# Elasticity of Wood-Based Materials

All of the materials presented above have three mutually perpendicular principal axes, defining their orthogonal anisotropy. Usually, it is assumed, however, that the thickness of boards is small in relation to other dimensions, and the elastic prop­erties of the wood-based materials in question adopt extreme values in the mutually orthogonal directions, while perpendicular to the thickness of the board. In other words, in two mutually perpendicular directions, the elastic properties of wood-based materials assume extreme values that is the largest in one direction and the smallest in the other direction. These directions correspond to the axes of symmetry of elastic properties, and it has been adopted to call them the main directions of anisotropy. Materials with such properties are called orthotropic. Therefore, Hooke’s law for the orthotropic body in a system of rectangular Cartesian coordinates, overlapping with the main directions of anisotropy (Fig. 4.20), can be written in the form:

where

Ex, Ey linear elasticity modules in the main directions of anisotropy, vx, vy Poisson’s ratios in the main directions of anisotropy,

Gxy shear elasticity module in the anisotropic plane,

ex, sy normal strains in the main directions of anisotropy and

Yxy shear strains in the anisotropic plane,

whereby the following dependency applies here

mxy myx

EX= Ey;

Tables 4.5, 4.6 and 4.7 show examples of values of mechanical properties of different types of wood-based materials.

 Property Unit Value Density kg/m3 550-680 Young’s modulus Ey MPa 3080 Young’s modulus Ex MPa 2530 Poisson’s ratio vyx 0.207 Poisson’s ratio vxy 0.282 Shear modulus Gyx MPa 794 Bending strength kgy MPa 13.3 Bending strength kgx MPa 11.9 Splitting strength kr MPa 0.15-0.20
 Table 4.5 Properties of chipboards (Wilczynski and Kociszewski 2000)

 Property Unit Value Density kg/m3 510-710 Young’s modulus Ey MPa 4000 Young’s modulus Ex MPa 3850 Poisson’s ratio vyx 0.30 Poisson’s ratio vxy 0.30 Shear modulus Gyx MPa G = E/2(1 + v) Bending strength kgy MPa 34.5 Bending strength kgx MPa 32.2 Splitting strength kr MPa 0.31
 Table 4.6 Properties of MDF boards (Wilczynski et al. 2001; Schulte and Fruhwald 1996)
 Property Unit Value Density kg/m3 550-800 Young’s modulus Ey MPa 3500-10,000 Young’s modulus Ex MPa 500-700 Poisson’s ratio vyx 0.439 Poisson’s ratio vxy 0.031 Shear modulus Gyx MPa 822, G = E/2(1 + v) Bending strength kgy MPa 30-100 Stretching strength krx MPa 30-60 Shearing strength kgy MPa 25-50
 Table 4.7 Properties of plywood

Updated: September 29, 2015 — 9:29 am