Re: [PSUBS-MAILIST] Ribs

[Jon Wallace <jonw@psubs.org>]

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=false

1) 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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