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

Abstract

Stress 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.