Figure 44
Design of tool for cleavage test.
The specimens are 2" × 2" × 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" × 3". Some of the blocks are cut radially and some tangentially, as indicated in the figure.
Figure 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" × 2.5" × 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" × 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.)