Upscaling of Friction Welding of Wood for structural Applications

The present work deals with the large-scale application of the friction welding bonding technology on timber. The investigations that are described in the following were carried out on a prototype of a welding machine that was previously developed for the purpose of this research. By means of this machine timber elements can be welded in a scale that is comparable to structural applications. Detailed experimental experience with the welding of large-sized specimens was not available at the beginning of this work. Preliminary results of initial tests on welded spruce boards indicate a correlation between the size of the sample and the occurrence of larger areas within the interface where no or only an incomplete welding result is attained. Therefore, these scaling effects were investigated in more detail in a first step. The results show that this phenomenon is associated with an internal gas pressure, which is generated at the interface during the evaporation of moisture content and other gaseous by-products. The latter result from the thermal decomposition of wooden cell wall molecules during the friction phase. It can be shown that the scaling effects can be restricted through a reduction of the moisture content or by the arrangement of gas evacuation channels at the interface, which leads to a decrease of the internal gas pressure. Subsequently the potential of the technology with regard to structural applications was estimated and evaluated by means of mechanical studies. First, investigations on the load bearing behaviour of parallel welded connections in form of double lap joints, and in particular the influence of the overlap length on the load capacity, are described. The results show that due to the extremely rigid behaviour of the bond so-called size effects occur, a phenomenon that describes a decrease in nominal strength with increasing size of a load-bearing structure. In addition to the load tests, a numerical strength prediction method is presented, which is based on a probabilistic approach and which allows for a precise prediction of the experimental results and the size effects. Furthermore, it is shown that timber boards with perpendicular fibre orientation can be welded in order to form stiff and resistant connections. Therefore, specimens of different sizes were prepared and tested for their moment transmitting capacity. In this context, a new method was introduced for the determination of a failure criterion of cross-welded joints is presented. Probabilistically determined strength values, which are based on this criterion, show good agreement with the experimental results. The non-linear relationship between the size of the specimens and the nominal strength is confirmed by the results of the numerical calculation. The results from the experiments on cross-wise welded bonds show that the strength of those connections is suitable for an application in timber construction. For this reason small scale prototypes of cross-lamin