Wood is mainly composed of oriented crystalline cellulose (~50%). This component has an elastic linear behavior with a Young's modulus around 130GPa [1], which gives wood its high stiffness. As a cristal, its Young's modulus is supposed to be time independent. The viscoelastic behavior of wood is then assumed to come from the matrix behavior and/or cinematic conditions at the interface between microfibrils and matrix, or between fibers, ... Studying the cinematic of crystals of cellulose in wood during a mechanical loading, and comparing it with wood cinematic at higher scale can produce valuable information to understand better the source wood macroscopic behavior. This methodology has already been used to understand the source of elastic behavior of wood using FTIR [2] or X-Ray diffraction [3], but not until now for the investigation of the viscous behavior of wood. The methodology of this study is to determine the cellulose crystals longitudinal strain (in average) using X-Ray diffraction technique from compression side to tension side during a four point bending creep test, and to compare with the longitudinal macroscopic strain on the same specimen. For this study, the tested specimens have been submitted to a constant load over time (~20h) in flexion, both in dry and wet conditions. For the dry specimens, the results show that there is a roughly linear relationship between macroscopic strain and the cellulose crystal strain during creep, with a ratio close to 1 between those two strains. This suggests that all of the creep macroscopic strain is visible in the cellulose crystals, so all of the compartments responsible of viscous behavior in wood are parallel to cellulose. For the Wet specimens, this same ratio is clearly higher than 1, which implies that cellulose strains more during creep than wood itself. This finding allows to propose rheological models of the wood viscoelastic behavior.
