Experimental characterization of hardwood bonded joints effectiveness in Mode I crack opening in normal conditions and after temperature exposure

Adhesive bonding governs the structural performance of engineered wood products (EWPs), yet the fracture behavior of hardwood bonded joints and their response to normal and elevated temperatures remain insufficiently understood. This study investigates the Mode I fracture toughness of beech and chestnut specimens bonded with melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF) adhesives using Double Cantilever Beam tests adapted from ASTM D5528. A full R-curve approach was adopted to capture crack initiation and propagation mechanisms. Half of the specimens were exposed to 150 °C prior to testing to assess and quantify any potential degradation. Results show a strong dependence of the fracture behavior on hardwood type/adhesive category combination. MUF exhibited higher fracture toughness under ambient conditions but showed significant degradation after thermal exposure, whereas PRF demonstrated greater thermal stability. Fracture surface analysis revealed that temperature alters the dominant failure mechanisms, shifting from adhesive to cohesive fracture depending on the wood species. The findings demonstrate the suitability of fracture mechanics methods for assessing bond-line quality in hardwood joints and provide insight for adhesive selection in applications subjected to elevated temperatures exposure.