Enhancing the repairability of fibre-reinforced vitrimers through hybrid composite formation

Fibre-reinforced vitrimers represent a promising class of healable structural composites; however, the influence of fibre type and healing conditions on interlaminar performance remains insufficiently understood. In this work, flax-fibre-, carbon-fibre-, and hybrid-fibre-reinforced benzoxazine-based vitrimer composites were investigated, with particular emphasis on interlaminar shear strength (ILSS) recovery as a function of healing temperature and pressure. All composite systems recovered at least 80% of their initial ILSS under at least one set of healing conditions, though the optimal conditions varied significantly between fibre types. The healing of carbon-fibre composites is governed by temperature and its interaction with pressure, while flax-fibre composites are sensitive primarily to pressure, with high pressure causing fibre damage during healing. In hybrid composites, a clear synergy between flax and carbon fibres is observed: healing efficiency becomes progressively independent of temperature, pressure, and their interaction as flax content increases. Hybrid composites containing two flax plies exhibited ILSS recovery above 74% across all conditions tested, and were successfully healed under pressure-free conditions at 150°C with efficiencies of 79 ± 3% and 85 ± 10%, enabled by the thermal expansion mismatch between flax and carbon plies generating internal compressive stress without external load. Additionally, a hybrid configuration with flax as the outermost layer exhibited a shift in dominant failure mechanism upon healing, resulting in apparent healing efficiencies exceeding 100%. These results demonstrate that fibre type and hybridisation play a key role in governing both damage recovery and process sensitivity, providing actionable guidelines for the design-for-repair of vitrimer-based composites.