Brian
The number of lobes for failure is not the number of ribs. In “Fundamentals
of Construction and Stability of Naval Ships”, by Thomas Gillmer, on page 207,
he describes the types of failure for stiffened cylindrical shells. The text shows pictures of each of these
failure modes but Gillmer’s description should give you a image of the lobes. At the end of this note, I have given a link to some pictures of failure modes.
"There are three primary modes of failure of a stiffened cylindrical
shell. These are buckling of the shell between rings stiffeners, identified by
the forming of dimples or lobes around the periphery of the shell platting as illustrated
in Fig. 29; yielding of the shell between ring stiffeners , usually appearing
as an axisymmetric accordion pleat as shown in Fig. 40 rather than as lobes;
and general instability, characterized by large dished-in portions of the stiffened
cylinder wherein the shell and the ring stiffeners defect bodily as a unit as
shown in Fig 41. This last mode of collapse is sensitive to the spacing
of the rigid bulkheads, wing bulkheads, or deep frames and may occur if the length
between them is too long, or the supporting ring frames are too small. Shell buckling and shell yield are analogous
to the behavior of a long slender column and a short stubby column, respectively.
One results from elastic or elastic-plastic instability while the other depends
on yield stress. The column length for the shell for both modes of
failure is, in effect, the unsupported shell length between adjacent transverse
frames.
When the shell is relatively heavy, and the fame spacing is coarse, the shell
will fail in yield. However, if the shell is relatively thin and the
frames widely spaced, the shell may buckle in lobes. These mechanisms of
collapse are obtained with an ideally perfect structure. In actual structures,
however, slight eccentricities weaken the ring strength and this magnifies the tendency
for general instability to develop. The effective compartment length
(e.g., between bulkheads) of a stiffened shell bears the same relation to general
instability as the frame spacing bears to local shell instability. For optimum
design, i.,e., minimum weight, the shell should be designed to fail by
yielding, while the frames should have the minimum size necessary to prevent premature
failure by general instability."
In "Buckling of thin Metal
Shells by By J. G. Teng, J. Michael Rotter. See Figure 11.1
at http://books.google.com/books?id=rv0QXKI0HvMC&pg=PA288&lpg=PA288&dq=failure+modes+for+stiffened+cylindrical+shells+pressure+hulls&source=bl&ots=WYLCbtL-U4&sig=B9Z-NTwzBMYq-HiOetBdWuO8StE&hl=en&ei=i38BS-XEKdKonQfezd0X&sa=X&oi=book_result&ct=result&resnum=2&ved=0CA0Q6AEwATgK#v=onepage&q=failure%20modes%20for%20stiffened%20cylindrical%20shells%20pressure%20hulls&f=false
for a picture of lobes or dimples of an actual stiffened shell that has
failed. Also see Figure 11.2 for a picture of general instability failure
and Figure 11.3 for the yielding of the shell between ring stiffeners.
Cliff