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

|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 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