Influence of mechanical surface preparation methods on the bonding of southern pine and spotted gum: Tensile shear strength of lap joints

Southern pine and spotted gum are two of Australia’s most important locally produced commercial timbers. However, internationally, they are amongst the most problematic species to glue cost-effectively, especially for sawn-laminate-based structural engineered wood products, such as glulam and cross-laminated timber. This study investigated the efficacy of different pre-gluing wood surface machining preparations on the tensile shear strength of lap shear samples prepared from both species. Surface machining methods tested included planing, face milling, and sanding post-planing with 40 and 80 grit sandpaper. Wood face milling is not currently used commercially in Australia and has not previously been adequately tested on Australian commercial timbers to improve wood adhesion. Planing is currently the most common method used internationally for preparing wood surfaces for gluing. For both species, face milling with fast feed speed (45 m/min), slow cutter speed (57 m/s), and sanding treatments post-planing resulted in significantly higher tensile shear strength compared to planing for lap shear samples that had been subjected to an accelerated weathering process. Performance differences in tensile shear strength between surface machining methods are likely to be related to the effects of these machining methods on surface roughness, fibrillation, and sub-surface cell damage.