Advancing impact and fatigue resistance in bio-adhesive joints: the combined effect of adhesive penetration and tough-layer techniques

• Bio-based adhesive joints reinforced with novel dual-toughening strategies. • Combination of adhesive penetration and tough-layer methods improves durability. • Hybrid oak–pine joints achieved ∼25 % higher fatigue resistance than pine joints. • Significant enhancement of residual strength under cyclic impact fatigue loading. • Approach supports sustainable bonded structures for demanding applications. Adhesive joints are essential in engineering applications, yet their durability under fatigue and impact loading remains a challenge. Bio-based adhesives provide sustainability benefits, but their limited toughness compared to petroleum-based alternatives reduces long-term performance. The aim of this study is to improve the durability of bio-based adhesive joints through substrate toughening strategies. Single-lap joints (SLJs) were fabricated using a moisture-curing polyurethane bio-adhesive with two wood substrates: pine (softwood) and oak (hardwood). Two reinforcement approaches were investigated: (i) adhesive penetration, enhancing mechanical interlocking within wood fibers, and (ii) the tough-layer method, producing hybrid joints with oak outer layers and a pine core to optimize stress distribution. Mechanical performance was evaluated under quasi-static, dynamic loads. Results demonstrated that hybrid joints consistently outperformed their reference counterparts. In particular, hybrid oak joints with a pine core exhibited the highest fatigue resistance, maintaining load ratios approximately 25 % higher than pine joints and 15 % higher than oak joints. These improvements were attributed to enhanced stress distribution by hardwood outer layers and controlled energy dissipation through the pine core. The effectiveness of substrate toughening strategies in enhancing the structural integrity and durability of bio-based adhesive joints, supporting the development of sustainable bonded structures for demanding engineering applications.