ELc = Pp/AeL (2:8)
The value of the modulus of elasticity so determined corresponds to the 'true' value of modulus of elasticity discussed under static bending. Values of the modulus of elasticity from compression-parallel-to-grain tests are not published but may be approximated by 10 percent to the apparent values shown under static bending in table 2-6.
A multiplying factor of 1.1 has been inserted in various formulas throughout this bulletin to convert EL values as shown in tables 2-6 and 2-7 to ELc values in formulas involving direct stress.
2.1211. Fiber stress at proportional limit (Fcp). The plotted points from which early portions of the curves of figure 2-4 (b) were drawn lie approximately on a straight line, showing that the deformation within the gage length is proportional to hee load. The point at Which this proportionality ceases to exist, is known as the proportional limit and the stress corresponding to the load at proportional limit is the fiber stress at proportional limit. It is calculated by
Fcp= Pp/A
2.1212. Maximum crushing strength (Fcu). The maximum crushing strength is computed by the same formula as in computing fiber stress at proportional limit except that
maximum load is used in place of load at proportional limitt.
2.122. Compression perpendicular to grain. The specimen for the compression-perpendicular-to-grain test is 2 by 2 inches in cross section and 6 inches long. Pressure is applied through a steel plate 2 inches wide placed across the center of the specimen and at right angles to its length. Hence the plate covers one-third of the surface. The standard placement of the specimen is with the growth rings vertical. The standard rate of descent of the movable head is 0.024 inch per minute. Simultaneous readings of load and compression are taken are taken until the test is discontinued at 0.1 inch compression.
Figure 2 4 (c) shows a test set-up, and typical load deflection curves for Sitka spruce and yellow birch.
The principal property determined is the stress at proportional limit (Fept) which is calculated by
Fept = Load at proportional limit/Width of plate X width of specimen (2.10)
Tests indicate that the stress at proportional limit when the growth rings are placed horizontal does not differ greatly from that when the growth rings are vertical. For design purposes, therefore, the values of strength in compression perpendicular to grain as given in tables 2-6 and 2-7 may be used regardless of ring placement.
2.123. Shear parallel to grain (Fsu) The shear-parallel-to-grain test is made by applying force to a 2- by 2-inch lip projecting 3/4 inch from a block 2 1/2 inches long. The block is placed in a special tool having a plate that is seated on the lip and moved downward at a rate of 0.015 inch per minute. The specimen is supported at the base so that a 1/8-inch offset exists between the outer edge of the support and the inner edge of the loading plate.
The shear tool has an adjustable seat in the plate to insure uniform lateral distribution of the load. Specimens are so cut that a radial surface of failure is obtained in some and a tangential surface of failure in others.
The property obtained from the test is the maxinium shearing strength parallel to grain. It is computed by
Fsu=Pmax/A
The value of Fsu as found when the surface of failure is in a tangential plane does not differ greatly from that found when the surface of failure is in a radial plane, and the two values have been combined to give the values shown in column 14 of tables 2-6 and 2-7.
2.124. Hardness. Hardness is measured by the load required to embed a 0.444-inch ball to one half its diameter in the wood. (The diameter of the ball is such that its projected area is one square centimeter.) The rate of penetration of the ball is 0.25 inch per minute. Two penetrations are made on each end, two on a radial, and two on a tangential surface of the specimen. A specil tool makes it easy to determine when the proper penetration of the ball has been reached. The accompanying load is recorded as the hardness value.
Values of radial and tangential hardness as determined by the standard test have been averaged to give the values of side hardness in tables 2-6 and 2-7.
2.125