The Mechanical Properties of Wood Part 14

_Indentation_: The tool used for this test consists of a punch with a hemispherical end or steel ball having a diameter of 0.444 inch, giving a surface area of one-fourth square inch. It is fitted with a guard plate, which works loosely until the penetration has progressed to a depth of 0.222 inch, whereupon it tightens. (See Fig. 43.) The effect is that of sinking a ball half its diameter into the specimen. This apparatus is fitted into the movable head of the static testing machine.

[Ill.u.s.tration: FIG. 43.--Design of tool for testing the hardness of woods by indentation.]

The wood to be tested is cut square with the grain into rectangular blocks measuring 2" X 2" X 6". A block is placed on the platform and the end of the punch forced into the wood at the rate of 0.25 inch per minute. The operator keeps moving the small handle of the guard plate back and forth until it tightens. At this instant the load is read and recorded.

Two penetrations each are made on the tangential and radial surfaces, and one on each end of every specimen tested.

In choosing the places on the block for the indentations, effort should be made to get a fair average of heartwood and sapwood, fine and coa.r.s.e grain, early and late wood.

Another method of testing by indentation involves the use of a right-angled cone instead of a ball. For details of this test as used in New South Wales see _loc. cit._, pp. 86-87.

CLEAVAGE TEST

A static testing machine and a special cleavage testing device are required. (See Fig. 44.) The latter consists essentially of two hooks, one of which is suspended from the centre of the top of the cage, the other extended above the movable head.

[Ill.u.s.tration: FIG. 44.--Design of tool for cleavage test.]

The specimens are 2" X 2" X 3.75". At one end a one-inch hole is bored, with its centre equidistant from the two sides and 0.25 inch from the end. (See Fig. 45.) This makes the cross section to be tested 2" X 3". Some of the blocks are cut radially and some tangentially, as indicated in the figure.

[Ill.u.s.tration: FIG. 45.--Design of cleavage test specimen.]

The free ends of the hooks are fitted into the notch in the end of the specimen. The movable head of the machine is then made to descend at the rate of 0.25 inch per minute, pulling apart the hooks and splitting the block. The maximum load only is taken and the result expressed in pounds per square inch of width. A piece one-half inch thick is split off parallel to the failure and used for moisture determination.

TENSION TEST PARALLEL TO THE GRAIN

Since the tensile strength of wood parallel to the grain is greater than the compressive strength, and exceedingly greater than the shearing strength, it is very difficult to make satisfactory tension tests, as the head and shoulders of the test specimen (which is subjected to both compression and shear) must be stronger than the portion subjected to a pure tensile stress.

Various designs of test specimens have been made. The one first employed by the Division of Forestry[61] was prepared as follows: Sticks were cut measuring 1.5" X 2.5" X 16". The thickness at the centre was then reduced to three-eighths of an inch by cutting out circular segments with a band saw. This left a breaking section of 2.5" X 0.375". Care was taken to cut the specimen as nearly parallel to the grain as possible, so that its failure would occur in a condition of pure tension. The specimen was then placed between the plane wedge-shaped steel grips of the cage and the movable head of the static machine and pulled in two. Only the maximum load was recorded. (See Fig. 46, No. 1.)

[Ill.u.s.tration: FIG. 46.--Designs of tension test specimens used in United States.]

[Footnote 61: Bul. No. 8: Timber physics, 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.

[Ill.u.s.tration: 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".

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: 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 a.s.sumed which are sufficiently exact for practical cases. The formulae 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.