The entire deformed shell sector between two nodes of minimum displacement is what I would refer to as a "lobe", but the terminology really doesn't matter, as the number of minimum displacement nodes is equal to the number of maximum displacement points in a cylindrical shell. No matter what you count, you get the same number.
-Sean
On Nov 16, 2009, Brian Cox <ojaivalleybeefarm@dslextreme.com> wrote:
So the lobe is not really the center point of deformation. In a triange it
would be the 3 points away from the center?
brian
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org]On Behalf Of Sean T.
Stevenson
Sent: Monday, November 16, 2009 11:40 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Ribs
According to Section 6 of the ABS Rules, cylindrical hulls are assessed
based
on the following criteria:
1) inter-stiffener strength
2) longitudinal stress at frame
3) overall buckling strength
4) stiffeners, with particular checks for:
a) stress limits
b) stiffener tripping
c) local buckling
d) inertia requirements
5) exostructure loads
So, changing your stiffener geometry will affect the result of one or more
of
these failure modes, but the maximum allowable working pressure will not
change unless your geometry change affects the particular stress value that
was
the limiting factor. What this effectively means is that a geometry change
which does not change the maximum allowable working pressure implies that
some
aspect of the geometry was overdesigned. For the most efficient design
possible, the allowable working pressures for each possible failure mode
should be similar, although it is desireable to have one which is somewhat
dominant, so that the actual failure mode of your hull is deterministic.
The number of lobes expected at failure is not the number of ribs. Rather,
it
is the number of distinct radial sectors present at shell collapse, or if
you
like, the number of "folds" that occur in the radial direction when the
shell
collapses. Similar to an inter-stiffener strength failure which would
produce
a characteristic "accordion" shape between stiffeners, a shell collapse in
overall buckling folds in the other direction - imagine that instead of a
cylinder, which has a circle as its cross-sectional shape, you had a
triangular cross-section - at collapse, this would force three lobes at
failure. Similarly for a hexagonal shape, you would have six lobes. With a
perfect circle, it is not so easy to predict what shape it will assume at
collapse. Have a look at Figure 4 in Section 6 of the ABS Rules - this
gives
you some idea of what to expect, but in fact you should repeat your stress
calculation for a number of values of n (number of overall instability
lobes),
and use the one which gives the lowest maximum allowable pressure.
-Sean
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