Upgrading Low-Density Black Poplar Wood with Oak and Ash Facings: Bending Strength and a Laminated-Beam Design Framework

Black poplar (Populus nigra L.) is a fast-growing hardwood whose use in furniture and joinery is limited by its low density and strength. This study combined bending tests with a laminated-beam analysis to quantify and generalize the effect of symmetric oak (Quercus robur L.) and ash (Fraxinus excelsior L.) veneer facings on poplar cores. 45 specimens (15 per group) were tested in three-point bending. Facing increased the density from 432 kg/m3 to 504 kg/m3 (oak) and 494 kg/m3 (ash) and the bending strength (MOR, 12% moisture content) from 63.5 MPa to 86.8 MPa (+37%) and 77.1 MPa (+21%) (Welch’s ANOVA, p < 0.001). A dimensionless transformed-section framework was developed in which strength gain equals the product of stiffness amplification, determined by the face fraction and the face-to-core modulus ratio, and the face-to-core ratio of failure strains. Inverse analysis yielded in situ face strengths of 97 MPa (oak) and 91 MPa (ash) and failure-strain ratios of 1.06 and 0.81, which explains why the stiffer ash facings produced the smaller strength gain. The framework predicts face-governed failure, in agreement with the observations, and shear stresses below the shear strength of poplar. For components of fixed depth, the mass-specific strength is maximized at face fractions of 0.15–0.25, where faced poplar outperforms both unfaced poplar and solid oak or ash; the tested oak-faced elements were close to this optimum. Closed-form conditions and design charts generalize these results to other face–core combinations.