Influence of growth stresses and material properties on distortion of sawn timber – numerical investigation

Issue		Ann. For. Sci. Volume 66, Number 6, September 2009


Article Number		604
Number of page(s)		10
DOI		10.1051/forest/2009045
Published online		01 September 2009

Ann. For. Sci. 66 (2009) 604
Number of pages: 10
DOI: 10.1051/forest/2009045

Influence of growth stresses and material properties on distortion of sawn timber – numerical investigation

Marie Johansson¹ and Sigurdur Ormarsson²

¹ School of Technology and Design, Växjö University, 35195 Växjö, Sweden
² Department of Civil Engineering Technical, University of Denmark, 2800 Lyngby, Denmark

Received 28 November 2008; Accepted 23 February 2009 ; Published online 1 September 2009

Abstract
• The board distortion that occurs during the sawing and the drying process causes major problems in the utilisation of sawn timber. The distortion is highly influenced by parameters such as spiral grain angle, modulus of elasticity, shrinkage, growth stresses and sawing pattern.
• In this study a finite element simulation of log sawing and timber drying was performed to study how these parameters interact to affect board distortion. A total of 81 logs with different material combinations were simulated. From each simulated log four boards with different annual ring orientation were studied.
• The results showed that the elastic modulus, shrinkage coefficient and growth stresses had a large influence on the final bow and spring deformation. After sawing of the log into boards, the release of growth stresses was the main contributor to the bow and spring deformation. For boards with low modulus of elasticity, the bending distortion became larger than for the boards with high modulus of elasticity. The twist deformation was very small after sawing but increased significantly during drying of the boards. The results showed that spiral grain angle and the board location within the log were the main contributors to the twist deformation.

Résumé - Influence des contraintes de croissance et des propriétés du matériau sur la déformation des sciages. Analyse numérique.
• La déformation des planches qui se produit au cours du sciage et du séchage conduit à des difficultés importantes pour l'utilisation des sciages. Les déformations sont très dépendantes de paramètres tels que l'angle du fil, le module d'élasticité, les retraits, les contraintes de croissance et le plan de débit.
• Dans cette étude nous avons réalisé une simulation par éléments finis du sciage de billons et de leur séchage pour étudier comment ces paramètres interagissent et affectent la distorsion des planches. Nous avons simulé un total de 81 billons avec différentes combinaisons des propriétés du matériau. Pour chaque billon quatre planches sont simulées avec différentes orientations des limites de cernes.
• Les résultats montrent que le module d'élasticité, les coefficients des retraits et les contraintes de croissance ont une grande influence sur la courbure finale de la face et sur la courbure de chant. Après le sciage des billons en planches, la libération des contraintes de croissance est le principal contributeur des deux déformations précédentes. Enfin, on montre que l'angle du fil et la position de la planche dans le billon sont les principales causes du gauchissement.

Key words: modulus of elasticity / longitudinal shrinkage / spiral grain angle / shape stability

Mots clés : module d'élasticité / retrait longitudinal / angle du fil / stabilité de la forme

Corresponding author: marie.johansson@vxu.se

© INRA, EDP Sciences 2009

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