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ABS cannon explosion - pulled from the cold case files

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ABS cannon explosion - pulled from the cold case files

Unread postAuthor: SpudBlaster15 » Tue Jul 31, 2007 9:37 pm

Many of you have seen my thread detailing the failure of my ABS combustion launcher, and know that I assumed the cause was a defective section of pipe. However, today I dug up the old chamber, and had another look. The diagram below shows the nature of the primary chamber failure.

Image

As you can see, both cracks branched outward from the propane meter mounting point, and one of the cracks traveled directly to the hole I had drilled, through one layer of pipe, in the center of the chamber. The strange thing is, only the pipe failed. The 4" female adapter did not crack in either location, only the pipe beneath it. This leads me to believe that one or more of the following is true.

1)The solvent cement did not effectively bond the pipe and fitting in this area, and thus the pressure generated within the chamber expanded the pipe within the fitting, causing it to crack.
2)The crack formed at the screw threaded into the center of the chamber, and branched towards the propane meter hole. Upon the next firing, the now weakened pipe formed another crack, which branched from the propane meter hole.
3)When the propane meter fitting was threaded into the pipe, the solvent weld partially separated (it had only been curing for about 1 hour), and the pipe was flexed downward, thus putting large amounts of stress upon it, which eventually formed the cracks.

If any of these are true, several lessens should be noted.

1)Always thread screws through the area in which a coupler and the pipe overlap, never directly into the chamber.
2)Allow the solvent welds to cure completely before drilling/tapping any holes.
3)Apply enough cement to fill the gap between the pipe and fitting.
4)Use a coupler and a cleanout adapter rather than a female adapter, thus allowing you to tap the meter fitting into a location with large amounts of double-layered material surrounding it.

Please post your thoughts, including potential alternative explanations, and any faults in my reasoning.
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Unread postAuthor: Fnord » Tue Jul 31, 2007 9:48 pm

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.
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Unread postAuthor: jimmy101 » Wed Aug 01, 2007 11:22 am

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.
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Unread postAuthor: hi » Wed Aug 01, 2007 12:40 pm

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.
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Unread postAuthor: TurboSuper » Wed Aug 01, 2007 12:45 pm

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

K, thanks. I said that from the beginning. :D
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Unread postAuthor: spud yeti » Wed Aug 01, 2007 1:49 pm

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


Not for combustion, but you may from a pneumatic. :wink: Be careful what you say (from the beginning) :D <--[joke]
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really good quote/phrase here
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Unread postAuthor: noname » Wed Aug 01, 2007 3:28 pm

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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Unread postAuthor: SpudBlaster15 » Wed Aug 01, 2007 5:07 pm

ABS solvent welds will only fail if you don't apply enough cement, use old and jelled cement, or allow the cement too long to dry before assembling the parts.
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Unread postAuthor: dongfang » Wed Aug 01, 2007 5:24 pm

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
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Unread postAuthor: TurboSuper » Wed Aug 01, 2007 11:34 pm

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. :wink: Be careful what you say (from the beginning) :D <--[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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Unread postAuthor: SpudBlaster15 » Wed Aug 01, 2007 11:55 pm

I believe the failure was more or less my fault. I don't believe a propane powered combustion launcher made from ABS pipe is truly a death trap.
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Unread postAuthor: CannonCreator » Sat Aug 18, 2007 5:48 pm

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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Unread postAuthor: SpudBlaster15 » Sat Aug 18, 2007 9:53 pm

If I was using too much fuel, combustion would create less pressure, or the fuel might not ignite at all.
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