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.

[Illustration: 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 illustrated.

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 assumed to be the same as already
recorded for the beams. In specimens not so taken, these
quantities 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.

[Illustration: 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