Influence of surface shapes on the mechanical behaviour of friction welded wood bonds

Friction welding of wood is a fast and adhesive-free bonding technology for the production of timber joints with good mechanical characteristics making this type of connection interesting for applications in timber construction. However, the moisture stability is reduced due to swelling and shrinking deformations of the wood with changing climate conditions. These deformations lead to high internal stresses and thus irreversible crack formation within the bond. This paper describes experimental investigations carried out in order to improve the behaviour of welded interfaces under changing moisture contents by the use of interlocking surface shapes. Both, the influence of these modifications on the initial as well as on the residual joint strength was examined. The investigations revealed that the load bearing capacity of friction welded joints disappears completely if no additional measures, such as the geometrical modifications of the surface proposed herein, are taken. As a result the joint strength could be maintained even after a certain number of climate variations from very dry to very humid conditions. The described approach constitutes a practical solution to overcome the main drawback of the technology for industrial application.