Presence of knots and associated fiber deviation are crucial for engineering properties of sawn timber. Yet, there is a notable absence of a thoroughly calibrated and verified mathematical model for fiber directions. This gap is largely due to the lack of comprehensive and detailed experimental data on growth surface geometry and 3D fiber orientation. Such data, ideally extracted at the sawn timber level, should include diverse information related to single knots, multiple knots, knot clusters, and both live and dead knots. This study presents a comprehensive laboratory examination of a full-size Norway spruce timber board. The extraction of knots, growth surfaces, and full-volume 3D fiber directions was successfully achieved, yielding highly detailed experimental data. The method developed comprises X-ray computed tomography for 3D knot and growth surface geometry, and optical scanning utilizing the tracheid effect for in-plane fiber directions. A limitation was identified when the normal vector of growth surfaces and the normal vector of the optically scanned board surface are orthogonal but a sensitivity analysis revealed that an angle error introduced to the in-plane fiber directions has limited impact on the computed 3D fiber vectors when the angle between the two normal vectors is below 60 degrees. The 3D knot, growth surface geometries, and fiber patterns observed in this study clearly align with the patterns revealed by a previous micro-CT study. The method and data obtained are valuable for the subsequent development of a more refined and rigorously calibrated fiber angle model than those currently available.

Distance to the pith is a parameter that is known to be correlated with the mechanical properties of wood, but it is not utilized in strength grading machines. This study aimed to investigate how different the mechanical properties and grading yields are for Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) boards with small and large distances to the pith, respectively, and whether the distance to the pith could be an interesting parameter to use for strength grading in combination with other predictor variables. For this purpose, 221 boards were scanned to obtain fiber orientation and local density. Their dynamic modulus of elasticity and distance to the pith were measured, and they were finally tested in bending. The boards were classified into two categories: corewood if a board’s cross-section was entirely located within a radius of 200 mm from the pith, and outerwood otherwise. The results show that corewood presents lower mechanical properties than outerwood, explained especially by the higher knottiness of corewood. Distance to the pith improves the grading yields of a machine based on fiber orientation measurements, but using the dynamic modulus of elasticity rather than the distance to the pith leads to better results. Distance to the pith can be used as a single or secondary parameter to predict timber strength if the dynamic modulus of elasticity is not used.

To increase and optimize the use of wood in structural elements, a deep understanding of its mechanical behavior is necessary. The transverse material properties of wood are particularly important for mass timber construction and for utilizing wood as a strengthening material in timber connections. This study experimentally determined the stiffness and strength of Scots pine wood under compression perpendicular to the grain and rolling shear loading, as well as their dependence on the annual ring structure. A previously established biaxial test configuration was employed for this purpose. The modulus of elasticity in the radial direction was found to be about twice that in the tangential direction (687 vs. 372 N/mm²), although the strength in the tangential direction (5.19 N/mm²) was comparatively higher than that in the radial direction (4.70 N/mm²). For rolling shear, especially for the rolling shear modulus, a large variation was found, and its relationship with annual ring structure was assessed. The obtained RS modulus ranged from 50 to 254 N/mm², while RS strength was found to be between 2.14 and 4.61 N/mm². The results aligned well with previous findings.

The mechanical properties of structural timber largely depend on the occurrence of knots and on fibre deviation in their vicinities. In recent strength grading machines, lasers and cameras are used to detect surface characteristics such as the size and position of knots and local fibre orientation. Since laser dot scanning only gives reliable information about the fibre orientation in the plane of board surfaces, simple assumptions are usually made to define the inner fibre orientation to model timber boards. Those models would be improved by better insight into real fibre deviation around knots. In the present work, a laboratory method is developed to evaluate growth layers geometries and fibre orientation, solely based on the fact that the fibers are parallel to the tree rings and without any further assumptions. The method simply relies on color scans and laser dot scans of Douglas fir (Pseudotsuga menziesii) timber specimen sections revealed by successive planing. The proposed method provides data on fibre orientation in 3D with an accuracy that is relevant for the calibration of detailed models.

This paper proposes a one-dimensional convolutional long short-term memory (1D-CNN-LSTM) model for estimating the pith position and average ring width in Norway spruce timber boards. The model predicts these crosssectional parameters by processing sequences of light-intensity signals derived from optical scans of the board's four surfaces. The dataset used for training the model consists of synthetic boards sawn from simulated 3D logs. The model was evaluated on a dataset consisting of 552 end cross-sections from actual Norway spruce boards. Comparisons between the automatic and manual pith and ring width estimations demonstrated a very good accuracy. The computational speed of the model was more than twice as fast as the quickest method available in the literature. A large set of boards was then used to determine the advantages of incorporating the automatically determined average ring width in formulating indicating properties for machine strength grading. This evaluation revealed that the average ring width could, in certain situations, compensate for unknown variables such as density or resonance frequency in predicting the tensile and bending strength of Norway spruce boards.

Cross-Laminated Timber (CLT) is an engineered wood product composed of solid layers of glued sawn timber. In this study, essential material stiffness parameters for CLT made from Norway spruce and Scots pine are evaluated. Specifically, the longitudinal modulus of elasticity (MoE) for longitudinally oriented layers and the effective rolling shear modulus for transversely oriented layers are the focus. By combining finite element (FE) analysis with four-point, out-of-plane bend- ing tests using digital image correlation (DIC), a robust assessment of the effective rolling shear modulus of CLT layers is achieved. Additionally, eigenvalue analysis, applied to an FE model, along with resonance frequencies obtained from dynamic excitation of CLT, enables stable and simultaneous assessment of the dynamic longitudinal MoE and effective rolling shear modulus. Notably, while the dynamic MoE of longitudinal CLT layers is only 4% higher than the quasi-static local MoE, the dynamic effective rolling shear modulus of CLT layers is 40% higher than the quasi-static effective rolling shear modulus. This finding indicates a tangible viscoelastic behavior of wood concerning rolling shear.

The strength of cross laminated timber (CLT) depends on the stiffness and strength of the lamellas and on thestrength of the finger joints. A model for how stiffness and strength vary along and between lamellas is used incombination with a finite element model of CLT and Monte Carlo simulations to calculate out-of-plane bendingstrength of homogeneous and inhomogeneous CLT. Calculated and experimentally obtained results of characteristicbending strengths, coefficient of variation of bending strength and the proportion of finger joint failures,agree very well for both types of CLT. The characteristic out-of-plane bending strength and the mean bendingstiffness were 23% and 16% higher, respectively, for inhomogeneous CLT with outer layer lamellas graded in thestrength class C35, compared to homogeneous CLT with all lamellas graded in the class C24. Simulation resultsgive basis for simple equations by which bending strength of CLT can be determined as function of the layup, thestrength class of outer layer lamellas and characteristic strength of the finger joints. Furthermore, system effectsare investigated. For inhomogeneous CLT, with outer layer lamellas of high strength class, the system effects turnout to be quite different from those of ordinary, homogeneous CLT.