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Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: BigBang J » Mon Jul 12, 2010 10:31 pm

Take 304 stainless steel for example. The yield pressure rating is about 30,000 psi, while the tensile strength is 75,000 psi. The difference is the yield strength is the amount of constant pressure it can handle before it begins to flex. The tensile strength is the point at which the metal actually tears appart.


What is pinging in an ice, and what causes it? Also what does DDT mean?
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Unread postAuthor: Technician1002 » Mon Jul 12, 2010 10:47 pm

The recent failure of the pipe cap on my Marshmallow cannon is a prime example of how PVC does not like repeated strikes. The cannon held up fine for a year and under normal use the PVC pipe cap had degraded enough that it failed. Just becasue PVC survives a static pressure test and a few test firings does not mean that it will continue to service the loads placed on it for a long time. It weakens with use.

http://www.spudfiles.com/forums/pvc-is-fragile-t21334.html

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Unread postAuthor: D_Hall » Tue Jul 13, 2010 11:36 am

BigBang J wrote:Take 304 stainless steel for example. The yield pressure rating is about 30,000 psi, while the tensile strength is 75,000 psi. The difference is the yield strength is the amount of constant pressure it can handle before it begins to flex. The tensile strength is the point at which the metal actually tears appart.

Not true.

Yield stress is the point at which the material starts to PERMANENTLY deform. It will "flex" at much lower stresses right down to 1 psi (just because the deflection is very very small doesn't mean it isn't there). The thing is, below yield stress the material will return to its original shape. Above yield stress it will not.

And as long as I'm typing I'll throw in this nit pick... Ultimate stress is the point at which the material rips apart; not tensile strength. Both yield stress and ultimate stress are measures of tensile strength of materials. I will, however, concede that "tensile strength" is commonly misused even in some sales literatures. Thus why I'm calling this a nit pick.

What is pinging in an ice, and what causes it? Also what does DDT mean?

Pinging is the name given to DDT when it happens in an ICE. Why? Because that's the sound it makes as heard by the driver... "Ping!"

DDT = Deflagration to Detonation Transition (or something like that... the point is you're transitioning from a burn to a detonation).
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Unread postAuthor: Ragnarok » Tue Jul 13, 2010 12:32 pm

D_Hall wrote:Ultimate stress is the point at which the material rips apart...

I recognise that my knowledge in this area is inferior to yours, but my understanding was that ultimate stress was defined by apparent stress (i.e not compensating for the narrowing diameter of the material), not actual stress.
And therefore, the maximum achievable apparent stress normally falls immediately before necking* begins in the material - so, it's not necessarily the point at which the material actually ruptures.

*If it's a ductile material. If it's brittle, it obviously wouldn't neck.

Of course, if I am suffering a brain fart, feel free to correct me.
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Unread postAuthor: D_Hall » Tue Jul 13, 2010 1:29 pm

You're correct in that ultimate is apparent stress, but at that point it's largely semantics. Realistically once your part starts to neck significantly things happen so quickly that you'll not see a difference unless you're running an instrom or similar aparatus.
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Unread postAuthor: saefroch » Tue Jul 13, 2010 6:26 pm

DDT is a very significant phenomenon, because the pressure wave merges with the flame front, creating a pressure spike that is far more powerful (destructive might be a better word) than a normal deflagration we're used to dealing with. http://www.youtube.com/watch?v=nCgQDjiotG0 watch how the potato begins to travel down the barrel long before the flame front reaches it. The pressure wave travels far faster than the flame front in a normal deflagration.
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