Porous Structure of Wood and its Relaxation Modulus

Abstract

The aim of this study is to investigate the viscoelastic anisotropy in the transverse direction of wood in terms of its porous structure.Considering wood as a porous material consisting of the substance and the void, the following relation is given:logE=nlogθ+logES, where E is the apparent modulus, ES is the modulus of wood substance, θ is the volume fraction of wood substance and the numerical value of n is called “form exponent”. The contribution of the porous structures such as geometry and distribution of cells to the modulus of wood can be evaluated by the two factors, θ and n.The dependence of grain angle and angle of annual ring on n is discussed qualitatively as follows; we find that n in any direction is larger than three principal directions, and especially in RT-plane the angle of annual ring which is 45° takes the largest value of about 4 for n.The effect of the thickness of the sample on Young's modulus has been examined and it is found that the discrepancy between the theoretical and the experimental value of form exponent can be explained in terms of the effect of the thickness of the specimen.Finally, on the relaxation modulus at 20°C, 45% R.H., we find that n takes the value of about 1.1 in radial direction and about 1.5 in the tangential direction, and also that they are both independent of time.