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Aviation History
1934
1934 - 1407.PDF
MARCH 29, 1934 19 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT where F1= 1 + ~D(tfd) D= 7^/8^/ 12(1 - <?)) = 0-368 d = r^/8 = " the bulge "' <j> = angle subtended by are of panel. If (fc/f)2 is small in comparison with rjt the formula reduces to that for a flat panel p=[w2/ 3(l-a') ]E(«/6)* (4) For slightly curved surfaces equation (4) must be modified to where F2= [1 + y1 1 -+- ""* W«)2 ]/2. For slightly curved panels it is advisable to use formula (3), although the critical stress will be the same if formula (5) is used. The graph shows the relationship between the multiplying factors Fx and F2 and the djt ratios. It is stated in the text of the paper that the theoretical critical stress given by formula (3) above was in many cases 80-90 per cent, of the collapsing load of some actual tests ; in this connection, it must be remembered that the critical stress is the stress at which instability will occur, but that it is possible for a test specimen to take additional loads after the critical stress has been reached. There is generally a discrepancy between the theoretical and actual critical stresses for most cases of instability; this is mainly due to the difficulty in reproducing the exact theoretical boundary conditions on the specimens. Another cause which is often overlooked is that in most cases the formulae are only valid providing the limit of proportionality Fig. 1, Appendix II Method of Test, in shear. Square sheets is not exceeded. The question of plastic instability is of importance in certain phases of structural design and has, as yet, received little consideration. Fig. 2, Appendix II : Sequence of sheets tested. Types of failure. 306 c
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