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
 
The Rotary Engine NewsLetter.  Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/            
Copyright 1998-2004 All world wide rights reserved.