Brian, you should design the
stiffened hull so that the failure mode is yielding, either at the mid bay point
between two inner stiffeners or yielding of the shell right at the inner stiffener.
This yielding of course will correspond to the depth where the stress at these
points just reaches the yield stress. You can think of this depth as the
crush depth but in fact it is the first plastic deformation of the hull.
You then pick a design depth that is some fraction of this depth. In my
boat , the calculations says I reach the yield point at 1100 ft but my max operating
depth is 300 ft so I have a factor of safety of over 3.5. The
reason you want the pick yielding rather than bucking (either local or general instability)
is that there is far more uncertainty in the equations for buckling as compared
to yielding. As Sean mentioned in an earlier post on this thread, you want
to make each of the maximum allowable working pressures (MAWP) associated with
each failure mode as close as possible to the same value but also pick one of the failure
modes associated with yielding and
make it slightly less. Another way to say this is you don't want to make
buckling (lobes or dents) between stiffeners or general instability (shell and
inner stiffeners buckle inward as unit) to be the first failure mode because these
failure modes are more unpredictable.
-----Original Message-----
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org]On Behalf Of Cliff Redus
Sent: Monday, November 16, 2009 8:50 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] RibsBrian
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