 |
"(3) A high degree of steaming is injurious to wood in strength and spike-holding power. The degree of steaming at which pronounced harm results will depend upon the quality of the wood and its degree of seasoning, and upon the pressure (temperature) of steam and the duration of its application. For loblolly pine the limit of safety is certainly 30 pounds for 4 hours, or 20 pounds for 6 hours."[52]
[Footnote 52: Ibid., p. 21. See also Cir. 108, p. 19, table 5.]
Experiments made at the Yale Forest School showed that steaming above 30 pounds' gauge pressure reduces the strength of wood permanently while wet from 25 to 75 per cent.
PRESERVATIVES
The exact effects of chemical impregnation upon the mechanical properties of wood have not been fully determined, though they have been the subject of considerable investigation.[53] More depends upon the method of treatment than upon the preservatives used. Thus preliminary steaming at too high pressure or for too long a period will materially weaken the wood, (See TEMPERATURE, above.)
[Footnote 53: Hatt, W. K.: Experiments on the strength of treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]
The presence of zinc chloride does not weaken wood under static loading, although the indications are that the wood becomes brittle under impact. If the solution is too strong it will decompose the wood.
Soaking in creosote oil causes wood to swell, and accordingly decreases the strength to some extent, but not nearly so much so as soaking in water.[54]
[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p. 7.]
Soaking in kerosene seems to have no significant weakening effect.[55]
[Footnote 55: Tiemann, H.D.: Effect of moisture upon the strength and stiffness of wood. Bul. 70, U. S. Forest Service, 1907, pp. 122-123, tables 43-44.]
PART III TIMBER TESTING[56]
[Footnote 56: The methods of timber testing described here are for the most part those employed by the U. S. Forest Service. See Cir. 38 (rev. ed.), 1909.]
WORKING PLAN
Preliminary to making a series of timber tests it is very important that a working plan be prepared as a guide to the investigation. This should embrace: (1) the purpose of the tests; (2) kind, size, condition, and amount of material needed; (3) full description of the system of marking the pieces; (4) details of any special apparatus and methods employed; (5) proposed method of analyzing the data obtained and the nature of the final report. Great care should be taken in the preparation of this plan in order that all problems arising may be anticipated so far as possible and delays and unnecessary work avoided. A comprehensive study of previous investigations along the same or related lines should prove very helpful in outlining the work and preparing the report. (For sample working plan see Appendix.)
FORMS OF MATERIAL TESTED
In general, four forms of material are tested, namely: (1) large timbers, such as bridge stringers, car sills, large beams, and other pieces five feet or more in length, of actual sizes and grades in common use; (2) built-up structural forms and fastenings, such as built-up beams, trusses, and various kind of joints; (3) small clear pieces, such as are used in compression, shear, cleavage, and small cross-breaking tests; (4) manufactured articles, such as axles, spokes, shafts, wagon-tongues, cross-arms, insulator pins, barrels, and packing boxes.
As the moisture content is of fundamental importance (see WATER CONTENT, above.), all standard tests are usually made in the green condition. Another series is also usually run in an air-dry condition of about 12 per cent moisture. In all cases the moisture is very carefully determined and stated with the results in the tables.
SIZE OF TEST SPECIMENS
The size of the test specimen must be governed largely by the purpose for which the test is made. If the effect of a single factor, such as moisture, is the object of experiment, it is necessary to use small pieces of wood in order to eliminate so far as possible all disturbing factors. If the specimens are too large, it is impossible to secure enough perfect pieces from one tree to form a series for various tests. Moreover, the drying process with large timbers is very difficult and irregular, and requires a long period of time, besides causing checks and internal stresses which may obscure the results obtained.
On the other hand, the smaller the dimensions of the test specimen the greater becomes the relative effect of the inherent factors affecting the mechanical properties. For example, the effect of a knot of given size is more serious in a small stick than in a large one. Moreover, the smaller the specimen the fewer growth rings it contains, hence there is greater opportunity for variation due to irregularities of grain.
Tests on large timbers are considered necessary to furnish designers data on the probable strength of the different sizes and grades of timber on the market; their coefficients of elasticity under bending (since the stiffness rather than the strength often determines the size of a beam); and the manner of failure, whether in bending fibre stress or horizontal shear. It is believed that this information can only be obtained by direct tests on the different grades of car sills, stringers, and other material in common use.
When small pieces are selected for test they very often are clear and straight-grained, and thus of so much better grade than the large sticks that tests upon them may not yield unit values applicable to the larger sizes. Extensive experiments show, however, (1) that the modulus of elasticity is approximately the same for large timbers as for small clear specimens cut from them, and (2) that the fibre stress at elastic limit for large beams is, except in the weakest timbers, practically equal to the crushing strength of small clear pieces of the same material.[57]
[Footnote 57: Bul. 108, U. S. Forest Service: Tests of structural timbers, pp. 53-54.]
MOISTURE DETERMINATION
In order for tests to be comparable, it is necessary to know the moisture content of the specimens at the zone of failure. This is determined from disks an inch thick cut from the timber immediately after testing.
In cases, as in large beams, where it is desirable to know not only the average moisture content but also its distribution through the timber, the disks are cut up so as to obtain an outside, a middle, and an inner portion, of approximately equal areas. Thus in a section 10" x 12" the outer strip would be one inch wide, and the second one a little more than an inch and a quarter. Moisture determinations are made for each of the three portions separately.
The procedure is as follows:
(1) Immediately after sawing, loose splinters are removed and each section is weighed.
(2) The material is put into a drying oven at 100 deg. C. (212 deg. F.) and dried until the variation in weight for a period of twenty-four hours is less than 0.5 per cent.
(3) The disk is again carefully weighed.
(4) The loss in weight expressed in per cent of the dry weight indicates the moisture content of the specimen from which the specimen was cut.
MACHINE FOR STATIC TESTS
The standard screw machines used for metal tests are also used for wood, but in the case of wood tests the readings must be taken "on the fly," and the machine operated at a uniform speed without interruption from beginning to end of the test. This is on account of the time factor in the strength of wood. (See SPEED OF TESTING MACHINE, below.)
The standard machines for static tests can be used for transverse bending, compression, tension, shear, and cleavage. A common form consists of three main parts, namely: (1) the straining mechanism, (2) the weighing apparatus, and (3) the machinery for communicating motion to the screws.
The straining mechanism consists of two parts, one of which is a movable crosshead operated by four (sometimes two or three) upright steel straining screws which pass through openings in the platform and bear upward on the bed of the machine upon which the weighing platform rests as a fulcrum. At the lower ends of these screws are geared nuts all rotated simultaneously by a system of gears which cause the movable crosshead to rise and fall as desired.
The stationary part of the straining mechanism, which is used only for tension and cleavage tests, consists of a steel cage above the movable crosshead and rests directly upon the weighing platform. The top of the cage contains a square hole into which one end of the test specimen may be clamped, the crosshead containing a similar clamp for the other end, in making tension tests.
For testing long beams a special form of machine with an extended platform is used. (See Fig. 29.)
The weighing platform rests upon knife edges carried by primary levers of the weighing apparatus, the fulcrum being on the bed of the machine, and any pressure upon it is directly transmitted through a series of levers to the weighing beam. This beam is adjusted by means of a poise running on a screw. In operation the beam is kept floating by means of another poise moved back and forth by a screw which is operated by a hand wheel or automatically. The larger units of stress are read from the graduations along the side of the beam, while the intermediate smaller weights are observed on the dial on the rear end of the beam.
The machine is driven by power from a shaft or a motor and is so geared that various speeds are obtainable. One man can operate it.
In making tests the operation of the straining screws is always downward so as to bring pressure to bear upon the weighing platform. For tests in tension and cleavage the specimen is placed between the top of the stationary cage and the movable head and subjected to a pull. For tests in transverse bending, compression, and cleavage the specimen is placed between the movable head and the platform, and a direct compression force applied.
Testing machines are usually calibrated to a portion of their capacity before leaving the factory. The delicacy of the weighing levers is verified by determining the number of pounds necessary to move the beam between the stops while a load of 1,000 pounds rests on the platform. The usual requirement is that ten pounds should accomplish this movement.
The size of machine suitable for compression tests on 2" X 2" sticks or for 2" X 2" beams with 26 to 36-inch span has a capacity of 30,000 pounds.
SPEED OF TESTING MACHINE
In instructions for making static tests the rate of application of the stress, i.e., the speed of the machine, is given because the strength of wood varies with the speed at which the fibres are strained. The speed of the crosshead of the testing machine is practically never constant, due to mechanical defects of the apparatus and variations in the speed of the motor, but so long as it does not exceed 25 per cent the results will not be appreciably affected. In fact, a change in speed of 50 per cent will not cause the strength of the wood to vary more than 2 per cent.[58]
[Footnote 58: See Tiemann, Harry Donald: The effect of the speed of testing upon the strength and the standardization of tests for speed. Proc. Am. Soc. for Testing Materials, Vol. VIII, Philadelphia, 1908.]
Following are the formulae used in determining the speed of the movable head of the machine in inches per minute (n):
(1) For endwise compression n = Z l
Z l^{2} (2) For beams (centre loading) n = ————- 6h
Z l^{2} (3) For beams (third-pointloading) n = ————- 5.4h
Z = rate of fibre strain per inch of fibre length. l = span of beam or length of compression specimen. h = height of beam.
The values commonly used for Z are as follows:
Bending large beams Z = 0.0007 Bending small beams Z = 0.0015 Endwise compression-large specimens Z = 0.0015 Endwise compression-small " Z = 0.003 Right-angled compression-large " Z = 0.007 Right-angled compression-small " Z = 0.015 Shearing parallel to the grain Z = 0.015
Example: At what speed should the crosshead move to give the required rate of fibre strain in testing a small beam 2" X 2" X 30". (Span = 28".) Substituting these values in equation (2) above: (0.0015 X 28^2) n = ————————- = 0.1 inch per minute. (6 X 2)
In order that tests may be intelligently compared, it is important that account be taken of the speed at which the stress was applied. In determining the basis for a ratio between time and strength the rate of strain, which is controllable, and not the ratio of stress, which is circumstantial, should be used. In other words, the rate at which the movable head of the testing machine descends and not the rate of increase in the load is to be regulated. This ratio, to which the name speed-strength modulus has been given, may be expressed as a coefficient which, if multiplied into any proportional change in speed, will give the proportional change in strength. This ratio is derived from empirical curves. (See Table XVII.)
- TABLE XVII TABLE XVII - SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, loc. cit.) (Values in parentheses are approximate) - Rate of fibre strain. Ten-thousandths inch 2/3 2 6 18 54 162 486 per minute per inch -+ -+ -+ -+ -+ -+ -+ - C Speed of crosshead. O Inches per minute 0.000383 0.00115 0.00345 0.0103 0.0310 0.0931 .279 M P -+ -+ -+ -+ -+ -+ -+ - R Specimens Wet Dry All Wet Dry All Wet Dry All Wet Dry All Wet Dry All Wet Dry All Wet Dry All E -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ - S Relative S crushing 100.0 100.0 100.0 103.4 100.8 101.5 107.5 102.7 103.8 113.9 105.5 107.9 121.3 108.3 116.4 128.8 110.0 118.9 I strength O N Speed-strength 0.017 (0.006) (0.009) 0.033 0.012 0.016 0.047 0.021 0.029 0.053 0.027 0.039 0.060 0.023 0.049 (0.052) (0.015) (0.040) modulus, T -+ -+ -+ -+ -+ -+ -+ -+ - Speed of crosshead. Inches per minute 0.0072 0.0216 0.0648 0.194 0.583 1.75 5.25 B E -+ -+ -+ -+ -+ -+ -+ - N Specimens Wet Dry All Wet Dry All Wet Dry All Wet Dry All Wet Dry All Wet Dry All Wet Dry All D -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ - I Relative N crushing 97.4 99.0 98.2 100.0 100.0 100.0 105.1 102.1 103.7 111.3 105.8 108.1 117.9 108.6 112.7 123.7 109.6 116.3 126.3 110.3 118.9 G strength Speed-strength (0.014) (0.005) 0.012 0.033 0.014 0.026 0.049 0.026 0.037 0.053 0.033 0.038 0.049 0.014 0.035 0.038 0.006 0.025 (0.023) (0.004) (0.014) modulus, T - NOTE. The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively. -
BENDING LARGE BEAMS
Apparatus: A static bending machine (described above), with a special crosshead for third-point loading and a long platform bearing knife-edge supports, is required. (See Fig. 29.)
Preparing the material: Standard sizes and grades of beams and timbers in common use are employed. The ends are roughly squared and the specimen weighed and measured, taking the cross-sectional dimensions midway of the length. Weights should be to the nearest pound, lengths to the nearest 0.1 inch, and cross-sectional dimensions to the nearest 0.01 inch.
Marking and sketching: The butt end of the beam is marked A and the top end B. While facing A, the top side is marked a, the right hand b, the bottom c, the left hand d. Sketches are made of each side and end, showing (1) size, location, and condition of knots, checks, splits, and other defects; (2) irregularities of grain; (3) distribution of heartwood and sapwood; and on the ends: (4) the location of the pith and the arrangement of the growth rings, (5) number of rings per inch, and (6) the proportion of late wood.
The number of rings per inch and the proportion of late wood should always be determined along a radius or a line normal to the rings. The average number of rings per inch is the total number of rings divided by the length of the line crossing them. The proportion of late wood is equal to the sum of the widths of the late wood crossed by the line, divided by the length of the line. Rings per inch should be to the nearest 0.1; late wood to the nearest 0.1 per cent.
Since in large beams a great variation in rate of growth and relative amount of late wood is likely in different parts of the section, it is advisable to consider the cross section in three volumes, namely, the upper and lower quarters and the middle half. The determination should be made upon each volume separately, and the average for the entire cross section obtained from these results.
At the conclusion of the test the failure, as it appears on each surface, is traced on the sketches, with the failures numbered in the order of their occurrence. If the beam is subsequently cut up and used for other tests an additional sketch may be desirable to show the location of each piece.
Adjusting specimen in machine: The beam is placed in the machine with the side marked a on top, and with the ends projecting equally beyond the supports. In order to prevent crushing of the fibre at the points where the stress is applied it is necessary to use bearing blocks of maple or other hard wood with a convex surface in contact with the beam. Roller bearings should be placed between the bearing blocks and the knife edges of the crosshead to allow for the shortening due to flexure. (See Fig. 29.) Third-point loading is used, that is, the load is applied at two points one-third the span of the beam apart. (See Fig. 30.) This affords a uniform bending moment throughout the central third of the beam.
Measuring the deflection: The method of measuring the deflection should be such that any compression at the points of support or at the application of the load will not affect the reading. This may be accomplished by driving a small nail near each end of the beam, the exact location being on the neutral plane and vertically above each knife-edge support. Between these nails a fine wire is stretched free of the beam and kept taut by means of a rubber band or coiled spring on one end. Behind the wire at a point on the beam midway between the supports a steel scale graduated to hundredths of an inch is fastened vertically by means of thumb-tacks or small screws passing through holes in it. Attachment should be made on the neutral plane.
The first reading is made when the scale beam is balanced at zero load, and afterward at regular increments of the load which is applied continuously and at a uniform speed. (See SPEED OF TESTING MACHINE, above.) If desired, however, the load may be read at regular increments of deflection. The deflection readings should be to the nearest 0.01 inch. To avoid error due to parallax, the readings may be taken by means of a reading telescope about ten feet distant and approximately on a level with the wire. A mirror fastened to the scale will increase the accuracy of the readings if the telescope is not used. As in all tests on timber, the strain must be continuous to rupture, not intermittent, and readings must be taken "on the fly." The weighing beam is kept balanced after the yield point is reached and the maximum load, and at least one point beyond it, noted.
Log of the test: The proper log sheet for this test consists of a piece of cross-section paper with space at the margin for notes. (See Fig. 32.) The load in some convenient unit (1,000 to 10,000 pounds, depending upon the dimensions of the specimen) is entered on the ordinates, the deflection in tenths of an inch on the abscissae. The increments of load should be chosen so as to furnish about ten points on the stress-strain diagram below the elastic limit.
As the readings of the wire on the scale are made they are entered directly in their proper place on the cross-section paper. In many cases a test should be continued until complete failure results. The points where the various failures occur are indicated on the stress-strain diagram. A brief description of the failure is made on the margin of the log sheet, and the form traced on the sketches.
Disposal of the specimen: Two one-inch sections are cut from the region of failure to be used in determining the moisture content. (See MOISTURE DETERMINATION, above.) A two-inch section may be cut for subsequent reference and identification, and possible microscopic study. The remainder of the beam may be cut into small beams and compression pieces.
Calculating the results: The formulae used in calculating the results of tests on large rectangular simple beams loaded at third points of the span are as follows:
0.75 P (1) J = ———— b h
l (P_{1} + 0.75 W) (2) r = —————————— b h^{2}
l (P + 0.75 W) (3) R = ———————— b h^{2}
P_{1} l^{3} (4) E = ———————- 4.7 D b h^{3}
0.87 P_{1} D (5) S = ——————— 2 V
b, h, l = breadth, height, and span of specimen, inches. D = total deflection at elastic limit, inches. P = maximum load, pounds. P_{1} = load at elastic limit, pounds. E = modulus of elasticity, pounds per square inch. r = fibre stress at elastic limit, pounds per sq. inch. R = modulus of rupture, pounds per square inch. S = elastic resilience or work to elastic limit, inch-pounds per cu. in. J = greatest calculated longitudinal shear, pounds per square inch. V = volume of beam, cubic inches. W = weight of the beam.
In large beams the weight should be taken into account in calculating the fibre stress. In (2) and (3) three-fourths of the weight of the beam is added to the load for this reason.
BENDING SMALL BEAMS
Apparatus: An ordinary static bending machine, a steel I-beam bearing two adjustable knife-edge supports to rest on the platform, and a special deflectometer, are required. (See Fig. 31.)
Preparing the material: The specimens may be of any convenient size, though beams 2" X 2" X 30" tested over a 28-inch span, are considered best. The beams are surfaced on all four sides, care being taken that they are not damaged by the rollers of the surfacing machine. Material for these tests is sometimes cut from large beams after failure. The specimens are carefully weighed in grams, and all dimensions measured to the nearest 0.01 inch. If to be tested in a green or fresh condition the specimens should be kept in a damp box or covered with moist sawdust until needed. No defects should be allowed in these specimens.
Marking and sketching: Sketches are made of each end of the specimen to show the character of the growth, and after testing, the manner of failure is shown for all four sides. In obtaining data regarding the rate of growth and the proportion of late wood the same procedure is followed as with large beams.
Adjusting specimen in machine: The beam should be correctly centred in the machine and each end should have a plate with roller bearings between it and the support. Centre loading is used. Between the movable head of the machine and the specimen is placed a bearing block of maple or other hard wood, the lower surface of which is curved in a direction along the beam, the curvature of which should be slightly less than that of the beam at rupture, in order to prevent the edges from crushing into the fibres of the test piece.
Measuring the deflection: The method of measuring deflection of large beams can be used for small sizes, but because of the shortness of the span and consequent slight deformation in the latter, it is hardly accurate enough for good work. The special deflectometer shown in Fig. 31 allows closer reading, as it magnifies the deflection ten times. It rests on two small nails driven in the beam on the neutral plane and vertically above the supports. The fine wire on the wheel at the base of the indicator is attached to another small nail driven in the beam on the neutral plane midway between the end nails. All three nails should be in place before the beam is put into the machine. The indicator is adjustable by means of a thumb-screw at the base and is set at zero before the load is applied. Deflections are read to the nearest 0.001 inch. For rate of application of load see SPEED OF TESTING MACHINE, above. The speed should be uniform from start to finish without stopping. Readings must be made "on the fly."
Log of the test: The log sheets used for small beams (see Fig. 32) are the same as for large sizes and the procedure is practically identical. The stress-strain diagram is continued to or beyond the maximum load, and in a portion of the tests should be continued to six-inch deflection or until the specimen fails to support a load of 200 pounds. Deflection readings for equal increments of load are taken until well beyond the elastic limit, after which the scale beam is kept balanced and the load read for each 0.1 inch deflection. The load and deflection at first failure, the maximum load, and any points of sudden change should be shown on the diagram, even though they do not occur at one of the regular points. A brief description of the failure and the nature of any defects is entered on the log sheet.
Calculating the results: The formulae used in calculating the results of tests on small rectangular simple beams are as follows:
0.75 P (1) J = ———— b h
1.5 P_{1} l (2) r = ——————- b h^{2}
1.5 P l (3) R = ————- b h^{2}
P_{1} l^{3} (4) E = ——————- 4 D b h^{3}
P_{1} D (5) S = ————- 2 V
The same legend is used as in BENDING LARGE BEAMS. The weight of the beam itself is disregarded.
ENDWISE COMPRESSION
Apparatus: An ordinary static testing machine and a compressometer are required. (See Fig. 33.)
Preparing the material: Two classes of specimens are commonly used, namely, (1) posts 24 inches in length, and (2) small clear blocks approximately 2" X 2" X 8". The specimens are surfaced on all four sides and both ends squared smoothly and evenly. They are carefully weighed, measured, rate of growth and proportion of late wood determined, as in bending tests. After the test a moisture section is cut and weighed. Ordinarily these specimens should be free from defects.
Sketching: Sketches are made of each end of the specimens to show the character of the growth. After testing, the manner of failure is shown for all four sides, and the various parts of the failure are numbered in the order of their occurrence.
Adjusting specimen in machine: The compressometer collars are adjusted, the distance between them being 20 inches for the posts and 6 inches for the blocks. If the two ends of the blocks are not exactly parallel a ball-and-socket block can be placed between the upper end of the specimen and the movable head of the machine to overcome the irregularity. If the blocks are true they can simply be stood on end upon the platform and the movable head allowed to press directly upon the upper end.
Measuring the deformation: The deformation is measured by a compressometer. (See Fig. 33.) The latter registers to 0.001 inch. In the case of posts the compression between the collars is communicated to the four points on the arms by means of brass rods; with short blocks, as in Fig. 33, the points of the arms are in direct contact with the collars. The operator lowers the fulcrum of the apparatus by moving the micrometer screws at such a rate that the set-screw in the rear end of the upper lever is kept barely touching the fixed arm below it, being guided by a bell operated by electric contact.
Log of the test: The load is applied continuously at a uniform rate of speed. (See SPEED OF TESTING MACHINE, above.) Readings are taken from the scale of the compressometer at regular increments of either load or compression. The stress-strain diagram is continued to at least one deformation point beyond the maximum load, and in event of sudden failure, the direction of the curve beyond the maximum point is indicated. A brief description of the failure is entered on the log sheet. (See Fig. 34.)
In short specimens the failure usually occurs in one or several planes diagonal to the axis of the specimen. If the ends are more moist than the middle a crushing may occur on the extreme ends in a horizontal plane. Such a test is not valid and should always be culled. If the grain is diagonal or the stress is unevenly applied a diagonal shear may occur from top to bottom of the test specimen. Such tests are also invalid and should be culled. When the plane (or several planes) of failure occurs through the body of the specimen the test is valid. It may sometimes be advantageous to allow the extreme ends to dry slightly before testing in order to bring the planes of failure within the body. This is a perfectly legitimate procedure provided no drying is allowed from the sides of the specimen, and the moisture disk is cut from the region of failure.
Calculating the results: The formulae used in calculating the results of tests on endwise compression are as follows:
P (1) C = ——- A
P_{1} (2) c = ———- A
P_{1} l (3) E = ————- A D
P D (4) S = ——- 2 V
C = crushing strength, pounds per square inch. c = fibre strength at elastic limit, pounds per square inch. A = area of cross section, square inches. l = distance between centres of collars, inches. D = total shortening at elastic limit, inches. V = volume of specimen, cubic inches.
Remainder of legend as in BENDING LARGE BEAMS, above.
COMPRESSION ACROSS THE GRAIN
Apparatus: An ordinary static testing machine, a bearing plate, and a deflectometer are required. (See Fig. 35.)
Preparing the material: Two classes of specimens are used, namely, (1) sections of commercial sizes of ties, beams, and other timbers, and (2) small, clear specimens with the length several times the width. Sometimes small cubes are tested, but the results are hardly applicable to conditions in practice. In (2) the sides are surfaced and the ends squared. The specimens are then carefully measured and weighed, defects noted, rate of growth and proportion of late wood determined, as in bending tests. (See BENDING LARGE BEAMS, above.) After the test a moisture section is cut and weighed.
Sketching: Sketches are made as in endwise compression tests. (See ENDWISE COMPRESSION, above.)
Adjusting specimen in machine: The specimen is laid horizontally upon the platform of the machine and a steel bearing plate placed on its upper surface immediately beneath the centre of the movable head. For the larger specimens this plate is six inches wide; for the smaller sizes, two inches wide. The plate in all cases projects over the edges of the test piece, and in no case should the length of the latter be less than four times the width of the plate.
Measuring the deformation: The compression is measured by means of a deflectometer (see Fig. 35), which, after the first increment of load is applied, is adjusted (by means of a small set screw) to read zero. The actual downward motion of the movable head (corresponding to the compression of the specimen) is multiplied ten times on the scale from which the readings are made.
Log of the test: The load is applied continuously and at uniform speed (see SPEED OF TESTING MACHINE, above), until well beyond the elastic limit. The compression readings are taken at regular load increments and entered on the cross-section paper in the usual way. Usually there is no real maximum load in this case, as the strength continually increases as the fibres are crushed more compactly together.
Calculating the results: Ordinarily only the fibre stress at the elastic limit (c) is computed. It is equal to the load at elastic limit (P{1}) divided by the area under the plate (B). { P{1} } { c = ———- } { B }
SHEAR ALONG THE GRAIN
Apparatus: An ordinary static testing machine and a special tool designed for producing single shear are required. (See Figs. 36 and 37.) This shearing apparatus consists of a solid steel frame with set screws for clamping the block within it firmly in a vertical position. In the centre of the frame is a vertical slot in which a square-edged steel plate slides freely. When the testing block is in position, this plate impinges squarely along the upper surface of the tenon or lip, which, as vertical pressure is applied, shears off.
Preparing the material: The specimens are usually in the form of small, clear, straight-grained blocks with a projecting tenon or lip to be sheared off. Two common forms and sizes are shown in Figure 38. Part of the blocks are cut so that the shearing surface is parallel to the growth rings, or tangential; others at right angles to the growth rings, or radial. It is important that the upper surface of the tenon or lip be sawed exactly parallel to the base of the block. When the form with a tenon is used the under cut is extended a short distance horizontally into the block to prevent any compression from below.
In designing a shearing specimen it is necessary to take into consideration the proportions of the area of shear, since, if the length of the portion to be sheared off is too great in the direction of the shearing face, failure would occur by compression before the piece would shear. Inasmuch as the endwise compressive strength is sometimes not more than five times the shearing strength, the shearing surface should be less than five times the surface to which the load is applied. This condition is fulfilled in the specimens illustrated.
Shearing specimens are frequently cut from beams after testing. In this case the specific gravity (dry), proportion of late wood, and rate of growth are assumed to be the same as already recorded for the beams. In specimens not so taken, these quantities are determined in the usual way. The sheared-off portion is used for a moisture section.
Adjusting specimen in machine: The test specimen is placed in the shearing apparatus with the tenon or lip under the sliding plate, which is centred under the movable head of the machine. (See Fig. 39.) In order to reduce to a minimum the friction due to the lateral pressure of the plate against the bearings of the slot, the apparatus is sometimes placed upon several parallel steel rods to form a roller base. A slight initial load is applied to take up the lost motion of the machinery, and the beam balanced.
Log of the test: The load is applied continuously and at a uniform rate until failure, but no deformations are measured. The points noted are the maximum load and the length of time required to reach it. Sketches are made of the failure. If the failure is not pure shear the test is culled.
The shearing strength per square inch is found by dividing the { P } maximum load by the cross-sectional area. { Q = —- } { A }
IMPACT TEST
Apparatus: There are several types of impact testing machines.[59] One of the simplest and most efficient for use with wood is illustrated in Figure 40. The base of the machine is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500 pounds. Two upright columns, each 8 feet long, act as guides for the striking head. At the top of the column is the hoisting mechanism for raising or lowering the striking weights. The power for operating the machine is furnished by a motor set on the top. The hoisting-mechanism is all controlled by a single operating lever, shown on the side of the column, whereby the striking weight may be raised, lowered, or stopped at the will of the operator. There is an automatic safety device for stopping the machine when the weight reaches the top.
[Footnote 59: For description of U.S. Forest Service automatic and autographic impact testing machine, see Proc. Am. Soc. for Testing Materials, Vol. VIII, 1908, pp. 538-540.]
The weight is lifted by a chain, one end of which passes over a sprocket wheel in the hoisting mechanism. On the lower end of the chain is hung an electro-magnet of sufficient magnetic strength to support the heaviest striking weights. When it is desired to drop the striking weight the electric current is broken and reversed by means of an automatic switch and current breaker. The height of drop may be regulated by setting at the desired height on one of the columns a tripping pin which throws the switch on the magnet and so breaks and reverses the current.
There are four striking weights, weighing respectively 50, 100, 250, and 500 pounds, any one of which may be used, depending upon the desired energy of blow. When used for compression tests a flat steel head six inches in diameter is screwed into the lower end of the weight. For transverse tests, a well-rounded knife edge is screwed into the weight in place of the flat head. Knife edges for supporting the ends of the specimen to be tested, are securely bolted to the base of the machine.
The record of the behavior of the specimen at time of impact is traced upon a revolving drum by a pencil fixed in the striking head. (See Fig. 41.) When a drop is made the pencil comes in contact with the drum and is held in place by a spring. The drum is revolved very slowly, either automatically or by hand. The speed of the drum can be recorded by a pencil in the end of a tuning fork which gives a known number of vibrations per second.
One size of this machine will handle specimens for transverse tests 9 inches wide and 6-foot span; the other, 12 inches wide and 8-foot span. For compression tests a free fall of about 6.5 feet may be obtained. For transverse tests the fall is a little less, depending upon the size of the specimen.
The machine is calibrated by dropping the hammer upon a copper cylinder. The axial compression of the plug is noted. The energy used in static tests to produce this axial compression under stress in a like piece of metal is determined. The external energy of the blow (i.e., the weight of the hammer X the height of drop) is compared with the energy used in static tests at equal amounts of compression. For instance:
Energy delivered, impact test 35,000 inch-pounds Energy computed from static test .26,400 " " Efficiency of blow of hammer .75.3 per cent.
Preparing the material: The material used in making impact tests is of the same size and prepared in the same way as for static bending and compression tests. Bending in impact tests is more commonly used than compression, and small beams with 28-inch span are usually employed.
Method: In making an impact bending test the hammer is allowed to rest upon the specimen and a zero or datum line is drawn. The hammer is then dropped from increasing heights and drum records taken until first failure. The first drop is one inch and the increase is by increments of one inch until a height of ten inches is reached, after which increments of two inches are used until complete failure occurs or 6-inch deflection is secured.
The 50-pound hammer is used when with drops up to 68 inches it is reasonably certain it will produce complete failure or 6-inch deflection in the case of all specimens of a species; for all other species a 100-pound hammer is used.
Results: The tracing on the drum (see Fig. 41) represents the actual deflection of the stick and the subsequent rebounds for each drop. The distance from the lowest point in each case to the datum line is measured and its square in tenths of a square inch entered as an abscissa on cross-section paper, with the height of drop in inches as the ordinate. The elastic limit is that point on the diagram where the square of the deflection begins to increase more rapidly than the height of drop. The difference between the datum line and the final resting point after each drop represents the set the material has received.
The formulae used in calculating the results of impact tests in bending when the load is applied at the centre up to the elastic limit are as follows:
3 W H l (1) r = —————- D b h^{2}
F S l^{2} (2) E = —————- 6 D h
W H (3) S = ———- l b h
H = height of drop of hammer, including deflection, inches. S = modulus of elastic resilience, inch-pounds per cubic inch. W = weight of hammer, pounds.
Remainder of legend as in BENDING LARGE BEAMS, above.
HARDNESS TEST: ABRASION AND INDENTATION
Abrasion: The machine used by the U.S. Forest Service is a modified form of the Dorry abrasion machine. (See Fig. 42.) Upon the revolving horizontal disk is glued a commercial sandpaper, known as garnet paper, which is commonly employed in factories in finishing wood.
A small block of the wood to be tested is fixed in one clamp and a similar block of some wood chosen as a standard, as sugar maple, at 10 per cent moisture, in the opposite, and held against the same zone of sandpaper by a weight of 26 pounds each. The size of the section under abrasion for each specimen is 2" X 2". The conditions for wear are the same for both specimens. The speed of rotation is 68 revolutions a minute.
The test is continued until the standard specimen is worn a specified amount, which varies with the kind of wood under test. A comparison of the wear of the two blocks affords a fair idea of their relative resistance to abrasion.
Another method makes use of a sand blast to abrade the woods and is the one employed in New South Wales.[60] The apparatus consists essentially of a nozzle through which sand can be propelled at a high velocity against the test specimen by means of a steam jet.
[Footnote 60: See Warren, W.H.: The strength, elasticity, and other properties of New South Wales hardwood timbers. Dept. For., N.S.W., Sydney, 1911, pp. 88-95.]
The wood to be tested is cut into blocks 3" X 3" X 1', and these are weighed to the nearest grain just before placing in the apparatus. Steam from the boiler at a pressure of about 43 pounds per square inch is ejected from a nozzle in such a way that particles of fine quartz sand are caught up and thrown violently against the block which is being rotated. Only superheated steam strikes the block, thus leaving the wood dry. The test is continued for two minutes, after which the specimen is removed and immediately weighed.
By comparison with the original weight the loss from abrasion is determined, and by comparison with a certain wood chosen as a standard, a coefficient of wear-resistance can be obtained. The amount of wear will vary more or less according to the surface exposed, and in these tests quarter-sawed material was used with the edge grain to the blast.
Indentation: The tool used for this test consists of a punch with a hemispherical end or steel ball having a diameter of 0.444 inch, giving a surface area of one-fourth square inch. It is fitted with a guard plate, which works loosely until the penetration has progressed to a depth of 0.222 inch, whereupon it tightens. (See Fig. 43.) The effect is that of sinking a ball half its diameter into the specimen. This apparatus is fitted into the movable head of the static testing machine.
The wood to be tested is cut square with the grain into rectangular blocks measuring 2" X 2" X 6". A block is placed on the platform and the end of the punch forced into the wood at the rate of 0.25 inch per minute. The operator keeps moving the small handle of the guard plate back and forth until it tightens. At this instant the load is read and recorded.
Two penetrations each are made on the tangential and radial surfaces, and one on each end of every specimen tested.
In choosing the places on the block for the indentations, effort should be made to get a fair average of heartwood and sapwood, fine and coarse grain, early and late wood.
Another method of testing by indentation involves the use of a right-angled cone instead of a ball. For details of this test as used in New South Wales see loc. cit., pp. 86-87.
CLEAVAGE TEST
A static testing machine and a special cleavage testing device are required. (See Fig. 44.) The latter consists essentially of two hooks, one of which is suspended from the centre of the top of the cage, the other extended above the movable head.
The specimens are 2" X 2" X 3.75". At one end a one-inch hole is bored, with its centre equidistant from the two sides and 0.25 inch from the end. (See Fig. 45.) This makes the cross section to be tested 2" X 3". Some of the blocks are cut radially and some tangentially, as indicated in the figure.
The free ends of the hooks are fitted into the notch in the end of the specimen. The movable head of the machine is then made to descend at the rate of 0.25 inch per minute, pulling apart the hooks and splitting the block. The maximum load only is taken and the result expressed in pounds per square inch of width. A piece one-half inch thick is split off parallel to the failure and used for moisture determination.
TENSION TEST PARALLEL TO THE GRAIN
Since the tensile strength of wood parallel to the grain is greater than the compressive strength, and exceedingly greater than the shearing strength, it is very difficult to make satisfactory tension tests, as the head and shoulders of the test specimen (which is subjected to both compression and shear) must be stronger than the portion subjected to a pure tensile stress.
Various designs of test specimens have been made. The one first employed by the Division of Forestry[61] was prepared as follows: Sticks were cut measuring 1.5" X 2.5" X 16". The thickness at the centre was then reduced to three-eighths of an inch by cutting out circular segments with a band saw. This left a breaking section of 2.5" X 0.375". Care was taken to cut the specimen as nearly parallel to the grain as possible, so that its failure would occur in a condition of pure tension. The specimen was then placed between the plane wedge-shaped steel grips of the cage and the movable head of the static machine and pulled in two. Only the maximum load was recorded. (See Fig. 46, No. 1.)
[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.]
The difficulty of making such tests compared with the minor importance of the results is so great that they are at present omitted by the U.S. Forest Service. A form of specimen is suggested, however, and is as follows: "A rod of wood about one inch in diameter is bored by a hollow drill from the stick to be tested. The ends of this rod are inserted and glued in corresponding holes in permanent hardwood wedges. The specimen is then submitted to the ordinary tension test. The broken ends are punched from the wedges."[62] (See Fig. 46, No. 2.)
[Footnote 62: Cir. 38: Instructions to engineers of timber tests, 1906, p. 24.]
The form used by the Department of Forestry of New South Wales[63] is as shown in Fig. 47. The specimen has a total length of 41 inches and is circular in cross section. On each end is a head 4 inches in diameter and 7 inches long. Below each head is a shoulder 8.5 inches long, which tapers from a diameter of 2.75 inches to 1.25 inches. In the middle is a cylindrical portion 1.25 inches in diameter and 10 inches long.
[Footnote 63: Warren, W.H.: The strength, elasticity, and other properties of New South Wales hardwood timbers, 1911, pp. 58-62.]
In making the test the specimen is fitted in the machine, and an extensometer attached to the middle portion and arranged to record the extension between the gauge points 8 inches apart. The area of the cross section then is 1.226 square inches, and the tensile strength is equal to the total breaking load applied divided by this area.
TENSION TEST AT RIGHT ANGLES TO THE GRAIN
A static testing machine and a special testing device (see Fig. 48) are required. The latter consists essentially of two double hooks or clamps, one of which is suspended from the centre of the top of the cage, the other extended above the movable head. The specimens are 2" X 2" X 2.5". At each end a one-inch hole is bored with its centre equidistant from the two sides and 0.25 inch from the ends. This makes the cross section to be tested 1" X 2".
The free ends of the clamps are fitted into the notches in the ends of the specimen. The movable head of the machine is then made to descend at the rate of 0.25 inch per minute, pulling the specimen in two at right angles to the grain. The maximum load only is taken and the result expressed in pounds per inch of width. A piece one-half inch thick is split off parallel to the failure and used for moisture determination.
TORSION TEST[64]
[Footnote 64: Wood is so seldom subjected to a pure stress of this kind that the torsion test is usually omitted.]
Apparatus: The torsion test is made in a Riehle-Miller torsional testing machine or its equivalent. (See Fig. 49.)
Preparation of material: The test pieces are cylindrical, 1.5 inches in diameter and 18 inches gauge length, with squared ends 4 inches long joined to the cylindrical portion with a fillet. The dimensions are carefully measured, and the usual data obtained in regard to the rate of growth, proportion of late wood, location and kind of defects. The weight of the cylindrical portion of the specimen is obtained after the test.
Making the test: After the specimen is fitted in the machine the load is applied continuously at the rate of 22 deg. per minute. A troptometer is used in measuring the deformation. Readings are made until failure occurs, the points being entered on the cross-section paper. The character of the failure is described. Moisture determinations are made by the disk method.
Results: The conditions of ultimate rupture due to torsion appear not to be governed by definite mathematical laws; but where the material is not overstrained, laws may be assumed which are sufficiently exact for practical cases. The formulae commonly used for computations are as follows:
5.1 M (1) T = ———- c^{3}
114.6 T f (2) G = —————- a c
a = angle measured by troptometer at elastic limit, in degrees. c = diameter of specimen, inches. f = gauge length of specimen, inches. G = modulus of elasticity in shear across the grain, pounds per square inch. M = moment of torsion at elastic limit, inch-pounds. T = outer fibre torsional stress at elastic limit, pounds per square inch.
SPECIAL TESTS
Spike-pulling Test
Spike-pulling tests apply to problems of railroad maintenance, and the results are used to compare the spike-holding powers of various woods, both untreated and treated with different preservatives, and the efficiency of various forms of spikes. Special tests are also made in which the spike is subjected to a transverse load applied repetitively by a blow.
For details of tests and results see:
Cir. 38, U.S.F.S.: Instructions to engineers of timber tests, p. 26. Cir. 46, U.S.F.S.: Holding force of railroad spikes in wooden ties. Bul. 118, U.S.F.S,: Prolonging the life of cross-ties, pp. 37-40.
Packing Boxes
Special tests on the strength of packing boxes of various woods have been made by the U.S. Forest Service to determine the merits of different kinds of woods as box material with the view of substituting new kinds for the more expensive ones now in use. The methods of tests consisted in applying a load along the diagonal of a box, an action similar to that which occurs when a box is dropped on one of its corners. The load was measured at each one-fourth inch in deflection, and notes were made of the primary and subsequent failures.
For details of tests and results, see:
Cir. 47, U.S.F.S.: Strength of packing boxes of various woods. Cir. 214, U.S.F.S.: Tests of packing boxes of various forms.
Vehicle and Implement Woods
Tests were made by the U.S. Forest Service to obtain a better knowledge of the mechanical properties of the woods at present used in the manufacture of vehicles and implements and of those which might be substituted for them. Tests were made upon the following materials: hickory buggy spokes (see Fig. 5); hickory and red oak buggy shafts; wagon tongues; Douglas fir and southern pine cultivator poles.
Details of the tests and results may be found in:
Cir. 142, U.S.F.S.: Tests on vehicle and implement woods.
Cross-arms
In tests by the U.S. Forest Service on cross-arms a special apparatus was devised in which the load was distributed along the arm as in actual practice. The load was applied by rods passing through the pinholes in the arms. Nuts on these rods pulled down on the wooden bearing-blocks shaped to fit the upper side of the arm. The lower ends of these rods were attached to a system of equalizing levers, so arranged that the load at each pinhole would be the same. In all the tests the load was applied vertically by means of the static machine.
See Cir. 204, U.S.F.S.: Strength tests of cross-arms.
Other Tests
Many other kinds of tests are made as occasion demands. One kind consists of barrels and liquid containers, match-boxes, and explosive containers. These articles are subjected to shocks such as they would receive in transit and in handling, and also to hydraulic pressure.
One of the most important tests from a practical standpoint is that of built-up structures such as compounded beams composed of small pieces bolted together, mortised joints, wooden trusses, etc. Tests of this kind can best be worked out according to the specific requirements in each case.
APPENDIX
SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE
MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES
Working Plan No. 124
PURPOSE OF WORK
It is the general purpose of the work here outlined to provide:
(a) Reliable data for comparing the mechanical properties of various species;
(b) Data for the establishment of correct strength functions or working stresses;
(c) Data upon which may be based analyses of the influence on the mechanical properties of such factors as:
Locality;
Distance of timber from the pith of the tree;
Height of timber in the tree;
Change from the green to the air-dried condition, etc.
The mechanical properties which will be considered and the principal tests used to determine them are as follows:
Strength and stiffness— Static bending; Compression parallel to grain; Compression perpendicular to grain; Shear.
Toughness— Impact bending; Static bending; Work to maximum load and total work.
Cleavability— Cleavage test.
Hardness— Modification of Janka ball test for surface hardness.
MATERIAL
Selection and Number of Trees
The material will be from trees selected in the forest by one qualified to determine the species. From each locality, three to five dominant trees of merchantable size and approximately average age will be so chosen as to be representative of the dominant trees of the species. Each species will eventually be represented by trees from five to ten localities. These localities will be so chosen as to be representative of the commercial range of the species. Trees from one to three localities will be used to represent each species until most of the important species have been tested.
The 16-foot butt log will be taken from each tree selected and the entire merchantable hole of one average tree for each species.
Field Notes and Shipping Instructions
Field notes as outlined in Form—a Shipment Description, Manual of the Branch of Products, will be fully and carefully made by the collector. The age of each tree selected will be recorded and any other information likely to be of interest or importance will also be made a part of these field notes. Each log will have the bark left on. It will be plainly marked in accordance with directions given under Detailed Instructions. All material will be shipped to the laboratory immediately after being cut. No trees will be cut until the collector is notified that the laboratory is ready to receive the material.
DETAILED INSTRUCTIONS
Part of Tree to be Tested
(a) For determining the value of tree and locality and the influence on the mechanical properties of distance from the pith, a 4-foot bolt will be cut from the top end of each 16-foot butt log.
(b) For investigating the variation of properties with the height of timber in the tree, all the logs from one average tree will be used.
(c) For investigating the effect of drying the wood, the bolt next below that provided for in (a) will be used in the case of one tree from each locality.
Marking and Grouping of Material
The marking will be standard except as noted. Each log will be considered a "piece." The piece numbers will be plainly marked upon the butt end of each log by the collector. The north side of each log will also be marked.
When only one bolt from a tree is used it will be designated by the number of the log from which it is cut. Whenever more than one bolt is taken from a tree, each 4-foot bolt or length of trunk will be given a letter (mark), a, b, c, etc., beginning at the stump.
All bolts will be sawed into 2-1/2" X 2-1/2" sticks and the sticks marked according to the sketch, Fig. 50. The letters N, E, S, and W indicate the cardinal points when known; when these are unknown, H, K, L, and M will be used. Thus, N5, K8, S7, M4 are stick numbers, the letter being a part of the stick number.
Only straight-grained specimens, free from defects which will affect their strength, will be tested.
Care of Material
No material will be kept in the bolt or log long enough to be damaged or disfigured by checks, rot, or stains.
Green material: The material to be tested green will be kept in a green state by being submerged in water until near the time of test. It will then be surfaced, sawed to length, and stored in damp sawdust at a temperature of 70 deg.F. (as nearly as practicable) until time of test. Care should be taken to avoid as much as possible the storage of green material in any form.
Air-dry material: The material to be air-dried will be cut into sticks 2-1/2" X 2-1/2" X 4'. The ends of these sticks will be paraffined to prevent checking. This material will be so piled as to leave an air space of at least one-half inch on each side of each stick, and in such a place that it will be protected from sunshine, rain, snow, and moisture from the ground. The sticks will be surfaced and cut to length just previous to test.
Order of Tests
The order of tests in all cases will be such as to eliminate so far as possible from the comparisons the effect of changes of condition of the specimens due to such factors as storage and weather conditions.
The material used for determining the effect of height in tree will be tested in such order that the average time elapsing from time of cutting to time of test will be approximately the same for all bolts from any one tree.
Tests on Green Material
The tests on all bolts, except those from which a comparison of green and dry timber is to be gotten, will be as follows:
Static bending: One stick from each pair. A pair consists of two adjacent sticks equidistant from the pith, as N7 and N8, or H5 and H6.
Impact bending: Four sticks; one to be taken from near the pith; one from near the periphery; and two representative of the cross section.
Compression parallel to grain: One specimen from each stick. These will be marked "1" in addition to the number of the stick from which they are taken.
Compression perpendicular to grain: One specimen from each of 50 per cent of the static bending sticks. These will be marked "2" in addition to the number of the stick from which they are cut.
Hardness: One specimen from each of the other 50 per cent of the static bending sticks. These specimens will be marked "4."
Shear: Six specimens from sticks not tested in bending or from the ends cut off in preparing the bending specimens. Two specimens will be taken from near the pith; two from near the periphery; and two that are representative of the average growth. One of each two will be tested in radial shear and the other in tangential shear. These specimens will have the mark "3."
Cleavage: Six specimens chosen and divided just as those for shearing. These specimens will have the mark "5." (For sketches showing radial and tangential cleavage, see Fig. 45.)
When it is impossible to secure clear specimens for all of the above tests, tests will have precedence in the order in which they are named.
Tests to Determine the Effect of Air-drying
These tests will be made on material from the adjacent bolts mentioned in "c" under Part of Tree to be Tested. Both bolts will be cut as outlined above. One-half the sticks from each bolt will be tested green, the other half will be air-dried and tested. The division of green and air-dry will be according to the following scheme:
STICK NUMBERS
Lower bolt, 1, 4, 5, 8, 9, } Tested etc. } green Upper bolt, 2, 3, 6, 7, 10, }
Lower bolt, 2, 3, 6, 7, 10, } Air-dried etc. } and Upper bolt, 1, 4, 5, 8, 9, } tested
All green sticks from these two bolts will be tested as if they were from the same bolt and according to the plan previously outlined for green material from single bolts. The tests on the air-dried material will be the same as on the green except for the difference of seasoning.
The material will be tested at as near 12 per cent moisture as is practicable. The approximate weight of the air-dried specimens at 12 per cent moisture will be determined by measuring while green 20 per cent of the sticks to be air-dried and assuming their dry gravity to be the same as that of the specimens tested green. This 20 per cent will be weighed as often as is necessary to determine the proper time of test.
Methods of Test
All tests will be made according to Circular 38 except in case of conflict with the instructions given below:
Static bending: The tests will be on specimens 2" X 2" X 30" on 28-inch span. Load will be applied at the centre.
In all tests the load-deflection curve will be carried to or beyond the maximum load. In one-third of the tests the load-deflection curve will be continued to 6-inch deflection, or till the specimen fails to support a 200-pound load. Deflection readings for equal increments of load will be taken until well past the elastic limit, after which the scale beam will be kept balanced and the load read for each 0.1-inch deflection. The load and deflection at first failure, maximum load and points of sudden change, will be shown on the curve sheet even if they do not occur at one of the regular load or deflection increments.
Impact bending: The impact bending tests will be on specimens of the same size as those used in static bending. The span will be 28 inches.
The tests will be by increment drop. The first drop will be 1 inch and the increase will be by increments of 1 inch till a height of 10 inches is reached, after which increments of 2 inches will be used until complete failure occurs or 6-inch deflection is secured.
A 50-pound hammer will be used when with drops up to 68 inches it is practically certain that it will produce complete failure or 6-inch deflection in the case of all specimens of a species. For all other species, a 100-pound hammer will be used.
In all cases drum records will be made until first failure. Also the height of drop causing complete failure or 6-inch deflection will be noted.
Compression parallel to grain: This test will be on specimens 2" X 2" X 8" in size. On 20 per cent of these tests load-compression curves for a 6-inch centrally located gauge length will be taken. Readings will be continued until the elastic limit is well passed. The other 80 per cent of the tests will be made for the purpose of obtaining the maximum load only.
Compression perpendicular to grain: This test will be on specimens 2" X 2" X 6" in size. The bearing plates will be 2 inches wide. The rate of descent of the moving head will be 0.024 inch per minute. The load-compression curve will be plotted to 0.1 inch compression and the test will then be discontinued.
Hardness: The tool shown in Fig. 43 (an adaptation of the apparatus used by the German investigator, Janka) will be used. The rate of descent of the moving head will be 0.25 inch per minute. When the penetration has progressed to the point at which the plate "a" becomes tight, due to being pressed against the wood, the load will be read and recorded.
Two penetrations will be made on a tangential surface, two on a radial, and one on each end of each specimen tested. The choice between the two radial and between the two tangential surfaces and the distribution of the penetrations over the surfaces will be so made as to get a fair average of heart and sap, slow and fast growth, and spring and summer wood. Specimens will be 2" X 2" X 6".
Shear: The tests will be made with a tool slightly modified from that shown in Circular 38. The speed of descent of head will be 0.015 inch per minute. The only measurements to be made are those of the shearing area. The offset will be 1/8 inch. Specimens will be 2" X 2" X 2-1/2" in size. (For definition of offset and form of test specimen, see Fig. 38.)
Cleavage: The cleavage tests will be made on specimens of the form and size shown in Fig. 45. The apparatus will be as shown in Fig. 44. The maximum load only will be taken and the result expressed in pounds per inch of width. The speed of the moving head will be 0.25 inch per minute.
Moisture Determinations
Moisture determinations will be made on all specimens tested except those to be photographed or kept for exhibit. A 1-inch disk will be cut from near the point of failure of bending and compression parallel specimens, from the portion under the plate in the case of the compression perpendicular specimens, and from the centre of the hardness test specimens. The beads from the shear specimens will be used as moisture disks. In the case of the cleavage specimens a piece 1/2 inch thick will be split off parallel to the failure and used as a moisture disk.
RECORDS
All records will be standard.
PHOTOGRAPHS
Cross Sections
Just before cutting into sticks, the freshly cut end of at least one bolt from each tree will be photographed. A scale of inches will be shown in this photograph.
Specimens
Three photographs will be made of a group consisting of four 2" X 2" X 30" specimens chosen from the material from each locality. Two of these specimens will be representative of average growth, one of fast and one of slow growth. These photographs will show radial, tangential, and end surfaces for each specimen.
Failures
Typical and abnormal failures of material from each site will be photographed.
Disposition of Material
The specimens photographed to show typical and abnormal failures will be saved for purposes of exhibit until deemed by the person in charge of the laboratory to be of no further value.
SHRINKAGE AND SPECIFIC GRAVITY
Appendix to Working Plan 124
PURPOSE OF WORK
It is the purpose of this work to secure data on the shrinkage and specific gravity of woods tested under Project 124. The figures to be obtained are for use as average working values rather than as the basis for a detailed study of the principles involved.
MATERIAL
The material will be taken from that provided for mechanical tests.
RADIAL AND TANGENTIAL SHRINKAGE
Specimens
Preparation: Two specimens 1 inch thick, 4 inches wide, and 1 inch long will be obtained from near the periphery of each "d" bolt. These will be cut from the sector-shaped sections left after securing the material for the mechanical tests or from disks cut from near the end of the bolt. They will be taken from adjoining pieces chosen so that the results will be comparable for use in determining radial and tangential shrinkage. (When a disk is used, care must be taken that it is green and has not been affected by the shrinkage and checking near the end of the bolt.)
One of these specimens will be cut with its width in the radial direction and will be used for the determination of radial shrinkage. The other will have its width in the tangential direction and will be used for tangential shrinkage. These specimens will not be surfaced.
Marking: The shrinkage specimens will retain the shipment and piece numbers and marks of the bolts from which they are taken, and will have the additional mark 7R or 7T according as their widths are in the radial or tangential direction.
Shrinkage measurements: The shrinkage specimens will be carefully weighed and measured soon after cutting. Rings per inch, per cent sap, and per cent summer wood will be measured. They will then be air-dried in the laboratory to constant weight, and afterward oven-dried at 100 deg.C. (212 deg.F.), when they will again be weighed and measured.
VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY
Specimens
Selection and preparation: Four 2" X 2" X 6" specimens will be cut from the mechanical test sticks of each "d" bolt; also from each of the composite bolts used in getting a comparison of green and air-dry. One of these specimens will be taken from near the pith and one from near the periphery; the other two will be representative of the average growth of the bolt. The sides of these specimens will be surfaced and the ends smooth sawn.
Marking: Each specimen will retain the shipment, piece, and stick numbers and mark of the stick from which it is cut, and will have the additional mark "S."
Manipulation: Soon after cutting, each specimen will be weighed and its volume will be determined by the method described below. The rings per inch and per cent summer wood, where possible, will be determined, and a carbon impression of the end of the specimen made. It will then be air-dried in the laboratory to a constant weight and afterward oven-dried at 100 deg.C. When dry, the specimen will be taken from the oven, weighed, and a carbon impression of its end made. While still warm the specimen will be dipped in hot paraffine. The volume will then be determined by the following method:
On one pan of a pair of balances is placed a container having in it water enough for the complete submersion of the test specimen. This container and water is balanced by weights placed on the other scale pan. The specimen is then held completely submerged and not touching the container while the scales are again balanced. The weight required to balance is the weight of water displaced by the specimen, and hence if in grams is numerically equal to the volume of the specimen in cubic centimetres. A diagrammatic sketch of the arrangement of this apparatus is shown in Fig. 51.
Air-dry specimens will be dipped in water and then wiped dry after the first weighing and just before being immersed for weighing their displacement. All displacement determinations will be made as quickly as possible in order to minimize the absorption of water by the specimen.
STRENGTH VALUES FOR STRUCTURAL TIMBERS
(From Cir. 189, U.S. Forest Service)
The following tables bring together in condensed form the average strength values resulting from a large number of tests made by the Forest Service on the principal structural timbers of the United States. These results are more completely discussed in other publications of the Service, a list of which is given in BIBLIOGRAPHY, PART III.
The tests were made at the laboratories of the U.S. Forest Service, in cooperation with the following institutions: Yale Forest School, Purdue University, University of California, University of Oregon, University of Washington, University of Colorado, and University of Wisconsin.
Tables XVIII and XIX give the average results obtained from tests on green material, while Tables XX and XXI give average results from tests on air-seasoned material. The small specimens, which were invariably 2" X 2" in cross section, were free from defects such as knots, checks, and cross grain; all other specimens were representative of material secured in the open market. The relation of stresses developed in different structural forms to those developed in the small clear specimens is shown for each factor in the column headed "Ratio to 2" X 2"." Tests to determine the mechanical properties of different species are often confined to small, clear specimens. The ratios included in the tables may be applied to such results in order to approximate the strength of the species in structural sizes, and containing the defects usually encountered, when tests on such forms are not available.
A comparison of the results of tests on seasoned material with those from tests on green material shows that, without exception, the strength of the 2" X 2" specimens is increased by lowering the moisture content, but that increase in strength of other sizes is much more erratic. Some specimens, in fact, show an apparent loss in strength due to seasoning. If structural timbers are seasoned slowly, in order to avoid excessive checking, there should be an increase in their strength. In the light of these facts it is not safe to base working stresses on results secured from any but green material. For a discussion of factors of safety and safe working stresses for structural timbers see the Manual of the American Railway Engineering Association, Chicago, 1911. A table from that publication, giving working unit stresses for structural timber, is reproduced in this book, see Table XXII.
- TABLE XVIII TABLE XVIII - BENDING TESTS ON GREEN MATERIAL - Sizes F.S. at E.L. M. of R. M. of E. Calculated - Num- Per Rings shear Species ber cent per - - - - Cross Span of mois- inch Average Ratio Average Ratio Average Ratio Average Ratio Section tests ture per sq. to 2" per sq. to 2" per sq. to 2" per sq. to 2" inch by 2" inch by 2" inch by 2" inch by 2" - - - - - - - - - - - - 1,000 Inches Ins. Lbs. Lbs. lbs. Lbs. Longleaf pine 12 by 12 138 4 28.6 9.7 4,029 0.83 6,710 0.74 1,523 0.99 261 0.86 10 by 16 168 4 26.8 16.7 6,453 .85 6,453 .71 1,626 1.05 306 1.01 8 by 16 156 7 28.4 14.6 3,147 .64 5,439 .60 1,368 .89 390 1.29 6 by 16 132 1 40.3 21.8 4,120 .83 6,460 .71 1,190 .77 378 1.25 6 by 10 180 1 31.0 6.2 3,580 .72 6,500 .72 1,412 .92 175 .58 6 by 8 180 2 27.0 8.2 3,735 .75 5,745 .63 1,282 .83 121 .40 2 by 2 30 15 33.9 14.1 4,950 1.00 9,070 1.00 1,540 1:00 303 1.00 Douglas fir 8 by 16 180 191 31.5 11.0 3,968 .76 5,983 .72 1,517 .95 269 .81 5 by 8 180 84 30.1 10.8 3,693 .71 5,178 .63 1,533 .96 172 .52 2 by 12 180 27 35.7 20.3 3,721 .71 5,276 .64 1,642 1.03 256 .77 2 by 10 180 26 32.9 21.6 3,160 .60 4,699 .57 1,593 1.00 189 .57 2 by 8 180 29 33.6 17.6 3,593 .69 5,352 .65 1,607 1.01 171 .51 2 by 2 24 568 30.4 11.6 5,227 1.00 9,070 1.00 1,540 1.00 303 1.00 Douglas fir (fire-killed) 8 by 16 180 30 36.8 10.9 3,503 .80 4,994 .64 1,531 .94 330 1.19 2 by 12 180 32 34.2 17.7 3,489 .80 5,085 .66 1,624 .99 247 .89 2 by 10 180 32 38.9 18.1 3,851 .88 5,359 .69 1,716 1.05 216 .78 2 by 8 180 31 37.0 15.7 3,403 .78 5,305 .68 1,676 1.02 169 .61 2 by 2 30 290 33.2 17.2 4,360 1.00 7,752 1.00 1,636 1.00 277 1.00 Shortleaf pine 8 by 16 180 12 39.5 12.1 3,185 .73 5,407 .70 1,438 1.03 362 1.40 8 by 14 180 12 45.8 12.7 3,234 .74 5,781 .75 1,494 1.07 338 1.31 8 by 12 180 24 52.2 11.8 3,265 .75 5,503 .71 1,480 1.06 277 1.07 5 by 8 180 24 47.8 11.5 3,519 .81 5,732 .74 1,485 1.06 185 .72 2 by 2 30 254 51.7 13.6 4,350 1.00 7,710 1.00 1,395 1.00 258 1.00 Western larch 8 by 16 180 32 51.0 25.3 3,276 .77 4,632 .64 1,272 .97 298 1.11 8 by 12 180 30 50.3 23.2 3,376 .79 5,286 .73 1,331 1.02 254 .94 5 by 8 180 14 56.0 25.6 3,528 .83 5,331 .74 1,432 1.09 169 .63 2 by 2 28 189 46.2 26.2 4,274 1.00 7,251 1.00 1,310 1.00 269 1.00 Loblolly pine 8 by 16 180 17 15.8 6.1 3,094 .75 5,394 .69 1,406 .98 383 1.44 5 by 12 180 94 60.9 5.9 3,030 .74 5,028 .64 1,383 .96 221 .83 2 by 2 30 44 70.9 5.4 4,100 1.00 7,870 1.00 1,440 1.00 265 1.00 Tamarack 6 by 12 162 15 57.6 16.6 2,914 .75 4,500 .66 1,202 1.05 255 1.11 4 by 10 162 15 43.5 11.4 2,712 .70 4,611 .68 1,238 1.08 209 .91 2 by 2 30 82 38.8 14.0 3,875 1.00 6,820 1.00 1,141 1.00 229 1.00 Western hemlock 8 by 16 180 39 42.5 15.6 3,516 .80 5,296 .73 1,445 1.01 261 .92 2 by 2 28 52 51.8 12.1 4.406 1.00 7,294 1.00 1,428 1.00 284 1.00 Redwood 8 by 16 180 14 86.5 19.9 3,734 .79 4,492 .64 1,016 .96 300 1.21 6 by 12 180 14 87.3 17.8 3,787 .80 4,451 .64 1,068 1.00 224 .90 7 by 9 180 14 79.8 16.7 4,412 .93 5,279 .76 1,324 1.25 199 .80 3 by 14 180 13 86.1 23.7 3,506 .74 4,364 .62 947 .89 255 1.03 2 by 12 180 12 70.9 18.6 3,100 .65 3,753 .54 1,052 .99 187 .75 2 by 10 180 13 55.8 20.0 3,285 .69 4,079 .58 1,107 1.04 169 .68 2 by 8 180 13 63.8 21.5 2,989 .63 4,063 .58 1,141 1.08 134 .54 2 by 2 28 157 75.5 19.1 4,750 1.00 6,980 1.00 1,061 1.00 248 1.00 Norway pine 6 by 12 162 15 50.3 12.5 2,305 .82 3,572 .69 987 1.03 201 1.17 4 by 12 162 18 47.9 14.7 2,648 .94 4,107 .79 1,255 1.31 238 1.38 4 by 10 162 16 45.7 13.3 2,674 .95 4,205 .81 1,306 1.36 198 1.15 2 by 2 30 133 32.3 11.4 2,808 1.00 5,173 1.00 960 1.00 172 1.00 Red spruce 2 by 10 144 14 32.5 21.9 2,394 .66 3,566 .60 1,180 1.02 181 .80 2 by 2 26 60 37.3 21.3 3,627 1.00 5,900 1.00 1,157 1.00 227 1.00 White spruce 2 by 10 144 16 40.7 9.3 2,239 .72 3,288 .63 1,081 1.08 166 .83 2 by 2 26 83 58.3 10.2 3.090 1.00 5,185 1.00 998 1.00 199 1.00 - Note. Following is an explanation of the abbreviations used in the foregoing tables: F.S. at E.L. = Fiber stress at elastic limit. M. of E. = Modulus of elasticity. M. of R. = Modulus of rupture. Cr. str. at E.L. = Crushing strength at elastic limit. Cr. str. at max. ld. = Crushing strength at maximum load. -
- TABLE XIX TABLE XIX - COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL - Compression Compression Shear parallel to grain perpendicular to grain - Cr. Cr. Cr. Species Per str. M. of str. Per str. Per Size of No. cent at E. at max. Stress No. cent at max. No. cent Shear specimen of of E. L. per ld.,. area Height of of ld., of of strength tests mois- per square per tests mois- per tests mois- ture square inch square ture square ture inch inch inch - - - - - - - - 1,000 Inches Lbs. lbs. Lbs. Inches Inches Lbs. Lbs. Longleaf pine 4 by 4 46 26.3 3,480 4,800 4 by 4 4 22 25.3 568 44 21.8 973 2 by 2 14 34.7 4,400 Douglas fir 6 by 6 515 30.7 2,780 1,181 3,500 4 by 8 16 259 30.3 570 531 29.7 765 5 by 6 170 30.9 2,720 2,123 3,490 2 by 2 902 29.8 3,500 1,925 4,030 Douglas fir (fire-killed) 6 by 6 108 34.8 2,620 1,801 3,290 6 by 8 16 24 33.7 368 77 35.8 631 2 by 2 204 37.9 3,430 Shortleaf pine 6 by 6 95 41.2 2,514 1,565 3,436 5 by 8 16 12 37.7 361 179 47.0 704 5 by 8 23 43.5 2,241 1,529 3,423 5 by 8 14 12 42.8 366 2 by 2 281 51.4 3,570 5 by 8 12 24 53.0 325 5 by 5 8 24 47.0 344 2 by 2 2 277 48.5 400 Western larch 6 by 6 107 49.1 2,675 1,575 3,510 6 by 8 16 22 43.6 417 179 40.7 700 2 by 2 491 50.6 3,026 1,545 3,696 6 by 8 12 20 40.2 416 4 by 6 6 53 52.8 478 4 by 4 4 30 50.4 472 Loblolly pine 8 by 8 14 63.4 1,560 365 2,140 8 by 4 8 16 67.2 392 121 83.2 630 4 by 8 18 60.0 2,430 691 3,560 4 by 4 8 38 44.6 546 2 by 2 53 74.0 3,240 Tamarack 6 by 7 4 49.9 2,332 1,432 3,032 24 39.2 668 4 by 7 6 27.7 2,444 1,334 3,360 2 by 2 165 36.8 3,190 Western hemlock 6 by 6 82 46.6 2,905 1,617 3,355 6 by 4 6 30 48.7 434 54 65.7 630 2 by 2 131 55.6 2,938 1,737 3,392 Redwood 6 by 6 34 83.6 3,194 1,240 3,882 6 by 8 16 13 86.7 473 148 84.2 742 2 by 2 143 36.8 3,490 1,222 3,980 6 by 6 12 14 83.0 424 6 by 7 9 13 74.7 477 6 by 3 14 13 75.6 411 6 by 2 12 12 66.5 430 6 by 2 10 11 55.0 423 6 by 2 8 12 56.7 396 2 by 2 2 186 75.5 569 Norway pine 6 by 7 5 29.0 1,928 905 2,404 20 26.7 589 4 by 7 8 28.4 2,154 1,063 2,652 2 by 2 178 26.8 2,504 Red spruce 2 by 2 58 35.4 2,750 2 by 2 2 43 31.8 310 30 32.0 758 White spruce 2 by 2 84 61.0 2,370 2 by 2 2 46 50.4 270 40 58.0 651 - Note. Following is an explanation of the abbreviations used in the foregoing tables: F.S. at E.L. = Fiber stress at elastic limit. M. of E. = Modulus of elasticity. M. of R. = Modulus of rupture. Cr. str. at E.L. = Crushing strength at elastic limit. Cr. str. at max. ld. = Crushing strength at maximum load. -
- TABLE XX TABLE XX - BENDING TESTS ON AIR-SEASONED MATERIAL - Sizes F.S. at E.L. M. of R. M. of E. Calculated - Num- Per Rings shear Species ber cent per - - - - Cross Span of mois- inch Average Ratio Average Ratio Average Ratio Average Ratio Section tests ture per sq. to 2" per sq. to 2" per sq. to 2" per sq. to 2" inch by 2" inch by 2" inch by 2" inch by 2" - - - - - - - - - - - - 1,000 Inches Ins. Lbs. Lbs. lbs. Lbs. Longleaf pine 8 by 16 180 5 22.2 16.0 3,390 0.50 4,274 0.37 1,747 1.00 288 0.75 6 by 16 132 1 23.4 17.1 3,470 .51 6,610 .57 1,501 .86 388 1.01 6 by 10 177 2 19.0 8.8 4,560 .68 7,880 .68 1,722 .99 214 .56 4 by 11 180 1 18.4 23.9 3,078 .46 8,000 .69 1,660 .95 251 .66 6 by 8 177 6 20.0 13.7 4,227 .63 8,196 .71 1,634 .94 177 .46 2 by 2 30 17 15.9 13.9 6,750 1.00 11,520 1.00 1,740 1.00 383 1.00 Douglas fir 8 by 16 180 91 20.8 13.1 4,563 .68 6,372 .61 1,549 .91 269 .64 5 by 8 180 30 14.9 12.2 5,065 .76 6,777 .65 1,853 1.09 218 .52 2 by 2 24 211 19.0 16.4 6,686 1.00 10,378 1.00 1,695 1.00 419 1.00 Shortleaf pine 8 by 16 180 3 17.0 12.3 4,220 .54 6,030 .50 1,517 .85 398 .98 8 by 14 180 3 16.0 12.3 4,253 .55 5,347 .44 1,757 .98 307 .76 8 by 12 180 7 16.0 12.4 5,051 .65 7,331 .60 1,803 1.01 361 .89 5 by 8 180 6 12.2 22.5 7,123 .92 9,373 .77 1,985 1.11 301 .74 2 by 2 30 67 14.2 13.7 7,780 1.00 12,120 1.00 1,792 1.00 404 1.00 Western larch 8 by 16 180 23 18.3 21.9 3,343 .57 5,440 .53 1,409 .90 349 .96 8 by 12 180 29 17.8 23.4 3,631 .62 6,186 .60 1,549 .99 295 .81 5 by 8 180 10 13.6 27.6 4,730 .80 7,258 .71 1,620 1.04 221 .61 2 by 2 30 240 16.1 26.8 5,880 1.00 10,254 1.00 1,564 1.00 364 1.00 Loblolly pine 8 by 16 180 14 20.5 7.4 4,195 .81 6,734 .72 1,619 1.10 462 1.45 6 by 16 126 4 20.2 5.0 2,432 .47 4,295 .46 1,324 .90 266 .84 6 by 10 174 3 21.3 4.7 3,100 .60 6,167 .66 1,449 .99 173 .54 4 by 12 174 4 19.8 4.7 2,713 .52 5,745 .61 1,249 .85 185 .58 8 by 8 180 9 22.9 4.9 2,903 .56 4,557 .48 1,136 .77 93 .29 6 by 7 144 2 21.1 5.0 2,990 .58 4,968 .53 1,286 .88 116 .36 4 by 8 132 8 19.5 9.1 3,384 .65 6,194 .66 1,200 .82 196 .62 2 by 2 30 123 17.6 6.6 5,170 1.00 9,400 1.00 1,467 1.00 318 1.00 Tamarack 6 by 12 162 5 23.0 15.1 3,434 .45 5,640 .43 1,330 .82 318 .75 4 by 10 162 4 14.4 9.7 4,100 .54 5,320 .41 1,386 .84 252 .59 2 by 2 30 47 11.3 16.2 7,630 1.00 13,080 1.00 1,620 1.00 425 1.00 Western hemlock 8 by 16 180 44 17.7 17.8 4,398 .69 6,420 .62 1,737 1.04 406 1.06 2 by 2 28 311 17.9 19.4 6,333 1.00 10,369 1.00 1,666 1.00 382 1.00 Redwood 8 by 16 180 6 26.3 22.4 3,797 .79 4,428 .57 1,107 .96 294 1.05 6 by 12 180 6 16.1 17.7 3,175 .66 3,353 .43 728 .64 167 .60 7 by 9 180 6 15.9 15.2 3,280 .69 4,002 .51 1,104 .96 147 .53 3 by 14 180 6 13.1 24.4 5,033 .64 291 1.04 2 by 12 180 5 13.8 14.4 3,928 .82 5,336 .68 1,249 1.09 260 .93 2 by 10 180 5 13.8 24.8 3,757 .79 4,606 .59 1,198 1.05 186 .67 2 by 8 180 6 13.7 20.7 4,314 .90 5,050 .65 1,313 1.15 166 .60 2 by 2 28 122 15.2 18.8 4,777 1.00 7,798 1.00 1,146 1.00 279 1.00 Norway pine 6 by 12 162 5 16.7 8.1 2,968 .56 5,204 .61 1,123 .97 286 1.02 4 by 10 162 5 13.7 12.0 5,170 .98 6,904 .82 1,712 1.48 317 1.13 2 by 2 30 60 14.9 11.2 5,280 1.00 8,470 1.00 1,158 1.00 281 1.00 - Note. Following is an explanation of the abbreviations used in the foregoing tables: F.S. at E.L. = Fiber stress at elastic limit. M. of E. = Modulus of elasticity. M. of R. = Modulus of rupture. Cr. str. at E.L. = Crushing strength at elastic limit. Cr. str. at max. ld. = Crushing strength at maximum load. -
- TABLE XXI TABLE XXI - COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL - Compression Compression Shear parallel to grain perpendicular to grain - Cr. Cr. Cr. Species Per str. M. of str. Per str. Per Size of No. cent at E. at max. Stress No. cent at max. No. cent Shear specimen of of E. L. per ld.,. area Height of of ld., of of strength tests mois- per square per tests mois- per tests mois- ture square inch square ture square ture inch inch inch - - - - - - - - 1,000 Inches Lbs. lbs. Lbs. Inches Inches Lbs. Lbs. Longleaf pine 4 by 5 46 26.3 3,480 4,800 4 by 5 4 22 25.1 572 52 20.2 984 Douglas fir 6 by 6 259 20.3 3,271 1,038 4,258 4 by 8 16 44 20.8 732 465 22.1 822 2 by 2 247 18.7 3,842 1,084 5,002 4 by 8 10 32 18.1 584 4 by 4 8 51 20.2 638 4 by 4 6 49 24.0 613 4 by 4 4 29 24.8 603 Shortleaf pine 6 by 6 29 15.7 4,070 1,951 6,030 8 by 5 16 4 17.8 725 85 1,135 2 by 2 57 14.2 6,380 8 by 5 14 3 16.3 757 8 by 5 12 5 15.1 730 5 by 5 8 6 13.0 918 2 by 2 2 57 13.9 926 Western larch 6 by 6 112 16.0 5,445 8 by 6 16 17 18.8 491 193 15.0 905 4 by 4 81 14.7 6,161 8 by 6 12 18 17.6 526 2 by 2 270 14.8 5,934 5 by 4 8 22 13.3 735 Loblolly pine 6 by 6 23 3,357 1,693 5,005 8 by 5 16 12 19.8 602 156 11.3 1,115 5 by 5 10 22.4 2,217 545 2,950 8 by 5 8 7 22.9 679 4 by 8 8 19.4 3,010 633 3,920 4 by 5 8 8 19.5 715 2 by 2 69 5,547 Tamarack 6 by 7 3 15.7 2,257 1,042 3,323 2 by 2 2 57 16.2 697 60 14.0 879 4 by 7 3 13.6 3,780 1,301 4,823 4 by 4 57 14.9 3,386 1,353 4,346 2 by 2 66 14.6 4,790 Western hemlock 6 by 6 102 18.6 4,840 2,140 5,814 7 by 6 15 25 18.2 514 131 17.7 924 2 by 2 463 17.0 4,560 1,923 5,403 6 by 6 6 26 16.8 431 4 by 4 4 6 15.9 488 Redwood 6 by 6 18 16.9 4,276 8 by 6 16 5 25.4 548 95 12.4 671 2 by 2 115 14.6 5,119 6 by 6 12 6 14.7 610 7 by 6 9 5 14.8 500 3 by 6 14 2 12.6 470 2 by 6 12 2 16.2 498 2 by 6 10 4 14.3 511 2 by 6 8 2 13.2 429 2 by 2 2 145 13.8 564 Norway pine 6 by 7 4 15.2 2,670 1,182 4,212 2 by 2 2 36 10.0 924 44 11.9 1,145 4 by 7 2 22.2 3,275 1,724 4,575 4 by 4 55 16.6 3,048 1,367 4,217 2 by 2 44 11.2 7,550 - Note. Following is an explanation of the abbreviations used in the foregoing tables: F.S. at E.L. = Fiber stress at elastic limit. M. of E. = Modulus of elasticity. M. of R. = Modulus of rupture. Cr. str. at E.L. = Crushing strength at elastic limit. Cr. str. at max. ld. = Crushing strength at maximum load. - |
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