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The fracture behavior of bond lines in hardwood has been studied. The joints of a phenol resorcinol formaldehyde (PRF) resin and a one-component polyurethane (PUR) adhesive with European beech wood ( Fagus sylvatica  L.) adherends were examined for their fracture toughness ( K  c ). The initial crack tip was placed directly in the adhesive as a thin silicon film. Thereby, the examination of the bond line, and not the solid wood, can be assured. Five different load angles were applied, from fracture mode 1 (M1) to mode 2 (M2), with an Arcan test mount. Additionally, three sample series conditioned at the relative humidities (RH) of 50%, 65%, and 95% of the surrounding air were tested. The results clearly show an increasing  K  c  of both adhesives with increasing shear stresses. This observation is valid for all RHs, but the differences decrease with increasing RH. The moisture dependency is more pronounced in PUR than in PRF glue joints. PUR generally shows a lower  K  c  than PRF, with the only exceptions being  K  I,c  and  K  II,c  in dry climate. The subsequent crack propagation in the PRF samples mainly takes place in the wood adherend, whereas, in the PUR samples, the cracks remain within the bond line (adhesive failure). Nevertheless, the performance of PUR glue joints is not worse than that of the solid wood, which can be attributed to the ductile behavior of the adhesive.

Mixed-mode fracture toughness of bond lines of PRF and PUR adhesives in European beech wood