Long-term durability of flax-glass hybrid FRP-timber composite structures subjected to hygrothermal environment: Experimental and simulation

This paper focuses on the experimental and numerical analysis of long-term performance of flax-glass hybrid FRP (HFRP)-laminated veneer lumber (LVL) joints and beams subjected to hygrothermal environment (50℃ and 95 %RH) for six months. The joints and beams with different fibre fabric stacking sequences of HFRP exposed at different exposure intervals (0, 1, 2, 3 and 6 months) were tested under block shear and four-point bending, respectively. The tensile properties of epoxy and HFRP composites under those exposure intervals were also examined to explore degradation mechanisms of HFRP in LVL-HFRP beams. Tensile strength and strain of HFRP showed a major reduction (26.7 – 32.1 %) in the first month of exposure. Hydrolysis and oxidation of epoxy were found to have insignificant effects on HFRP tensile properties, based on Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) results. A significant decrease (34.7 – 35.7 %) of LVL-HFRP joints in their shear strength was attributed to weakened hydrogen bonds between cellulose and lignin-hemicellulose matrix, along with the degradation and softening of hemicellulose. LVL-GF beams in which the glass fibre layer of HFRP was adhered to LVL exhibited a major reduction in bending strength (23.4 %) after the first month of exposure. In LVL-FG beams where the flax fibre layer was adhered to LVL, a major decrease in bending strength (25.8 %) was observed after two-month exposure. The postponed reduction in LVL-FG beams compared with LVL-GF beams was caused by the slower moisture diffusion in HFRP of LVL-FG beams than that in LVL-GF beams. A diffusion–stress coupled finite element (FE) model was developed, incorporating moisture diffusion and moisture-dependent mechanical properties for both the timber and HFRP components. Based on this model, the flexural response of LVL–HFRP beams after hygrothermal exposure was simulated, showing satisfactory agreement with experimental results. This research developed a step towards the long-term performance evaluation of HFRP-timber composite structures with different fabric stacking sequences of HFRP.