Re: [PSUBS-MAILIST] Concrete submarine

["Mark Roberts - Harbortronics" <Mark@Harbortronics.com>]

Concrete is a hard, brittle material, with a significantly higher 
compressive strength than tensile strength.    Let's take this to the 
ultimate extreme in this class of material... glass.

If the stresses are entirely compressive, then life is good... glass 
(concrete?) get's even stronger when compressed... sounds weird to me, 
compressive strength is compressive strength... I digress.  However, unless 
you have built a perfectly shaped hull, the stresses are NOT all going to be 
compressive. Without mathematically perfect shaping, some of the stress of 
compression will result in tensile stresses in localized areas, and these 
areas are likely to rupture long long before the compressive stresses get 
too high.  If you add a viewing port, obviously essential for concrete, not 
for glass, then your design must carefully transfer stresses from the port 
to the hull.  Increasing the material thicknesss won't necessarily eliminate 
the problem at the interfaces.  Glass tubes and spheres are great, but once 
you put a hole in them, the depth rating can drop dramatically.

Metal is really nice in that the compressive / tensile stresses are about 
equal, so you don't have to sweat the details quite so much.  It's 
interesting tha so many sources say the compressive strength of concrete is 
so high... perhaps in comparison to more primitive construction materials, 
but standard concrete has a compressive strength of 2000-3000 PSI.  Cheap 
steel has a strength of what, 50,000 PSI?  High strength concrete starts at 
about 6000 PSI.

Mark@Harbortronics.com


----- Original Message ----- 
From: "Brian Cox" <ojaivalleybeefarm@dslextreme.com>
To: <personal_submersibles@psubs.org>
Sent: Thursday, January 18, 2007 1:21 PM
Subject: Re: [PSUBS-MAILIST] Concrete submarine


> A steel  renforced concrete structure will have millions of small cracks 
> which will not be detrimental to the overall structure.  As long as the 
> hull is sealed, or if it is thick enough it wouldn't matter, but if it was 
> sealed with epoxy there would be no problem.  The two materials are 
> completly different and could not be compared in the same way.  The 
> concrete hull would need to be very much thicker to be able to function as 
> compression structure.  At a certain thickness the concrete tube would be 
> able to transfer all the compression loads equally and would become 
> stronger as more load was applied.  Where as steel would buckle at some 
> point.




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