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Aluminum failure characteristics

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Aluminum failure characteristics

Unread postAuthor: Moonbogg » Tue Oct 06, 2009 1:08 am

Has anyone done any study or have any reference links/material as to what the characteristics of actual failure in aluminum pipe is? For what its being used for in most spud guns its difficult to imagine it exploding, but I am curious. Does it tear, crack, bend or blow apart? How brittle is it?
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 06, 2009 1:30 am

Why not put a hybrid mix in a soda can and find out for yourself :D
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Unread postAuthor: drac » Tue Oct 06, 2009 1:48 am

Tensile strength my dear friend, it's all in the tensile strength of a material.

There was a calculator that would determine the tensile strength of a material based upon wall thickens, material type and the like. Can't remember where it was, but if I stumble across it, I'll link you.

I'm assuming aluminum would blow apart, but it also depends on which surface the pressure is applied. If the outward force of the deflagration is applied to the concave surface of the pipe (inside), the outward force would probably shatter it, since Aluminum has great tensile strength, but as we know it's not very flexible.

There are too many factors to take into account.

Good luck with the soda can hybrid. They normally have between 30-50PSI in them, but once again, wall thickness makes a difference.

Thin wall will tear, but a thick wall will more than likely shatter.

Please, correct me if I'm wrong. Also, D_Hall may be the go to guy for this kind of thing.

EDIT: My point for bringing up tensile strength is that you need a massive farking amount of force to overcome the tensile strength of a metal, and generally a force that tends to be that strong could be considered catastrophic.

I'm trying to draw an analogy here. Think of a balloon. Set off gunpowder inside a balloon, it explodes with a big bang but relatively no damage to the surrounding areas. That's because latex has a low tensile strength and high flexability/resiliancy. It can stretch and absorb a certain amount of force before rupturing.

Set off a pipe bomb and what happens?
Metal doesn't expand like latex, so when said gunpowder ignites, the pressure generated is so intense because the volume of gas expands exponentially faster than the metal can expand.

Interesting question, since I think personally that holding a pipe at 5000 PSI indefinitely would be better than repeated shocks of 5000 PSI.

Sorry if non of this seems to make sense, but this was a train of thought post. Hope it stimulates some conversation!
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Unread postAuthor: Moonbogg » Tue Oct 06, 2009 2:16 am

I have design software that helps give an understanding of the yields of a particular design. I am curious to see what actually happens when it goes. I have never seen a video of any kind with aluminum breaking in any fashion. I was curious about this study in regard to ABS a while back as well and very little turned up there, and ABS is more common here than aluminum, so I am afraid this inquiry may not "yield" results.
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Unread postAuthor: Technician1002 » Tue Oct 06, 2009 3:12 am

Warning graphic photos of a serious accident from an aluminum O2 cylinder failure.

http://www.youtube.com/watch?v=9lw_fhNAIQc
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Unread postAuthor: Moonbogg » Tue Oct 06, 2009 4:26 am

Damn, guess I got what I asked for. I read every comment posted for that video and there is no conclusion as to what could have caused that to happen.
200 BAR of O2 is enough pressure to kill you even without an explosion let alone with one. I can only speculate as to how this happened. WD40 on the threads in an effort to loosen them?
In regard to the aluminum, it appeared to tear and fail as a result of too much tensile force pulling it apart. The explosion must have been very spectacular to create the pressure to simply play tug of war with the cylinder and win.
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Unread postAuthor: Technician1002 » Tue Oct 06, 2009 5:09 am

I can only speculate as to how this happened. WD40 on the threads in an effort to loosen them?


There is more information online regarding several aluminum home medical Oxygen tanks and regulators. The comments on the video is not from the forensics experts.

Aluminum is highly flamable. Burning aluminum burns very hot. Solid aluminum in Oxygen at only 35 PSI will burn just fine. Heat from moving a tight valve can cause ignition. High speed particle impacts on aluminum are a known cause of aluminum regulator fires. Loosening the valve either caused ignition from a hot spot from friction, or the flow caused a particle impact ignition. Once lit the inside of the aluminum cylinder caught fire. This is evidenced by the texture of the inside of the neck area. Failure from rapid heat and pressure caused the rupture shortly after. It is likely that once the flash fire started the threads burned out on the neck and the valve was ejected just prior to the tank rupture.

For further reading here is a bunch of forensics files on several auminum regulator fires, some which burned back into the tank.
http://www.usfa.dhs.gov/downloads/pdf/publications/tr-107.pdf
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Unread postAuthor: Solar » Sat Oct 10, 2009 2:27 am

Tensile strength calculation only is part of the story. Extruded is poor strength and even extruded over mandrel has seams. Drawn over PORTED mandrell is the best and the only one certified for over 150psi even though tensile might be 2400psi for 1/8" wall since aluminum can have inclusions. Think of an inclusion as a chunk of flour that didn't mix right in the bread. It will leave a weak spot.
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Unread postAuthor: btrettel » Sat Oct 10, 2009 8:04 am

Aluminum is a classic ductile (i.e. not brittle) material.

Pure aluminum is more ductile than alloys. Anything that increases tensile strength (alloying, work hardening, etc.) also makes the material more brittle. Basically, if you plot a stress-strain curve, the point of failure gets higher in the stress direction but lower in the strain direction. Let me reiterate that this is only in general--some alloys have the same tensile strength with less brittleness than another alloy with the same tensile strength.

Generally you can find all sorts of information about what material you're working with by a Google search. Some aluminums are better than others in this respect; you can find a ton about 6061-6 but I'm still looking for anything about the specific variety of 1100 of an aluminum pressure vessel I got a while back. If you can't find anything online, where you got the material usually can help. Alternatively you could do your own testing... but that's getting into a whole other realm that I'm not particularly qualified to talk about.
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All spud gun related projects are currently on hold.

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Unread postAuthor: Moonbogg » Sat Oct 10, 2009 8:19 am

I would love to do actual testing, but that means you have to have hundreds to throw away in purposely damaging a combustion chamber, or at least trying to damage it.
I avoid relying on the aluminum to be imperfection free. I other words I use a safety factor assuming there are holes in the chamber, and in both cases my cannons do have holes in the chamber. The chambers would be much stronger without holes of course, but for 2-300 psi aluminum seems to be able to handle it quite easily.
I would love to make a chamber and do closed combustion tests with different end cap thicknesses, chamber wall thicknesses etc. I am in no mood to risk damaging an expensive cannon just to ease my curiosity though.
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Unread postAuthor: inonickname » Sat Oct 10, 2009 8:24 am

I can burst some aluminum pipe for you if you would like, but it won't be with air/propane etc. (i.e. non sf legal).

Though the medium I'm using may sway the results slightly.
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Unread postAuthor: cannon monkey » Sat Oct 10, 2009 11:27 am

i would think it go off like a bomb sending chunks of metal in every way it can
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Unread postAuthor: inonickname » Sat Oct 10, 2009 11:47 am

I don't know. A lot of aluminum when machined is extremely gummy. The higher quality stuff behaves like an easy to work steel and isn't too common.

Gummy hardware store stuff I would say with 90% confidence would fail more like abs than pvc.. i.e. tear.

I can test it for you, though the only forces I can test it with are hundreds of times greater and faster than any standard combustion could muster. The Heat it generates is also transferred much more efficiently. I have no way of saying how accurate the test would be.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 10, 2009 11:59 am

Steel cased hand grenade exploding:

[youtube]http://www.youtube.com/watch?v=8SiD5EAXz_Y[/youtube]
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Unread postAuthor: Technician1002 » Sat Oct 10, 2009 1:39 pm

inonickname wrote:I don't know. A lot of aluminum when machined is extremely gummy. The higher quality stuff behaves like an easy to work steel and isn't too common.

Gummy hardware store stuff I would say with 90% confidence would fail more like abs than pvc.. i.e. tear.

I can test it for you, though the only forces I can test it with are hundreds of times greater and faster than any standard combustion could muster. The Heat it generates is also transferred much more efficiently. I have no way of saying how accurate the test would be.


Is anyone set up to do high pressure hydrostatic testing?
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