Technological heterogeneity of adhesive bonds in wood joints

 Gluing of wood is among the most effective methods for the permanent joining of furniture elements or building woodwork manufactured from wood. Technological errors occurring during the preparation process of the glue material may lead to variations in the strength of adhesive/wood joints. The purpose of the described research project was to investigate the effect of the heterogeneity of the glue bond on the distribution of tangential stresses in furniture joints, especially the effect of gas cavities, faulty glue bonds and glue outflows on the distribution of tangential stresses in adhesive bonded overlap, cross and angle wood joints. Using developed numerical models, it was shown that shear stresses in bonds of cross and angle joints reach their maximum values in corners of joints. The torsion center of cross joints is situated in the geometrical center of the bond, while in angle joints – it is found half-way through the length of one of the perpendicular edges of the joint. It was also proven that gas cavities present in the glue bond contribute to increased stresses in the neighbourhood of the source of heterogeneity. This phenomenon initiates a process of de-cohesion and, hence, reduces the overall strength of the joint. Faulty gluing, similar to gas cavities, constitutes a potential source of stress-breaking processes and reduces the strength of joints. On the other hand, glue outflows present in wood bonds increase their strength by expanding the initiation threshold of fractures even in situations where technological heterogeneity of the glue bond occurs. In furniture constructions as well as in large-size building woodwork constructions or, wherever grace and elegance of the finished product is of lesser importance, glue outflows can be treated as a positive and desirable phenomenon. Received 13 March 1999