In this study, the effects of the surface treatment, orientation, and moisture content of wood on the mechanical properties of adhesive bonded joints with polyurethane adhesive were evaluated in beech wood using double cantilever beam mechanical tests (DCB) and end-notch flexure mechanical tests supplemented by dynamic mechanical tests and acoustic emission analysis (AE). A portion of the specimens were tested at lower (EMC1 = 12%) and higher equilibrium moisture content (EMC2 = 18%), and also subjected to artificially accelerated ageing (UPS). The adhesive joints with a sanded surface obtained the best results in all tests. We found that the mechanical properties of the adhesive joints were negatively affected by the increased moisture content of the wood. In the case of the planed surface, the positive influence of using a primer before bonding was confirmed by a calculated higher fracture energy only in the DCB test. The inhomogeneous composition of the test specimens, due to the different orientation of the wood tissue in the cross-section of the lamellae, had negative effects on the mechanical properties of the bonded joints even after the UPS process. The UPS process caused structural defects in the lamellae and adhesive joints and reduced the number of suitable test specimens, so the effects of the process could not be quantified. A prolonged ultrasonic time of flight through the adhesive joint was observed in the aged specimens, but this could not be directly related to the results of the static mechanical tests. The longitudinal modulus of elasticity for beech wood was determined using the vibration resonance method in the longitudinal direction. The same method was used for the bending mode to confirm the correlation between the apparent dynamic (EB') and static modulus of elasticity (ES’).
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