Growth layer and fibre orientation around knots in Norway spruce: a laboratory investigation

Abstract

The strength of structural timber largely depends on the occurrence of knots and on the local material directions in the surroundings of such knots. There is, however, a lack of methods for establishing a full dataset of the local material directions. The present research aims at the development and application of a laboratory method to assess the geometry of growth layers and the orientation of fibres in a high-resolution 3D grid within wood specimens containing knots. The laboratory method was based on optical flatbed scanning and laser scanning, the former resulting in surface images and the latter, utilizing the tracheid effect, resulting in in-plane fibre angles determined in high-resolution grids on scanned surfaces. A rectangular solid wood specimen containing a single knot was cut from a tree in such a way that it could be assumed that a plane of symmetry existed in the specimen. By splitting the specimen through this plane through the centre line of the knot, two new specimens with assumed identical but mirrored properties were achieved. On one of the new specimens, the longitudinal-radial plane was subsequently scanned, and the longitudinal-tangential plane was scanned on the other. Then, by repeatedly planing off material on both specimens followed by scanning of the new surfaces that gradually appeared, 3D coordinate positions along different growth layers and 3D orientation of fibres in a 3D grid were obtained. Comparisons between detected fibre orientation and growth layer geometry were used for the assessment of the accuracy obtained regarding 3D fibre orientation. It was shown that the suggested method is well suited to capture growth layer surfaces and that it provides reliable information on 3D fibre orientation close to knots. Such knowledge is of great importance for understanding the properties of timber including

Influence of Fiber Deviation on Strength of Thin Birch (Betula pendula Roth.) Veneers

“…The full curve could still reassemble the behaviour mentioned by literature. Based on findings in the literature [31,42], it was expected that the deviation out-of-plane α1 also exhibits high variability due to knots. However, the observed variability (see Figure 4) is rather low compared to the in-plane deviation α2.…”

“…The fiber deviation around knots is a very complex but common occurrence within wood. Mainly effected by the growth of the tree [29,30] fibers in the vicinity of knots deviate heavily from the dominating fiber direction in-as well as out-of-plane [31]. Trunk curvature or crookedness either occurs due to genetic disposition or as a result of external forces [32].…”

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

“…A low resolution or increased strain reference length is required to minimise the noise and make it small compared to the strain measured. A lower resolution allows a lower level of noise, because of the set accuracy with which the DIC system can measure displacements (Hu et al 2018). Strain calculated over 4-5 mm corresponded to a noise level in strain of approx.…”

Modelling local bending stiffness based on fibre orientation in sawn timber

“…However, they did not present any data from laboratory investigations to confirm the accuracy of calculated fibre orientation. Hu et al (2018) presented results of the 3D fibre orientation around a knot in sawn timber of Norway spruce based on a close laboratory investigation. Matthews and Beech (1976) patented a method based on directing a spot of light to a timber surface to recognize knots and other defects.…”