Effects of mechanical and chemical surface preparation on adhesively bonded wooden joints

Tensile shear tests according to EN 302-1 (2013) [4] for load bearing timber constructions were performed using a one-component polyurethane (PUR) and a phenol resorcinol formaldehyde adhesive. Prior to bonding, the adherends’ surfaces were prepared with different machining techniques (planing, face-milling, and sanding). Additionally, the surfaces of the PUR specimens were chemically treated with two different priming substances. The results indicate that the priming of the surfaces can improve both tensile shear strength (TSS) and wood failure percentage (WFP) of PUR bondings. However, no effect of the different mechanical surface preparation techniques could be found. Additionally, the suitability of the wood failure proved to be questionable, as the highest WFP was accompanied by the lowest TSS and vice versa, even though the same wood was used for all tests. The inspection of the fracture paths indicated that a failure of the bonding close to the bondline (creating a low WFP) might be even more advantageous than the traditionally favored failure in the adherend part, as the adhesive itself could help dissipate energy, resulting in a higher TSS.