Z. Nonaxisymmetric or asymmetric collapse of the shell between 

 adjacent ring frames. This is usually referred to as shell or lobar 

 buckling, and since in any good design, the shell is intended to be stressed 

 beyond the elastic limit at design collapse, it is in reality, inelastic 

 asymmetric shell instability between frames. This mode is characterized 

 by inward-outward lobes which may or may not develop around the entire 

 periphery, and which may or may not occur in more than one bay of the 

 cylinder; a typical case is shown in Figure 9. 



The basic difference between Modes 1 and 2, therefore, is in their 

 axisymmetric and asymmetric patterns, respectively, and the question as 

 to which may be the more critical of the two is governed by the thickness- 

 radius ratio, frame spacing-radius ratio, frame cross- sectional area to 

 shell cross-sectional area ratio of the geometric configuration, and the 

 shape of the uniaxial stress- strain curve of the shell material. Imperfect 

 circularity of the shell and deviations from straightness of the cylinder 

 generators may also play some role in the interbay collapse of a ring- 

 stiffened cylinder. 



3. Overall asymmetric collapse of the shell and frames together, 

 which may extend over the entire length of the ring- stiffened cylinder. In 

 addition to the parameters mentioned above, the occurrence of this mode 

 is strongly influenced by the moment of inertia of the ring frames and the 

 ratio of the overall length to the radius of the cylinder. Imperfect circu- 

 larity of the ring frames plays an important role here since it can precipi- 



12 



