SpudFiles

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I've seen other abs cannons form cracks/splits where holes were drilled.
I can't find a link right now, but I remember other sites where people had abs fail around a hole (They weren't on spudfiles)
I made sure not to drill into any pressure-bearing area when I made mine.

Failure analysis is a really hard thing to do properly.

You really can't tell which way the crack propagated. It is entirely possible that the crack that stops at the hole in the chamber was propagating that way (instead of starting at the hole and propagating towards the fitting). In other words, it is possible that the hole in the chamber stopped the crack from growing, it did not cause the crack.

I would think that the greatest stress in these types of structures occurs where fittings end. PVC pipe will stretch a fair amount before it fails. But, pipe can't stretch where it is constrained by a fitting. When you pressurize the pipe + fitting assemble the maximum stress is where the expanding/stretching pipe meets the non-expanding/stretching pipe + fitting junction.

Once a crack starts it will tend to propagate to the weakest point in the gun, hence the cracks running towards the hole for the meter and the hole out in the chamber.

i had a combustion like yours (nowhere near as nice) and had both the reducer and the female adaptor fail because of glue.

i applyed it evenly and was generous with the glue, but ABS glue simply doesnt bond like PVC glue.

i glued it back together with epoxy and it worked just fine after that.

So we aren't all going to die by using ABS after all?

K, thanks. I said that from the beginning.

So we aren't all going to die by using ABS after all?

Not for combustion, but you may from a pneumatic. Be careful what you say (from the beginning) <--[joke]

Hi, in this case, you are simply an idiot. ABS cement is made to bond ABS. PVC cement is made to bond PVC. PVC cement is not made to bond ABS. If your ABS cement doesn't work, the answer is simple: get a different brand.

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Hi,

And I would like to add: Screwing tapered threaded things into too small holes adds stress to the material around the hole. Do you know how much; is the material in fact just about to fail? I don't know...
I'd say always cut a thread. OR drill a hole large enough that the fitting does not have to be jammed in, and then accept having to seal with silicone, epoxy or something else.

This reminds me of the history of the Comet jet airliner: Britain could have been the major source of jet passenger aircraft today .. of not their first one, the Comet, which was operational before the 707, had a design fault: They fell apart mid-air, seemingly without cause or warning. After recovery of one (or was it several) wreck, a lot of careful engineering analysis, and water-tank tests, the cause was determined to be the shape of one window that overstressed the material. From the corner of that window - the only square one in the aircraft - a crack would slowly develop -- and after a certain number of pressurization cycles, it would grow rapidly - until so rapidly that the pressurized plane would explode like a toy baloon.

Regards
Soren

spud yeti wrote:

So we aren't all going to die by using ABS after all?

Not for combustion, but you may from a pneumatic. Be careful what you say (from the beginning) <--[joke]

Sheesh, you people are such sticklers for details:

So we won't die from DWV ABS used in a launcher powered by combustible gases with an equal to or weaker combustion pressure than C3H8, provided it's at STP with no compression or added oxidizer?

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well if you made the hole a lot smaller then the meter fitting that screws into the cannon, It probably could of put a large amout of stain on the ABS fibers around that hole, and maybe some microscopic damage to the fibers. Which after time from use, those damaged fibers break from high pressures(Explosions in chamber), cuasing that crack.

But if it wasn't from a small screw in hole, then maybe ur using to much gas in your shots, and the area around the metering plugin to the chamber takes strain, and eventually cracks.

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