meet the requirements in many engineering applications. It has been used especially in applications for the automotive industry due to the several excellent specific properties which are similar to composites manufactured with glass fibres. However, due to the hydrophilic nature of flax fibre, this material is prone to absorb water and moisture from the environment, and therefore, they are not reliable for outdoor applications. The growing demand for flax composites in the field of bio-material research enforces the researchers to have in-depth knowledge in their processing into a useful final product. However, the correlation between the environmental effects and the performance of the flax fibre bio-composites is not well established. The main focus of this study is to address the specific environmental issues and their effects on flax fibre composites manufactured by the vacuum infusion method. First of all, the effect of the moisture content in the fibres on the mechanical performance and overall quality of the manufactured composites is studied. Flax composites are manufactured with fibres having different moisture content and they are compared to composites manufactured with dry fibres. This work begins with measuring moisture absorption kinetics of the flax fibres. Flax fibres absorb moisture almost linearly from 0% to 70% RH (relative humidity) environments, however, this absorption almost double from 70% to 95% RH. The tensile strength of the flax fibre reinforced composites increases until a certain RH value (50% RH) and then it decreases as the RH of the environment is raised up to the maximum value (95% RH). However, the tensile modulus, flexural strength and modulus of the composite samples continuously decrease with the increasing RH values. The microstructural tests of the composites confirmed the presence of micro-cracks and pores and debonding in the fibre-matrix interface. In addition, a study about the dynamic response of the flax composites manufactured with fibres stored at different RH levels ensures that dry composites have better vibration and damping properties. The damping ratios of the flax composites manufactured with fibres with different RH values are slightly increased with the increasing rates of moisture absorption although this change is not notable. Moreover, the mechanical properties of the flax composites manufactured with fibres at different RH environments are predicted by analytical models. The geometric and displacement potential function approaches are proposed to estimate the tensile properties of the flax composites. The proposed models demonstrate greater accuracy when validated with experimental observations. Secondly, the consequence of different environmental conditions on the performance and durability of flax composites is analysed. Composites made by vacuum infusion using dry fibres are immersed in water, exposed to warm humid environments and subjected to freeze-thaw (F/T) cycles. During exposure to different environments, the mechanical performance (tensile and flexural properties), moisture content and physical changes (dimensional stability) of the composites are analysed. The water absorption trend is partially Fickian for the samples immersed in water at room temperature and these samples are greatly affected and degraded. Furthermore, the mechanical properties of the water saturated (WS) samples are clearly degraded in contrast to the as manufactured (AM) samples. Compared to AM samples, the tensile strength and modulus are decreased approximately by 9% and 57%, respectively for WS samples and their flexural strength and modulus is decreased by 64% and 70%, respectively. However, some of these properties could be recovered, to some extent, by drying the WS samples. On the other hand, the reduction of tensile strength and modulus for the humidity saturated (HS) samples is only 0.8% and 3%, respectively, when compared to the AM composite samples. It was found that freezing-thawing cycles have almost no effects on the flax bio-composites.