Part II
., 1893, p. 7.]
The difficulty of making such tests compared with the minor importance of the results is so great that they are at present omitted by the U.S. Forest Service. A form of specimen is suggested, however, and is as follows: "A rod of wood about one inch in diameter is bored by a hollow drill from the stick to be tested. The ends of this rod are inserted and glued in corresponding holes in permanent hardwood wedges. The specimen is then submitted to the ordinary tension test. The broken ends are punched from the wedges."[62] (See Fig. 46, No. 2.)
[Footnote 62: Cir. 38: Instructions to engineers of timber tests, 1906, p. 24.]
The form used by the Department of Forestry of New South Wales[63] is as shown in Fig. 47. The specimen has a total length of 41 inches and is circular in cross section. On each end is a head 4 inches in diameter and 7 inches long. Below each head is a shoulder 8.5 inches long, which tapers from a diameter of 2.75 inches to 1.25 inches. In the middle is a cylindrical portion 1.25 inches in diameter and 10 inches long.
[Illustration: FIG. 47.--Design of tension test specimen used in New South Wales.]
[Footnote 63: Warren, W.H.: The strength, elasticity, and other properties of New South Wales hardwood timbers, 1911, pp. 58-62.]
In making the test the specimen is fitted in the machine, and an extensometer attached to the middle portion and arranged to record the extension between the gauge points 8 inches apart. The area of the cross section then is 1.226 square inches, and the tensile strength is equal to the total breaking load applied divided by this area.
TENSION TEST AT RIGHT ANGLES TO THE GRAIN
A static testing machine and a special testing device (see Fig. 48) are required. The latter consists essentially of two double hooks or clamps, one of which is suspended from the centre of the top of the cage, the other extended above the movable head. The specimens are 2" X 2" X 2.5". At each end a one-inch hole is bored with its centre equidistant from the two sides and 0.25 inch from the ends. This makes the cross section to be tested 1" X 2".
[Illustration: FIG. 48.--Design of tool and specimen for testing tension at right angles to the grain.]
The free ends of the clamps are fitted into the notches in the ends of the specimen. The movable head of the machine is then made to descend at the rate of 0.25 inch per minute, pulling the specimen in two at right angles to the grain. The maximum load only is taken and the result expressed in pounds per inch of width. A piece one-half inch thick is split off parallel to the failure and used for moisture determination.
TORSION TEST[64]
[Footnote 64: Wood is so seldom subjected to a pure stress of this kind that the torsion test is usually omitted.]
_Apparatus_: The torsion test is made in a Riehle-Miller torsional testing machine or its equivalent. (See Fig. 49.)
[Illustration: FIG. 49.--Making a torsion test on hickory.]
_Preparation of material_: The test pieces are cylindrical, 1.5 inches in diameter and 18 inches gauge length, with squared ends 4 inches long joined to the cylindrical portion with a fillet. The dimensions are carefully measured, and the usual data obtained in regard to the rate of growth, proportion of late wood, location and kind of defects. The weight of the cylindrical portion of the specimen is obtained after the test.
_Making the test_: After the specimen is fitted in the machine the load is applied continuously at the rate of 22° per minute. A troptometer is used in measuring the deformation. Readings are made until failure occurs, the points being entered on the cross-section paper. The character of the failure is described. Moisture determinations are made by the disk method.
_Results_: The conditions of ultimate rupture due to torsion appear not to be governed by definite mathematical laws; but where the material is not overstrained, laws may be assumed which are sufficiently exact for practical cases. The formulæ commonly used for computations are as follows:
5.1 M (1) T = ------- c^{3}
114.6 T f (2) G = ----------- a c
a = angle measured by troptometer at elastic limit, in degrees. c = diameter of specimen, inches. f = gauge length of specimen, inches. _G_ = modulus of elasticity in shear across the grain, pounds per square inch. M = moment of torsion at elastic limit, inch-pounds. T = outer fibre torsional stress at elastic limit, pounds per square inch.
SPECIAL TESTS
_Spike-pulling Test_
Spike-pulling tests apply to problems of railroad maintenance, and the results are used to compare the spike-holding powers of various woods, both untreated and treated with different preservatives, and the efficiency of various forms of spikes. Special tests are also made in which the spike is subjected to a transverse load applied repetitively by a blow.
For details of tests and results see:
Cir. 38, U.S.F.S.: Instructions to engineers of timber tests, p. 26. Cir. 46, U.S.F.S.: Holding force of railroad spikes in wooden ties. Bul. 118, U.S.F.S,: Prolonging the life of cross-ties, pp. 37-40.
_Packing Boxes_
Special tests on the strength of packing boxes of various woods have been made by the U.S. Forest Service to determine the merits of different kinds of woods as box material with the view of substituting new kinds for the more expensive ones now in use. The methods of tests consisted in applying a load along the diagonal of a box, an action similar to that which occurs when a box is dropped on one of its corners. The load was measured at each one-fourth inch in deflection, and notes were made of the primary and subsequent failures.
For details of tests and results, see:
Cir. 47, U.S.F.S.: Strength of packing boxes of various woods. Cir. 214, U.S.F.S.: Tests of packing boxes of various forms.
_Vehicle and Implement Woods_
Tests were made by the U.S. Forest Service to obtain a better knowledge of the mechanical properties of the woods at present used in the manufacture of vehicles and implements and of those which might be substituted for them. Tests were made upon the following materials: hickory buggy spokes (see Fig. 5); hickory and red oak buggy shafts; wagon tongues; Douglas fir and southern pine cultivator poles.
Details of the tests and results may be found in:
Cir. 142, U.S.F.S.: Tests on vehicle and implement woods.
_Cross-arms_
In tests by the U.S. Forest Service on cross-arms a special apparatus was devised in which the load was distributed along the arm as in actual practice. The load was applied by rods passing through the pinholes in the arms. Nuts on these rods pulled down on the wooden bearing-blocks shaped to fit the upper side of the arm. The lower ends of these rods were attached to a system of equalizing levers, so arranged that the load at each pinhole would be the same. In all the tests the load was applied vertically by means of the static machine.
See Cir. 204, U.S.F.S.: Strength tests of cross-arms.
_Other Tests_
Many other kinds of tests are made as occasion demands. One kind consists of barrels and liquid containers, match-boxes, and explosive containers. These articles are subjected to shocks such as they would receive in transit and in handling, and also to hydraulic pressure.
One of the most important tests from a practical standpoint is that of built-up structures such as compounded beams composed of small pieces bolted together, mortised joints, wooden trusses, etc. Tests of this kind can best be worked out according to the specific requirements in each case.
APPENDIX
SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE
MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES
Working Plan No. 124
PURPOSE OF WORK
It is the general purpose of the work here outlined to provide:
(_a_) Reliable data for comparing the mechanical properties of various species;
(_b_) Data for the establishment of correct strength functions or working stresses;
(_c_) Data upon which may be based analyses of the influence on the mechanical properties of such factors as:
Locality;
Distance of timber from the pith of the tree;
Height of timber in the tree;
Change from the green to the air-dried condition, etc.
The mechanical properties which will be considered and the principal tests used to determine them are as follows:
Strength and stiffness-- Static bending; Compression parallel to grain; Compression perpendicular to grain; Shear.
Toughness-- Impact bending; Static bending; Work to maximum load and total work.
Cleavability-- Cleavage test.
Hardness-- Modification of Janka ball test for surface hardness.
MATERIAL
_Selection and Number of Trees_
The material will be from trees selected in the forest by one qualified to determine the species. From each locality, three to five dominant trees of merchantable size and approximately average age will be so chosen as to be representative of the dominant trees of the species. Each species will eventually be represented by trees from five to ten localities. These localities will be so chosen as to be representative of the commercial range of the species. Trees from one to three localities will be used to represent each species until most of the important species have been tested.
The 16-foot butt log will be taken from each tree selected and the entire merchantable hole of one average tree for each species.
_Field Notes and Shipping Instructions_
Field notes as outlined in Form--_a_ Shipment Description, Manual of the Branch of Products, will be fully and carefully made by the collector. The age of each tree selected will be recorded and any other information likely to be of interest or importance will also be made a part of these field notes. Each log will have the bark left on. It will be plainly marked in accordance with directions given under Detailed Instructions. All material will be shipped to the laboratory immediately after being cut. No trees will be cut until the collector is notified that the laboratory is ready to receive the material.
DETAILED INSTRUCTIONS
_Part of Tree to be Tested_
(_a_) For determining the value of tree and locality and the influence on the mechanical properties of distance from the pith, a 4-foot bolt will be cut from the top end of each 16-foot butt log.
(_b_) For investigating the variation of properties with the height of timber in the tree, all the logs from one average tree will be used.
(_c_) For investigating the effect of drying the wood, the bolt next below that provided for in (_a_) will be used in the case of one tree from each locality.
_Marking and Grouping of Material_
The marking will be standard except as noted. Each log will be considered a "piece." The piece numbers will be plainly marked upon the butt end of each log by the collector. The north side of each log will also be marked.
When only one bolt from a tree is used it will be designated by the number of the log from which it is cut. Whenever more than one bolt is taken from a tree, each 4-foot bolt or length of trunk will be given a letter (mark), _a, b, c,_ etc., beginning at the stump.
All bolts will be sawed into 2-1/2" X 2-1/2" sticks and the sticks marked according to the sketch, Fig. 50. The letters _N, E, S,_ and _W_ indicate the cardinal points when known; when these are unknown, _H, K, L,_ and _M_ will be used. Thus, _N5, K8, S7, M4_ are stick numbers, the letter being a part of the stick number.
[Illustration: FIG. 50.--Method of cutting and marking test specimens.]
Only straight-grained specimens, free from defects which will affect their strength, will be tested.
_Care of Material_
No material will be kept in the bolt or log long enough to be damaged or disfigured by checks, rot, or stains.
_Green material_: The material to be tested green will be kept in a green state by being submerged in water until near the time of test. It will then be surfaced, sawed to length, and stored in damp sawdust at a temperature of 70°F. (as nearly as practicable) until time of test. Care should be taken to avoid as much as possible the storage of green material in any form.
_Air-dry material_: The material to be air-dried will be cut into sticks 2-1/2" X 2-1/2" X 4'. The ends of these sticks will be paraffined to prevent checking. This material will be so piled as to leave an air space of at least one-half inch on each side of each stick, and in such a place that it will be protected from sunshine, rain, snow, and moisture from the ground. The sticks will be surfaced and cut to length just previous to test.
_Order of Tests_
The order of tests in all cases will be such as to eliminate so far as possible from the comparisons the effect of changes of condition of the specimens due to such factors as storage and weather conditions.
The material used for determining the effect of height in tree will be tested in such order that the average time elapsing from time of cutting to time of test will be approximately the same for all bolts from any one tree.
_Tests on Green Material_
The tests on all bolts, except those from which a comparison of green and dry timber is to be gotten, will be as follows:
_Static bending_: One stick from each pair. A pair consists of two adjacent sticks equidistant from the pith, as _N_7 and _N_8, or _H_5 and _H_6.
_Impact bending_: Four sticks; one to be taken from near the pith; one from near the periphery; and two representative of the cross section.
_Compression parallel to grain_: One specimen from each stick. These will be marked "1" in addition to the number of the stick from which they are taken.
_Compression perpendicular to grain_: One specimen from each of 50 per cent of the static bending sticks. These will be marked "2" in addition to the number of the stick from which they are cut.
_Hardness_: One specimen from each of the other 50 per cent of the static bending sticks. These specimens will be marked "4."
_Shear_: Six specimens from sticks not tested in bending or from the ends cut off in preparing the bending specimens. Two specimens will be taken from near the pith; two from near the periphery; and two that are representative of the average growth. One of each two will be tested in radial shear and the other in tangential shear. These specimens will have the mark "3."
_Cleavage_: Six specimens chosen and divided just as those for shearing. These specimens will have the mark "5." (For sketches showing radial and tangential cleavage, see Fig. 45.)
When it is impossible to secure clear specimens for all of the above tests, tests will have precedence in the order in which they are named.
_Tests to Determine the Effect of Air-drying_
These tests will be made on material from the adjacent bolts mentioned in "_c_" under Part of Tree to be Tested. Both bolts will be cut as outlined above. One-half the sticks from each bolt will be tested green, the other half will be air-dried and tested. The division of green and air-dry will be according to the following scheme:
STICK NUMBERS
Lower bolt, 1, 4, 5, 8, 9, } Tested etc. } green Upper bolt, 2, 3, 6, 7, 10, }
Lower bolt, 2, 3, 6, 7, 10, } Air-dried etc. } and Upper bolt, 1, 4, 5, 8, 9, } tested
All green sticks from these two bolts will be tested as if they were from the same bolt and according to the plan previously outlined for green material from single bolts. The tests on the air-dried material will be the same as on the green except for the difference of seasoning.
The material will be tested at as near 12 per cent moisture as is practicable. The approximate weight of the air-dried specimens at 12 per cent moisture will be determined by measuring while green 20 per cent of the sticks to be air-dried and assuming their dry gravity to be the same as that of the specimens tested green. This 20 per cent will be weighed as often as is necessary to determine the proper time of test.
_Methods of Test_
All tests will be made according to Circular 38 except in case of conflict with the instructions given below:
_Static bending_: The tests will be on specimens 2" X 2" X 30" on 28-inch span. Load will be applied at the centre.
In all tests the load-deflection curve will be carried to or beyond the maximum load. In one-third of the tests the load-deflection curve will be continued to 6-inch deflection, or till the specimen fails to support a 200-pound load. Deflection readings for equal increments of load will be taken until well past the elastic limit, after which the scale beam will be kept balanced and the load read for each 0.1-inch deflection. The load and deflection at first failure, maximum load and points of sudden change, will be shown on the curve sheet even if they do not occur at one of the regular load or deflection increments.
_Impact bending_: The impact bending tests will be on specimens of the same size as those used in static bending. The span will be 28 inches.
The tests will be by increment drop. The first drop will be 1 inch and the increase will be by increments of 1 inch till a height of 10 inches is reached, after which increments of 2 inches will be used until complete failure occurs or 6-inch deflection is secured.
A 50-pound hammer will be used when with drops up to 68 inches it is practically certain that it will produce complete failure or 6-inch deflection in the case of all specimens of a species. For all other species, a 100-pound hammer will be used.
In all cases drum records will be made until first failure. Also the height of drop causing complete failure or 6-inch deflection will be noted.
_Compression parallel to grain_: This test will be on specimens 2" X 2" X 8" in size. On 20 per cent of these tests load-compression curves for a 6-inch centrally located gauge length will be taken. Readings will be continued until the elastic limit is well passed. The other 80 per cent of the tests will be made for the purpose of obtaining the maximum load only.
_Compression perpendicular to grain_: This test will be on specimens 2" X 2" X 6" in size. The bearing plates will be 2 inches wide. The rate of descent of the moving head will be 0.024 inch per minute. The load-compression curve will be plotted to 0.1 inch compression and the test will then be discontinued.
_Hardness_: The tool shown in Fig. 43 (an adaptation of the apparatus used by the German investigator, Janka) will be used. The rate of descent of the moving head will be 0.25 inch per minute. When the penetration has progressed to the point at which the plate "_a_" becomes tight, due to being pressed against the wood, the load will be read and recorded.
Two penetrations will be made on a tangential surface, two on a radial, and one on each end of each specimen tested. The choice between the two radial and between the two tangential surfaces and the distribution of the penetrations over the surfaces will be so made as to get a fair average of heart and sap, slow and fast growth, and spring and summer wood. Specimens will be 2" X 2" X 6".
_Shear_: The tests will be made with a tool slightly modified from that shown in Circular 38. The speed of descent of head will be 0.015 inch per minute. The only measurements to be made are those of the shearing area. The offset will be 1/8 inch. Specimens will be 2" X 2" X 2-1/2" in size. (For definition of offset and form of test specimen, see Fig. 38.)
_Cleavage_: The cleavage tests will be made on specimens of the form and size shown in Fig. 45. The apparatus will be as shown in Fig. 44. The maximum load only will be taken and the result expressed in pounds per inch of width. The speed of the moving head will be 0.25 inch per minute.
_Moisture Determinations_
Moisture determinations will be made on all specimens tested except those to be photographed or kept for exhibit. A 1-inch disk will be cut from near the point of failure of bending and compression parallel specimens, from the portion under the plate in the case of the compression perpendicular specimens, and from the centre of the hardness test specimens. The beads from the shear specimens will be used as moisture disks. In the case of the cleavage specimens a piece 1/2 inch thick will be split off parallel to the failure and used as a moisture disk.
RECORDS
All records will be standard.
PHOTOGRAPHS
_Cross Sections_
Just before cutting into sticks, the freshly cut end of at least one bolt from each tree will be photographed. A scale of inches will be shown in this photograph.
_Specimens_
Three photographs will be made of a group consisting of four 2" X 2" X 30" specimens chosen from the material from each locality. Two of these specimens will be representative of average growth, one of fast and one of slow growth. These photographs will show radial, tangential, and end surfaces for each specimen.
_Failures_
Typical and abnormal failures of material from each site will be photographed.
_Disposition of Material_
The specimens photographed to show typical and abnormal failures will be saved for purposes of exhibit until deemed by the person in charge of the laboratory to be of no further value.
SHRINKAGE AND SPECIFIC GRAVITY
Appendix to Working Plan 124
PURPOSE OF WORK
It is the purpose of this work to secure data on the shrinkage and specific gravity of woods tested under Project 124. The figures to be obtained are for use as average working values rather than as the basis for a detailed study of the principles involved.
MATERIAL
The material will be taken from that provided for mechanical tests.
RADIAL AND TANGENTIAL SHRINKAGE
_Specimens_
_Preparation_: Two specimens 1 inch thick, 4 inches wide, and 1 inch long will be obtained from near the periphery of each "_d_" bolt. These will be cut from the sector-shaped sections left after securing the material for the mechanical tests or from disks cut from near the end of the bolt. They will be taken from adjoining pieces chosen so that the results will be comparable for use in determining radial and tangential shrinkage. (When a disk is used, care must be taken that it is green and has not been affected by the shrinkage and checking near the end of the bolt.)
One of these specimens will be cut with its width in the radial direction and will be used for the determination of radial shrinkage. The other will have its width in the tangential direction and will be used for tangential shrinkage. These specimens will not be surfaced.
_Marking_: The shrinkage specimens will retain the shipment and piece numbers and marks of the bolts from which they are taken, and will have the additional mark _7_R or _7_T 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.
VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY
_Specimens_
_Selection and preparation_: Four 2" X 2" X 6" specimens will be cut from the mechanical test sticks of each "_d_" bolt; also from each of the composite bolts used in getting a comparison of green and air-dry. One of these specimens will be taken from near the pith and one from near the periphery; the other two will be representative of the average growth of the bolt. The sides of these specimens will be surfaced and the ends smooth sawn.
_Marking_: Each specimen will retain the shipment, piece, and stick numbers and mark of the stick from which it is cut, and will have the additional mark "_S_."
_Manipulation_: Soon after cutting, each specimen will be weighed and its volume will be determined by the method described below. The rings per inch and per cent summer wood, where possible, will be determined, and a carbon impression of the end of the specimen made. It will then be air-dried in the laboratory to a constant weight and afterward oven-dried at 100°C. When dry, the specimen will be taken from the oven, weighed, and a carbon impression of its end made. While still warm the specimen will be dipped in hot paraffine. The volume will then be determined by the following method:
On one pan of a pair of balances is placed a container having in it water enough for the complete submersion of the test specimen. This container and water is balanced by weights placed on the other scale pan. The specimen is then held completely submerged and not touching the container while the scales are again balanced. The weight required to balance is the weight of water displaced by the specimen, and hence if in grams is numerically equal to the volume of the specimen in cubic centimetres. A diagrammatic sketch of the arrangement of this apparatus is shown in Fig. 51.
[Illustration: FIG. 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.
STRENGTH VALUES FOR STRUCTURAL TIMBERS
(From Cir. 189, U.S. Forest Service)
The following tables bring together in condensed form the average strength values resulting from a large number of tests made by the Forest Service on the principal structural timbers of the United States. These results are more completely discussed in other publications of the Service, a list of which is given in BIBLIOGRAPHY,