13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Identification of fracture process under moisture variation in wood materials Frédéric Dubois1,*, Rostand Moutou Pitti2,3,4, Eric Fournely2,3, Destrebecq JF2,3 1 Heterogeneous Material Research Group, Civil Engineering and Durability department, University of Limoges, Egletons, 19300, France 2 Clermont Université, Université Blaise Pascal, Institut Pascal, BP 10448, F-63000 CLERMONT-FERRAND, France 3 CNRS, UMR 6602, IP, F-6317, Aubière, France F-63000, Clermont-Ferrand, France 4 CENAREST, IRT, BP 14070, Libreville, Gabon * Corresponding author: frédéric.dubois@unilim.fr Abstract The instantaneous fracture of wood material is commonly the source of important accident in the mechanical industries and civil engineering structures. This fact proves that it is necessary to propose a model capable to identify or predict the crack initiation in such materials. In this work, an analytical and numerical model taking into account the stress relaxation coupled with hygrothermal effects and an ageing viscoelastic behavior is proposed. This approach includes shrinkage – swelling effects with orthotropy properties in the radial and transversal directions. This model is applied to a green wood slice during the natural drying process. The viscoelastic properties inducing a partial hydric stress relaxation and a finite element model is proposed based on a orthotropic generalized Kelvin Voigt model taking into account a moisture content dependence on its rheological properties as mechano-sorptive effects. The fracture criterion is introduced according to the Tsai-Wu or Hill approaches rewritten in terms of energy criterion and the numerical model allows to focusing the crack growth initiation localization. Some experimental results are compared with numerical modeling to predict the shrinkage process integrating the crack growth initiation. Keywords Green wood drying, experimental characterization, finite element method, crack growth initiation 1. Introduction Aided by its environmental benefits, the wood material is widely used in engineering structures and also, in civil and industrial buildings. As structure element, this material is often machined after drying but also immediately after cutting as green wood. However, the natural or artificial shrinkage due to drying process is commonly responsible of deformations and crack appears in wood pieces or timber structures [1]. Among these deformations, the cup, twist, crook and bow are defects due to the moisture variations, and are even more marked on the wood pieces cut up and stored outdoor during a long time. These behaviors are mainly reinforced by the environmental effects, but also by the orthotropic nature, the heterogeneous aspect, and the anisotropic character of the wood material. In this fact, it is essential, for the lumber, to investigate the different processes that lead to these phenomena in this shape of green wood during the drying. After the tree felling, the wood log starts a drying phase. Initially, we note the diffusion of the free water molecules to reach the fiber saturation point. In a second step, the bound water migrates from the wood heart until the surface. During this phase, wood is subject to shrinkage swelling effect. The orthotropic properties in the radial tangential plane causes strain blockings inducing hygrothermal stresses. Moreover, in the hygroscopic domain, elastic properties are characterized by a moisture content dependence. The coupling between a tension stress increasing and the elastic property evolutions induce a mechano-sorptive process. Simultaneously, during the tree growth, it is usually observed a mechanical state distribution providing maturation stresses. The moisture content of freshly felled timber varies enormously, so that in the literature, the deformations and the evolution of drying-induced stresses in wood are studied according numerous methods. Thus, analytical approaches have been developed by Cave [2]. Also, the theoretical expression taking into account the drying time of thin lumber has been developed by Bekhta et al. [3]. Dubois et al. [4] have proposed the analytical approach of elastic response in wood under
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