ramses wrote: Heck even a hunk of PVC pipe rated at say 200 PSI will take an air shock pressure that is at least ten times that pressure.
NO! or at least, not with PVC
Some materials can take the spike better than static, but some will shatter with spikes well below what they can take statically.
DYI's (I Think) ETG produced peak chamber pressures he estimates to be 2x greater than the theoretical failure pressure ( SF=1). He got a bunch of plastic deformation, but nothing exploded. It was made of metal, either SS or brass, which tolerates shock fairly well. The mass probably allowed for some inertial confinement, which can in some cases contain fusion pressures in nothing more than a very low pressure gas.
PVC on the other hand, doesn't like shock. It is brittle.
Case in point: brittle things don't like shock. Ductile things don't mind shock too much, but fatigue and fail eventually.
We have to be careful because we are drifting into discussion of things which are generally treated differently. Smacking a piece of PVC with a hammer produces a shock load. It is a fairly mild total load that is concentrated on a small area and the local pressure can be extremely high. A hammer blow could easily put as mush as a couple thousand PSI onto a small spot of the PVC. That is in excess of what the PVC will handle but it is also much higher than what the PVC is rated for in static pressure. If you hit the PVC with a hammer but don't exceed it's pressure/in^2 rating it'll probably be fine.
I wasn't referring to that kind of shock though. I was referring to the shock wave produced by a
detonation event. That type of shock wave is supersonic (often tens to hundreds of times the speed of sound). The duration of the pressure spike depends on how it is generated. A detonation event produces a single shock event with a very short duration. Over pressures of tens of thousands of PSI can be produced but the damage those waves produce is much less than the damage that a static pressure of that same amplitude would produce. So, in general, when talking about
shock wave loads the effective pressure at failure of say a container is typically much high for a shock wave than it is for static pressure.
I suspect that "ping" events in an ICE produce local shock waves of perhaps 10,000 psi, compared to the "static" peak pressure of what many hundred PSI? The shock peak pressure probably pushes, or even greatly exceeds what the engine is designed for but it still takes many (probably thousands) of ping events to do any damage to the engine.
So it all depends on what you mean by "shock". A hammer puts a very high force on a very small area (a high pressure) but the event also lasts a "long time" (relatively speaking), perhaps as much as a tenth of a second. A detonation shock wave often produces much higher peak pressures but the duration of the shock wave is very short, I would expect it is typically less than 1/1000 second. At a sufficient short shock duration the
mass of the pipe starts to contribute significantly to the strength of the pipe. Not because mass=strength but because mass="pipe doesn't want to move" and it can't fail if it doesn't move (deform, stretch...). With a static pressure (defined as any pressure that lasts for say a tenth of a second) the mass of the pipe is irrelevant since there is more than enough time for the pipe to deform until it fails.
So, if a spud gunners concern is DDT giving a shock load in excess of the pipes rating then they need to think carefully about exactly what the shock wave failure load of the pipe is. It generally is not the same as the static pressure failure pressure and is usually considerably higher.
Personally though, I would say that no spud gun, regardless of what it is made of, should be operating at DDT. As tech pointed out with the car piston, a single event might not be a problem but if you keep repeating it you will fatigue the pipe and it's rating will drop. (Most cars ping occasionally, very few cars blast a hole through a piston of throw a bearing or do any of the other things that a chronically pining engine will do.)
So now that is clear.
