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

is a measure of the wearing ability of wood and is an important consideration in the use of wood for floors, paving blocks, bearings, and rollers. While resistance to indentation is dependent mostly upon the density of the wood, the wearing qualities may be governed by other factors such as toughness, and the size, cohesion, and arrangement of the fibres. In use for floors, some woods tend to compact and wear smooth, while others become splintery and rough. This feature is affected to some extent by the manner in which the wood is sawed; thus edge-grain pine flooring is much better than flat-sawn for uniformity of wear. |-------------------------------------------------------------------| | TABLE XII | |-------------------------------------------------------------------| | HARDNESS OF 32 WOODS IN GREEN CONDITION, | | AS INDICATED BY THE LOAD REQUIRED TO IMBED | | A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER | | (Forest Service Cir. 213) | |-------------------------------------------------------------------| | COMMON NAME OF SPECIES | Average | End | Radial | Tangential | | | | surface | surface | surface | |------------------------+---------+---------+---------+------------| | | Pounds | Pounds | Pounds | Pounds | | | | | | | | Hardwoods | | | | | | | | | | | | 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 | | 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 | | 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 | | 4 White oak | 1,164 | 1,183 | 1,163 | 1,147 | | 5 Post oak | 1,099 | 1,139 | 1,068 | 1,081 | | 6 Black oak | 1,069 | 1,093 | 1,083 | 1,031 | | 7 Red oak | 1,043 | 1,107 | 1,020 | 1,002 | | 8 White ash | 1,046 | 1,121 | 1,000 | 1,017 | | 9 Beech | 942 | 1,012 | 897 | 918 | | 10 Sugar maple | 937 | 992 | 918 | 901 | | 11 Rock elm | 910 | 954 | 883 | 893 | | 12 Hackberry | 799 | 829 | 795 | 773 | | 13 Slippery elm | 788 | 919 | 757 | 687 | | 14 Yellow birch | 778 | 827 | 768 | 739 | | 15 Tupelo | 738 | 814 | 666 | 733 | | 16 Red maple | 671 | 766 | 621 | 626 | | 17 Sycamore | 608 | 664 | 560 | 599 | | 18 Black ash | 551 | 565 | 542 | 546 | | 19 White elm | 496 | 536 | 456 | 497 | | 20 Basswood | 239 | 273 | 226 | 217 | | | | | | | | Conifers | | | | | | | | | | | | 1 Longleaf pine | 532 | 574 | 502 | 521 | | 2 Douglas fir | 410 | 415 | 399 | 416 | | 3 Bald cypress | 390 | 460 | 355 | 354 | | 4 Hemlock | 384 | 463 | 354 | 334 | | 5 Tamarack | 384 | 401 | 380 | 370 | | 6 Red pine | 347 | 355 | 345 | 340 | | 7 White fir | 346 | 381 | 322 | 334 | | 8 Western yellow pine | 328 | 334 | 307 | 342 | | 9 Lodgepole pine | 318 | 316 | 318 | 319 | | 10 White pine | 299 | 304 | 294 | 299 | | 11 Engelmann pine | 266 | 272 | 253 | 274 | | 12 Alpine fir | 241 | 284 | 203 | 235 | |-------------------------------------------------------------------| | NOTE.--Black locust and hickory are not included in this table, | | but their position would be near the head of the list. | |-------------------------------------------------------------------| Tests for either form of hardness are of comparative value only. Tests for indentation are commonly made by penetrations of the material with a steel punch or ball.[16] Tests for abrasion are made by wearing down wood with sandpaper or by means of a sand blast. [Footnote 16: See articles by Gabriel Janka listed in bibliography, pages 151-152.] CLEAVABILITY _Cleavability_ is the term used to denote the facility with which wood is split. A splitting stress is one in which the forces act normally like a wedge. (See Fig. 21.) The plane of cleavage is parallel to the grain, either radially or tangentially. [Illustration: FIG. 21.--Cleavage of highly elastic wood. The cleft runs far ahead of the wedge.] This property of wood is very important in certain uses such as firewood, fence rails, billets, and squares. Resistance to splitting or low cleavability is desirable where wood must hold nails or screws, as in box-making. Wood usually splits more readily along the radius than parallel to the growth rings though exceptions occur, as in the case of cross grain. Splitting involves transverse tension, but only a portion of the fibres are under stress at a time. A wood of little stiffness and strong cohesion across the grain is difficult to split, while one with great stiffness, such as longleaf pine, is easily split. The form of the grain and the presence of knots greatly affect this quality. |---------------------------------------------| | TABLE XIII | |---------------------------------------------| | CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF | | 32 WOODS IN GREEN CONDITION | | (Forest Service Cir. 213) | |---------------------------------------------| | | When | When | | COMMON NAME | surface of | surface of | | OF SPECIES | failure is | failure is | | | radial | tangential | |-------------------+------------+------------| | | Lbs. per | Lbs. per | | | sq. inch | sq. inch | | | | | | Hardwoods | | | | | | | | Ash, black | 275 | 260 | | white | 333 | 346 | | Bashwood | 130 | 168 | | Beech | 339 | 527 | | Birch, yellow | 294 | 287 | | Elm, slippery | 401 | 424 | | white | 210 | 270 | | Hackberry | 422 | 436 | | Locust, honey | 552 | 610 | | Maple, red | 297 | 330 | | sugar | 376 | 513 | | Oak, post | 354 | 487 | | red | 380 | 470 | | swamp white | 428 | 536 | | white | 382 | 457 | | yellow | 379 | 470 | | Sycamore | 265 | 425 | | Tupelo | 277 | 380 | | | | | | Conifers | | | | | | | | Arborvitæ | 148 | 139 | | Cypress, bald | 167 | 154 | | Fir, alpine | 130 | 133 | | Douglas | 139 | 127 | | white | 145 | 187 | | Hemlock | 168 | 151 | | Pine, lodgepole | 142 | 140 | | longleaf | 187 | 180 | | red | 161 | 154 | | sugar | 168 | 189 | | western yellow | 162 | 187 | | white | 144 | 160 | | Spruce, Engelmann | 110 | 135 | | Tamarack | 167 | 159 | |---------------------------------------------| PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD INTRODUCTION Wood is an organic product--a structure of infinite variation of detail and design.[17] It is on this account that no two woods are alike--in reality no two specimens from the same log are identical. There are certain properties that characterize each species, but they are subject to considerable variation. Oak, for example, is considered hard, heavy, and strong, but some pieces, even of the same species of oak, are much harder, heavier, and stronger than others. With hickory are associated the properties of great strength, toughness, and resilience, but some pieces are comparatively weak and brash and ill-suited for the exacting demands for which good hickory is peculiarly adapted. [Footnote 17: For details regarding the structure of wood see Record, Samuel J.: Identification of the economic woods of the United States. New York, John Wiley & Sons, 1912.] It follows that no definite value can be assigned to the properties of any wood and that tables giving average results of tests may not be directly applicable to any individual stick. With sufficient knowledge of the intrinsic factors affecting the results it becomes possible to infer from the appearance of material its probable variation from the average. As yet too little is known of the relation of structure and chemical composition to the mechanical and physical properties to permit more than general conclusions. RATE OF GROWTH To understand the effect of variations in the rate of growth it is first necessary to know how wood is formed. A tree increases in diameter by the formation, between the old wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots. Under ordinary conditions one layer is formed each year and in cross section as on the end of a log they appear as rings--often spoken of as _annual rings_. These growth layers are made up of wood cells of various kinds, but for the most part fibrous. In timbers like pine, spruce, hemlock, and other coniferous or softwood species the wood cells are mostly of one kind, and as a result the material is much more uniform in structure than that of most hardwoods. (See Frontispiece.) There are no vessels or pores in coniferous wood such as one sees so prominently in oak and ash, for example. (See Fig. 22.) [Illustration: FIG. 22.--Cross sections of a ring-porous hardwood (white ash), a diffuse-porous hardwood (red gum), and a non-porous or coniferous wood (eastern hemlock). X 30. _Photomicrographs by the author._] The structure of the hardwoods is more complex. They are more or less filled with vessels, in some cases (oak, chestnut, ash) quite large and distinct, in others (buckeye, poplar, gum) too small to be seen plainly without a small hand lens. In discussing such woods it is customary to divide them into two large classes--_ring-porous_ and _diffuse-porous_. (See Fig. 22.) In ring-porous species, such as oak, chestnut, ash, black locust, catalpa, mulberry, hickory, and elm, the larger vessels or pores (as cross sections of vessels are called) become localized in one part of the growth ring, thus forming a region of more or less open and porous tissue. The rest of the ring is made up of smaller vessels and a much greater proportion of wood fibres. These fibres are the elements which give strength and toughness to wood, while the vessels are a source of weakness. In diffuse-porous woods the pores are scattered throughout the growth ring instead of being collected in a band or row. Examples of this kind of wood are gum, yellow poplar, birch, maple, cottonwood, basswood, buckeye, and willow. Some species, such as walnut and cherry, are on the border between the two classes, forming a sort of intermediate group. If one examines the smoothly cut end of a stick of almost any kind of wood, he will note that each growth ring is made up of two more or less well-defined parts. That originally nearest the centre of the tree is more open textured and almost invariably lighter in color than that near the outer portion of the ring. The inner portion was formed early in the season, when growth was comparatively rapid and is known as _early wood_ (also spring wood); the outer portion is the _late wood_, being produced in the summer or early fall. In soft pines there is not much contrast in the different parts of the ring, and as a result the wood is very uniform in texture and is easy to work. In hard pine, on the other hand, the late wood is very dense and is deep-colored, presenting a very decided contrast to the soft, straw-colored early wood. (See Fig. 23.) In ring-porous woods each season's growth is always well defined, because the large pores of the spring abut on the denser tissue of the fall before. In the diffuse-porous, the demarcation between rings is not always so clear and in not a few cases is almost, if not entirely, invisible to the unaided eye. (See Fig. 22.) [Illustration: FIG. 23.--Cross section of longleaf pine showing several growth rings with variations in the width of the dark-colored late wood. Seven resin ducts are visible. X 33. _Photomicrograph by U.S. Forest Service_] If one compares a heavy piece of pine with a light specimen it will be seen at once that the heavier one contains a larger proportion of late wood than the other, and is therefore considerably darker. The late wood of all species is denser than that formed early in the season, hence the greater the proportion of late wood the greater the density and strength. When examined under a microscope the cells of the late wood are seen to be very thick-walled and with very small cavities, while those formed first in the season have thin walls and large cavities. The strength is in the walls, not the cavities. In choosing a piece of pine where strength or stiffness is the important consideration, the principal thing to observe is the comparative amounts of early and late wood. The width of ring, that is, the number per inch, is not nearly so important as the proportion of the late wood in the ring. It is not only the proportion of late wood, but also its quality, that counts. In specimens that show a very large proportion of late wood it may be noticeably more porous and weigh considerably less than the late wood in pieces that contain but little. One can judge comparative density, and therefore to some extent weight and strength, by visual inspection. The conclusions of the U.S. Forest Service regarding the effect of rate of growth on the properties of Douglas fir are summarized as follows: "1. In general, rapidly grown wood (less than eight rings per inch) is relatively weak. A study of the individual tests upon which the average points are based shows, however, that when it is not associated with light weight and a small proportion of summer wood, rapid growth is not indicative of weak wood. "2. An average rate of growth, indicated by from 12 to 16 rings per inch, seems to produce the best material. "3. In rates of growths lower than 16 rings per inch, the average strength of the material decreases, apparently approaching a uniform condition above 24 rings per inch. In such slow rates of growth the texture of the wood is very uniform, and naturally there is little variation in weight or strength. "An analysis of tests on large beams was made to ascertain if average rate of growth has any relation to the mechanical properties of the beams. The analysis indicated conclusively that there was no such relation. Average rate of growth [without consideration also of density], therefore, has little significance in grading structural timber."[18] This is because of the wide variation in the percentage of late wood in different parts of the cross section. [Footnote 18: Bul. 88: Properties and uses of Douglas fir, p. 29.] Experiments seem to indicate that for most species there is a rate of growth which, in general, is associated with the greatest strength, especially in small specimens. For eight conifers it is as follows:[19] [Footnote 19: Bul. 108, U. S. Forest Service: Tests of structural timbers, p. 37.] Rings per inch Douglas fir 24 Shortleaf pine 12 Loblolly pine 6 Western larch 18 Western hemlock 14 Tamarack 20 Norway pine 18 Redwood 30 No satisfactory explanation can as yet be given for the real causes underlying the formation of early and late wood. Several factors may be involved. In conifers, at least, rate of growth alone does not determine the proportion of the two portions of the ring, for in some cases the wood of slow growth is very hard and heavy, while in others the opposite is true. The quality of the site where the tree grows undoubtedly affects the character of the wood formed, though it is not possible to formulate a rule governing it. In general, however, it may be said that where strength or ease of working is essential, woods of moderate to slow growth should be chosen. But in choosing a particular specimen it is not the width of ring, but the proportion and character of the late wood which should govern. In the case of the ring-porous hardwoods there seems to exist a pretty definite relation between the rate of growth of timber and its properties. This may be briefly summed up in the general statement that the more rapid the growth or the wider the rings of growth, the heavier, harder, stronger, and stiffer the wood. This, it must be remembered, applies only to ring-porous woods such as oak, ash, hickory, and others of the same group, and is, of course, subject to some exceptions and limitations. In ring-porous woods of good growth it is usually the middle portion of the ring in which the thick-walled, strength-giving fibres are most abundant. As the breadth of ring diminishes, this middle portion is reduced so that very slow growth produces comparatively light, porous wood composed of thin-walled vessels and wood parenchyma. In good oak these large vessels of the early wood occupy from 6 to 10 per cent of the volume of the log, while in inferior material they may make up 25 per cent or more. The late wood of good oak, except for radial grayish patches of small pores, is dark colored and firm, and consists of thick-walled fibres which form one-half or more of the wood. In inferior oak, such fibre areas are much reduced both in quantity and quality. Such variation is very largely the result of rate of growth. Wide-ringed wood is often called "second-growth," because the growth of the young timber in open stands after the old trees have been removed is more rapid than in trees in the forest, and in the manufacture of articles where strength is an important consideration such "second-growth" hardwood material is preferred. This is particularly the case in the choice of hickory for handles and spokes. Here not only strength, but toughness and resilience are important. The results of a series of tests on hickory by the U.S. Forest Service show that "the work or shock-resisting ability is greatest in wide-ringed wood that has from 5 to 14 rings per inch, is fairly constant from 14 to 38 rings, and decreases rapidly from 38 to 47 rings. The strength at maximum load is not so great with the most rapid-growing wood; it is maximum with from 14 to 20 rings per inch, and again becomes less as the wood becomes more closely ringed. The natural deduction is that wood of first-class mechanical value shows from 5 to 20 rings per inch and that