Robert Collet

Numerical study of the influence of veneer lathe checks on the elastic mechanical properties of laminated veneer lumber (LVL) made of beech

Abstract Laminated veneer lumber (LVL) is a well-known high-performance engineered wood product suitable for structural applications. However, the peeling process can induce lathe checks of the veneer with various depth and spatial frequencies. In this study, a finite element model (FEM) is proposed to describe and to analyze the influence of veneer lathe checks on the elastic properties of LVL. Firstly, the typical lathe check depths and frequencies were determined by means of different compression rates of the pressure bar when peeling. These experimental results served as input to the model to compare the influence of check depth and frequency on the elastic behavior of an LVL beam in four-point bending. The checks were modeled as free spaces in the cross-section that can be partially filled with glue. The results show that the longitudinal modulus of elasticity is marginally affected by checking, while the shear rigidity of the LVL beam is significantly reduced in edgewise bending if checks are not glued. Gluing checks, even under consideration of a low Young’s modulus of glue, highly reduces the effect of checking on the elastic mechanical properties of LVL.

Evaluation of the visual stress grading standard on French Spruce (Picea excelsa) and Douglas-fir (Pseudotsuga menziesii) sawn timber

Abstract• In this paper an evaluation of the visual grading standard for softwood sawn timber was made.• In order to do so, visual grading according to EN 518 and theoretical grading according to EN 338 and EN 384 (measurements of MOE, MOR and density) were applied to lumber. Two batches of 111 and 102 French boards were graded, respectively, of Spruce (Picea excelsa) and Douglas-fir (Pseudotsuga menziesii). For the visual grading the most discriminant criterion was noted: knots, cracks, wane, etc.• Finally, the results of the two grading methods were compared, and it was shown that the visual stress grading gave quite low results for our two French species.Résumé• Cette étude a pour objectif d’évaluer la pertinence des résultats du classement visuel de la norme EN 518 sur des sciages de bois français.• Pour cela 111 pièces d’épicéa (Picea excelsa) et 102 pièces de Douglas (Pseudotsuga menziesii) ont été classées en suivant d’une part le classement visuel de la norme EN 518 et en réalisant d’autre part un classement dit théorique suivant les normes EN 338 and EN 384 (mesure du Module de Young (MOE), contrainte de rupture (MOR) et masse volumique). Pour le classement visuel, le critère le plus discriminent a été relevé pour chaque planche : nœuds, fentes, flaches.• Finalement une comparaison entre les résultats des deux techniques de classements a été réalisée, et le faible rendement du classement visuel sur nos deux essences à été montré.

Analysis of chip formation mechanisms and modelling of slabber process

During the primary transformation in wood industry, logs are faced with conical rough milling cutters commonly named slabber or canter heads. Chips produced consist of raw materials for pulp paper and particleboard industries. The process efficiency of these industries partly comes from particle size distribution. However, chips formation is greatly dependent on milling conditions and material variability. Thus, this study aims at better understanding and predicting chips production in wood milling. The different mechanisms of their formation were studied through orthogonal cutting experiments at high cutting speed for beech and Douglas fir. Under these conditions, ejection of free water inside wood can be observed during fragmentation, particularly on beech. As previously seen in quasi-static experiments, chip thickness is proportional to the nominal cut thickness. Moreover, the grain orientation has a great influence on the cutting mechanisms, so as the nominal cut and the growth ring widths. This chip fragmentation study finally allows the improvement of the cutting conditions in rough milling. In order to optimize machine design as well as cutting geometry, a geometrical model of a generic slabber head was developed. This model allows the study of the effective cutting kinematics, the log-cutting edges interactions and the effective wood grain direction during cutting. This paper describes the great influence of the carriage position on cutting conditions. The results obtained here can be directly used by milling machine manufacturers.

Utilization of temperature kinetics as a method to predict treatment intensity and corresponding treated wood quality: Durability and mechanical properties of thermally modified wood

Wood heat treatment is an attractive alternative to improve decay resistance of wood species with low natural durability. However, this improvement of durability is realized at the expense of the mechanical resistance. Decay resistance and mechanical properties are strongly correlated to thermal degradation of wood cells wall components. Mass loss resulting from this degradation is a good indicator of treatment intensity and final treated wood properties. However, the introduction of a fast and accurate system for measuring this mass loss on an industrial scale is very difficult. Nowadays, many studies are conducted on the determination of control parameters which could be correlated with the treatment conditions and final heat treated wood quality such as decay resistance. The aim of this study is to investigate the relations between kinetics of temperature used during thermal treatment process representing heat treatment intensity, mass losses due to thermal degradation and conferred properties to heat treated wood. It might appear that relative area of treatment temperature curves is a good indicator of treatment intensity. Heat treatment with different treatment conditions (temperature-time) have been performed under vacuum, on four wood species (one hardwood and three softwoods) in order to obtain thermal degradation mass loses of 8, 10 and 12%. For each experiment, relative areas corresponding to temperature kinetics, mass loss, decay resistance and mechanical properties have been determined. Results highlight the statement that the temperature curves’ area constitutes a good indicator in the prediction of needed treatment intensity, to obtain required wood durability and mechanical properties such as bending resistance and Brinell hardness.

Index of efficiency for strength-grading machines for solid wood

The settings of strength-grading machine for structural pieces of wood are checked according to the EN 14081 standard. However, different machines have different performances depending on the accuracy of the estimation of the board’s properties, and there is no easy way to compare the efficiency of these machines especially if the machine does not use the same sampling. In this paper, we introduce an index called index of efficiency for grading machines. This parameter is in the range of 0–100% and allows to compare performances of different machines for a given set of grades. The computation of this index is based on the cost matrix method of the EN 14081 and requires to have the size matrix of a setting to be computed. Les réglages des machines de classement mécanique des bois de structure sont établis en accord avec la norme EN 14081. Cependant, en fonction de la précision de l’estimation des propriétés mécaniques, les machines auront des performances différentes. Il n’y a toutefois pas de méthode simple pour comparer l’efficience de ces différentes machines surtout si elles ne sont pas étalonnées avec le même échantillonnage. Dans cet article, nous introduisons un indice que nous appelons indice d’efficience pour machines de classement. Cet indice variant de 0 à 100% permet de comparer les performances de différentes machines pour une combinaison de classes donnée. Le calcul de cet indice est basé sur la méthode de la matrice coûts de l’EN 14081 et nécessite la connaissance de la matrice de contingence de chaque machine.

Experimental study of locally loaded timber in compression perpendicular to the grain

ABSTRACT Since the test method for compression strength perpendicular to the grain was changed from a partial area to a full area loading test, strength values for softwood in EN 338 were significantly reduced. Consequently, a change in Eurocode 5 was necessary to ensure an equivalent resistance level in compression perpendicular to the grain. Experimental results are presented with wood in compression perpendicular to the grain. A numerical model is also presented. This model was used to establish a rule of calculation proposed at the P21A-Committee of the technical and construction equipment standardization office (BNTEC).