The Mechanical Properties of Wood Part 16

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.


_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 70F. (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:


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.

Chapter end

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