Stress on thin cylinder ends

[d***@gmail.com]

Hello... I am trying to calculate the stress on the ends of a thin
cylinder (I already know how to calculate the circumferential
stress).

Specifically, I am making tubes for a type of inflatable kite out of
polyester film and I'm wondering how to calculate the stress at the
end closure. These tubes have a fineness ratio of about 4:1 and are
made from thin film (about 38 microns). The ends are closed with a
type of clamp - that is, I just gather the film together at the end,
cover with sheath of felt to protect the surface and seal the end with
a type of hose clamp. So the ends have extra, gathered film - not the
prettiest thing, but I thought it would work.

However, when I pressurize one of these to destruction using a pump
(just to see if my spreadsheet is set up right), the film actually
pops near one of the two clamps, not along the circumference of the
tube where I was expecting the most stress.

Even when the tube is drum-tight along the length of it, the ends are
still puffy and don't seem to be under a lot of stress (due to the
extra film there). So why would it fail there and not along the
length, like a soda bottle does? The place where it breaks is usually
*not* along a seam - my seams are actually stronger than the film
itself. Beyond why it would fail, how would I actually calculate the
stresses at this point? And is the gathered (i.e., extra) film
actually a bad thing or does it have no effect (other than adding
useless mass)?

Thanks!

[gawazo]

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Best Regards,
Sambo Yobo Tutu

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