13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fracture Behavior in Timber Element Under Climatic Variations Frédéric Lamy 1, Frédéric Dubois 1,*, Octavian Pop 1, Mokhfi Takarli 1, Nicolas Angellier 1, Nicolas Larcher 1 1 Heterogeneous Material Research Group, Civil Engineering and Durability department, University of Limoges, Egletons, 19300, France * Corresponding author: frédéric.dubois@unilim.fr Abstract Timber elements placed in outdoor conditions are solicited by mechanical loadings and climatic variations. Joints or notched beams are subjected to stress concentrations which can cause a crack growth process due to drying phases. This paper deals with the experimental and analytical approaches about the fracture process in timber element subjected to different moisture content conditions. Two lots of SEN specimens are conditioned in different air conditions and tested in opening mode. A quasi-brittle fracture behavior is studied using a thermodynamic approach taking into account the dissipation based on the crack surface separation and the development of a process zone around the crack tip. The model includes the fracture and damage energy release rate concepts. Based on fracture and damage approaches, the dissipation separation is realized using the coupling of a finite element approach and image analysis tracking the visible crack tip during the experimental test. Experimental results and numerical treatments allow highlighting the moisture content effect on damage and crack growth with an increase of the ductility for high moisture content levels caused by the increase of the process zone size. This work allows envisaging the comprehension of the crack growth process under climatic variations introducing mechano-sorptive aspects. Keywords Fracture mechanic, Viscoelastic behavior, Mechano-sorptive effects 1. Introduction Timber elements placed in outdoor conditions are solicited by mechanical loadings and climatic variations. Around knots and joints, stress concentrations can induce crack growth initiation until fracture. Around these singularities, moisture content can be concentrated accentuated with climatic variations, .This coupling is usually treated by a mechano-sorptive approach coupling mechanical fields and moisture content gradients. In the past, a lot of works has been developed in order to understand this interaction at a material scale and for bending behavior. If we take the example of timber joints, the toughness can be accelerate by moisture content variations by provoking a crack initiation and the crack propagation until a partial collapse [1]. The one difficulty meet by the scientist community is the illustration of the mechano-sorptive effects in the fracture mechanic kinetic. Several scientific explorations have shown that coupling between moisture content variations and mechanical loading is subject to shrinkage-swelling effects and a modification of elastic or viscoelastic properties. In terms of thermodynamic visions, the last phenomenon is traduced, for constant mechanical loadings, by an increase of compliance properties during moistening phases and a blocking of strains during drying process corresponding to a partial storage of the free energy. These last observations request to put in evidence the local fracture behavior in moistening and drying phases. However, before to highlight the mechano-sorptive effects during wetting and drying phases, it is necessary to characterize the mechanical properties for dry and wet conditions. The first section deals with the experimental setup based on a double cantilever beam specimen in Douglas fire using a electromechanical testing machine associated with a regulated environment chamber. These tests allow the determination of the specimen compliance and the critical energy release rate versus the crack tip position for dry and wet conditions. Moisture content variations are stud in the second session. This part allows showing effects of a drying phase on the crack growth initiation under a creep loading.
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