Part 13 (1/2)

_Apparatus_: An ordinary static testing machine and a special tool designed for producing single shear are required. (See Figs. 36 and 37.) This shearing apparatus consists of a solid steel frame with set screws for clamping the block within it firmly in a vertical position. In the centre of the frame is a vertical slot in which a square-edged steel plate slides freely.

When the testing block is in position, this plate impinges squarely along the upper surface of the tenon or lip, which, as vertical pressure is applied, shears off.

[Ill.u.s.tration: Fig. 36.--Vertical section of shearing tool.]

[Ill.u.s.tration: FIG. 37.--Front view of shearing tool with test specimen and steel plate in position for testing.]

_Preparing the material_: The specimens are usually in the form of small, clear, straight-grained blocks with a projecting tenon or lip to be sheared off. Two common forms and sizes are shown in Figure 38. Part of the blocks are cut so that the shearing surface is parallel to the growth rings, or tangential; others at right angles to the growth rings, or radial. It is important that the upper surface of the tenon or lip be sawed exactly parallel to the base of the block. When the form with a tenon is used the under cut is extended a short distance horizontally into the block to prevent any compression from below.

[Ill.u.s.tration: FIG. 38.--Two forms of shear test specimens.]

In designing a shearing specimen it is necessary to take into consideration the proportions of the area of shear, since, if the length of the portion to be sheared off is too great in the direction of the shearing face, failure would occur by compression before the piece would shear. Inasmuch as the endwise compressive strength is sometimes not more than five times the shearing strength, the shearing surface should be less than five times the surface to which the load is applied. This condition is fulfilled in the specimens ill.u.s.trated.

Shearing specimens are frequently cut from beams after testing.

In this case the specific gravity (dry), proportion of late wood, and rate of growth are a.s.sumed to be the same as already recorded for the beams. In specimens not so taken, these quant.i.ties are determined in the usual way. The sheared-off portion is used for a moisture section.

_Adjusting specimen in machine_: The test specimen is placed in the shearing apparatus with the tenon or lip under the sliding plate, which is centred under the movable head of the machine.

(See Fig. 39.) In order to reduce to a minimum the friction due to the lateral pressure of the plate against the bearings of the slot, the apparatus is sometimes placed upon several parallel steel rods to form a roller base. A slight initial load is applied to take up the lost motion of the machinery, and the beam balanced.

[Ill.u.s.tration: FIG. 39.--Making a shearing test.]

_Log of the test_: The load is applied continuously and at a uniform rate until failure, but no deformations are measured.

The points noted are the maximum load and the length of time required to reach it. Sketches are made of the failure. If the failure is not pure shear the test is culled.

The shearing strength per square inch is found by dividing the { P } maximum load by the cross-sectional area. { Q = --- } { A }

IMPACT TEST

_Apparatus_: There are several types of impact testing machines.[59] One of the simplest and most efficient for use with wood is ill.u.s.trated in Figure 40. The base of the machine is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500 pounds. Two upright columns, each 8 feet long, act as guides for the striking head. At the top of the column is the hoisting mechanism for raising or lowering the striking weights. The power for operating the machine is furnished by a motor set on the top. The hoisting-mechanism is all controlled by a single operating lever, shown on the side of the column, whereby the striking weight may be raised, lowered, or stopped at the will of the operator. There is an automatic safety device for stopping the machine when the weight reaches the top.

[Footnote 59: For description of U.S. Forest Service automatic and autographic impact testing machine, see Proc. Am. Soc. for Testing Materials, Vol. VIII, 1908, pp. 538-540.]

[Ill.u.s.tration: FIG. 40.--Impact testing machine.]

The weight is lifted by a chain, one end of which pa.s.ses over a sprocket wheel in the hoisting mechanism. On the lower end of the chain is hung an electro-magnet of sufficient magnetic strength to support the heaviest striking weights. When it is desired to drop the striking weight the electric current is broken and reversed by means of an automatic switch and current breaker. The height of drop may be regulated by setting at the desired height on one of the columns a tripping pin which throws the switch on the magnet and so breaks and reverses the current.

There are four striking weights, weighing respectively 50, 100, 250, and 500 pounds, any one of which may be used, depending upon the desired energy of blow. When used for compression tests a flat steel head six inches in diameter is screwed into the lower end of the weight. For transverse tests, a well-rounded knife edge is screwed into the weight in place of the flat head.

Knife edges for supporting the ends of the specimen to be tested, are securely bolted to the base of the machine.

The record of the behavior of the specimen at time of impact is traced upon a revolving drum by a pencil fixed in the striking head. (See Fig. 41.) When a drop is made the pencil comes in contact with the drum and is held in place by a spring. The drum is revolved very slowly, either automatically or by hand. The speed of the drum can be recorded by a pencil in the end of a tuning fork which gives a known number of vibrations per second.

[Ill.u.s.tration: FIG. 41.--Drum record of impact bending test.]

One size of this machine will handle specimens for transverse tests 9 inches wide and 6-foot span; the other, 12 inches wide and 8-foot span. For compression tests a free fall of about 6.5 feet may be obtained. For transverse tests the fall is a little less, depending upon the size of the specimen.

The machine is calibrated by dropping the hammer upon a copper cylinder. The axial compression of the plug is noted. The energy used in static tests to produce this axial compression under stress in a like piece of metal is determined. The external energy of the blow (_i.e._, the weight of the hammer X the height of drop) is compared with the energy used in static tests at equal amounts of compression. For instance:

Energy delivered, impact test 35,000 inch-pounds Energy computed from static test .26,400 ” ”