Improving strength grading of timber by grain angle measurement and mechanical modeling

Abstract

Timber strength grading has become a major issue in the European Union during the last years, due to the introduction of the Eurocode 5 and all its related standards. Currently, the most performing strength grading machines are able to locally detect the boards' knots sizes and positions and interpret this information through adapted grading models. The best lead to improve their accuracy seems to be the introduction of new information about the boards and adapt the mechanical model to take them in account. Small grain angle causes high reduction of clear wood's mechanical properties; local value of slope of grain appears to be of high interest. The aim of this study is to quantify the additional accuracy that grain angle information can bring to an optical scanner used as a strength grading machine. A specific grading model has been developed accordingly, and the results obtained for different machine/model/loading combinations are presented. These results show that slope of grain measurement can significantly improve the accuracy of the optical scanner, for both modulus of elasticity and modulus of rupture estimations.

Automatic measurement of wood fiber orientation and knot detection using an optical system based on heating conduction

“…With both methods the angles measured are 2D angles. Actually, to classify mechanically a board of wood using scattering method, the existing methods propose to compute the grain angle on the two main faces of the board and calculate the mean between them [12,21]. In the following, first are presented the results for different wood species, then for different surface finishes (before and after planing operation).…”

“…Indeed, wood is made of highly elongated cells of a diameter of a few tens of micrometers and a length that can be of several millimeters. Variations in slope of grain are especially great around the knots [11,12], because of the simultaneous growth of the trunk and a branch in different directions.…”

“…To predict bending strength, most of these grading techniques are extremely dependent on the correlation between the MOR and the density or the MOE, and thus only partially take into account local singularities. A recent study [12] shows that for a species with large knots (such as douglas fir produced in southern Europe), a mechanical model based on local measurements of singularities, and more specifically grain slope measurements, significantly improves MOR prediction, since timber strength decreases non-linearly as the slope of grain increases [13].…”

Strength grading on the basis of high resolution laser scanning and dynamic excitation: a full scale investigation of performance

“…Among these is a research group at TU Wien which, on the basis of X-ray and reconstruction of knots in 3D, recently has suggested new IPs for strength grading, for example, Lukacevic et al (2015). Another group, based in France and involving several universities and institutes, has suggested grading on the basis of X-ray in combination with fibre orientation information from tracheid effect scanning, for example Viguier et al (2015). Further, a group in Munich has recently published work on assessment of grading methods in use and on derivation of settings, for example Rais and Van de Kuilen (2015).…”

Wood fiber orientation assessment based on punctual laser beam excitation: A preliminary study

“…An accurate estimation of the wood fiber orientation is an interesting information to model the mechanical behaviour of timber [1]. In the state-of-the-art on Non-Destructive Testing (NDT), a method to assess the orientation of the fiber known as the capacitance technique has been proposed [2].…”