Re: pressure hull design (Fwd)

["Marsee Skidmore" <heyred@email.msn.com>]

I beg to differ on one point. External pressure vessels are barely mentioned
in most Engineering Mechanics texts. The topic is generally brushed over
with a quick derivation of the theoretical formulae for critical pressure.
As you say, this is darn near worthless, because of the effect of
manufacturing imperfections. The "Practical Stress Analysis in Engineering
Design" book listed in the Sources section of the website is a good source,
but I have yet to find a text that quantifies the effects of
out-of-roundness, thickness variations and residual stresses. That book
references a book by von Karman and Tsien that sounds enticing, but I don't
have the complete reference info. Does Nash get a rope around the effects of
tolerances? Are the tolerances of forged hemis used for internal pressure
vessels way too icky for external use? :-o Joe
-----Original Message-----
From: John Brownlee <jonnie@chronic.lpl.arizona.edu>
To: personal_submersibles@psubs.org <personal_submersibles@psubs.org>
Date: Monday, September 14, 1998 1:39 PM
Subject: Re: pressure hull design (Fwd)


> You go down to the library and check out the following book(s):
>
> Roark, Raymond J. Formulas for Stress and Strain. McGraw-Hill,
>1975. I think the current printing is the 5th or 6th edition.
>
> Nash, Dr. William J. Hydrostatically Loaded Structures:The
>Structural Mechanics, Analysis and Design of Powered Submersibles.
>Pergamon Press, 1995.
>
> Fluuge, Wilhelm . Handbook of Engineering Mechanics. McGraw-Hill
>Handbooks, 1962.
>
> I recommend the first two, and have only used the third for
>back-of-the-envelope calculations at a library a few times. Others have
>quoted it, so I include it here. Also you might grab a good text and/or
>class on engineering mechanics/statics/strengths of materials.
>
> You might notice that these books, save the middle one, are older.
>That is because this problem is one of the classics of materials
>strengths, and should be pretty commonplace in any good statics or
>engineering mechanics text. In particular, the Nash book has a pretty good
>treatment of the effect out-of-roundness has on crush depth and treats
>each hull design in a seperate chapter. It's always been hard to find, but
>Barnes and Noble's mail-order shop has had a recent surplus of them.
>
><DISCLAIM>
> -Do not- ever use these theoretical calculations of the crush
>depth for a perfect sphere or cyllinder with hemi-heads to determine your
>operational depth. Period. Ever. They are the absolute best-case, and real
>spheres are always imperfect particuarly so if welded or machined without
>relieving the strain produced during such fabbing. These imperfections,
>which are not limited to geometry alone, cause significant decreases from
>the theoretical crush depth.
></DISCLAIM>
>
> Curious, I don't know anyone who fabs spheres or hemi's big enough
>for this sort of exercise with the sort of out-of-round tolerance a
>submersible of any depth necessitating spheres would call for. Anyone out
>there have a shop to recommend? I'm a transplanted Easterner in Arizona,
>and my network out here is pretty thin for big fab stuff in the west.
> Okay, I guess being in land-locked Tucson doesn't help either :)
>
> John
>
>John Brownlee
>Lunar and Planetary Lab
>University of Arizona
>jonnie @ lpl . arizona . edu
>
>
>