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  • 1.
    Habite, Tadios
    et al.
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Florisson, Sara
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Vessby, Johan
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Numerical Simulation of Moisture-Induced Crack Propagation in Dowelled Timber Connection Using XFEM2018In: 2018 World Conference on Timber Engineering (WCTE), August 20-23, 2018, Seoul, Republic of Korea, World Conference on Timber Engineering (WCTE) , 2018Conference paper (Refereed)
    Abstract [en]

    At times dowelled glulam timber connections experience crack development in the fibre direction. The main reason for this is moisture variation in the timber elements which induces a stress perpendicular to the fibre direction. The aim of this paper is to study the influence of different moisture conditions and vertical dowel spacing on crack development through numerical simulations by use of the finite element method in three dimensions. A transient non-linear Fickian moisture diffusion model is implemented to simulate the moisture state within the glulam beam. The moisture gradient in the diffusion model was created by adopting a physical scenario by assuming what conditions the considered glulam beam will go through, from the factory up to installation. Further, an extended finite element method (XFEM) for two different vertical dowel spacing, 100 mm and 300 mm, with a linear elastic fracture mechanics (LEFM) approach was applied for the crack simulation. The results reveal that the moisture variation in combination with unfavourable placement of dowels can cause a crack to develop in the glulam timber beam. Moreover, it was shown that a moisture induced crack development may be modelled successfully by use of an Extended Finite Element Method (XFEM) approach.

  • 2.
    Habite, Tadios
    et al.
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Olsson, Anders
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Oscarsson, Jan
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Automatic detection of pith location along boards of Norway spruce on the basis of data from optical scanning of longitudinal surfaces2019In: CompWood 2019 - International Conference on Computational Methods in Wood Mechanics - from material properties to Timber Structures, Växjö, Sweden, June 17-19, 2019, Växjö: Lnu Press , 2019, p. 64-64Conference paper (Refereed)
    Abstract [en]

    Different mechanical and physical properties of wood are related to the location of pith. Norway spruce wood from the centre of logs, close to the pith, is characterized by lower longitudinal MOE, larger spiral grain angle, and larger longitudinal shrinkage coefficient than what wood farther away from the pith is [1]. Thus, knowledge of pith location along timber boards may play an important role in both appearance grading and in assessment of mechanical properties such as strength [2]. The current work aims to develop an algorithm which is capable of automatically estimating the pith location of Norway spruce boards, along the boards’ length direction, by utilizing optical scanning of longitudinal surfaces. The initial step of the algorithm is to identify defect free sections along the timber board. This is done by utilizing data from tracheid effect scanning of the four sides of the timber board. Thereafter, a continuous wavelet transform (CWT), similar to fast Fourier transform, is applied on grey scale images from scanning, to analyse the variation of light intensity across the four surfaces at selected positions along the board. Obtained local frequencies correspond to the local annular ring pattern on surfaces. Then, assuming that annular growth rings are concentric circles with the pith in the centre, detected local annular ring wavelengths (using CWT) and artificial annual ring wavelengths corresponding to different hypothetical locations of pith are compared, and an optimization procedure is used to identify the location of pith that minimizes the discrepancy between the detected and artificial sets of annular ring wavelengths. Figure 1 shows grey scale images of short segments of longitudinal surfaces, graphs of the detected local annual ring widths, and a photograph of the board cross section where the determined location of pith is marked out. Preliminary results reveal that data from optical scanners and the suggested method allow for accurate detection of annular ring width and location of pith along boards.

  • 3.
    Habite, Tadios
    et al.
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Olsson, Anders
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Oscarsson, Jan
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Detection of Pith Location of Norway Spruce Timber Boards on the Basis of Optical Scanning2019In: Proceedings, 21st international nondestructive testing and evaluation of wood symposium: Freiburg, Germany / [ed] Xiping Wang; Udo H. Sauter; Robert J. Ross, Madison, U.S.A: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory , 2019, p. 268-275Conference paper (Refereed)
    Abstract [en]

    Optical scanners are used in the woodworking industry to detect various defects, such as dead and live knots, cracks, and fibre distortions, which are important for the visual appearance grading of wood. Data from scanning is also used to assess mechanical properties such as bending and tensile strength, for the purpose of machine strength grading of sawn timbers. Knowledge of annular ring width and location of pith in relation to board cross-sections, and how these properties vary in the longitudinal direction of boards, is relevant for many purposes, such as assessment of shape stability and mechanical properties of timber. Therefore, the purpose of the present research is to evaluate possibilities to determine annular ring width and location of pith on the basis of scanning of surfaces parallel to the longitudinal board direction. The first step of this novel method is to identify clear wood sections, free of defects along boards. Then time-frequency analysis is applied to assess the variation of light intensity over surfaces of these sections, such that local wavelengths, related to the annular ring width patterns are detected on all four surfaces around the board. Finally, the location of pith is calculated by comparing annular ring width distributions on the different surfaces, and assuming that annular rings are concentric circles with the pith in the centre. Results indicate that optical scanners and the suggested method allow for accurate detection of annular ring width and location of pith along boards.

  • 4.
    Vessby, Johan
    et al.
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Florisson, Sara
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Habite, Tadios
    Linnaeus University, Faculty of Technology, Department of Building Technology.
    Numerical simulation of moisture driven fracture in mechanical timber connection using XFEM2017In: CompWood 2017 ECCOMAS Thematic Conference on Computational Methods in Wood Mechanics – from Material Properties to Timber Structures, Vienna, Austria, June 7-9, 2017, TuVerlag , 2017, p. 25-25Conference paper (Refereed)
    Abstract [en]

    Structural timber and glulam elements are an appealing alternative when it comes to choosing between structural elements as load bearing parts in e.g. halls, arenas and residential buildings. The wooden material is relatively strong in respect to its weight and its stiffness is sufficient enough to allow its use in a wide range of applications. However, there are also challenges associated with handling the material, one of which is the dimensional instability associated with moisture changes. The effect of climate variations on moisture induced deformations, stresses and failure in timber structures has already been addressed by several researchers, see e.g. [1] and [2]. A numerical model developed in the finite element package Abaqus is proposed herein to simulate crack propagation caused by variation in climate. In mechanical connections moisture induced strains in combination with boundary conditions that introduces constraints can lead to crack development and in turn weakening of wooden structures. Previous application of fracture mechanics typically focused on crack development caused by pure mechanical loading, see e.g. [3] for methods summarized and typical applications. Within the scope of the current work a numerical model is presented to simulate moisture driven crack growth within the beam/column dowel group connection shown in Figure 1. The model consists of two dimensional hygro-mechanical plane stress and XFEM analysis coupled to a nonlinear transient moisture flow analysis. A visualization of the considered problem is given in Figure 1. This figure shows a beam to column connection, which is exposed to natural climate variation (a). A schematic description of the problem is shown in Figure 1 (b). Figure 1 (c) shows simulated moisture content gradient and significant cracked beam because of the deformation constraints imposed by the dowels. The transient non-linear moisture flow was modelled using Fick’s law of orthotropic diffusion, using different diffusion coefficient in the two main directions, the length direction of the beam (assumed parallel to the fibers) and the direction perpendicular to that. The moisture transport in parallel direction was taken to be dominant. The shrinkage coefficients experience different values in perpendicular and parallel direction, αperp and αpar, respectively. For the fracture model, the critical energy release rate, GIC, is set to 300 J/m2, the strength in the perpendicular direction, ft,perp, to 2.5 MPa and the stiffness perpendicular and parallel to the length directions of the fibres are Eperp= 500 MPa and Epar= 10 000 MPa respectively.

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