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° C. (212° 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 formulæ 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.)

[Illustration: FIG. 29.--Static bending test on large beam. Note arrangement of wire and scale for measuring deflection; also method of applying load at "third-points."]

_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.

[Illustration: FIG. 30.--Two methods of loading a beam, namely, third-point loading (upper), and centre loading (lower).]

_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 abscissæ. 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 formulæ 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.)

[Illustration: FIG. 31.--Static bending test on small beam. Note the use of the deflectometer with indicator and dial for measuring the deflection; also roller bearings between beam and supports.]

_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.

[Illustration: FIG. 32.--Sample log sheet, giving full details of a transverse bending test on a small pine beam.]

_Calculating the results_: The formulæ 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.)

[Illustration: FIG. 33.--Endwise compression test, showing method of measuring the deformation by means of a compressometer.]

_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.)

[Illustration: FIG. 34.--Sample log sheet of an endwise compression test on a short pine column.]

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 formulæ 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.)

[Illustration: FIG. 35.--Compression across the grain. Note method of measuring the deformation by means of a deflectomoter.]

_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.

[Illustration: Fig. 36.--Vertical section of shearing tool.]

[Illustration: FIG. 37.--Front view of shearing tool with test specimen and steel plate in position for testing.]

_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.

[Illustration: FIG. 38.--Two forms of shear test specimens.]

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.

[Illustration: FIG. 39.--Making a shearing test.]

_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.]

[Illustration: FIG. 40.--Impact testing machine.]

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.

[Illustration: FIG. 41.--Drum record of impact bending test.]

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 formulæ 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.

[Illustration: FIG. 42.--Abrasion machine for testing the wearing qualities of woods.]

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.

[Illustration: FIG. 43.--Design of tool for testing the hardness of woods by indentation.]

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.

[Illustration: FIG. 44.--Design of tool for cleavage test.]

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.

[Illustration: FIG. 45.--Design of cleavage test specimen.]

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.)

[Illustration: FIG. 46.--Designs of tension test specimens used in United States.]

[Footnote 61: Bul. No. 8: Timber physics,