For a given species of wood, relative density is perhaps the best single predictor of relative strength. It seems logical that the higher the relative density, the harder and stronger the wood. This is illustrated by the following comparison of figures for a typical white oak with those for a typical species of poplar.
Timber Relative Modulus of Modulus of density elasticity rupture
Quercus spp. 0.68 11 700MN/m2 105 MN/m2 1.7 X 106 psi 15 200 psi
Populus spp. 0.34 7600 MN/m2 47 MN/m2 1.1 X 106 psi 6800 psi
Within a given species of wood, there is striking superiority of strength parallel to grain compared with that perpendicular to the grain. For example, among softwoods with average relative densities in the range of approximately 0.3-0.55, the tensile strength parallel to grain of air-dry woods is in the range of approximately 70-140 MN/m2 (10 000-20 000 psi), perpendicular-to-grain tensile strength averages only about 3-8% as great. Compression strength parallel to grain of dry softwoods is in the range of 30-60 MN/m2 (4000-9000 psi), perpendicular to grain only 8-25% of this value.
Strength properties for common species of cabinet wood are presented in Table 2.1.
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