RESEARCH DATA
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Development of Friction Welding of Wood Physical, Mechanical and Chemical Studies Funding: EPFL, FNS Status: Completed, 2003 – 2006 Doctor: Bernhard Stamm, engineer IBOIS Thesis director: Prof. Yves Weinand, architect and engineer, IBOIS Partners: Prof. G. Wegener, engineer, TU München ; Fischer Kunststoff-Schweisstechnik, Berkatal DE |
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Upscaling Friction Welding of Wood for Structural Applications Funding: EPFL, FNS Status: Completed, 2006 – 2016 Doctor: Benjamin Hahn, engineer IBOIS Thesis director: Prof. Yves Weinand, architect and engineer, IBOIS Partners: BFH Bern University of Applied Science, Architecture, Wood and Civil Engineering ; LERMAB-ENSTIB Laboratoire d’Etude et de Recherches sur le Matériau Bois : EMPA Swiss Federal Laboratories for Material Science and Technology. |
RESEARCH ABSTRACT
Wood is a natural polymeric substrate and the most abundant regenerative material on earth. Its dimensions, however, are relatively limited because of the form and the diameter of the tree. In order to be able to build more complex structures efficient connections between wood pieces become necessary. For instance, two established connection types are used commonly. One the one hand there are metallic connectors like screws, threaded rods or nails and on the other hand we have chemical glues creating powerful planar joints. A big advantage of the latter type of connections is the high stiffness in comparison to metallic connectors. On the other hand a long and high pressure has to be applied on the glued joints to obtain a successful hardening. In addition the use of these adhesives still implicates concerns about their impact on health and environment during application as well for later disposal.
In order to overcome the geometrical limitations of timber by maintaining its physical and ecologic properties, adhesive-free friction welded bonds are a promising alternative to glued connections. Friction welding technologies are already commonly used for thermoplastic materials and metals. Research at the Laboratory for Timber Construction IBOIS started to apply the principle on small wood samples. In order to achieve a laminar wood-to-wood bond, the interface between two timber boards is heated by a fast and short oscillating frictional movement combined with pressure. The introduction of heat energy leads to a thermal decomposition of the polymeric compounds in the wood cell material. The chemical products of this degradation process form a viscous layer of thermally softened material, which hardens when the friction movement is stopped and the interface is cooling down, while a certain cooling pressure is applied. In principle any kind of wood can be welded.
The ongoing research project aims to evaluate the potential of this technology for application in prefabrication processes for the building industry. In a first step challenges and technical requirements for the enhancement of this technology from small specimens to samples of structural scale had to be addressed. Inhomogeneities in bonding quality and joint strength appeared, especially if larger spruce boards were welded. These inhomogeneities, herein referred to as scale effects, could significantly be reduced by facilitating the evaporation of vapour from the interface during welding.
Actually research is focussed on the fabrication and application of friction welded massive timber panels. Small scale samples have been produced and successfully tested for bending and shear. Nevertheless, the long-term resistance of the connection has to be ensured. In fact, the biggest challenge is the susceptibility of the weld with regard to varying moisture contents. The relatively brittle bond is highly sensitive to swelling and shrinkage movements of the wood. Changing climatic conditions can lead to cracks within the interface.
Furthermore, the structural design of friction welded components for timber construction requires a calculation tool for strength prediction. First Investigations within the scope of this research work used probabilistic methods to determine the load bearing capacity of welded double lap joints with parallel and perpendicular fibre direction. A good agreement between calculated and experimentally determined strength could be found. Further research will extend these investigations on more complex systems, in order to simulate more realistic scenarios closer to full scale elements.
Link to publications – Stamm
Link to publications – Hahn
Link to thesis – Stamm
Link to thesis – Hahn