The purpose of this study was to investigate the vibrational properties and corresponding anisotropicity in wood during different states of moisture sorption. Samples of maple (Acer spp.) and red oak (Quercus rubra Michx.f.) were moisture conditioned by the adsorption process from an ovendried state and by the desorption process from a water-saturated state. The dynamic modulus of elasticity (DMOE) and logarithmic decrement (8) were examined as a function of grain orientation during moisture change processes and under constant moisture contents (MC). It was observed that regardless of species and grain direction, the DMOE and [delta] were lower and higher, respectively, during the moisture change process compared with those measured without a change in MC. The increase in 8 value during adsorption was greater than that during desorption. These results suggest that wood in an unstable state shows lower elasticity and strength and higher damping properties than wood in an equilibrium state. Furthermore, results of this study demonstrate that a greater adsorption rate leads to greater destabilization during an adsorption process. The anisotropy in vibrational properties was found to vary between two species.
Wood is a complex biocomposite consisting of principally three structural components: cellulose, hemicellulose, and lignin. From the point of view of structure, wood is composed of elongated cells whose walls have a complex multilayered structure. In each layer, cellulose molecules are grouped together in long filaments called microfibrils embedded in a matrix substance composed of lignin and hemicelluloses with some amorphous cellulose around the surface of the microfibrils. As wood is a viscoelastic material, its mechanical properties are highly dependent on the elasticity of, as well as the internal friction within, the cell-wall polymers and matrix.
One convenient way of studying viscoelastic properties of wood is through vibrational analysis. A damped free vibration test was carried out with a cantilever beam vibration testing apparatus. The dynamic modulus of elasticity (DMOE) and internal friction (logarithmic decrement [[delta]]) measurements can be used to study the viscoelastic nature of wood. The DMOE is related to sound velocity and [delta] to sound absorption or damping within the wood so that, in themselves, both quantities are important as far as technological applications are concerned.
Because wood is a hygroscopic material, its physical and mechanical properties are highly affected by the change of its moisture content (MC). Since environmental conditions never remain stable and ambient relative humidity (RH) and temperature change continuously, repeated adsorption and desorption of moisture take place, and MC of wood seldom reaches an equilibrium level with the surroundings. It has been observed by several researchers that wood behaves differently during adsorption and desorption, and some of its physical and mechanical properties are influenced by the state of sorption (Goulet and Hernandez 1991; Hernandez 1993a, 1993b; Obataya et al. 1998; Naderi and Hernandez 1999; Chauhan and Aggarwal 2004; Takahashi et al. 2006). It is well known that under the same RH and temperature conditions, equilibrium moisture content (EMC) is found to be higher during desorption compared with adsorption and thus is...