Abstract Understanding the Mode II fracture behavior of adhesive joints in hardwood species is essential for reliable structural design in timber engineering. This study investigates the shear fracture energy (G II ) of polyurethane (PUR)-bonded European beech wood ( Fagus sylvatica L.) under two different moisture contents. Three-point bending end-notched flexure (3ENF) tests were conducted on bonded specimens conditioned at 12% and 18% moisture content (MC), with three types of bonding surface preparation: planed, planed + sanded, planed + primer prior to bonding. Crack initiation and propagation were monitored using 3D digital image correlation (DIC) and fracture energy was calculated using the compliance-based beam method (CBBM). The results reveal a clear influence of both moisture content and surface treatment on Mode II fracture behavior. Higher moisture content consistently reduced G c and stress values, while specimens with different lamellae orientations achieved the highest stresses before adhesive joint failure occurred. Different bond failures and their influence on fracture behavior are also discussed. The critical Mode II strain energy release rate, G IIc , was defined at the maximum applied load corresponding to the onset of unstable crack propagation, as confirmed by synchronized DIC measurements. The findings contribute to an improved understanding of shear behavior in PUR-bonded hardwood joints and offer valuable input for cohesive zone modelling and structural performance prediction.
Capuder et al. (Tue,) studied this question.