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

| white | 58 | .594 | .704 | 15.8 | 6.2 | 8.3 | | " | 62 | .603 | .696 | 14.3 | 4.9 | 9.0 | | " | 78 | .600 | .708 | 16.0 | 4.8 | 9.2 | | yellow | 77 | .573 | .669 | 14.2 | 4.5 | 9.7 | | " | 80 | .550 | | | | | | Osage orange | 31 | .761 | .838 | 8.9 | | | | Sycamore | 81 | .454 | .526 | 13.5 | 5.0 | 7.3 | | Tupelo | 121 | .475 | .545 | 12.4 | 4.4 | 7.9 | |----------------------------------------------------------------------------| |----------------------------------------------------------------------------| | TABLE XIV (CONT.) | |----------------------------------------------------------------------------| | SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS | | (Forest Service Cir. 213) | |----------------------------------------------------------------------------| | | | Specific gravity | Shrinkage from green to | | | | oven-dry, based on | oven-dry condition | | COMMON NAME | |--------------------+---------------------------| | OF SPECIES | Mois- | Volume | Volume | In | | Tangen- | | | ture | when | when | volume | Radial | tial | | | content | green | oven-dry | | | | |-----------------+---------+---------+----------+--------+--------+---------| | | Per | | | Per | Per | Per | | | cent | | | cent | cent | cent | | | | | | | | | | Conifers | | | | | | | | | | | | | | | | Arborvitæ | 55 | .293 | .315 | 7.0 | 2.1 | 4.9 | | Cedar, incense | 80 | .363 | | | | | | Cypress, bald | 79 | .452 | .513 | 11.5 | 3.8 | 6.0 | | Fir, alpine | 47 | .306 | .321 | 9.0 | 2.5 | 7.1 | | amabilis | 117 | .383 | | | | | | Douglas | 32 | .418 | .458 | 10.9 | 3.7 | 6.6 | | white | 156 | .350 | .437 | 10.2 | 3.4 | 7.0 | | Hemlock (east.) | 129 | .340 | .394 | 9.2 | 2.3 | 5.0 | | Pine, lodgepole | 44 | .370 | .415 | 11.3 | 4.2 | 7.1 | | " | 58 | .371 | .407 | 10.1 | 3.6 | 5.9 | | longleaf | 63 | .528 | .599 | 12.8 | 6.0 | 7.6 | | red or Nor | 54 | .440 | .507 | 11.5 | 4.5 | 7.2 | | shortleaf | 52 | .447 | | | | | | sugar | 123 | .360 | .386 | 8.4 | 2.9 | 5.6 | | west yellow | 98 | .353 | .395 | 9.2 | 4.1 | 6.4 | | " " | 125 | .377 | .433 | 11.5 | 4.3 | 7.3 | | " " | 93 | .391 | .435 | 9.9 | 3.8 | 5.8 | | white | 74 | .363 | .391 | 7.8 | 2.2 | 5.9 | | Redwood | 81 | .334 | | | | | | " | 69 | .366 | | | | | | Spruce, | | | | | | | | Engelmann | 45 | .325 | .359 | 10.5 | 3.7 | 6.9 | | " | 156 | .299 | .335 | 10.3 | 3.0 | 6.2 | | red | 31 | .396 | | | | | | white | 41 | .318 | | | | | | Tamarack | 52 | .491 | .558 | 13.6 | 3.7 | 7.4 | |----------------------------------------------------------------------------| This weight divided by 62.43 gives the specific gravity per green volume. It is purely a fictitious quantity. To convert this figure into actual density or specific gravity of the dry wood, it is necessary to know the amount of shrinkage in volume. If S is the percentage of shrinkage from the green to the oven-dry condition, based on the green volume; D, the density of the dry wood per cubic foot while green; and d the actual D density of oven-dry wood, then ---------- = d. 1 - .0 S This relation becomes clearer from the following analysis: Taking V and W as the volume and weight, respectively, when green, and v and w as the corresponding volume and weight when w W V - v oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and v V V V - v s = ------- X 100, in which S is the percentage of shrinkage v from the green to the oven-dry condition, based on the green volume, and s the same based on the oven-dry volume. In tables of specific gravity or density of wood it should always be stated whether the dry weight per unit of volume when green or the dry weight per unit of volume when dry is intended, since the shrinkage in volume may vary from 6 to 50 per cent, though in conifers it is usually about 10 per cent, and in hardwoods nearer 15 per cent. (See Table XIV.) COLOR In species which show a distinct difference between heartwood and sapwood the natural color of heartwood is invariably darker than that of the sapwood, and very frequently the contrast is conspicuous. This is produced by deposits in the heartwood of various materials resulting from the process of growth, increased possibly by oxidation and other chemical changes, which usually have little or no appreciable effect on the mechanical properties of the wood. (See HEARTWOOD AND SAPWOOD, above.) Some experiments[28] on very resinous longleaf pine specimens, however, indicate an increase in strength. This is due to the resin which increases the strength when dry. Spruce impregnated with crude resin and dried is greatly increased in strength thereby. [Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126, appendix.] Since the late wood of a growth ring is usually darker in color than the early wood, this fact may be used in judging the density, and therefore the hardness and strength of the material. This is particularly the case with coniferous woods. In ring-porous woods the vessels of the early wood not infrequently appear on a finished surface as darker than the denser late wood, though on cross sections of heartwood the reverse is commonly true. Except in the manner just stated the color of wood is no indication of strength. Abnormal discoloration of wood often denotes a diseased condition, indicating unsoundness. The black check in western hemlock is the result of insect attacks.[29] The reddish-brown streaks so common in hickory and certain other woods are mostly the result of injury by birds.[30] The discoloration is merely an indication of an injury, and in all probability does not of itself affect the properties of the wood. Certain rot-producing fungi impart to wood characteristic colors which thus become criterions of weakness. Ordinary sap-staining is due to fungous growth, but does not necessarily produce a weakening effect.[31] [Footnote 29: See Burke, H.E.: Black check in western hemlock. Cir. No. 61, U.S. Bu. Entomology, 1905.] [Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.] [Footnote 31: See Von Schrenck, Hermann: The "bluing" and the "red rot" of the western yellow pine, with special reference to the Black Hills forest reserve. Bul. No. 36, U.S. Bu. Plant Industry, Washington, 1903, pp. 13-14. Weiss, Howard, and Barnum, Charles T.: The prevention of sapstain in lumber. Cir. 192, U.S. Forest Service, Washington, 1911, pp. 16-17.] CROSS GRAIN _Cross grain_ is a very common defect in timber. One form of it is produced in lumber by the method of sawing and has no reference to the natural arrangement of the wood elements. Thus if the plane of the saw is not approximately parallel to the axis of the log the grain of the lumber cut is not parallel to the edges and is termed diagonal. This is likely to occur where the logs have considerable taper, and in this case may be produced if sawed parallel to the axis of growth instead of parallel to the growth rings. Lumber and timber with diagonal grain is always weaker than straight-grained material, the extent of the defect varying with the degree of the angle the fibres make with the axis of the stick. In the vicinity of large knots the grain is likely to be cross. The defect is most serious where wood is subjected to flexure, as in beams. _Spiral grain_ is a very common defect in a tree, and when excessive renders the timber valueless for use except in the round. It is produced by the arrangement of the wood fibres in a spiral direction about the axis instead of exactly vertical. Timber with spiral grain is also known as "torse wood." Spiral grain usually cannot be detected by casual inspection of a stick, since it does not show in the so-called visible grain of the wood, by which is commonly meant a sectional view of the annual rings of growth cut longitudinally. It is accordingly very easy to allow spiral-grained material to pass inspection, thereby introducing an element of weakness in a structure. There are methods for readily detecting spiral grain. The simplest is that of splitting a small piece radially. It is necessary, of course, that the split be radial, that is, in a plane passing through the axis of the log, and not tangentially. In the latter case it is quite probable that the wood would split straight, the line of cleavage being between the growth rings. In inspection, the elements to examine are the rays. In the case of oak and certain other hardwoods these rays are so large that they are readily seen not only on a radial surface, but on the tangential as well. On the former they appear as flakes, on the latter as short lines. Since these rays are between the fibres it naturally follows that they will be vertical or inclined according as the tree is straight-grained or spiral-grained. While they are not conspicuous in the softwoods, they can be seen upon close scrutiny, and particularly so if a small hand magnifier is used. When wood has begun to dry and check it is very easy to see whether or not it is straight- or spiral-grained, since the checks will for the most part follow along the rays. If one examines a row of telephone poles, for example, he will probably find that most of them have checks running spirally around them. If boards were sawed from such a pole after it was badly checked they would fall to pieces of their own weight. The only way to get straight material would be to split it out. It is for this reason that split billets and squares are stronger than most sawed material. The presence of the spiral grain has little, if any, effect on the timber when it is used in the round, but in sawed material the greater the pitch of the spiral the greater is the defect. KNOTS _Knots_ are portions of branches included in the wood of the stem or larger branch. Branches originate as a rule from the central axis of a stem, and while living increase in size by the addition of annual woody layers which are a continuation of those of the stem. The included portion is irregularly conical in shape with the tip at the pith. The direction of the fibre is at right angles or oblique to the grain of the stem, thus producing local cross grain. During the development of a tree most of the limbs, especially the lower ones, die, but persist for a time--often for years. Subsequent layers of growth of the stem are no longer intimately joined with the dead limb, but are laid around it. Hence dead branches produce knots which are nothing more than pegs in a hole, and likely to drop out after the tree has been sawed into lumber. In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place.[32] [Footnote 32: See Standard classification of structural timber. Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303. Contains three plates showing standard defects.] Knots materially affect checking and warping, ease in working, and cleavability of timber. They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration. The weakening effect is much more serious where timber is subjected to bending and tension than where under compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fibre, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. The knot, especially (as is often the case) if there is a season check in it, offers little resistance to this tensile stress. Small, knots, however, may be so located in a beam along the neutral plane as actually to increase the strength by tending to prevent longitudinal shearing. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects. Extensive experiments by the U.S. Forest Service[33] indicate the following effects of knots on structural timbers: [Footnote 33: Bul. 108, pp. 52 _et seq._] (1) Knots do not materially influence the stiffness of structural timber. (2) Only defects of the most serious character affect the elastic limit of beams. Stiffness and elastic strength are more dependent upon the quality of the wood fibre than upon defects in the beam. (3) The effect of knots is to reduce the difference between the fibre stress at elastic limit and the modulus of rupture of beams. The breaking strength is very susceptible to defects. (4) Sound knots do not weaken wood when subject to compression parallel to the grain.[34] [Footnote 34: Bul. 115, U.S. Forest Service: Mechanical properties of western hemlock, p. 20.] FROST SPLITS A common defect in standing timber results from radial splits which extend inward from the periphery of the tree, and almost, if not always, near the base. It is most common in trees which split readily, and those with large rays and thin bark. The primary cause of the splitting is frost, and various theories have been advanced to explain the action. R. Hartig[35] believes that freezing forces out a part of the imbibition water of the cell walls, thereby causing the wood to shrink, and if the interior layers have not yet been cooled, tangential strains arise which finally produce radial clefts. [Footnote 35: Hartig, R.: The diseases of trees (trans. by Somerville and Ward), London and New York, 1894, pp. 282-294.] Another theory holds that the water is not driven out of the cell walls, but that difference in temperature conditions of inner and outer layers is itself sufficient to set up the strains, resulting in splitting. An air temperature of 14°F. or less is considered necessary to produce frost splits. A still more recent theory is that of Busse[36] who considers the mechanical action of the wind a very important factor. He observed: (_a_) Frost splits sometimes occur at higher temperatures than 14°F. (_b_) Most splits take place shortly before sunrise, _i.e._, at the time of lowest air and soil temperature; they are never heard to take place at noon, afternoon, or evening. (_c_) They always occur between two roots or between the collars of two roots, (_d_) They are most frequent in old, stout-rooted, broad-crowned trees; in younger stands it is always the stoutest members that are found with frost splits, while in quite young stands they are altogether absent, (_e_) Trees on wet sites are most liable to splits, due to difference in wood structure, just as difference in wood structure makes different species vary in this regard. (_f_) Frost splits are most numerous less than three feet above the ground. [Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss. Centralb., XXXII, 2, 1910, pp. 74-81.] When a tree is swayed by the wind the roots are counteracting forces, and the wood fibres are tested in tension and compression by the opposing forces; where the roots exercise tension stresses most effectively the effect of compression stresses is at a minimum; only where the pressure is in excess of the tension, _i.e._, between the roots, can a separation of the fibre result. Hence, when by frost a tension on the entire periphery is established, and the wind localizes additional strains, failure occurs. The stronger the compression and tension, the severer the strains and the oftener failures occur. The occurrence of reports of frost splits on wind-still days is believed by Busse to be due to the opening of old frost splits where the tension produced by the frost alone is sufficient. Frost splits may heal over temporarily, but usually open up again during the following winter. The presence of old splits is often indicated by a ridge of callous, the result of the cambium's effort to occlude the wound. Frost splits not only affect the value of lumber, but also afford an entrance into the living tree for disease and decay. 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