Strain Distributions in Adhesive Wood Joints

Abstract The influence of overlap length on the strain distribution in a double-lap joint was determined using a computer vision, white light speckle technique. The overlap lengths were 12.7 mm (0.5″) and 38.1 mm (1.5″). This range represents a one-tenth scale of overlap lengths in double-lap joints used in adhesive connections in timber trusses. The side members were 50.8 mm (2″) long and the center member was 88.9 mm (3.5″) long. All joint members were 25.4 mm × 25.4 mm (1″ × 1″) in cross section. The joint members were yellow-poplar (Liriodendron tulipifera) wood and the adhesive was a resorcinol-formaldehyde. Even though the connection was loaded in shear, both transverse and shear strains were shown to exist along the gluelines. These strains were significantly intensified at the ends of the overlap. The strain distributions were altered by the degree of overlap and the level of strain concentration was reduced with increased overlap. The full-field strain distributions as measured with the new computer vision technique revealed progressive failure development, the eventual failure mode and location, and knowledge of the actual distribution of the strains in the entire joint assembly.