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First, please understand that I'm lacking fundamentals in physics and engineering.

I'm trying to find the maximum pressure a submarine hull can resist. For the sake of simplicity I'm just considering a tube closed with magical caps that keep a different pressure inside and outside.

After much research and a few calculations, I think I've gone wrong somewhere, because I'm getting that a 2m diameter, 2m long steel tube 1cm thick will plastically deform under the pressure at 114m depth only.

From here (http://www.calctool.org/CALC/other/games/depth_press) I'm getting that the pressure at 114m depth is 1.24723 MPa = 1,247,230 Pascals or Newtons/square meter

My magical hull is a tube 2.02m outer diameter and 2m long, again without caps, and keeping the pressure difference, and I guess the inner pressure would be zero. So its surface area should be: D x pi x height = 2.02 x 3.14 x 2 = 12.69m2

That'd mean it's getting a total force exertion of 1,247,230 x 12.69 = 15,827,349 Newtons / 9.8 = 1,615,035 Kg = 1,615 metric tons. (Which seems ridiculously high)

Then I'm calculating the yield moment using this (http://en.wikipedia.org/wiki/Section_modulus#Elastic_section_modulus):

S =

pi x (oD^4 - iD^4)

-----------------------

32 x oD

pi x (2.02^4 - 2^4)

-----------------------

32 x 2.02

3.14 x (16.64 - 16)

------------------------

64.64

S= 0.031

If my hull is made of structural steel its got a yield strength of about 250MPa = 250,000,000 Newtons/square meter

Yield moment = S x yield strength = 0.031 x 250,000,000 = 7,893,642 Newtons/meter / 9.8 = 805,474 Kg/meter

Since my hull is 2m long, it should be able to withstand 1,610,947 Kg before plastic deformation occurs.

Where did I go wrong? And please remember that I have little physics/engineering background, so explain as you would to a child.

Thanks!

P.S.: Sorry for the format, but I can't bother to learn Latex or whatever it's used for writing equations just for one question.