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

velocity, and this velocity of the weight and beam gives to them an energy, or _vis viva_, which must now spend itself in overcoming an _excess_ of resistance over and above the imposed load, and the whole mass will not stop until the deflection (as well as the resistance) has come to be equal to _twice_ that corresponding to the static load imposed. Hence we say the effect of a suddenly imposed load is to produce twice the deflection and stress of the same load statically applied. It must be evident, however, that this case has nothing in common with either the ordinary 'static' tests of structural materials in testing-machines, or with impact tests."[13] [Footnote 13: Johnson, J.B.: The materials of construction, pp. 81-82.] (3) ~Impact, shock,~ or ~blow.~[14] There are various common uses of wood where the material is subjected to sudden shocks and jars or impact. Such is the action on the felloes and spokes of a wagon wheel passing over a rough road; on a hammer handle when a blow is struck; on a maul when it strikes a wedge. [Footnote 14: See Tiemann, Harry D.: The theory of impact and its application to testing materials. Jour. Franklin Inst., Oct., Nov., 1909, pp. 235-259, 336-364.] Resistance to impact is resistance to energy which is measured by the product of the force into the space through which it moves, or by the product of one-half the moving mass which causes the shock into the square of its velocity. The work done upon the piece at the instant the velocity is entirely removed from the striking body is equal to the total energy of that body. It is impossible, however, to get all of the energy of the striking body stored in the specimen, though the greater the mass and the shorter the space through which it moves, or, in other words, the greater the proportion of weight and the smaller the proportion of velocity making up the energy of the striking body, the more energy the specimen will absorb. The rest is lost in friction, vibrations, heat, and motion of the anvil. In impact the stresses produced become very complex and difficult to measure, especially if the velocity is high, or the mass of the beam itself is large compared to that of the weight. The difficulties attending the measurement of the stresses beyond the elastic limit are so great that commonly they are not reckoned. Within the elastic limit the formulæ for calculating the stresses are based on the assumption that the deflection is proportional to the stress in this case as in static tests. A common method of making tests upon the resistance of wood to shock is to support a small beam at the ends and drop a heavy weight upon it in the middle. (See Fig. 40.) The height of the weight is increased after each drop and records of the deflection taken until failure. The total work done upon the specimen is equal to the area of the stress-strain diagram plus the effect of local inertia of the molecules at point of contact. The stresses involved in impact are complicated by the fact that there are various ways in which the energy of the striking body may be spent: (_a_) It produces a local deformation of both bodies at the surface of contact, within or beyond the elastic limit. In testing wood the compression of the substance of the steel striking-weight may be neglected, since the steel is very hard in comparison with the wood. In addition to the compression of the fibres at the surface of contact resistance is also offered by the inertia of the particles there, the combined effect of which is a stress at the surface of contact often entirely out of proportion to the compression which would result from the action of a static force of the same magnitude. It frequently exceeds the crushing strength at the extreme surface of contact, as in the case of the swaging action of a hammer on the head of an iron spike, or of a locomotive wheel on the steel rail. This is also the case when a bullet is shot through a board or a pane of glass without breaking it as a whole. (_b_) It may move the struck body as a whole with an accelerated velocity, the resistance consisting of the inertia of the body. This effect is seen when a croquet ball is struck with a mallet. (_c_) It may deform a fixed body against its external supports and resistances. In making impact tests in the laboratory the test specimen is in reality in the nature of a cushion between two impacting bodies, namely, the striking weight and the base of the machine. It is important that the mass of this base be sufficiently great that its relative velocity to that of the common centre of gravity of itself and the striking weight may be disregarded. (_d_) It may deform the struck body as a whole against the resisting stresses developed by its own inertia, as, for example, when a baseball bat is broken by striking the ball. |-------------------------------------------------------| | TABLE X | |-------------------------------------------------------| | RESULTS OF IMPACT BENDING TESTS ON SMALL CLEAR BEAMS | | OF 34 WOODS IN GREEN CONDITION | | (Forest Service Cir. 213) | |-------------------------------------------------------| | | Fibre | | Work in | | COMMON NAME | stress at | Modulus of | bending | | OF SPECIES | elastic | elasticity | to | | | limit | | elastic | | | | | limit | |-------------------+-----------+------------+----------| | | | | In.-lbs. | | | Lbs. per | Lbs. per | per cu. | | | sq. in. | sq. in. | inch | | | | | | | Hardwoods | | | | | | | | | | Ash, black | 7,840 | 955,000 | 3.69 | | white | 11,710 | 1,564,000 | 4.93 | | Basswood | 5,480 | 917,000 | 1.84 | | Beech | 11,760 | 1,501,000 | 5.10 | | Birch, yellow | 11,080 | 1,812,000 | 3.79 | | Elm, rock | 12,090 | 1,367,000 | 6.52 | | slippery | 11,700 | 1,569,000 | 4.86 | | white | 9,910 | 1,138,000 | 4.82 | | Hackberry | 10,420 | 1,398,000 | 4.48 | | Locust, honey | 13,460 | 2,114,000 | 4.76 | | Maple, red | 11,670 | 1,411,000 | 5.45 | | sugar | 11,680 | 1,680,000 | 4.55 | | Oak, post | 11,260 | 1,596,000 | 4.41 | | red | 10,580 | 1,506,000 | 4.16 | | swamp white | 13,280 | 2,048,000 | 4.79 | | white | 9,860 | 1,414,000 | 3.84 | | yellow | 10,840 | 1,479,000 | 4.44 | | Osage orange | 15,520 | 1,498,000 | 8.92 | | Sycamore | 8,180 | 1,165,000 | 3.22 | | Tupelo | 7,650 | 1,310,000 | 2.49 | | | | | | | Conifers | | | | | | | | | | Arborvitæ | 5,290 | 778,000 | 2.04 | | Cypress, bald | 8,290 | 1,431,000 | 2.71 | | Fir, alpine | 5,280 | 980,000 | 1.59 | | Douglas | 8,870 | 1,579,000 | 2.79 | | white | 7,230 | 1,326,000 | 2.21 | | Hemlock | 6,330 | 1,025,000 | 2.19 | | Pine, lodgepole | 6,870 | 1,142,000 | 2.31 | | longleaf | 9,680 | 1,739,000 | 3.02 | | red | 7,480 | 1,438,000 | 2.18 | | sugar | 6,740 | 1,083,000 | 2.34 | | western yellow | 7,070 | 1,115,000 | 2.51 | | white | 6,490 | 1,156,000 | 2.06 | | Spruce, Engelmann | 6,300 | 1,076,000 | 2.09 | | Tamarack | 7,750 | 1,263,000 | 2.67 | |-------------------------------------------------------| Impact testing is difficult to conduct satisfactorily and the data obtained are of chief value in a relative sense, that is, for comparing the shock-resisting ability of woods of which like specimens have been subjected to exactly identical treatment. Yet this test is one of the most important made on wood, as it brings out properties not evident from other tests. Defects and brittleness are revealed by impact better than by any other kind of test. In common practice nearly all external stresses are of the nature of impact. In fact, no two moving bodies can come together without impact stress. Impact is therefore the commonest form of applied stress, although the most difficult to measure. _Failures in Timber Beams_ If a beam is loaded too heavily it will break or fail in some characteristic manner. These failures may be classified according to the way in which they develop, as tension, compression, and horizontal shear; and according to the appearance of the broken surface, as brash, and fibrous. A number of forms may develop if the beam is completely ruptured. Since the tensile strength of wood is on the average about three times as great as the compressive strength, a beam should, therefore, be expected to fail by the formation in the first place of a fold on the compression side due to the crushing action, followed by failure on the tension side. This is usually the case in green or moist wood. In dry material the first visible failure is not infrequently on the lower or tension side, and various attempts have been made to explain why such is the case.[15] [Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912, XXIII_{2}, pp. 12-13.] Within the elastic limit the elongations and shortenings are equal, and the neutral plane lies in the middle of the beam. (See TRANSVERSE OR BENDING STRENGTH: BEAMS, above.) Later the top layer of fibres on the upper or compression side fail, and on the load increasing, the next layer of fibres fail, and so on, even though this failure may not be visible. As a result the shortenings on the upper side of the beam become considerably greater than the elongations on the lower side. The neutral plane must be presumed to sink gradually toward the tension side, and when the stresses on the outer fibres at the bottom have become sufficiently great, the fibres are pulled in two, the tension area being much smaller than the compression area. The rupture is often irregular, as in direct tension tests. Failure may occur partially in single bundles of fibres some time before the final failure takes place. One reason why the failure of a dry beam is different from one that is moist, is that drying increases the stiffness of the fibres so that they offer more resistance to crushing, while it has much less effect upon the tensile strength. There is considerable variation in tension failures depending upon the toughness or the brittleness of the wood, the arrangement of the grain, defects, etc., making further classification desirable. The four most common forms are: (1)~Simple tension,~ in which there is a direct pulling in two of the wood on the under side of the beam due to a tensile stress parallel to the grain, (See Fig. 17, No. 1.) This is common in straight-grained beams, particularly when the wood is seasoned. [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 formulæ for horizontal shear. (See page 98 for this formulæ.) 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