Adhesion strength and wood failure relationship in wood-glue bonds

Six Burmeses hardwoods with specific gravity values ranging from 0.62 to 0. 90 were glued into laminates with three adhesives, phenol-resorcinol formaldehyde, urea formaldehyde, and casein. The block-shear strength of the glueline increased with species specific gravity up to 0.8 and then decreased sharply when the specific gravity exceeded 0.8. The wood failures at specific gravities less than 0.8 are in the range of 80 to 100per thousand. However, when the specific gravity of the species rose to more than 0.8, wood failure also decreased sharply. An intrinsic relationship between bond strength and wood failure was derived by computing adhesion strength as a percentage value of bond-and wood-strength difference divided by the wood strength alone. This relative strength was found to relate well to the percentage wood failure and was applicable to a population of 50 South Asian hardwood species with specific gravities ranging from 0.33 to 1.01, all bonded with five kinds of adhesives. This evidence indicates that if a sound wood substrate is used, the wood failure percentage is a good representaiton of adhesion strength, regardless of its direct or indirect correlation with absolute strength.