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.
Figure 36
Vertical section of shearing tool.
Figure 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.