SPECIAL TESTS

5.1M(1)T=-------c3114.6T f(2)G=-----------a ca=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.

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.

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.

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.

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.

Cir. 204,U.S.F.S.:Strength tests of cross-arms.

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.

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;

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.

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 as outlined in Form—aShipment 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.

(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.

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" × 2-1/2" sticks and the sticks marked according to the sketch,Fig. 50. The lettersN, E, S,andWindicate the cardinal points when known; when these are unknown,H, K, L,andMwill be used. Thus,N5, K8, S7, M4are stick numbers, the letter being a part of the stick number.

Figure 50

Method of cutting and marking test specimens.

Only straight-grained specimens, free from defects which will affect their strength, will be tested.

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°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" × 2-1/2" × 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.

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.

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, asN7 andN8, orH5 andH6.

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.

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 NUMBERSLower bolt,1,4,5,8,9,etc.}Tested greenUpper bolt,2,3,6,7,10,Lower bolt,2,3,6,7,10,etc.}Air-dried and testedUpper bolt,1,4,5,8,9,

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.

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" × 2" × 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 specimens2" × 2" × 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" × 2" × 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 inFig. 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" × 2" × 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" × 2" × 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 inFig. 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 will be made on all specimens tested except those to be photographed or kept for exhibit. A 1-inchdisk 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.

All records will be standard.

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.

Three photographs will be made of a group consisting of four 2" × 2" × 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.

Typical and abnormal failures of material from each site will be photographed.

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.

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.

The material will be taken from that provided for mechanical tests.

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 mark7R or7T 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°C. (212°F.), when they will again be weighed and measured.

Selection and preparation: Four 2" × 2" × 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°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 inFig. 51.

Figure 51

Diagram of specific gravity apparatus, showing a balance with container (c) filled with water in which the test block (b) is held submerged by a light rod (a) which is adjustable vertically and provided with a sharp point to be driven into the specimen.

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.

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 onpages 157-159.

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.

TablesXVIIIandXIXgive the average results obtained from tests on green material, while TablesXXandXXIgive average results from tests on air-seasoned material. The small specimens, which were invariably 2" × 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" × 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" × 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 areseasoned 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 XVIIIBENDING TESTS ON GREEN MATERIALSpeciesSizesNumber of testsPer cent of moistureRings per inchF.S. at E.L.M. of R.M. of E.Calculated shearCross SectionSpanAverage per sq. inchRatio to 2" by 2"Average per sq. inchRatio to 2" by 2"Average per sq. inchRatio to 2" by 2"Average per sq. inchRatio to 2" by 2"InchesIns.Lbs.Lbs.1,000lbs.Lbs.Longleaf pine12 by 12138428.69.74,0290.836,7100.741,5230.992610.8610 by 16168426.816.76,453.856,453.711,6261.053061.018 by 16156728.414.63,147.645,439.601,368.893901.296 by 16132140.321.84,120.836,460.711,190.773781.256 by 10180131.06.23,580.726,500.721,412.92175.586 by 8180227.08.23,735.755,745.631,282.83121.402 by 2301533.914.14,9501.009,0701.001,5401:003031.00Douglas fir8 by 1618019131.511.03,968.765,983.721,517.95269.815 by 81808430.110.83,693.715,178.631,533.96172.522 by 121802735.720.33,721.715,276.641,6421.03256.772 by 101802632.921.63,160.604,699.571,5931.00189.572 by 81802933.617.63,593.695,352.651,6071.01171.512 by 22456830.411.65,2271.009,0701.001,5401.003031.00Douglas fir (fire-killed)8 by 161803036.810.93,503.804,994.641,531.943301.192 by 121803234.217.73,489.805,085.661,624.99247.892 by 101803238.918.13,851.885,359.691,7161.05216.782 by 81803137.015.73,403.785,305.681,6761.02169.612 by 23029033.217.24,3601.007,7521.001,6361.002771.00Shortleaf pine8 by 161801239.512.13,185.735,407.701,4381.033621.408 by 141801245.812.73,234.745,781.751,4941.073381.318 by 121802452.211.83,265.755,503.711,4801.062771.075 by 81802447.811.53,519.815,732.741,4851.06185.722 by 23025451.713.64,3501.007,7101.001,3951.002581.00Western larch8 by 161803251.025.33,276.774,632.641,272.972981.118 by 121803050.323.23,376.795,286.731,3311.02254.945 by 81801456.025.63,528.835,331.741,4321.09169.632 by 22818946.226.24,2741.007,2511.001,3101.002691.00Loblolly pine8 by 161801715.86.13,094.755,394.691,406.983831.445 by 121809460.95.93,030.745,028.641,383.96221.832 by 2304470.95.44,1001.007,8701.001,4401.002651.00Tamarack6 by 121621557.616.62,914.754,500.661,2021.052551.114 by 101621543.511.42,712.704,611.681,2381.08209.912 by 2308238.814.03,8751.006,8201.001,1411.002291.00Western hemlock8 by 161803942.515.63,516.805,296.731,4451.01261.922 by 2285251.812.14.4061.007,2941.001,4281.002841.00Redwood8 by 161801486.519.93,734.794,492.641,016.963001.216 by 121801487.317.83,787.804,451.641,0681.00224.907 by 91801479.816.74,412.935,279.761,3241.25199.803 by 141801386.123.73,506.744,364.62947.892551.032 by 121801270.918.63,100.653,753.541,052.99187.752 by 101801355.820.03,285.694,079.581,1071.04169.682 by 81801363.821.52,989.634,063.581,1411.08134.542 by 22815775.519.14,7501.006,9801.001,0611.002481.00Norway pine6 by 121621550.312.52,305.823,572.699871.032011.174 by 121621847.914.72,648.944,107.791,2551.312381.384 by 101621645.713.32,674.954,205.811,3061.361981.152 by 23013332.311.42,8081.005,1731.009601.001721.00Red spruce2 by 101441432.521.92,394.663,566.601,1801.02181.802 by 2266037.321.33,6271.005,9001.001,1571.002271.00White spruce2 by 101441640.79.32,239.723,288.631,0811.08166.832 by 2268358.310.23.0901.005,1851.009981.001991.00Note.—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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