Russell Kent wrote:

Paul Lamar wrote:
[On the topic of a -10 psi versus +10 psi pressure differential across
the 4 inch square-cross-section tube, and its deformation response:]

Certainly will form a different shape there is no question about it.
What I am interested in is the magnitude of the wall deflection and

that

will be more or less the same.

Paul,
While you *may* be correct in this particular case (and I tend to
agree
with you on this point), in general it isn't true.  Witness the
behavior
of an aluminum soda can under pressure versus under suction.  I, too,
think it's worth investigating.

In addition your suggestion to weld end caps on the tube and test it
(paraphrased) is a good one.  Problem is, wouldn't the welded end caps
add strength?  Instead, maybe need to smooth the cut ends of the tube
and seal via wax against a smooth, inflexible surface (thick piece of
aluminum)?

Alas, I haven't the tools to test this.

Russell Kent

You certainly picked an extreme case there Russell. The only thing I
can think
of that would be worse than a soda can is a balloon :) -10 psi in a
soda
can would probably result in complete collapse despite it being round
and not square. While + 10 psi of pressure would hardly be noticed. You
could calculate the deflection from Roark's formula for a uniformly
loaded beam fixed at both ends.

Y = - (1/384) x (W x (l^2)) / (E x I)

l = length of the wall or four inches.
W = total load or load per inch times length.

Let me know what you come up with.

This would be worse case as a square tube tends to reduce
this type of wall deflection under both pressure and vacuum
due to balancing counter moments on each end of the beam.

Paul Lamar

Aluminum cans and balloons I cannot speak about.  But if you are
pressure testing your newly welded up fuel tank and you make it to 5
psi, run for cover.    Jerry

A lot depends on the size of the pressure vessel. If a fuel tank is
25 by 25 inches that is 625 square inches. At 5 psi that is a force
of over 3000 pounds. We are talking about 4 inches by 14 inches
or 56 square inches or 280 pounds for the square tube plenum. The next 
thing you need to take into consideration is the shape of the pressure 
vessel. A sphere is the strongest followed by a cylinder with a flat 
rectangular box (such as a fuel tank) the worst. A thick wall 
square tube is not that much worse than a round tube or cylinder.

Paul Lamar
 
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