A 3D model for knots and related fiber deviations in sawn timber for prediction of mechanical properties of boards

Lukacevic, Markus; Kandler, Georg; Hu, Min; Olsson, Anders; Füssl, Josef

doi:10.1016/j.matdes.2019.107617

Cited by 47 publications

(28 citation statements)

References 30 publications

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“…2) close to, or in a knot for every board except for boards c, f and i with no knots in this zone, and mean values were − 0.49% for unmodified and − 0.72% for modified boards. The location of ε x,max and ε x,min was clearly related to the presence of knots both for unmodified and modified boards, as was shown earlier for unmodified spruce (Oscarsson et al 2012;Hu et al 2015Hu et al , 2016Lukacevic et al 2019). No significant increase in the mean value of the pairwise difference of ε x,max was found, i.e.…”

Section: Apparent Strain and Checkssupporting

confidence: 70%

“…Checks in knots could have a large effect on bending stiffness as well as on strength (Kollmann 1968). Lukacevic et al (2019) for example showed that simulation of a reduced local stiffness at sites of knots, used to take account the effect of checks in knots, gave good correlation with experimental results.…”

Section: Introductionmentioning

confidence: 93%

“…Knots in timber are known to reduce bending strength considerably, but also affect stiffness (Kollmann 1968;Johansson 2003;Hu et al 2016), and are thus taken into account in modern machine strength grading by fibre direction or X-ray scanning (Hanhijärvi and Ranta-Maunus 2008;Olsson et al 2013;Olsson and Oscarsson 2017). Digital image correlation (DIC) has proven a valuable technique to show how strains measured on the surface of wood increase around knots in timber that is loaded (Oscarsson et al 2012;Hu et al 2015Hu et al , 2016Lukacevic et al 2019). These strain concentrations are due to geometrical irregularities as well as changes in fibre direction close to the knot.…”

Section: Introductionmentioning

confidence: 99%

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Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

et al. 2020

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The effect of thermal modification (TM) on the chemistry, anatomy and mechanical properties of wood is often investigated using small clear samples. Little is known on the effect of growth-related and processing defects, such as knots and checks, on the bending strength and stiffness of thermally modified timber (TMT). Nine boards of Norway spruce with different combinations of knot types were used to study the combined effects of checks and knots on the bending behaviour of TMT. Digital image correlation (DIC) measurements on board surfaces at sites of knots subjected to bending allowed to study strain distribution and localise cracks prior to and after TM, and to monitor development of fracture (around knots) in TMT to failure. DIC confirmed that checking in knots was increased after TM compared to kiln-dried timber, specifically for intergrown knots and intergrown parts of encased knots. Effects appear local and do not affect board bending stiffness at these sites. Bending failure in TMT initiated mainly at knot interfaces or besides knots and fractures often propagated from checks. Scanning electron microscopy analyses of fracture surfaces confirmed this, and fractures were typically initiated around knots and at knot interfaces due to crack propagation along the grain in the longitudinal-radial plane (TL fracture) under mixed mode I and II loading, such that boards failed in simple tension like unmodified timber. Images of fracture surfaces at the ultrastructural level revealed details of the brittle behaviour of TM wood. This was especially apparent from the smooth appearance of transwall failure under mode I loading across the grain.

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Section: Apparent Strain and Checkssupporting

confidence: 70%

Section: Introductionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

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Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

et al. 2020

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“…Nonetheless, pith location and annual ring width, and how these properties change in the longitudinal direction of a board, are relevant for assessment of stiffness and strength (Hu et al 2016) and shape stability (Ormarsson et al 1999) of sawn timber. Knowledge of pith location is also needed to establish detailed and accurate three-dimensional (3D) models of sawn timber, including geometry of knots and local fibre orientation on the basis of surface scanning, and attempts to develop such models have been made in Hu et al (2016) and Lukacevic et al (2019). Furthermore, pith location and annual ring width affect the visual appearance of wood products.…”

Section: Introductionmentioning

confidence: 99%

Automatic detection of annual rings and pith location along Norway spruce timber boards using conditional adversarial networks

2021

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In the woodworking industry, detection of annual rings and location of pith in relation to timber 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 prediction of mechanical properties of timber. The current work aims at developing a fast, accurate and operationally simple deep learning-based algorithm for automatic detection of surface growth rings and pith location along knot-free clear wood sections of Norway spruce boards. First, individual surface growth rings that are visible along the four longitudinal sides of the scanned boards are detected using trained conditional generative adversarial networks (cGANs). Then, pith locations are determined, on the basis of the detected growth rings, by using a trained multilayer perceptron (MLP) artificial neural network. The proposed algorithm was solely based on raw images of board surfaces obtained from optical scanning and applied to a total of 104 Norway spruce boards with nominal dimensions of $$45\times 145\times 4500\,\hbox {mm}^{3}$$ 45 × 145 × 4500 mm 3 . The results show that optical scanners and the proposed automatic method allow for accurate and fast detection of individual surface growth rings and pith location along boards. For boards with the pith located within the cross section, median errors of 1.4 mm and 2.9 mm, in the x- and y-direction, respectively, were obtained. For a sample of boards with the pith located outside the board cross section in most positions along the board, the median discrepancy between automatically estimated and manually determined pith locations was 3.9 mm and 5.4 mm in the x- and y-direction, respectively.

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“…There, an algorithm is presented for the reconstruction of knot geometries in timber boards, based on laser scanning measurements of fiber angles. The implementation of the obtained morphology to computational models for the prediction of strength of timber beams and boards is shown in reference [7].…”

Section: Introductionmentioning

confidence: 99%

Modelling Inhomogeneity of Veneer Laminates with a Finite Element Mapping Method Based on Arbitrary Grayscale Images

Zerbst

Liebold²,

Gereke

et al. 2020

Materials

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Failure and deformation behavior of veneer laminates of ring porous wood species vary with the individual arrangement of early- and latewood zones over a veneer sheet. Therefore, a method is presented, where local failure and damage modes are considered for finite element models with respect to forming simulations, during the development process of automotive interior trim parts. Within the mapping tool Envyo, a routine has been realized for the discretization of early- and latewood zones from ash wood veneer surfaces to finite element meshes. The routine performs the following steps: reading a grayscale image of known size and generation of a point cloud based on the number of pixels; transformation and scaling of the generated point cloud to align with a target finite element mesh; nearest neighbor search and transfer of grayscale values to the target mesh element centroids; assigning part and therefore material properties to the target elements based on the mapped grayscale value and user-defined grayscale ranges. Due to the absence of measurement data for early- and latewood, optimization was used to identify locally varying material constants. A set of material input parameters for early- and latewood was created, calibrating the force-displacement response of tensile test simulations to corresponding experimental curves. The numerical results gave a very good agreement to the failure behavior of tensile tests in the loading directions longitudinal and transverse to the fiber orientation. Furthermore, in a stochastic analysis the characteristic distribution of tensile strength and ultimate strain could be verified for the suggested procedure. The introduced modelling approach can be applied for the discrete implementation of inhomogeneity to numerical simulations.

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A 3D model for knots and related fiber deviations in sawn timber for prediction of mechanical properties of boards

Cited by 47 publications

References 30 publications

Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

Automatic detection of annual rings and pith location along Norway spruce timber boards using conditional adversarial networks

Modelling Inhomogeneity of Veneer Laminates with a Finite Element Mapping Method Based on Arbitrary Grayscale Images

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