Strength also includes the ability to resist shear. This is called "shearing strength." It is a measure of the adhesion of one part of the wood to an adjoining part. Shearing is what takes place when the portion of wood beyond a mortise near the end of a timber, A B C D, Fig. 43, is forced out by the tenon. In this case it would be shearing along the grain, sometimes called detrusion. The resistance of the portion A B C D, i.e., its power of adhesion to the wood adjacent to it on both sides, is its shearing strength. If the mortised piece were forced downward until it broke off the tenon at the shoulder, that would be shearing across the grain. The shearing resistance either with or across the grain is small compared with tension and compression. Green wood shears much more easily than dry, because moisture softens the wood and this reduces the adhesion of the fibers to each other.[7]
CLEAVABILITY OF WOOD.
Closely connected with shearing strength is cohesion, a property usually considered under the name of its opposite, cleavability, i.e., the ease of splitting.
When an ax is stuck into the end of a piece of wood, the wood splits in advance of the ax edge. See Handwork in Wood, Fig. 59, p. 52. The wood is not cut but pulled across the grain just as truly as if one edge were held and a weight were attached to the other edge and it were torn apart by tension. The length of the cleft ahead of the blade is determined by the elasticity of the wood. The longer the cleft, the easier to split. Elasticity helps splitting, and shearing strength and hardness hinder it.
A normal piece of wood splits easily along two surfaces, (1) along any radial plane, principally because of the presence of the pith rays, and, in regular grained wood like pine, because the cells are radially regular; and (2) along the annual rings, because the spring-wood separates easily from the next ring of summer-wood. Of the two, radial cleavage is 50 to 100 per cent. easier. Straight-grained wood is much easier to split than cross-grained wood in which the fibers are interlaced, and soft wood, provided it is elastic, splits easier than hard. Woods with sharp contrast between spring and summer wood, like yellow pine and chestnut, split very easily tangentially.
All these facts are important in relation to the use of nails. For instance, the reason why yellow pine is hard to nail and bass easy is because of their difference in cleavability.
ELASTICITY OF WOOD.
Elasticity is the ability of a substance when forced out of shape,—bent, twisted, compressed or stretched, to regain its former shape. When the elasticity of wood is spoken of, its ability to spring back from bending is usually meant. The opposite of elasticity is brittleness. Hickory is elastic, white pine is brittle.
Stiffness is the ability to resist bending, and hence is the opposite of pliability or flexibility. A wood may be both stiff and elastic; it may be even stiff and pliable, as ash, which may be made into splints for baskets and may also be used for oars. Willow sprouts are flexible when green, but quite brittle when dry.