2.111. Tension parallel to grain. Relatively few data are available on the tensile strength of various species parallel to grain. In the absence of sufficient tensile-test data upon which to base tension design values, the values used in design for modulus of rupture are used also for tension. While it is recognised that this is somewhat conservative, the pronounced effect of stress concentration, slope of grain (table 2-8) and other factors upon tensile strength makes the use of conservative values desirable.
Pending further investigation of the effects of stress concentration at bolt holes, it is recommended that the stress in the area remaining to resist tension at the critical section through a bolt hole not exceed two-thirds the modulus of rupture in static bending, when cross-banded reinforcing plates are used; otherwise one-half the modulus of rupture shall not be exceeded.
2.112. Tension perpendicular to grain. Values of strength of various species in tension perpendicular to grain have been included for use as a guide in estimating the adequacy of glued joints subject to such stresses. For example, the joints between the upper wing skin and wing framework are subjected to tensile stresses perpendicular to the grain by reason by reaso of the lift forces exerted on the upper skin surface.
Caution must be exercised in the use of these values, since little experience is available to serve as a guide in relating these design values to the average property. Considering the variability of this property , however, the possible discontinuity or lack of uniformity of glue joints, and the probable concentration of stress along the edges of such joints, the average test values for each species have been multiplied by a factor of 0.5 to obtain the values given in tables 2-6 and 2-7.
2.12. STANDARD TEST PROCEDURES
2.120. Static bending. In the static bending test, the resistance of a beam to slowly applied loads is measured. The bearn Is 2 by 2 inches in cross section and 30 inches long and is supported on roller bearings which rest on knife edges 28 inches apart. Load is applied at the center of the length through a hard maple block 3 13/16 inches wide, having a compouind curvature. The curvature has a radius of 3 inches over the central 2 1/8 inches of arc, and is joined by an arc of 2-inch radius on each side. The standard placement is with the annual rings of the specimen horizontal and the loading block bearing on the side of the piece neares, the pith. A constant rate of deflection (0.1 inch per minute) is maintained until the specimen fails. Load and deflection are read smultaneously at suitable intervals.
Figure 2-4 (a) shows a static-bending test set-up and typical load-deflection curves for Sitka spruce and yellow birch.
Data on a number of properties are obtained from this test. These are discussed as follows:
2.1200. Modulus of elasticity (E_{L}). The modulus of elasticity is determined from the slope of the straight line portion of the graph, the steeper the line, the higher being the modulus. Modulus of elasticity is computed by
[insert equation]
The standard static bending test is made under such conditions that shear deformations are responsible for approximately 10 percent, of the deflection. Values of E_{L}, from tests made under such conditions and calculated by the formula shown do not, therefore, represent, the true modulus of elasticity of the material, but an "apparent" modulus of elasticity.
The use of these values of apparent modulus of elasticity in the usual formulas will give the deflection of simple beams of ordinary length with but little error. For I- and box beams, where more exact computations are desired, and formulas are used that take into account the effect of shear deformations, a "true" value of the modulus of elasticity is necessary and may be had by adding 10 percent to the values in tables 2-6 and 2-7.
2.1201. Fiber stress at proportional limit (F_{bp}).
The plotted points from which the early portions of the curves of figure 2-4 (a) were drawn lie approximately on a straight line, showing that th deflection is proportional to the load. As the test progresses however, this proportionality between load and deflection ceases to exist. The
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