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

slower growth yields poorer stock. Thus the inspector or buyer of hickory should discriminate against timber that has more than 20 rings per inch. Exceptions exist, however, in the case of normal growth upon dry situations, in which the slow-growing material may be strong and tough."[20] [Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.] The effect of rate of growth on the qualities of chestnut wood is summarized by the same authority as follows: "When the rings are wide, the transition from spring wood to summer wood is gradual, while in the narrow rings the spring wood passes into summer wood abruptly. The width of the spring wood changes but little with the width of the annual ring, so that the narrowing or broadening of the annual ring is always at the expense of the summer wood. The narrow vessels of the summer wood make it richer in wood substance than the spring wood composed of wide vessels. Therefore, rapid-growing specimens with wide rings have more wood substance than slow-growing trees with narrow rings. Since the more the wood substance the greater the weight, and the greater the weight the stronger the wood, chestnuts with wide rings must have stronger wood than chestnuts with narrow rings. This agrees with the accepted view that sprouts (which always have wide rings) yield better and stronger wood than seedling chestnuts, which grow more slowly in diameter."[21] [Footnote 21: Bul. 53: Chestnut in southern Maryland, pp. 20-21.] In diffuse-porous woods, as has been stated, the vessels or pores are scattered throughout the ring instead of collected in the early wood. The effect of rate of growth is, therefore, not the same as in the ring-porous woods, approaching more nearly the conditions in the conifers. In general it may be stated that such woods of medium growth afford stronger material than when very rapidly or very slowly grown. In many uses of wood, strength is not the main consideration. If ease of working is prized, wood should be chosen with regard to its uniformity of texture and straightness of grain, which will in most cases occur when there is little contrast between the late wood of one season's growth and the early wood of the next. HEARTWOOD AND SAPWOOD Examination of the end of a log of many species reveals a darker-colored inner portion--the _heartwood_, surrounded by a lighter-colored zone--the _sapwood_. In some instances this distinction in color is very marked; in others, the contrast is slight, so that it is not always easy to tell where one leaves off and the other begins. The color of fresh sapwood is always light, sometimes pure white, but more often with a decided tinge of green or brown. Sapwood is comparatively new wood. There is a time in the early history of every tree when its wood is all sapwood. Its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the food prepared in the leaves. The more leaves a tree bears and the more thrifty its growth, the larger the volume of sapwood required, hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. Sometimes trees grown in the open may become of considerable size, a foot or more in diameter, before any heartwood begins to form, for example, in second-growth hickory, or field-grown white and loblolly pines. As a tree increases in age and diameter an inner portion of the sapwood becomes inactive and finally ceases to function. This inert or dead portion is called heartwood, deriving its name solely from its position and not from any vital importance to the tree, as is shown by the fact that a tree can thrive with its heart completely decayed. Some, species begin to form heartwood very early in life, while in others the change comes slowly. Thin sapwood is characteristic of such trees as chestnut, black locust, mulberry, Osage orange, and sassafras, while in maple, ash, gum, hickory, hackberry, beech, and loblolly pine, thick sapwood is the rule. There is no definite relation between the annual rings of growth and the amount of sapwood. Within the same species the cross-sectional area of the sapwood is roughly proportional to the size of the crown of the tree. If the rings are narrow, more of them are required than where they are wide. As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume. Sapwood is thicker in the upper portion of the trunk of a tree than near the base, because the age and the diameter of the upper sections are less. When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and be broken off. Subsequent growth of wood may completely conceal the stubs which, however, will remain as knots. No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle. Consequently the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the heartwood. Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties, it follows that sapwood, because of its position in the tree, may have certain advantages over heartwood. It is really remarkable that the inner heartwood of old trees remains as sound as it usually does, since in many cases it is hundreds of years, and in a few instances thousands of years, old. Every broken limb or root, or deep wound from fire, insects, or falling timber, may afford an entrance for decay, which, once started, may penetrate to all parts of the trunk. The larvæ of many insects bore into the trees and their tunnels remain indefinitely as sources of weakness. Whatever advantages, however, that sapwood may have in this connection are due solely to its relative age and position. If a tree grows all its life in the open and the conditions of soil and site remain unchanged, it will make its most rapid growth in youth, and gradually decline. The annual rings of growth are for many years quite wide, but later they become narrower and narrower. Since each succeeding ring is laid down on the outside of the wood previously formed, it follows that unless a tree materially increases its production of wood from year to year, the rings must necessarily become thinner. As a tree reaches maturity its crown becomes more open and the annual wood production is lessened, thereby reducing still more the width of the growth rings. In the case of forest-grown trees so much depends upon the competition of the trees in their struggle for light and nourishment that periods of rapid and slow growth may alternate. Some trees, such as southern oaks, maintain the same width of ring for hundreds of years. Upon the whole, however, as a tree gets larger in diameter the width of the growth rings decreases. It is evident that there may be decided differences in the grain of heartwood and sapwood cut from a large tree, particularly one that is overmature. The relationship between width of growth rings and the mechanical properties of wood is discussed under Rate of Growth. In this connection, however, it may be stated that as a general rule the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier. It follows that in a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log. After exhaustive tests on a number of different woods the U.S. Forest Service concludes as follows: "Sapwood, except that from old, overmature trees, is as strong as heartwood, other things being equal, and so far as the mechanical properties go should not be regarded as a defect."[22] Careful inspection of the individual tests made in the investigation fails to reveal any relation between the proportion of sapwood and the breaking strength of timber. [Footnote 22: Bul. 108: Tests of structural timber, p. 35.] In the study of the hickories the conclusion was: "There is an unfounded prejudice against the heartwood. Specifications place white hickory, or sapwood, in a higher grade than red hickory, or heartwood, though there is no inherent difference in strength. In fact, in the case of large and old hickory trees, the sapwood nearest the bark is comparatively weak, and the best wood is in the heart, though in young trees of thrifty growth the best wood is in the sap."[23] The results of tests from selected pieces lying side by side in the same tree, and also the average values for heartwood and sapwood in shipments of the commercial hickories without selection, show conclusively that "the transformation of sapwood into heartwood does not affect either the strength or toughness of the wood.... It is true, however, that sapwood is usually more free from latent defects than heartwood."[24] [Footnote 23: Bul. 80: The commercial hickories, p. 50.] [Footnote 24: _Loc. cit._] Specifications for paving blocks often require that longleaf pine be 90 per cent heart. This is on the belief that sapwood is not only more subject to decay, but is also weaker than heartwood. In reality there is no sound basis for discrimination against sapwood on account of strength, provided other conditions are equal. It is true that sapwood will not resist decay as long as heartwood, if both are untreated with preservatives. It is especially so of woods with deep-colored heartwood, and is due to infiltrations of tannins, oils, and resins, which make the wood more or less obnoxious to decay-producing fungi. If, however, the timbers are to be treated, sapwood is not a defect; in fact, because of the relative ease with which it can be impregnated with preservatives it may be made more desirable than heartwood.[25] [Footnote 25: Although the factor of heart or sapwood does not influence the mechanical properties of the wood and there is usually no difference in structure observable under the microscope, nevertheless sapwood is generally decidedly different from heartwood in its physical properties. It dries better and more easily than heartwood, usually with less shrinkage and little checking or honeycombing. This is especially the case with the more refractory woods, such as white oaks and _Eucalyptus globulus_ and _viminalis_. It is usually much more permeable to air, even in green wood, notably so in loblolly pine and even in white oak. As already stated, it is much more subject to decay. The sapwood of white oak may be impregnated with creosote with comparative ease, while the heartwood is practically impenetrable. These facts indicate a difference in its chemical nature.--H.D. Tiemann.] In specifications for structural timbers reference is sometimes made to "boxheart," meaning the inclusion of the pith or centre of the tree within a cross section of the timber. From numerous experiments it appears that the position of the pith does not bear any relation to the strength of the material. Since most season checks, however, are radial, the position of the pith may influence the resistance of a seasoned beam to horizontal shear, being greatest when the pith is located in the middle half of the section.[26] [Footnote 26: Bul. 108, U.S. Forest Service, p. 36.] WEIGHT, DENSITY, AND SPECIFIC GRAVITY From data obtained from a large number of tests on the strength of different woods it appears that, other things being equal, the crushing strength parallel to the grain, fibre stress at elastic limit in bending, and shearing strength along the grain of wood vary in direct proportion to the weight of dry wood per unit of volume when green. Other strength values follow different laws. The hardness varies in a slightly greater ratio than the square of the density. The work to the breaking point increases even more rapidly than the cube of density. The modulus of rupture in bending lies between the first power and the square of the density. This, of course, is true only in case the greater weight is due to increase in the amount of wood substance. A wood heavy with resin or other infiltrated substance is not necessarily stronger than a similar specimen free from such materials. If differences in weight are due to degree of seasoning, in other words, to the relative amounts of water contained, the rules given above will of course not hold, since strength increases with dryness. But of given specimens of pine or of oak, for example, in the green condition, the comparative strength may be inferred from the weight. It is not permissible, however, to compare such widely different woods as oak and pine on a basis of their weights.[27] [Footnote 27: The oaks for some unknown reason fall below the normal strength for weight, whereas the hickories rise above. Certain other woods also are somewhat exceptional to the normal relation of strength and density.] The weight of wood substance, that is, the material which composes the walls of the fibres and other cells, is practically the same in all species, whether pine, hickory, or cottonwood, being a little greater than half again as heavy as water. It varies slightly from beech sapwood, 1.50, to Douglas fir heartwood, 1.57, averaging about 1.55 at 30° to 35° C., in terms of water at its greatest density 4° C. The reason any wood floats is that the air imprisoned in its cavities buoys it up. When this is displaced by water the wood becomes water-logged and sinks. Leaving out of consideration infiltrated substances, the reason a cubic foot of one kind of dry wood is heavier than that of another is because it contains a greater amount of wood substance. ~Density~ is merely the weight of a unit of volume, as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre. ~Specific gravity~ or relative density is the ratio of the density of any material to the density of distilled water at 4° C. (39.2° F.). A cubic foot of distilled water at 4° C. weighs 62.43 pounds. Hence the specific gravity of a piece of wood with a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight per cubic foot when the specific gravity is given, simply multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric system, since the weight of a cubic centimetre of pure water is one gram, the density in grams per cubic centimetre has the same numerical value as the specific gravity. Since the amount of water in wood is extremely variable it usually is not satisfactory to refer to the density of green wood. For scientific purposes the density of "oven-dry" wood is used; that is, the wood is dried in an oven at a temperature of 100°C. (212°F.) until a constant weight is attained. For commercial purposes the weight or density of air-dry or "shipping-dry" wood is used. This is usually expressed in pounds per thousand board feet, a board foot being considered as one-twelfth of a cubic foot. Wood shrinks greatly in drying from the green to the oven-dry condition. (See Table XIV.) Consequently a block of wood measuring a cubic foot when green will measure considerably less when oven-dry. It follows that the density of oven-dry wood does not represent the weight of the dry wood substance in a cubic foot of green wood. In other words, it is not the weight of a cubic foot of green wood minus the weight of the water which it contains. Since the latter is often a more convenient figure to use and much easier to obtain than the weight of oven-dry wood, it is commonly expressed in tables of "specific gravity or density of dry wood." |----------------------------------------------------------------------------| | TABLE XIV | |----------------------------------------------------------------------------| | SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS | | (Forest Service Cir. 213) | |----------------------------------------------------------------------------| | | | Specific gravity | Shrinkage from green to | | | Mois- | oven-dry, based on | oven-dry condition | | COMMON NAME | ture |--------------------+---------------------------| | OF SPECIES | content | Volume | Volume | In | | Tangen- | | | | when | when | volume | Radial | tial | | | | green | oven-dry | | | | |-----------------+---------+---------+----------+--------+--------+---------| | | Per | | | Per | Per | Per | | | cent | | | cent | cent | cent | | | | | | | | | | Hardwoods | | | | | | | | | | | | | | | | Ash, black | 77 | 0.466 | | | | | | white | 38 | .550 | 0.640 | 12.6 | 4.3 | 6.4 | | " | 47 | .516 | .590 | 11.7 | | | | Basswood | 110 | .315 | .374 | 14.5 | 6.2 | 8.4 | | Beech | 61 | .556 | .669 | 16.5 | 4.6 | 10.5 | | Birch, yellow | 72 | .545 | .661 | 17.0 | 7.9 | 9.0 | | Elm, rock | 46 | .578 | | | | | | slippery | 57 | .541 | .639 | 15.5 | 5.1 | 9.9 | | white | 66 | .430 | | | | | | Gum, red | 71 | .434 | | | | | | Hackberry | 50 | .504 | .576 | 14.0 | 4.2 | 8.9 | | Hickory, | | | | | | | | big shellbark | 64 | .601 | | 17.6 | 7.4 | 11.2 | | " " | 55 | .666 | | 20.9 | 7.9 | 14.2 | | bitternut | 65 | .624 | | | | | | mockernut | 64 | .606 | | 16.5 | 6.9 | 10.4 | | " | 57 | .662 | | 18.9 | 8.4 | 11.4 | | " | 48 | .666 | | | | | | nutmeg | 76 | .558 | | | | | | pignut | 59 | .627 | | 15.0 | 5.6 | 9.8 | | " | 54 | .667 | | 15.3 | 6.3 | 9.5 | | " | 55 | .667 | | 16.9 | 6.8 | 10.9 | | " | 52 | .667 | | 21.2 | 8.5 | 13.8 | | shagbark | 65 | .608 | | 16.0 | 6.5 | 10.2 | | " | 58 | .646 | | 18.4 | 7.9 | 11.4 | | " | 64 | .617 | | | | | | " | 60 | .653 | | 15.5 | 6.5 | 9.7 | | water | 74 | .630 | | | | | | Locust, honey | 53 | .695 | .759 | 8.6 | | | | Maple, red | 69 | .512 | | | | | | sugar | 57 | .546 | .643 | 14.3 | 4.9 | 9.1 | | " | 56 | .577 | | | | | | Oak, post | 64 | .590 | .732 | 16.0 | 5.7 | 10.6 | | red | 80 | .568 | .660 | 13.1 | 3.7 | 8.3 | | swamp white | 74 | .637 | .792 | 17.7 | 5.5 | 10.6 | | tanbark | 88 | .585 | | | | |