In short specimens the failure usually occurs in one or several planes diagonal to the axis of the specimen. If the ends are more moist than the middle a crushing may occur on the extreme ends in a horizontal plane. Such a test is not valid and should always be culled. If the grain is diagonal or the stress is unevenly applied a diagonal shear may occur from top to bottom of the test specimen. Such tests are also invalid and should be culled. When the plane (or several planes) of failure occurs through the body of the specimen the test is valid. It may sometimes be advantageous to allow the extreme ends to dry slightly before testing in order to bring the planes of failure within the body. This is a perfectly legitimate procedure provided no drying is allowed from the sides of the specimen, and the moisture disk is cut from the region of failure.
Calculating the results: The formulæ used in calculating the results of tests on endwise compression are as follows:
| P | |||
| (1) | C | = | ----- |
| A | |||
| P1 | |||
| (2) | c | = | ------- |
| A | |||
| P1 l | |||
| (3) | E | = | --------- |
| A D | |||
| P D | |||
| (4) | S | = | ----- |
| 2 V | |||
| C | = | crushing strength, pounds per square inch. | |
| c | = | fibre strength at elastic limit, pounds per square inch. | |
| A | = | area of cross section, square inches. | |
| l | = | distance between centres of collars, inches. | |
| D | = | total shortening at elastic limit, inches. | |
| V | = | volume of specimen, cubic inches. |
Remainder of legend as on [page 98].
COMPRESSION ACROSS THE GRAIN
Apparatus: An ordinary static testing machine, a bearing plate, and a deflectometer are required. ([See Fig. 35].)
Figure 35
Compression across the grain. Note method of measuring the deformation by means of a deflectomoter.