RE: [PSUBS-MAILIST] rib span

["Alec Smyth" <Asmyth@changepoint.com>]

Hi Jon,

Your rationale is correct, but I'm afraid it's not the whole picture.
There's actually a number of independent ways a reinforced cylinder can
fail. Your reasoning holds for failures of the SHELL between stiffeners.
This could take two forms; 

1) Buckling, in which the shell between two stiffeners collapses into a
series of lobes. 
2) Yielding, in which the entire shell ring between two stiffeners
collapses simultaneously (sort of "sucks in").

The reason I asked for the overall length was that there is another
failure mode called "general instability", in which the shell and
stiffeners collapse TOGETHER over a large area spanning multiple
stiffeners. And as it turns out, that was the failure mode with the
lowest critical pressure in this instance. That probably explains the
difference between our results. You can avoid general instability by
beefing up the stiffeners, or by putting in an extra heavy one (a.k.a.
bulkhead) half way down the cylinder, effectively halving the overall
length used in the calculations, rather as you suggested. But either
solution implies a weight penalty.

Another thing to consider is that the weakest point of a K-350 might not
be the hull at all, but the flat window in the bow. I'm not sure about
that, it's just a hunch.


Rgds,

Alec

-----Original Message-----
From: Jonathan Wallace [mailto:jon@psubs.org] 
Sent: Thursday, August 21, 2003 2:32 PM
To: personal_submersibles@psubs.org
Subject: RE: [PSUBS-MAILIST] rib span

Quite a difference, yes.

Ok, let me ask this...the ribs effectively act as "hull thickeners" at
specific locations.  If ribs are used, isn't the weakest point of the
cylinder limited to the length of section between ribs (ignoring end
caps for the moment)?  If so, what is the relationship of the length
of the hull overall when ribs are used?  Or in other words, if I space
my ribs 12" apart why can't I just use 12" as length in the equation
to determine the crush depth of that weak point.

If I'm not making myself clear I'll try to reword it.

Jon


>That's quite a difference, no?