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This is Fig. 5 from the 2009 GENOPT paper. Shown here is the second axisymmetric bifurcation buckling mode shape (Mode 2) of the optimized isogrid-stiffened shell as computed by BIGBOSOR4.
The corresponding linear bifurcation buckling pressure according to BIGBOSOR4 is p(crit) = 1622.1 psi. The program STAGS obtains a linear bifurcation buckling pressure of 1612.3 psi for this second axisymmetric mode predicted by BIGBOSOR4. (See Fig. 9 of the 2009 GENOPT paper). In the STAGS model the second axisymmetric mode corresponds to the sixth STAGS eigenvalue, as listed in Table 41 of the 2009 GENOPT paper.
This second axisymmetric buckling mode is what is called in GENOPT jargon “mode 2”.
Plus and minus versions of “mode 2” are used as initial axisymmetric imperfection shapes in computations of the local skin and stiffener stresses and buckling load factors, axisymmetric collapse loads, and general nonlinear bifurcation buckling load factors.
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