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

glass expand equally in all directions, since they are homogeneous substances, while wood is a complicated structure. The coefficient of expansion of iron is .0000285, or nearly six times the coefficient of linear expansion of oak and seven times that of spruce[50]. [Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev. ed.), p. 75.] Under ordinary conditions wood contains more or less moisture, so that the application of heat has a drying effect which is accompanied by shrinkage. This shrinkage completely obscures the expansion due to the heating. Experiments made at the Yale Forest School revealed the effect of temperature on the crushing strength of wet wood. In the case of wet chestnut wood the strength decreases 0.42 per cent for each degree the water is heated above 60° F.; in the case of spruce the decrease is 0.32 per cent. The effects of high temperature on wet wood are very marked. Boiling produces a condition of great pliability, especially in the case of hardwoods. If wood in this condition is bent and allowed to dry, it rigidly retains the shape of the bend, though its strength may be somewhat reduced. Except in the case of very dry wood the effect of cold is to increase the strength and stiffness of wood. The freezing of any free water in the pores of the wood will augment these conditions. The effect of steaming upon the strength of cross-ties was investigated by the U.S. Forest Service in 1904. The conclusions were summarized as follows: "(1) The steam at pressure up to 40 pounds applied for 4 hours, or at a pressure of 20 pounds up to 20 hours, increases the weight of ties. At 40 pounds' pressure applied for 4 hours and at 20 pounds for 5 hours the wood began to be scorched. "(2) The steamed and saturated wood, when tested immediately after treatment, exhibited weaknesses in proportion to the pressure and duration of steaming. (See Table XVI.) If allowed to air-dry subsequently the specimens regained the greater part of their strength, provided the pressure and duration had not exceeded those cited under (1). Subsequent immersion in water of the steamed wood and dried specimens showed that they were weaker than natural wood similarly dried and resoaked."[51] [Footnote 51: Cir. 39. Experiments on the strength of treated timber, p. 18.] |------------------------------------------------------------------------------------------| | TABLE XVI | |------------------------------------------------------------------------------------------| | EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE | | (Forest Service, Cir. 39) | |------------------------------------------------------------------------------------------| | | Cylinder conditions | Strength | | |---------------------------------+--------------------------------------------| | | Steaming | Static | Impact | | | |---------------------------------+---------------------+----------| Average | | Treatment | | | | Bending | Compres- | Height | of the | | | | | | modulus | sion | of drop | three | | | Period | Pressure | Temperature | of | parallel | causing | strengths | | | | | | rupture | to grain | complete | | | | | | | | | failure | | |-----------+--------+----------+-------------+----------+----------+----------+-----------| | | | Lbs. per | | Per cent | Per cent | Per cent | Per cent | | | Hrs. | sq. inch | °F. | | | | | | | | | | Untreated wood = 100% | | | | | | | | | | | Steam, | 4 | | 230[a] | 91.3 | 79.1 | 96.4 | 88.9 | | at | 4 | 10 | 238 | 78.2 | 93.7 | 93.3 | 88.4 | | various | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 80.8 | | pressures | 4 | 30 | 269 | 80.4 | 78.4 | 89.8 | 82.9 | | | 4 | 40 | 283 | 78.1 | 74.4 | 74.0 | 75.5 | | | 4 | 50 | 292 | 75.8 | 71.5 | 63.9 | 70.4 | | | 4 | 100 | 337 | 41.4 | 65.0 | 55.2 | 53.9 | |-----------+--------+----------+-------------+----------+----------+----------+-----------| | Steam, | 1 | 20 | 257 | 100.6 | 98.6 | 86.7 | 95.3 | | for | 2 | 20 | 267 | 88.4 | 93.0 | 107.0 | 96.1 | | various | 3 | 20 | 260 | 90.0 | 93.6 | 84.1 | 89.2 | | periods | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 86.3 | | | 5 | 20 | 253 | 85.0 | 78.1 | 84.2 | 82.4 | | | 6 | 20 | 242 | 95.2 | 89.8 | 76.0 | 87.0 | | | 10 | 20 | 255 | 73.7 | 82.0 | 76.0 | 77.2 | | | 20 | 20 | 258 | 67.5 | 65.0 | 99.0 | 77.2 | |------------------------------------------------------------------------------------------| | [Footnote a: It will be noted that the temperature was 230°. This is the maximum | | temperature by the maximum-temperature recording thermometer, and is due to the handling | | of the exhaust valve. The average temperature was that of exhaust steam.] | |------------------------------------------------------------------------------------------| "(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° 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 |