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Shown here is a buckled axially compressed, axially stiffened cylindrical shell tested by Josef Singer and colleagues at the Technion in Israel.
The buckles are large because of the large “effective” axial bending stiffness of the stringer-stiffened shell wall. This behavior greatly reduces the imperfection sensitivity of stiffened shells compared with that for monocoque cylindrical shells under axial compression, as displayed on the next slide.
Also, the effect of an imperfection amplitude on the buckling load is related to the effective thickness of a shell. In other words, the imperfection sensitivity is a function of the imperfection amplitude divided by the effective thickness of the shell with stiffeners.
With a monocoque shell the effective thickness is just the shell thickness. With a stiffened shell the effective thickness is much bigger than the skin thickness. It is closer to the thickness of the skin of the shell plus the height of a stiffener. Therefore, the amplitude of an imperfection of a stiffened shell has to be much larger than that of an unstiffened shell in order to have the same effect on the shell buckling load. Imperfections with large amplitudes are easily detected. A shell with such an imperfection can be discarded as being "out of spec".
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