PVC, PLASTIC COKE BOTTELS & RUBBER SUBS???

[shawl@torchlake.com (Jonathan Shawl)]

Hi Phil,
OK, OK, OK, Please don't get frustrated, we are all still dry, and it's not
raining right now. :-) Your right, we are just talking theory here, that's all.
You definitely know way more about this than I do. And as we talk about it we
all learn something, I hope.  I was only talking about putting aluminum rings
on the outside of PVC pipe for battery housings. Welding not possible. In
theory now, all I was saying is that it will help to put them on the outside
verses nothing at all. I was not ever talking about steal pressure hulls for
human occupancy! You won't ever find me in a PVC sub, or a Coke bottle, or not
even a "rubber" sub designed by you. I'm not trying to start anything but
remember it was you that said you could make a sub out of anything even
"rubber" if you know the limits of the material and can live with and work with
it's limits, or something like that. I know you were not saying we should make
rubber subs, you were just making a point. That is all I was doing. Another
example of what I was saying is this. I used to work on vacuum furnace
equipment, sometimes we used a soft rubber hose to make connections. Some of
the hose would fail and flatten, but if you put a close fitting coil spring
around it, it would hold up just fine even when bent around corners. The spring
acts like an external nonattached rib to keep the hose from going oval and
flattening. maybe this only works until you get up near to the point that the
material will fail from compressive load anyway.  When I hooked a vacuum pump
to the thin Aluminum wing tank I made my first failed sub from, it started to
get soft a squishy feeling when you hit it with your hand. A little later at
around 15" Vac I was tapping on it some more and BANG it flattened faster than
"lightning", and it did get a lot wider the other way. When I saw that happen I
got serious about learning more about pressure hulls before I made anything
that would ever go under water again. Isn't this what is called thin walled
buckling failure? Doesn't this happen way before you got to the plastic state
of the material? I'm still learning today, and hopefully will never stop
learning.
Do you think there is a need for a report on the failure modes of plastic pop
bottles? Maybe a web page with photos would do.   ;-)
As always, thank you Phil for your input to the group!
Jonathan Shawl

P.S. If you have any photos of the failed hulls I would love to see them!
Please e-mail them to me if you can. Maybe the group would also like to see the
results of the pressure we are all trying to deal with. I get the feeling that
a lot of jaws would drop to see what can happen at the speed of lightning! I
wish now that I had taken photos of my failed hull.


Phil Nuytten wrote:

> Hi, Jon:
>         Arrrgghhh . .you're trying to use a postage stamp for an umbrella!
> Even if it did some little itsy-bitsy good in theory -  in practise you're
> gonna get pretty wet! Likewise ,if you slide some rings over a cylindrical
> hull and rely on them as stiffeners, you will also get wet ( and, possibly,
> flat . .). Why wouldn't you  weld them?? I don't for one second concede
> that the cylinder must 'get wider' to go into an asymetric buckling mode .
> .we are not talking about machined cylinders and lapped rings - at the
> point of plasticity, only a fraction of circumference  discontinuity will
> propagate like lightning into a catastrophic  failure. The hull 'folds'
> inwards. You are assumimg that there is some rigidity in the balance of the
> hull when the buckling portion has turned plastic and yielded . .why?  You
> ought to see some of the pressure housings that we've tested to destruction
> - and see them fail! Often it's so quick that you miss it and you have to
> look at the tapes in slow motion to see the failure process.
>         Must admit, tho', I don't know diddly-squat about the pressure
> integrity of plastic coke bottles!! (G)
>
> regards
> Phil Nuytten