_Stiffness, Elasticity, and Strength_--Rules for measuring the stiffness of timber are involved in mathematical formulas; but the practical quality of stiffness is not difficult to understand. Wood which does not bend easily is stiff. If it springs back to its original position after the removal of the force which bends it, the wood is elastic. The greatest load it can sustain without breaking, is the measure of its strength. The load required to produce a certain amount of bending is the measure of its stiffness. Flexibility, a term much used by certain classes of workers in wood, is the opposite of stiffness. A brittle wood is not necessarily weak. It may sustain a heavy load without breaking, but when it fails, the break is sudden and complete. A tough wood behaves differently, though it may not be as strong as a brittle one. When a tough wood breaks, the parts are inclined to adhere after they have ceased to sustain the load. Hickory is tough, and in breaking, the wood crushes and splinters. Mesquite is brittle, and a clean snap severs the stick at once.
Builders of houses and bridges, and the manufacturers of articles of wood, study with the greatest care the stiffness, elasticity, strength, toughness, and brittleness of timber. Its chief value may depend upon the presence or absence of one or more of these properties. Take away hickory’s toughness and elasticity and it would cease to be a great vehicle and handle material. Reduce the stiffness and strength of longleaf pine and Douglas fir and they would drop at once from the high esteem in which they are held as structural timbers. Destroy the brittleness of red cedar and it would lose one of the chief qualities which make it the leading lead pencil wood of the world.
There are recognized methods of measuring these important physical properties of woods, but they are expressed in language so technical that it means little to persons who are not specialists. For ordinary purposes, it is unnecessary to be more explicit than to state a certain wood is or is not strong, stiff, tough and elastic. Some species possess one or more of these properties to double the degree that others possess them. Different trees of the same species differ greatly, and even different parts of the same tree. Most tables of figures which show the various physical properties of woods, give averages only, not absolute values.