Hello Wil,
...The submarine steel pressure hulls are not only calculated for linear stress (some private people do so) but also checked for nonlinear stability failure (Buckling - sudden crash) this is very important because especially this inexpensive home made shells with no or less frames tends to buckle long ago befor the stress limit is reached. So for steel hulls a FE-analysis is highly recomended.
But Your hull has a very high thickness to diameter ratio so I would not believe that the will buckle at all. Due to the very smooth shape every cross cut is more or less a perfect circle and can stand for its own... what helps very well to deal with the pressure load.
So without sail and viewport cutouts such a shell would withstand external pressure until the concrete would reach its pressure limit.
The calculation for such a perfect rotational shape could be made with pen and paper and I'm sure You have done this very well and carfully.
B U T - the restrictions starts with leaving the perfect issue...
The sail is normally the biggest disturbance in stress distribution, followed by the viewports. An other very important point is the accuracy of roundness You are able to reach
... I know that there are special element types with special formula to take into consideration that the steel grid will take all the tension and the concrete can brittle and so one... I "simply" have to find out how it works in detail...
By the way .... are You able to make some practical test later to compare fist calculations with experiment? I would feel much more better with some hard facts behind the calculation -
....
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Hello Andre
I agree with your conclusion that in general due to the hull geometry the pressure limit of the concrete is what limits depth and there is no buckling, no folding in to expect until this limit is reached.
The sail in this design is not part of the pressure hull.
The roundness achievable in construcction should come close to max 2mm out of round.
Concrete is normal concrete (portlandcement PZ 275) so calculate B 500 resistance class - but it would be better not to use the table resistance as this is based on security factors - concrete lab average test resistance would be nearer to field findings.
Kindest Regards,
Wil
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