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The Mechanical Properties of Wood Part 4

[Illustration: FIG. 17.--Characteristic failures of simple beams.]

(2)~Cross-grained tension,~ in which the fracture is caused by a tensile force acting oblique to the grain. (See Fig. 17, No. 2.) This is a common form of failure where the beam has diagonal, spiral or other form of cross grain on its lower side. Since the tensile strength of wood across the grain is only a small fraction of that with the grain it is easy to see why a cross-grained timber would fail in this manner.

(3)~Splintering tension,~ in which the failure consists of a considerable number of slight tension failures, producing a ragged or splintery break on the under surface of the beam. (See Fig. 17, No. 3.) This is common in tough woods. In this case the surface of fracture is fibrous.

(4)~Brittle tension,~ in which the beam fails by a clean break extending entirely through it. (See Fig. 17, No. 4.) It is characteristic of a brittle wood which gives way suddenly without warning, like a piece of chalk. In this case the surface of fracture is described as brash.

~Compression failure~ (see Fig. 17, No. 5) has few variations except that it appears at various distances from the neutral plane of the beam. It is very common in green timbers. The compressive stress parallel to the fibres causes them to buckle or bend as in an endwise compressive test. This action usually begins on the top side shortly after the elastic limit is reached and extends downward, sometimes almost reaching the neutral plane before complete failure occurs. Frequently two or more failures develop at about the same time.

~Horizontal shear failure,~ in which the upper and lower portions of the beam slide along each other for a portion of their length either at one or at both ends (see Fig. 17, No. 6), is fairly common in air-dry material and in green material when the ratio of the height of the beam to the span is relatively large. It is not common in small clear specimens. It is often due to shake or season checks, common in large timbers, which reduce the actual area resisting the shearing action considerably below the calculated area used in the formulae for horizontal shear. (See page 98 for this formulae.) For this reason it is unsafe, in designing large timber beams, to use shearing stresses higher than those calculated for beams that failed in horizontal shear. The effect of a failure in horizontal shear is to divide the beam into two or more beams the combined strength of which is much less than that of the original beam. Fig. 18 shows a large beam in which two failures in horizontal shear occurred at the same end. That the parts behave independently is shown by the compression failure below the original location of the neutral plane.

[Illustration: FIG. 18.--Failure of a large beam by horizontal shear. _Photo by U. S, Forest Service._]

Table XI gives an analysis of the causes of first failure in 840 large timber beams of nine different species of conifers. Of the total number tested 165 were air-seasoned, the remainder green.

The failure occurring first signifies the point of greatest weakness in the specimen under the particular conditions of loading employed (in this case, third-point static loading).

----------------------------------------------------------- TABLE XI ----------------------------------------------------------- MANNER OF FIRST FAILURE OF LARGE BEAMS (Forest Service Bul. 108, p. 56) ----------------------------------------------------------- Total Per cent of total failing by COMMON NAME number ---------+-------------+------- OF SPECIES of Tension Compression Shear tests ------------------+--------+---------+-------------+------- Longleaf pine: green 17 18 24 58 dry 9 22 22 56 Douglas fir: green 191 27 72 1 dry 91 19 76 5 Shortleaf pine: green 48 27 56 17 dry 13 54 46 Western larch: green 62 23 71 6 dry 52 54 19 27 Loblolly pine: green 111 40 53 7 dry 25 60 12 28 Tamarack: green 30 37 53 10 dry 9 45 22 33 Western hemlock: green 39 21 74 5 dry 44 11 66 23 Redwood: green 28 43 50 7 dry 12 83 17 Norway pine: green 49 18 76 6 dry 10 30 60 10 ----------------------------------------------------------- NOTE.--These tests were made on timbers ranging in cross section from 4" x 10" to 8" x 16", and with a span of 15 feet. -----------------------------------------------------------

TOUGHNESS: TORSION

Toughness is a term applied to more than one property of wood.

Thus wood that is difficult to split is said to be tough. Again, a tough wood is one that will not rupture until it has deformed considerably under loads at or near its maximum strength, or one which still hangs together after it has been ruptured and may be bent back and forth without breaking apart. Toughness includes flexibility and is the reverse of brittleness, in that tough woods break gradually and give warning of failure. Tough woods offer great resistance to impact and will permit rougher treatment in manipulations attending manufacture and use.

Toughness is dependent upon the strength, cohesion, quality, length, and arrangement of fibre, and the pliability of the wood. Coniferous woods as a rule are not as tough as hardwoods, of which hickory and elm are the best examples.

The torsion or twisting test is useful in determining the toughness of wood. If the ends of a shaft are turned in opposite directions, or one end is turned and the other is fixed, all of the fibres except those at the axis tend to assume the form of helices. (See Fig. 19.) The strain produced by torsion or twisting is essentially shear transverse and parallel to the fibres, combined with longitudinal tension and transverse compression. Within the elastic limit the strains increase directly as the distance from the axis of the specimen. The outer elements are subjected to tensile stresses, and as they become twisted tend to compress those near the axis. The elongated elements also contract laterally. Cross sections which were originally plane become warped. With increasing strain the lateral adhesion of the outer fibres is destroyed, allowing them to slide past each other, and reducing greatly their power of resistance. In this way the strains on the fibres nearer the axis are progressively increased until finally all of the elements are sheared apart. It is only in the toughest materials that the full effect of this action can be observed. (See Fig.

20.) Brittle woods snap off suddenly with only a small amount of torsion, and their fracture is irregular and oblique to the axis of the piece instead of frayed out and more nearly perpendicular to the axis as is the case with tough woods.

[Illustration: FIG. 19.--Torsion of a shaft.]

[Illustration: FIG. 20.--Effect of torsion on different grades of hickory. _Photo by U. S. Forest Service._]

HARDNESS

The term _hardness_ is used in two senses, namely: (1) resistance to indentation, and (2) resistance to abrasion or scratching. In the latter sense hardness combined with toughness 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 Arborvitae 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.

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