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Circular cylindrical shells under axial compression and spherical shells under external pressure loading exhibit wide divergence in critical load values when nominally identical specimens are tested under similar conditions. The inadequacy of the classical small displacement theory of thin shells in explaining the buckling phenomenon for such bodies has long been established. Likewise it has become general belief that an explanation of their behavior can be given by means of nonlinear large deflection theory. The current interpretations of such theories when applied to these problems lean heavily upon a plausible but arbitrarily chosen energy criterion first given by Tsien, and predict that the initial buckling load of thin shells should be influenced by the test system stiffness. Five experiments to determine this influence are described, statistically analyzed, and discussed. Due to the fact that good cylinders are much more readily available than spheres, the test program was restricted to such vehicles. In all, some 315 tests were conducted, 215 on cylinders under axial compression, and 100 on cylinders under external pressure. Ranges in test machine extensional stiffness from 589,000 to 2400 lb/in. were considered; R/t values ranged from 226 to 1419, and Pcr/Pcl values from 0.249 to 0.77. L/D ratios were approximately constant in all cases. Three different methods of specimen manufacture were used and two different experimental techniques. The first technique was that of testing many nominally identical specimens under similar conditions—the second that of repeated testing of the same vehicle. The results provide overwhelming evidence that the Tsien criterion is inapplicable in all problems considered. This may be due to the invalidity of the criterion itself or to the inadequacy of the large displacement analysis. The consequences, however, are the same; scatter in experimental results is not due to the influence of test machine rigidity, and a new look at the large displacement analysis and the appropriate criterion is needed.
testing machines, compression members, shells (structure members), buckling, rigidity, stiffness
Horton, W. H.
Associate professor of aeronautics astronautics, Stanford University, Stanford, Calif
Bailey, S. C.
Research assistant, Stanford University, Stanford, Calif