Mechanical Properties.—The trunk and woody branches of a tree owe their strength and stiffness mainly to their wood. Accordingly, wood must possess strength to resist breakage or permanent damage by slow crushing, stretching, bending and twist ing; it must also resist shock in the form of sudden application of these stresses : it must possess sufficient stiffness to give the requisite pillar-strength to the trunk, and when a deforming stress is removed must return to its former shape.
In conformity with its difference of structure along and across the grain, wood differs in strength and stiffness in these two main directions. As regards crushing (compression) and stretching (tension) wood is strongest and stiffest along the grain, and weak est at right angles to this : but as regards shear it is strongest at right angles to the grain : a beam in bending is strongest and stiffest when the load is applied perpendicular to the grain. Consequently,• joists, beams, floor-boards, posts, axe-handles, wheel-spokes and so forth are strongest when their grain is straight and parallel to the sides of these objects, and they are weakened by cross-grain and considerable knots.
The remarkable mechanical property of wood is its combination of strength and lightness: for instance, if the crushing strength be divided by the density in the cases of constructional wood and iron, steel and other metals, the resultant value is highest in wood. Nevertheless, the tensile strength of steel or iron is so much higher than that of wood that one of the former is used in engineering construction for members required to resist rupture by tension, whereas wood is often used where resistance to bending or com pression is needed. (See STRENGTH OF MATERIALS.) The peculiar elastic properties of wood render possible its em ployment as resonance-wood, used for instance in the bellies of violins, and the sounding boards of pianos. Wood undergoing decay soon declines in strength and elasticity, and even when only partly decayed emits when struck a dull, in place of a sharp ringing sound.
Wood, thanks to its structure, elasticity, low tensile strength, and relative softness, can be riven or cleft along the grain and yields a more or less smooth surface.
structural peculiarities that represent defects from the mechanical point of view but may enhance the value because of the decorative effect. This is the case where deviations of the grain such as wavy or curly grain, or abundance of knots, weaken the wood.
Always decreasing the value of timber are long splits, termed shakes, directed along the grain of the standing tree. These in cross-section may run parallel with the annual rings (or contour of the trunk) and are termed cup-shakes, and may assume the form of short arcs or complete circles (ring-shake) : in the latter case when the log is sawn the central part separates from the outer part. But in cross-section the shake may run in a radial direction, it is then a heart-shake: such a shake is widest towards the centre and tapers outward, and contrasts with the radial splits induced by drying of the felled timber which are wider at the outside and taper inwards. Heart-shake may assume the form, in cross-section, of a single split traversing the centre and is described as simple, or this split may be crossed at right angles by another thus producing double heart-shake, or, finally a number of splits may radiate from the centre and thus constitute star-shake, which is often associated with discolouration and decay. Radial frost-shakes are frequently associated with "frost-ribs" that are visible on the growing tree as ridges running down one or more sides of the trunk. With these shakes contrast the internal radially directed splits that first arise in felled timber which has been so rapidly dried that "case-hardening" has resulted.
In coniferous wood resin-containing cavities, known as resin pockets or pitch-pockets, occasionally occur : they are of varied dimensions but usually of width quite considerable in comparison with the length. Sometimes longer resin-veins easily visible to the naked eye traverse the wood or this may clearly show a general excess of resin in the absence of veins or pockets. In some cases excess of resin or abnormal development of the ducts is due to wood-attacking fungi. In hardwoods so-called gum-veins are occasionally present in Australian species of Eucalyptus, including jarrah, and in true mahoganies.