Mode I Fracture and Cohesive-Zone Modeling of PUR-Bonded Chinese Fir–OSB Bilayers: Effect of Sandblasting–HMR Pretreatment

Hybrid cross-laminated timber combining plantation Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) with oriented strand board (OSB) is resource-efficient, but its bond line may govern delamination. Double cantilever beam (DCB) specimens bonded with one-component polyurethane were tested at four initial crack lengths (a0 = 40–100 mm), comparing untreated bond lines with bond lines pretreated by sandblasting followed by hydroxymethylated resorcinol (HMR). The arms differed in bending stiffness by a factor of about 3.7, so the specimen was an asymmetric bi-material DCB, and the reported values were apparent, predominantly Mode I quantities. Pretreatment raised the critical load by 17%–24% and the apparent initiation toughness by 18%–35%. A bilinear cohesive-zone model with one parameter set per surface condition reproduced all eight calibration groups to within 5% in critical load and load-point displacement; the calibrated cohesive energies (402 and 594 N/m) differed from the apparent toughness at a0 = 100 mm by −9.6% and +12.9%. Apparent toughness fell with crack length, but the measured compliances do not follow the cubic scaling expected of a DCB, so this trend is unexplained. The calibrated parameters describe the responses measured here; their transfer to other geometries remains to be verified.