Mechanical Behavior of Hybrid Adhesive Joints between Softwood and Hardwood

A series of static bending tests was conducted on specimens from four groups bonded with PVAc (D3) and PUR (D4) adhesives. Ultimate bending strength (σ), modulus of elasticity (E), specific strength, and fracture work (A) were evaluated. For the first time, it was demonstrated that the use of PUR adhesives in heterogeneous joints mitigates the anisotropic effect of load orientation: the difference between strength values under perpendicular and parallel loading in Group 2 was only 0.75 % (3.97 MPa vs. 4.00 MPa), whereas in PVAc systems this difference reached 44 % (5.43 MPa vs. 3.03 MPa). The highest static strength (10.6 MPa) and stiffness were achieved by the oak-maple combination bonded with PVAc; however, this system exhibited brittle failure with a fracture work of 65.0 J. The application of PUR adhesive in the pine–maple configuration resulted in a modulus of elasticity of up to 5016 MPa, which is 15–20 % higher than that of comparable PVAc systems. The maximum energy absorption capacity was recorded in Group 2 (PUR, perpendicular loading) at 93.3 J, which is 2.7 times higher than that of the least energy-absorbing configuration (Group 3, parallel loading, 34.1 J). PUR joints retained residual load-bearing capacity in the range of 3000-3400 N beyond peak load. The use of pine as a damping layer enabling displacement of up to 30 mm prior to complete failure was substantiated. The findings support differentiated selection of adhesive systems for furniture applications: PVAc for highly loaded static joints and PUR for joints requiring enhanced toughness and a controlled failure mechanism.