This study investigates the fracture behaviour of adhesive joints in beech wood (Fagus sylvatica L.) under Mode II loading. A three-point end-notched flexure test (3ENF) was used, which enables the determination of the critical strain energy release rate GIIc. To more precisely monitor crack propagation, the digital image correlation (DIC) method was employed, allowing for the calculation of local displacements and strains, as well as the determination of the Mode II cohesive law σ(wII). The study examined adhesive joints in beech wood bonded with polyurethane adhesive, considering different test specimen groups based on surface preparation and wood moisture content. The results of the 3ENF tests showed characteristic force–displacement curves and enabled the calculation of the Mode II energy release rate. Statistical analysis (ANOVA and Bonferroni post-hoc test) confirmed statistically significant differences between the individual groups. The DIC method enabled direct determination of the relationship between energy release rate and the crack tip shear displacement (CTSD), and the derivation of the Mode II cohesive law, which are crucial for numerical modelling of adhesive joints. The results confirm that specimen preparation conditions (moisture content, surface treatment, and lamella orientation) have a significant impact on the mechanical properties of adhesive joints. The findings contribute to a better understanding of the fracture behaviour of beech wood and provide useful input data for the development of more reliable numerical models.
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