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-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 [...]

Experimental and numerical study on structural behavior of a single timber textile module

Markus Hudert, Laurent Humbert, Meisam Sistaninia, Yves Weinand

The present work investigates an innovative class of timber structure with potential applications in roofing, facade and bridge construction, called Timberfabric. The development of Timberfabric structures originates from the approach of harnessing the structural, modular and qualities of textiles in timber construction (Weinand and Hudert, 2010) [9]. Timberfabric structures are comprised of repetitive arrangements of one or more structural unit cells called Textile Modules. When properly designed, one obtains a modular and lightweight structure with interesting and unusual geometrical and structural qualities.This paper focuses on the single timber Textile Module. Based on the finite element (FE) method, a reliable procedure is proposed for modeling the overall assembly process of the Textile Module. For comparison, Textile Module prototypes are constructed at two different scales (large and intermediate scales) with different assembly conditions. The proposed geometrically nonlinear FE model allows evaluation of the stresses that are induced during the construction process and which may affect the structural integrity of the module. In particular, the risk of failure during assembly is identified using the anisotropic Tsai-Hill criterion. The structural behavior of the timber Textile Module is then investigated through bending tests using the constructed prototypes. During the loading procedure, the vertical deflections are measured at different locations on the prototype surface by means of external displacement transducers. Using the FE model, the corresponding deformed shapes are simulated by applying the bending loads on the prestressed Textile Module. Experimental displacements and FE predictions are thus compared and found to be in good agreement.

Elsevier2013

Stochastic analysis of clear timber as a structural material

Alireza Farajzadeh Moshtaghin

Wood/timber has been widely used for house and bridge construction. It is a widely available natural material that necessitates low energy for the production, following simple processes. The environmentally friendliness, together with the low cost of raw material makes it an efficient building material. Moreover, timber possesses attractive mechanical properties such as high specific strength and stiffness. In contrast, timber constructions have, to a large extent, been based on experience and craftsmanship, which prevents taking full advantage of this material. There are several reasons for this. Timber has a complex mechanical behavior being a natural highly anisotropic fiber composite, with properties that are also affected by moisture content. For specific species, geographical location, local growth conditions and moisture content, the material properties depend, among others, on the age, the structural imperfections, the location of timber within the tree, and load history. Consequently, the mechanical properties of timber are, inherently, highly variable. Variability of timber properties includes statistical and spatial variabilities, referred to as random spatial variability (RSV). This entails adopting a probabilistic/stochastic approach to analysis of timer structures. The aim of this research is to understand and model the effect of the RSV on the clear timber mechanical properties, as well as the experimental characterization of RSV for clear timber, and also to develop a stochastic finite element framework for random response assessment of clear timber components. A size effect model was developed which takes into account the RSV in the strength field. The theory of random fields was used for this purpose. Using the spectral representation scheme, realizations of strength field in each specimen were generated. The stochastic response was obtained via the Monte Carlo method. The model results was compared to the existing experimental data in the literature. Also, an analytical expression was provided to facilitate the application of the model. Clear timber specimens of different lengths were fabricated for longitudinal tensile tests. Local deformations along the lengths of the specimens were recorded during the tests in order to characterize the RSV in longitudinal properties. A connection between the mesostructure of the clear wood and its local elastic modulus was observed. Statistics concerning the elastic modulus, strength and strain to failure and the effect of length change on these properties were extracted. The correlations between the strength, the elasticity and the density were obtained. Transverse properties were also investigated which are of particular importance in some applications such as mechanical and adhesively-bonded timber joints. Regularly positioned and randomly positioned specimens were cut from different timber boards. Statistics and size effects concerning the elastic modulus, strength and strain to failure as well as the correlation between the properties were studied. The spatial variability in the transverse elastic modulus, the tensile strength and the failure strain was also experimentally studied. Mesostructural patterns of clear timber were shown to have a direct effect on the local elastic modulus. Finally, a stochastic finite element framework was established by combining the spectral representation scheme for RSV modelling and the finite element software ABAQUS in a non-intrusive manner. This framework can be used for the stochastic structural response assessment of timber structural components made of clear timber. To show the applicability of the model in real applications, the failure of adhesively bonded double-lap timber joints were simulated under tensile loading. The effect of size on the strength was also taken into account. The results were in a fairly well agreement with the available experimental data in the literature.

EPFL2016

Ductility and strain rate-dependency of adhesively-bonded timber joints

Myrsini Angelidi

One of the most important characteristics of load-bearing structures is ductility, i.e. the ability of a material or a structure to sustain inelastic deformation prior to failure, without loss of resistance. The topic of this PhD Thesis was to develop ductile adhesively bonded timber joints, in order to compensate for the inherent lack of ductility of timber. After the rate-dependent true tensile and compressive properties of the acrylic adhesive had been established (including Poisson ratio, ductility and recovery), two types of adhesively-bonded timber joints were experimentally studied in tension and compression: stiff epoxy and ductile acrylic joints. The latter ones exhibited lower stiffness but higher capacity and ductility, as explained by the extracted stresses and strains too. Furthermore, finite element (FE) models, including these strain rate-dependent properties of the acrylics, were developed to validate all the experimental results obtained and study the parametric effect of the different applied displacement rates on the acrylic joints. Extensive simulation using the developed FE models has shown their capacity to accurately predict the rate effect on bonded joints' mechanical response: stiffness, yield, ductility, ultimate failure. Finally, the behavior of the developed bonded joints over mechanical joints was analytically compared, based on Eurocode and the results confirmed the promising potential of the developed joint concept. To summarize, if ductile adhesives are used, it is possible to create ductile timber joints, which can perform better compared to other existing assembling techniques (i.e. mechanical fasteners or epoxy-adhesives) and their complex, rate-dependent behavior can be modeled with precision with the help of FE. This knowledge can lead to a broader and more efficient use of timber, contributing subsequently to achieving more sustainable modern structures.