The Mechanical Properties of Wood / Including a Discussion of the Factors Affecting the Mechanical Properties, and Methods of Timber Testing

by Samuel J. Record

SHAKES, GALLS, PITCH POCKETS

Heart shake occurs in nearly all overmature timber, being more frequent in hardwoods (especially oak) than in conifers. In typical heart shake the centre of the hole shows indications of becoming hollow and radial clefts of varying size extend outward from the pith, being widest inward. It frequently affects only the butt log, but may extend to the entire hole and even the larger branches. It usually results from a shrinkage of the heartwood due probably to chemical changes in the wood.

When it consists of a single cleft extending across the pith it is termed simple heart shake. Shake of this character in straight-grained trees affects only one or two central boards when cut into lumber, but in spiral-grained timber the damage is much greater. When shake consists of several radial clefts it is termed star shake. In some instances one or more of these clefts may extend nearly to the bark. In felled or converted timber clefts due to heart shake may be distinguished from seasoning cracks by the darker color of the exposed surfaces. Such clefts, however, tend to open up more and more as the timber seasons.

Cup or ring shake results from the pulling apart of two or more growth rings. It is one of the most serious defects to which sound timber is subject, as it seriously reduces the technical properties of wood. It is very common in sycamore and in western larch, particularly in the butt portion. Its occurrence is most frequent at the junction of two growth layers of very unequal thickness. Consequently it is likely to occur in trees which have grown slowly for a time, then abruptly increased, due to improved conditions of light and food, as in thinning. Old timber is more subject to it than young trees. The damage is largely confined to the butt log. Cup shake is often associated with other forms of shake, and not infrequently shows traces of decay.

The causes of cup shake are uncertain. The swaying action of the wind may result in shearing apart the growth layers, especially in trees growing in exposed places. Frost may in some instances be responsible for cup shake or at least a contributing factor, although trees growing in regions free from frost often have ring shake. Shrinkage of the heartwood may be concentric as well as radial in its action, thus producing cup shake instead of, or in connection with, heart shake.

A local defect somewhat similar in effect to cup shake is known as rind gall. If the cambium layer is exposed by the removal of the entire bark or rind it will die. Subsequent growth over the damaged portion does not cohere with the wood previously formed by the old cambium. The defect resulting is termed rind gall. The most common causes of it are fire, gnawing, blazing, chipping, sun scald, lightning, and abrasions.

Heart break is a term applied to areas of compression failure along the grain found in occasional logs. Sometimes these breaks are invisible until the wood is manufactured into the finished article. The occurrence of this defect is mostly limited to the dense hardwoods, such as hickory and to heavy tropical species. It is the source of considerable loss in the fancy veneer industry, as the veneer from valuable logs so affected drops to pieces.

The cause of heart break is not positively known. It is highly probable, however, that when the tree is felled the trunk strikes across a rock or another log, and the impact causes actual failure in the log as in a beam.

Resin or pitch pockets are of common occurrence in the wood of larch, spruce, fir, and especially of longleaf and other hard pines. They are due to accumulations of resin in openings between adjacent layers of growth. They are more frequent in trees growing alone than in those of dense stands. The pockets are usually a few inches in greatest dimension and affect only one or two growth layers. They are hidden until exposed by the saw, rendering it impossible to cut lumber with reference to their position. Often several boards are damaged by a single pocket. In grading lumber, pitch pockets are classified as small, standard, and large, depending upon their width and length.

INSECT INJURIES[³⁷]

The larvæ of many insects are destructive to wood. Some attack the wood of living trees, others only that of felled or converted material. Every hole breaks the continuity of the fibres and impairs the strength, and if there are very many of them the material may be ruined for all purposes where strength is required.

Some of the most common insects attacking the wood of living trees are the oak timber worm, the chestnut timber worm, carpenter worms, ambrosia beetles, the locust borer, turpentine beetles and turpentine borers, and the white pine weevil.

The insect injuries to forest products may be classed according to the stage of manufacture of the material. Thus round timber with the bark on, such as poles, posts, mine props, and sawlogs, is subject to serious damage by the same class of insects as those mentioned above, particularly by the round-headed borers, timber worms, and ambrosia beetles. Manufactured unseasoned products are subject to damage from ambrosia beetles and other wood borers. Seasoned hardwood lumber of all kinds, rough handles, wagon stock, etc., made partially or entirely of sapwood, are often reduced in value from 10 to 90 per cent by a class of insects known as powder-post beetles. Finished hardwood products such as handles, wagon, carriage and machinery stock, especially if ash or hickory, are often destroyed by the powder-post beetles. Construction timbers in buildings, bridges and trestles, cross-ties, poles, mine props, fence posts, etc., are sometimes seriously injured by wood-boring larvæ, termites, black ants, carpenter bees, and powder-post beetles, and sometimes reduced in value from 10 to 100 per cent. In tropical countries termites are a very serious pest in this respect.