THE MECHANICAL PROPERTIES OF WOOD INCLUDING A DISCUSSION OF THE FACTORS AFFECTING THE MECHANICAL PROPERTIES, AND METHODS OF TIMBER TESTING TEXT

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] [Footnote 37: For detailed information regarding insect injuries, the reader is referred to the various publications of the U.S. Bureau of Entomology, Washington, D.C.] 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. MARINE WOOD-BORER INJURIES Vast amounts of timber used for piles in wharves and other marine structures are constantly being destroyed or seriously injured by marine borers. Almost invariably they are confined to salt water, and all the woods commonly used for piling are subject to their attacks. There are two genera of mollusks, _Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria, Chelura_, and _Sphoeroma_, that do serious damage in many places along both the Atlantic and Pacific coasts. These mollusks, which are popularly known as "shipworms," are much alike in structure and mode of life. They attack the exposed surface of the wood and immediately begin to bore. The tunnels, often as large as a lead pencil, extend usually in a longitudinal direction and follow a very irregular, tangled course. Hard woods are apparently penetrated as readily as soft woods, though in the same timber the softer parts are preferred. The food consists of infusoria and is not obtained from the wood substance. The sole object of boring into the wood is to obtain shelter. Although shipworms can live in cold water they thrive best and are most destructive in warm water. The length of time required to destroy an average barked, unprotected pine pile on the Atlantic coast south from Chesapeake Bay and along the entire Pacific coast varies from but one to three years. Of the crustacean borers, _Limnoria_, or the "wood louse," is the only one of great importance, although _Sphoeroma_ is reported destructive in places. _Limnoria_ is about the size of a grain of rice and tunnels into the wood for both food and shelter. The galleries extend inward radially, side by side, in countless numbers, to the depth of about one-half inch. The thin wood partitions remaining are destroyed by wave action, so that a fresh surface is exposed to attack. Both hard and soft woods are damaged, but the rate is faster in the soft woods or softer portions of a wood. Timbers seriously attacked by marine borers are badly weakened or completely destroyed. If the original strength of the material is to be preserved it is necessary to protect the wood from the borers. This is sometimes accomplished by proper injection of creosote oil, and more or less successfully by the use of various kinds of external coatings.[38] No treatment, however, has proved entirely satisfactory. [Footnote 38: See Smith, C. Stowell: Preservation of piling against marine wood borers. Cir. 128, U.S. Forest Service, 1908, pp. 15.] FUNGOUS INJURIES[39] [Footnote 39: See Von Schrenck, H.: The decay of timber and methods of preventing it. Bul. 14, U.S. Bu. Plant Industry, Washington, D.C., 1902. Also Buls. 32, 114, 214, 266. Meineoke, E.P.: Forest tree diseases common in California and Nevada, U.S. Forest Service, Washington, D.C., 1914. Hartig, R.: The diseases of trees. London and New York, 1894.] Fungi are responsible for almost all decay of wood. So far as known, all decay is produced by living organisms, either fungi or bacteria. Some species attack living trees, sometimes killing them, or making them hollow, or in the case of pecky cypress and incense cedar filling the wood with galleries like those of boring insects. A much larger variety work only in felled or dead wood, even after it is placed in buildings or manufactured articles. In any case the process of destruction is the same. The mycelial threads penetrate the walls of the cells in search of food, which they find either in the cell contents (starches, sugars, etc.), or in the cell wall itself. The breaking down of the cell walls through the chemical action of so-called "enzymes" secreted by the fungi follows, and the eventual product is a rotten, moist substance crumbling readily under the slightest pressure. Some species remove the ligneous matter and leave almost pure cellulose, which is white, like cotton; others dissolve the cellulose, leaving a brittle, dark brown mass of ligno-cellulose. Fungi (such as the bluing fungus) which merely stain wood usually do not affect its mechanical properties unless the attacks are excessive. It is evident, then, that the action of rot-causing fungi is to decrease the strength of wood, rendering it unsound, brittle, and dangerous to use. The most dangerous kinds are the so-called "dry-rot" fungi which work in many kinds of lumber after it is placed in the buildings. They are particularly to be dreaded because unseen, working as they do within the walls or inside of casings. Several serious wrecks of large buildings have been attributed to this cause. It is stated[40] that in the three years (1911-1913) more than $100,000 was required to repair damage due to dry rot. [Footnote 40: Dry rot in factory timbers, by Inspection Dept. Associated Factory Mutual Fire Insurance Cos., 31 Milk Street, Boston, 1913.] Dry rot develops best at 75°F. and is said to be killed by a temperature of 110°F.[41] Fully 70 per cent humidity is necessary in the air in which a timber is surrounded for the growth of this fungus, and probably the wood must be quite near its fibre saturation condition. Nevertheless _Merulius lacrymans_ (one of the most important species) has been found to live four years and eight months in a dry condition.[42] Thorough kiln-drying will kill this fungus, but will not prevent its redevelopment. Antiseptic treatment, such as creosoting, is the best prevention. [Footnote 41: Falck, Richard: Die Meruliusfaüle des Bauholzes, Hausschwammforschungen, 6. Heft., Jena, 1912.] [Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.] All fungi require moisture and air[43] for their growth. Deprived of either of these the fungus dies or ceases to develop. Just what degree of moisture in wood is necessary for the "dry-rot" fungus has not been determined, but it is evidently considerably above that of thoroughly air-dry timber, probably more than 15 per cent moisture. Hence the importance of free circulation of air about all timbers in a building. [Footnote 43: A culture of fungus placed in a glass jar and the air pumped out ceases to grow, but will start again as soon as oxygen is admitted.] Warmth is also conducive to the growth of fungi, the most favorable temperature being about 90°F. They cannot grow in extreme cold, although no degree of cold such as occurs naturally will kill them. On the other hand, high temperature will kill them, but the spores may survive even the boiling temperature. Mould fungus has been observed to develop rapidly at 130°F. in a dry kiln in moist air, a condition under which an animal cannot live more than a few minutes. This fungus was killed, however, at about 140° or 145°F.[44] [Footnote 44: Experiments in kiln-drying _Eucalyptus_ in Berkeley, U.S. Forest Service.] The fungus (_Endothia parasitica_ And.) which causes the chestnut blight kills the trees by girdling them and has no direct effect upon the wood save possibly the four or five growth rings of the sapwood.[45] [Footnote 45: See Anderson, Paul J.: The morphology and life history of the chestnut blight fungus. Bul. No. 7, Penna. Chestnut Tree Blight Com., Harrisburg, 1914, p. 17.] PARASITIC PLANT INJURIES.[46] [Footnote 46: See York, Harlan H.: The anatomy and some of the biological aspects of the "American mistletoe." Bul. 120, Sci. Ser. No. 13, Univ. of Texas, Austin, 1909. Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166, U.S. Bu. Plant Ind., Washington, 1910. Meinecke, E.P.: Forest tree diseases common in California and Nevada. U.S. Forest Service, Washington, 1914, pp. 54-58.] The most common of the higher parasitic plants damaging timber trees are mistletoes. Many species of deciduous trees are attacked by the common mistletoe (_Phoradendron flavescens_). It is very prevalent in the South and Southwest and when present in sufficient quantity does considerable damage. There is also a considerable number of smaller mistletoes belonging to the genus _Razoumofskya (Arceuthobium)_ which are widely distributed throughout the country, and several of them are common on coniferous trees in the Rocky Mountains and along the Pacific coast. One effect of the common mistletoe is the formation of large swellings or tumors. Often the entire tree may become stunted or distorted. The western mistletoe is most common on the branches, where it produces "witches' broom." It frequently attacks the trunk as well, and boards cut from such trees are filled with long, radial holes which seriously damage or destroy the value of the timber affected. LOCALITY OF GROWTH The data available regarding the effect of the locality of growth upon the properties of wood are not sufficient to warrant definite conclusions. The subject has, however, been kept in mind in many of the U.S. Forest Service timber tests and the following quotations are assembled from various reports: "In both the Cuban and longleaf pine the locality where grown appears to have but little influence on weight or strength, and there is no reason to believe that the longleaf pine from one State is better than that from any other, since such variations as are claimed can be found on any 40-acre lot of timber in any State. But with loblolly and still more with shortleaf this seems not to be the case. Being widely distributed over many localities different in soil and climate, the growth of the shortleaf pine seems materially influenced by location. The wood from the southern coast and gulf region and even Arkansas is generally heavier than the wood from localities farther north. Very light and fine-grained wood is seldom met near the southern limit of the range, while it is almost the rule in Missouri, where forms resembling the Norway pine are by no means rare. The loblolly, occupying both wet and dry soils, varies accordingly." Cir. No. 12, p. 6. " ... It is clear that as all localities have their heavy and their light timber, so they all share in strong and weak, hard and soft material, and the difference in quality of material is evidently far more a matter of individual variation than of soil or climate." _Ibid._, p.22 "A representative committee of the Carriage Builders' Association had publicly declared that this important industry could not depend upon the supplies of southern timber, as the oak grown in the South lacked the necessary qualities demanded in carriage construction. Without experiment this statement could be little better than a guess, and was doubly unwarranted, since it condemned an enormous amount of material, and one produced under a great variety of conditions and by at least a dozen species of trees, involving, therefore, a complexity of problems difficult enough for the careful investigator, and entirely beyond the few unsystematic observations of the members of a committee on a flying trip through one of the greatest timber regions of the world. "A number of samples were at once collected (part of them supplied by the carriage builders' committee), and the fallacy of the broad statement mentioned was fully demonstrated by a short series of tests and a more extensive study into structure and weight of these materials. From these tests it appears that pieces of white oak from Arkansas excelled well-selected pieces from Connecticut, both in stiffness and endwise compression (the two most important forms of resistance)." Report upon the forestry investigations of the U.S.D.A. 1877-1898, p. 331. See also Rep. of Div. of For., 1890, p. 209. "In some regions there are many small, stunted hickories, which most users will not touch. They have narrow sap, are likely to be birdpecked, and show very slow growth. Yet five of these trees from a steep, dry south slope in West Virginia had an average strength fully equal to that of the pignut from the better situation, and were superior in toughness, the work to maximum load being 36.8 as against 31.2 for pignut. The trees had about twice as many rings per inch as others from better situations. "This, however, is not very significant, as trees of the same species, age, and size, growing side by side under the same conditions of soil and situation, show great variation in their technical value. It is hard to account for this difference, but it seems that trees growing in wet or moist situations are rather inferior to those growing on fresher soil; also, it is claimed by many hickory users that the wood from limestone soils is superior to that from sandy soils. "One of the moot questions among hickory men is the relative value of northern and southern hickory. The impression prevails that southern hickory is more porous and brash than hickory from the north. The tests ... indicate that southern hickory is as tough and strong as northern hickory of the same age. But the southern hickories have a greater tendency to be shaky, and this results in much waste. In trees from southern river bottoms the loss through shakes and grub-holes in many cases amounts to as much as 50 per cent. "It is clear, therefore, that the difference in northern and southern hickory is not due to geographic location, but rather to the character of timber that is being cut. Nearly all of that from southern river bottoms and from the Cumberland Mountains is from large, old-growth trees; that from the north is from younger trees which are grown under more favorable conditions, and it is due simply to the greater age of the southern trees that hickory from that region is lighter and more brash than that from the north." Bul. 80, pp. 52-55. SEASON OF CUTTING It is generally believed that winter-felled timber has decided advantages over that cut at other seasons of the year, and to that cause alone are frequently ascribed much greater durability, less liability to check and split, better color, and even increased strength and toughness. The conclusion from the various experiments made on the subject is that while the time of felling may, and often does, affect the properties of wood, such result is due to the weather conditions rather than to the condition of the wood. There are two phases of this question. One is concerned with the physiological changes which might take place during the year in the wood of a living tree. The other deals with the purely physical results due to the weather, as differences in temperature, humidity, moisture, and other features to be mentioned later. Those who adhere to the first view maintain that wood cut in summer is quite different in composition from that cut in winter. One opinion is that in summer the "sap is up," while in winter it is "down," consequently winter-felled timber is drier. A variation of this belief is that in summer the sap contains certain chemicals which affect the properties of wood and does not contain them in winter. Again it is sometimes asserted that wood is actually denser in winter than in summer, as part of the wood substance is dissolved out in the spring and used for plant food, being restored in the fall. It is obvious that such views could apply only to sapwood, since it alone is in living condition at the time of cutting. Heartwood is dead wood and has almost no function in the existence of the tree other than the purely mechanical one of support. Heartwood does undergo changes, but they are gradual and almost entirely independent of the seasons. Sapwood might reasonably be expected to respond to seasonal changes, and to some extent it does. Just beneath the bark there is a thin layer of cells which during the growing season have not attained their greatest density. With the exception of this one annual ring, or portion of one, the density of the wood substance of the sapwood is nearly the same the year round. Slight variations may occur due to impregnation with sugar and starch in the winter and its dissolution in the growing season. The time of cutting can have no material effect on the inherent