Measuring the deformation:  The compression is measured by means of a deflectometer (see Fig. 35), which, after the first increment of load is applied, is adjusted (by means of a small set screw) to read zero.  The actual downward motion of the movable head (corresponding to the compression of the specimen) is multiplied ten times on the scale from which the readings are made.

Log of the test:  The load is applied continuously and at uniform speed (see SPEED OF TESTING MACHINE, above), until well beyond the elastic limit.  The compression readings are taken at regular load increments and entered on the cross-section paper in the usual way.  Usually there is no real maximum load in this case, as the strength continually increases as the fibres are crushed more compactly together.

Calculating the results:  Ordinarily only the fibre stress at the elastic limit (c) is computed.  It is equal to the load at elastic limit (P_{1}) divided by the area under the plate (B). { P_{1} } { c = ------- } { B }

SHEAR ALONG THE GRAIN

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.

[Illustration:  Fig. 36.—­Vertical section of shearing tool.]

[Illustration:  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.

[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.