Effects of Water Ageing on the Mechanical Properties of Flax and Glass Fibre Composites: Degradation and Reversibility

Mechanical properties of flax-fibre reinforced composites (FFRC) are strongly affected by water ageing compared to glass-fibre reinforced composites (GFRC). This study highlights the influence of water absorption during immersion at 30 degrees C on mechanical properties of unsaturated polyester reinforced composites. Flax-fibre composites showed a Fickian absorption behaviour and a water uptake 15 times higher than that of glass-fibre composites. GFRC's tensile modulus and maximum stress were slightly affected by water uptake while FFRC's tensile modulus decreased by 37 %, and ultimate stress increased by 34 %. A silane-based chemical treatment (1 % compared to flax fibres) was applied onto flax fibres. Water uptake was slightly reduced by 9 % while tensile modulus at saturation was enhanced by 22 % on treated FFRC compared to untreated ones. Moreover, the complete recovery of the tensile modulus after desiccation suggests that ageing was mainly reversible: fibre and matrix plasticizing phenomena occurred during immersion at 30 degrees C. No damage was noticed but composites' initial properties changed with the action of water: further crosslinking of matrix and release of fibre's small cell-walls components into water were observed. Finally, the drying conditions influenced the return to the initial state before ageing insofar the flax fibres partially lost their initial humidity.