The Mechanical Properties of Wood / Including a Discussion of the Factors Affecting the Mechanical Properties, and Methods of Timber Testing

by Samuel J. Record

MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES

Working Plan No. 124

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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" × 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.

Figure 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" × 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.

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 N7 and N8, or H5 and H6.

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,		etc.	}	Tested green
Upper bolt,		2,	3,			6,	7,			10,
Lower bolt,		2,	3,			6,	7,			10,	etc.	}	Air-dried and tested
Upper 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.

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" × 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 specimens 2" × 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 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" × 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 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" × 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.

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.