Mode Ⅰ interlaminar fracture toughness of epoxy- and polyurethane-bonded flax-glass hybrid FRP-timber interfaces

This study investigates the Mode I interlaminar fracture toughness of laminated veneer lumber (LVL) -hybrid fibre reinforced polymer composite (HFRP) interfaces using a combined experimental and numerical approach. Double cantilever beam tests were carried out on the interfaces between LVL and flax FRP, glass FRP, and flax-glass HFRP, which were bonded with epoxy (EP) and polyurethane (PUR) adhesives. Effects of crack length, adhesive type, fabric stacking sequence and FRP type on fracture toughness were statistically analysed using a linear mixed-effects model. The results show that wood failure dominated the fracture behaviour for both LVL-FRP and LVL-HFRP interfaces. In EP-bonded interfaces, limited flax fibre tearing failure led to adhesive failure. Fracture toughness increased consistently with crack length, which was attributed to progressive fibre bridging and crack deviation. PUR-bonded interfaces exhibited overall higher fracture toughness than EP-bonded interfaces, with this advantage strongly dependent on the fabric stacking sequence; notably, interfaces containing flax fibre plies showed higher fracture toughness than those with glass fibre plies. Statistical analysis further indicated that the effects of the examined factors on fracture toughness decreased in the following order: crack length, fabric stacking sequence, adhesive type, and FRP type. Finite element simulations incorporating a cohesive zone model with spatially varying fracture energy were performed to reproduce the experimental responses; the numerical results captured the main features of the load-displacement behaviour, including the peak load and post-peak softening response.