From: Jon Wallace <jonw@psubs.org>
To: personal_submersibles@psubs.org
Sent: Tue, November 17, 2009 12:37:04 PM
Subject: Re: [PSUBS-MAILIST] Ribs
I'd like to see Cliff or Sean do a presentation on the calculator at the next convention.
Based upon this discussion I went back and did some recalculating of my own and notice that some of the terms used in the text book and this discussion do not directly correlate to the hull calculator spreadsheet. I wonder if Cliff or Sean could connect the dots for me. For example, using the reference that Cliff gave us 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=false1) Where is local shell instability in the spreadsheet calculator? It looks
like it must be either lines 43-51, or 43-57? If local shell instability stops at 51, what are lines 53-57 providing? If local instability is 43-57, can you explain the difference between Inter-stiffener strength and longitudinal stress?
2) Does Inter-stiffener strength literally mean between the stiffeners, or does it include (physically) any two adjacent stiffeners?
3) What does "longitudinal stress at the frame" mean? Does this mean hull and stiffeners, just hull, just stiffeners? Or does it mean the stress at the flanges as measured between two adjacent ribs?
4) General Instability is lines 59-64, correct? I notice that for the numbers I am using for calculations that General Instability is quite large relative to inter-stiffener strength and longitudinal stress calculated just above it. I'm getting 5138psi for General Instability, 808psi for inter-stiffener, and 883psi for longitudinal
stress. I assume this means I'm likely never going to get to a General Instability failure because local instability will occur first. Am I right?
5) Where is "axisymmetric local plastic collapse" (from the book reference) located in the xls hull calculator?
6) What does "non-heavy stiffeners" (from the hull calculator) mean? Does this mean 1/4 inch rib webs are "non-heavy" whereas 1 inch rib webs are "heavy"?
7) What does "stiffener tripping" mean? I notice mine says "Tripping". This sounds bad.
8) What does "Inertia Requirement" mean?
9) Is "instability" and "buckling" the same thing and use interchangeably?
10) Just an observation given the photos in the book. The pictures give me the impression that the failures shown are not necessarily catastrophic (ie, hull ripping open and splitting). Instead, it appears as if a huge hammer has dented the cylinders. Can we
assume then that failure of the hull does not necessarily mean a hull breach. In other words, and I know I'll catch grief for this, but, acknowledging ahead of time that there are always exceptions, is it expected that buckling (instability?) typically does not translate to fatigue (in the sense that the metal splits and allows in water)? Or is that assuming too much.
Sorry for all the questions, but I agree with Brian that this is a fascinating topic once you get into it.
Jon
Brian Cox wrote:
> Cliff, Thank for the detailed information , I appreciate the time you put into explaining this. It's fascinating, and it's something I would like to get better educated on. These various modes of failure, I gather, all need to be balanced around the same ideal design pressure so you're not leaving yourself open to a mode of failure in one area. Does it make sence to have the first mode of
failure be the buckling of the shell between the rings stiffeners? That way you get some warning before you reach a general instablility? I know the senerio for that would mean that you have already gone beyond your operating depth, so maybe that should not really play into it. I guess the goal is to evenly design around the three modes of failure.
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