The effect of adhesive type and bond line thickness on the strength of mortise and tenon joints

This study was carried out in order to determine the effects of adhesive type, bond line thickness and moisture content on the strength of mortise and tenon joints. The polyvinyl acetate (PVAc), urea-formaldehyde (UF) and animal adhesives were used, combined with three different bond line thicknesses. At the same time the effect of the moisture content of the wood was investigated by using wood material conditioned to 8%, 12%, 15% moisture contents. The results of the experiments showed that the strength difference between tight fitting and loose fitting joints was 28% if animal adhesive was used, 11% with UF adhesive and negligible with PVAc adhesive. The effect of the moisture content of the wood when the joints were assembled did not depend on the adhesive used. If 12% is taken as the standard moisture content, increasing it to 15% weakened joints by 15% and decreasing it to 8% strengthened them by 6%, irrespective of the adhesive. Test results indicate that strongest joints are obtained when a close tolerance is maintained between the tenon and mortise.