Experimental Investigations and Probabilistic Strength Prediction of welded Double Lap Joints composed of Timber

Wood-to-wood bonds obtained by means of friction welding are a relatively new type of connections holding high potential to compete with existing structural adhesives, since a load bearing bond is formed almost instantaneously. This paper addresses the question to which extent welding of wood can be considered for structural joining of load bearing timber elements. For this purpose, experimental investigations in form of tensile tests on welded double-lap joints were carried-out, in which a set of parameters, including overlap length and layer thickness, has been varied. In addition to the experimental analysis, the stress-strain state inside of the connection was determined numerically, taking into account the mechanical complexity of the involved components. It could be concluded that (1) failure of the bond is triggered by a combination of shear and tensile stresses acting in the interface, but also that (2) joint strength is not linearly correlated to stress magnitudes. The mechanical behaviour of the welded interface was characterized by a series of off-axis tests, including its statistical components. The results from this characterization and the numerical modelling were combined in the framework of a strength prediction routine. Finally, experimental and numerical results are compared, allowing for a validation of the suggested approach

Experimental investigations on welded double lap joints composed of timber

Benjamin Hahn, Bernhard Stamm, Till Vallée, Yves Weinand

Fabrication of wood-to-wood connections by means of friction welding is an relatively new process holding high potential for development. To connect wooden pieces by means of friction welding, they are initially pressed against each other. The rapid movement of one of the pieces, applied simultaneously leads to friction, and heating generation, which is softening the material at the interface within a few seconds. This paper addresses the question to which extent welding of wood can be considered for structural joining of load bearing timber elements. For this purpose, experimental investigations on wood-welded double-lap joints were carried-out, in which a series of parameters, including overlap length, member thickness, etc., has been varied. This experimental campaign allowed for (a) the determination of the fracture mechanism of welded wood connections, (b) quantification of the influence of main design parameters on the strength of these and (c) delivering benchmark data related to the strength of welded wood joints, including its statistical component, for a subsequent joint strength prediction model. The experimental investigations were completed by numerical modelling of all investigated welded joints, which allowed to conclude that (1) failure of the investigated welded joints is triggered by a combination of shear and transversal stresses acting in the interface, but also that (2) joint strength is not linearly correlated to stress magnitudes.

Elsevier Science, Reg Sales Off, Customer Support Dept, 655 Ave Of The Americas, New York, Ny 10010 Usa2011

Numerical modelling and strength prediction of welded double lap joints made of timber

Benjamin Hahn, Bernhard Stamm, Till Vallée, Yves Weinand

Welding of wood is a relatively new method of joining load bearing elements, which holds the potential to compete with structural adhesion, since it allows for an almost instantaneous bond. First experimental and numerical investigations showed that failure of wood-welded connections is linked to a combination of shear and transverse-tensile stresses in the welded interface. One of the main conclusions drawn from previous investigations is the inability of simple stress-based approaches to allow for an estimate of strength, which is due to the huge stress peaks generated in such joints. To achieve a reliable dimensioning of wood-welded joints, it is paramount to determine the stress-strain state inside the connection, taking into account the mechanical complexity of the involved components, to determine the mechanical resistance towards the acting stresses and to combine both in the framework of a strength prediction routine. This paper addresses the aforementioned issues by summarizing experimental and numerical results on wood-welded joints, suggesting an experimental method to characterize the strength of the welded interface, and combining the previous results in the form of a strength prediction method. Lastly, experimental and numerical results are compared, which allows for a validation of the suggested approach.

Elsevier Ltd. Selection2011

Upscaling of Friction Welding of Wood for structural Applications

Benjamin Hahn

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-laminated timber panels were fabricated and examined for their load bearing capacity. For this purpose the most critical load case of in-plane shear is regarded mainly since the load is transmitted only by the moment resisting behaviour of the welded bond. Based on the previously developed probabilistic strength prediction method, the load bearing capacity of the panels can be calculated numerically [...]

EPFL2014

Upscaling of Friction Welding of Wood for structural Applications

Benjamin Hahn

EPFL2014