Wood quality of Eucalyptus from plantations : II. End spliting and sawing distorsion

End splits, distortions and cracks during sawing and drying frequently depreciate the quality of eucalypt solid wood products from fast growing plantations. These defects are mainly due to : 1) the release of growth strains when processing (=self balanced pre-stresses field in the standing trees : longitudinal tension in the outer part of the bole and longitudinal compression in the inner part), and 2) moisture content variations which alter wood structure (shrinkage or swelling). These defects affect the performance of wood structures and are related to the intensity of initial pre-stresses or shrinkages when drying and also, mainly, to the heterogeneity of these parameters. In this study, we are only presenting the consequences of growth strains. Mechanical models show that the cross cutting releases locally longitudinal pre-stresses but generates, on the open face of the log, tangential over-stresses similar to the failure stress. The more the longitudinal stress in the outer part and the compression in the core are important, the higher is the transversal over-stress near the pith when sawing. Experimental data underscore a close relation between the level of growth strains and an index which allows to quantification of end splits. However, this relation depends simultaneously on the natural weakness in the radial longitudinal plane and the grain angle. When sawing, modifications of the field of self-balarced stresses in a standing tree can generate distortions of wood. These distortions can be modelled using basic concepts of strength of materials, applied to long length beams according the axial direction. Tests on opening eucalypt logs in quarters has allowed establishing a relation between sawing distortions and growth strains measured on the surface of logs before sawing. Elaborating the model to take into account radial variations of the longitudinal modulus of elasticity and the circumferential asymmetry of growth strains has allowed obtaining good agreement between modelled values and measured values. (Résumé d'auteur