RE: [PSUBS-MAILIST] Ribs

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