Re: [PSUBS-MAILIST] acrylic cylinder

["Sean T. Stevenson" <cast55@telus.net>]

Using your numbers, I get about the same value for eta: 20 000, 
but Pc = eta * 0.035 = 20 000 * 0.035 = 700 psi (unless you are using a 
different E value).  This is the short term critical pressure, and your maximum 
allowable working pressure will of course be some factor lower than that.

Knowing the number of lobes expected at failure is useful when you come to 
perform overall buckling calculations, as the anticipated failure mode will 
affect the overall buckling strength.  This is easy to demonstrate - hold a 
spent toilet paper roll core in your hand and squeeze it as though to crush 
the cylinder - failure mode is two lobes, and it happens pretty easily.  Now, 
do the same thing again with three fingers spaced equally around the roll - 
this way you force n=3 and it is more difficult to crush.  In some instances, by 
using a slightly smaller L/D or larger t/D, you can force the anticipated 
failure mode to a higher number of lobes which is advantageous from a buckling 
perspective.

Have a look at "ABS Rules for Building and Classing Underwater Vehicles, 
Systems and Hyperbaric Facilities", section 6.19.13, and the associated Figure 
4 for an example.

-Sean


July 14, 2009 19:25:26 Jon Wallace wrote:

> Sean,
>
> Thanks for that, I see the integer identifier on 753 now.  I still have
> some confusion however since I'm not sure what information the number of
> lobes at collapse is supposed to tell me from a design perspective.  I
> can extrapolate that there are more lobes at collapse for a smaller L/D
> value and a given t/D value.  But so what?  Does it matter how many
> lobes there are at collapse?
>
> Looking at page 753, can you tell me if I am reading the chart correctly
> with the following information.
> t/D = .04
> L/D = .5
> n=20,000 (approximately depending on how straight your eyes are)
> psi = 770
>
> Jon
>
> Sean T. Stevenson wrote:
> > L in this case is the spacing between two adjacent stiffeners.  There is
> > no consideration of total hull length.  (which makes sense, as a
> > sufficiently stiff cylinder / stiffener combination can be repeated as
> > many times as you like to create a hull of indeterminate length -
> > buckling considerations aside).  Thus, elastic instability of a cylinder
> > supported by stiffeners spaced L apart considers only two stiffeners in
> > all cases - at either end of an otherwise unsupported length of cylinder.
> >  The integer identifier above each arc segment on the graph is indeed the
> > number of lobes anticipated at collapse, and you will note it is labeled
> > as such in the plot on page 753.
> >
> > -Sean




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