Influence of surface preparation and surface topography on tensile shear strength of polyurethane adhesively bonded beech wood single-lap joints: a finite element method approach

In this study, the mechanical properties of bonded single-lap joints are analysed by tensile lap shear tests on beech wood. A one-component polyurethane adhesive was used, and three different methods of surface preparation were applied: planing, sanding along the grain, and sanding perpendicular to the grain. Prior to bonding, the wooden lamellae underwent laser scanning to obtain surface profiles, which were then analysed for surface roughness. Scanned surface topographies with their features were integrated into the finite element analysis (FEA) software COMSOL Multiphysics to simulate the lap shear bonding area for different surface profiles and roughness. The FEA model implements linear material models, which represent the adherend and thin adhesive layer, combined with a modified local cohesive zone model for the adhesive bond interfacial forces. The experimental tests were conducted in a dry environment, where a higher surface roughness achieved by sanding correlated with a higher tensile shear strength. This increased surface roughness was attributed to the enhanced mechanical interlocking mechanism. This finding aligns with the FE analysis, which showed that increased surface roughness, micropillars and indentations, led to variations in stress concentration and distribution compared to a smooth surface bond.