Shown here is the nonlinear equilibrium state obtained from STAGS of the PANDA2-optimized stiffened cylindrical shell at a load factor, PA=1.005. (PA = 1.0 corresponds to the design load, which in this case is uniform axial compression, Nx = -3000 lb/in.)

The imperfect shell has two initial buckling modal imperfection shapes: the general buckling mode displayed two slides ago with amplitude, Wimp1=+0.0625 inch and the stringer bending-torsion buckling mode shown in the previous slide with amplitude, Wimp2= -0.0005 inch.

Prebuckling bending of the imperfect shell causes redistribution of stresses among the shell skin and the stiffener segments. Also, prebuckling bending gives rise to “flattened” regions with an “effective” circumferential radius of curvature that causes early general buckling. (See the right-most expanded insert).

This figure shows the outer fiber effective stress (psi) at axial load, Nx= -3000 x 1.005 lb/in. (Nx= -3000 lb/in is the design load used in the PANDA2 optimization, and 1.005 is the STAGS load factor, PA.)

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