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 Under water crush depth. Post Reply Forum
 Posted by: Mylo42 ® 08/23/2005, 10:50:12 Author Profile eMail author Edit I am building a submersible in which the pressure hull is made of 28" steel pipe with a wall thickness of 1/2" (140lb/ft weight of pipe). Assuming that the flat end caps are as integral as the pipe due to reinforcement. How can I calculate the crush depth ? This # would obviously be halved to obtain a safe operating max. depth.Thanks for the help.Mylo42

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 Re: Under water crush depth. Re: Under water crush depth. -- Mylo42 Post Reply Top of thread Forum
 Posted by: swearingen ® 08/23/2005, 18:34:18 Author Profile eMail author Edit Using Roark's and plate theory, I get that the end plate should begin to yield at about 85' of depth (37psi). I thought that maybe it isn't that deep until you calculate the total load on the plate - 22,700lb - over 11 tons! It'll hold up a little deeper until it passes the hump on the stress-strain curve, then it's a goner. This does not take into account the increase in pressure on the inside from the decreasing volume, so that'll add a few feet of depth. To be on the safe side, I wouldn't take it down deeper than 40' or so.If you dome the ends you can get much deeper...

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 Re: Under water crush depth. Re: Re: Under water crush depth. -- swearingen Post Reply Top of thread Forum
 Posted by: dino snider ® 01/23/2006, 23:17:38 Author Profile eMail author Edit Yes, end domes would improve the crush depth. However, see http://www.mech.port.ac.uk/CTFR/Silsub3.html. Strangely, concave vs convex end domes make sense.Go to pbsubs.org. Their .xls program will do what you want. Anybody care to run said calc for me? I don't have Office.I'm writing scifi story using seacrete concrete domes. Compressive strength is great.

 Re: Under water crush depth. Re: Re: Under water crush depth. -- swearingen Post Reply Top of thread Forum
 Posted by: Mylo42 ® 08/24/2005, 01:12:19 Author Profile eMail author Edit Swearingen, Thanks. What is the formula for Roark's plate theory ? Incidently, as a result in revising my hull structure plan to occomodate for better bouyancy, my pressure hull is a 16' aluminum pipe 30" in diameter, 3/8" wall thickness, bulkeads at 7' from one end, and 3' from the other with the end caps and bulkheads at 1/2" thickness. I need to find out where I can find hemispherical aluminum end caps of the size I need. I do realize the added strength this will provide. I am looking to obtain a safe operational depth of 30M (Just shy of 3 atmospheres. 99'), which means, the hull must retain it's integrety to twice that depth....to be safe. I knew I should have studies hydrodynamical engineering instead of goofing around.Thanks again,Myles.

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 Re: Under water crush depth. Re: Re: Under water crush depth. -- Mylo42 Post Reply Top of thread Forum
 Posted by: swearingen ® 08/24/2005, 16:43:37 Author Profile eMail author Edit I worked with one of our mechanical engineers and modeled this one on a code calculating pressure vessel program. It appears that for 6061 aluminum, the minimum required wall thickness would be 5/8". This will get you almost to 200' depth. The problem is your caps need to be a good bit thicker than 1/2" unless they're domed.The Roark's plate stress equations are pretty involved. Find someone that has a copy of Roark's Formulas for Stress & Strain and look them up. It's based on a closed form solution, so it's straightforward plug-and-chug, but you have to watch your units. That will give you moments which you have to compare to a bending stress calc.