Determination of Stresses in Drying Wood by Means of a Viscoelastic Relaxation Model

International audienceStress states caused in wood by drying periods are often the source of considerable structural disorders, when this material is used as structural material. The origin of these stresses is generally due to the viscoelastic behaviour of the wood combined with the dimensional variations (shrinkage) related to the moisture content. In order to better understand this phenomenon, a slice of green wood is submitted to natural drying in stable environmental conditions. The wood slice is placed on an electronic balance so as so to measure the moisture content variation during the drying period. Simultaneously, the displacements caused in the slice by the drying are captured by a video camera. An incremental relaxation model based on the generalized Maxwell's chain is used to analyze the evolution of the stresses induced by the drying process within the wood slice. Numerical results show the development of tensile stresses in the material. Analyzing these results leads to the conclusion that the stresses are due to the orthotropic behaviour of the wood material combined with anisotropic drying shrinkage. A tensile stress concentration is evidenced in a zone where a crack was finally observed during the test. Keywords Green wood • Relaxation stresses • Drying process • Finite element method 5.1 Introduction The natural or the artificial drying of wood is commonly responsible for large strain and can lead to the cracking and the final collapse of the timbers structures. These defects are much more marked on the wood pieces cut and stored outdoors the long periods ago [1]. The timber is composed of polymers sensitive to the temperature and the humidity, therefore, its drying involves dimensional changes due to the shrinkage. Hence, the behaviour of wood during drying depends strongly in the one part, of the geometry, the moisture content gradient, the temperature, the sorption of wood history, but also, of the heterogeneous character, the orthotropic behaviour, and the viscoelastic effects of this material. It appears that, the phenomenon of shrinkage and the mechanical behaviour of wood during drying play an important role in the development of wood defects. In this context, it is essential to understand the contribution of different processes in the strain of the green wood during the drying phase. In the literature, various studies concerning the strain and the stress evolutions caused by drying of wood have been conducted. These works are generally based on complex analytical approaches solved by finite element models taking into account the changes of the mechanical properties of wood during the drying phase [2, 3]. In addition, several authors have presented drying models that can assess the distribution of moisture inside the wood during the shrinkage with moisture content near the fiber saturation point (FSP) [4]. Also, the mechanical behaviour of wood and tree has been proposed in order to know the impact of the biomechanical and the microstructures parameters in the old history [5]. Recently, the studies on the rheological behaviour of wood during growth phases have shown the effect of the viscoelastic effects in the recovery of the tree during its growth [6] without insist on the contribution of moisture content in the manifestation of this complex phenomenon