Author Topic: Proof testing? (Read 1274 times)

Calamity Jane · « **on:** July 11, 2003, 09:06:20 AM »

Is there any valid reason to proof test a new cannon barrel?

In the "old days" an item would be tested to "maximum load" to test its integrity.

Then somebody figured out that testing to extremes could actually create problems that would lead to later failures. :shock:

So, there are two sides to the coin. One it to test a little above normal loads and check for signs of possible damage. The other is to test VERY high to see if it fails. :?

What is current practice among intelligent cannon builders?

CJ

Cat Whisperer · « **Reply #1 on:** July 11, 2003, 04:41:37 PM »

Proof testing -

Is there any reason - yes, if it blows up it's better where that's planned for and not in a crowd. Sorry to be so blunt. The cannon I saw explode (at 10 or 15 feet away) we (5 of us) were standing behind it (most fortuneately).

You've raised some good questions.

Let's review what we know about metals and stressing them. Somebody's theroem (Poison function?) says that within a range the metal will flex and return to the original shape - a function used extensively for springs. But if flexed past a point, the metal will deform and not return fully to the original shape.

So the question is, how far can we push the metal (what chamber pressure) that assures us of repeated lesser pressure excursions without affecting the strength of the vessel?

When I was in the Artillary, we logged EVERY round fired from the howitizers. The type/weight of the round and the charge. The tube was good for 4,000 rounds at maximum (charge 7). Anything less than charge 7, four rounds were equivalent to one at charge 7. At the end of the life of the tube, the tube was inspected (for cracks at the muzzel) after EVERY round as it approached 4,000 equivalent charge 7 rounds.

WHat margin of safety have we calculated? Should not the tube handle a certain pressure without damage (which we've planned for to be a multiple of what we're going to subject it to).

Should we not be able to subject the tube, without damage, to the maximum calculated pressure it should handle (for proof testing)?

What method of testing should/can we use to determine if the proof round has damaged the tube? Magnaflux? Measuring for stretching?

We should have some confidence in looking to see what is used in practice that survives long term use and abuse.

No answers, just more questions.

Double D · « **Reply #2 on:** July 12, 2003, 01:13:35 AM »

The gun factories don't even proof test there guns any more. The basic design is proof tested but the production guns are only function tested.

The proofing of black powder guns is witchcraft anyway. Triple charge and triple ball, then tie the tube to an old truck tire, long fuse and run like L.

If I use modern materials and proper manufacturing techiques I just get on with it and start shootng.

When I build a custom rifle for a customer, the last thing I do before delivery is fire 5 factory rounds through the rifle. I place the fired brass in an envelope and label it with all the information on the gun and save. I also purposely leave the bore dirty so the customer can see that it has been fired. The gun companies do the same thing. I never fire a proof round and was taught in school that proofing was done only on test guns to test the design.

Douglas

Calamity Jane · « **Reply #3 on:** July 12, 2003, 02:37:28 AM »

Good thesis Cat! Nice to know how "the pros" do it.

What was the limiting factor on military howitizers? Wear? Fatigue? Stretch?

You mention cracks at the muzzle - I would have expected the first signs of "age" to show up at the breech where pressures are highest ?

Are military howitizers made from a high strength (carbon or silicon)alloy?

DD: Thanks for the info.

I will do some type of test firing at maximum load to guard against any "manufacturing defects".

I woud like to "map the bore" - if I can find an inside micrometer that is 22" long! - before it's fired the first time (record the diameter at many different points) so I know where it started.

I'd be inclined to mount the barrel on a test stand (why risk the carriage!) and fire a few rounds down a deserted range. Then take it home, scrub it out, and re-map the barrel. Compairing the two sets of measurements would tell me if the load had exceeded the elastic limits of the steel.

I WISH I could find a good set of figures for chamber pressures for blackpowder loads! When I started this project, all I could find was one paper on interior bullistics that compared the pressure curves for BP v.s. smokeless - and it didn't even give load details (bore, charge, etc.) - and that's where I got the figure of 30,000 PSI peak.

Somebody somewhere must have more complete info on BP interior bullistics!

Thanks for the thoughts gentlemen! I am still reaming the bronze barrel to accept the sleeve - we are at 1.970 now, headed for 1.998 - but this is the kind of stuff I think about while doing the boring boring :shock:

CJ

Cat Whisperer · « **Reply #4 on:** July 12, 2003, 05:31:29 PM »

The limiting factor on our howitzers is at the muzzel. The breach is well built but at the end of the tube you have a stress point. It's because you have material at every other point in the barrel supporting adjacent metal, but not at the end. At the breach it's tremendously reinforced but at the muzzel there is only minor reinforcing. Usually it's a swell to the outside, but sometimes it's a relief cut inside - effectively adding metal forward of the point of exit (& drop in pressure).

I don't know what our tubes are made from. Must be good though just gauging from the powder they consume and the pressures (judging from the velocity and range of the projectiles - much like rifle bullets) and from the intensity of the report.

Two flavors of powder for separate loading cannon. White bag and green bag. Green bag would shoot further. (Hence the common reference to strong coffee as green bag coffee.)

Tank cannon would be made from especially good steel. To penetrate armour or bunkers one thing is needed - velocity. So cannons on tanks are direct fire and high velocity. (As opposed to howitizers which lob the shells and often use air bursts to cover a wide area with shrapenal.)

Often people will find shortish lengths of 20mm 75mm 105mm etc. cannon that are rifled and make blackpowder cannon from them.

Artilleryman · « **Reply #5 on:** January 23, 2006, 02:06:49 PM »

I just recently joined this board, and I have been going back through old posts such as this one to see what I missed. Calamity Jane said that she heard that proofing could cause problems later on.

It is my understanding that when they proofed muzzleloading cannon barrels they were testing the design and manufacturing process used to produce a new model. If it didn't blow up with a huge overload then they would manufacture it. The proofed barrel went back into the furnace because they considered it damaged or unsafe for continued use.

Someone may have already provided this information in one of the posts that I haven't read yet.

Cat Whisperer · « **Reply #6 on:** January 23, 2006, 02:32:52 PM »

Quote from: Artilleryman

I just recently joined this board, and I have been going back through old posts such as this one to see what I missed. Calamity Jane said that she heard that proofing could cause problems later on.

It is my understanding that when they proofed muzzleloading cannon barrels they were testing the design and manufacturing process used to produce a new model. If it didn't blow up with a huge overload then they would manufacture it. The proofed barrel went back into the furnace because they considered it damaged or unsafe for continued use.

Someone may have already provided this information in one of the posts that I haven't read yet.

THe concept of proving the design by proof testing vs. proving the manufacturing processes by other flavors of measurement is certainly a valid method employed in manufacturing today!

GGaskill · « **Reply #7 on:** January 23, 2006, 09:59:17 PM »

I think a cannon builder should "proof" test every piece with a full service powder charge and weight of shot. Depending on the design, it might be good to even go above these numbers somewhat (maybe 10%.)

The process of autofrettage is used to strengthen cannon barrels by stressing the interior beyond the proportional limit. This creates counter stresses from the surrounding metal that actually makes for a stronger gun barrel. While the home builder does not really have the machines and instrumentation to do a full job of autofrettage, a slight expansion in the internal dimensions of the chamber area (without a corresponding exterior increase) would not be a bad sign.

Artilleryman · « **Reply #8 on:** January 24, 2006, 06:44:16 AM »

I think that if you use a previously approved design with the proper materials that you don't need to proof your gun. This would be especially true I would think for those people who did not have the knowledge or ablility to determine if they have damaged their gun during proofing.

kappullen · « **Reply #9 on:** January 24, 2006, 05:05:09 PM »

Quote Artilleryman:

I think that if you use a previously approved design with the proper materials that you don't need to proof your gun. This would be especially true I would think for those people who did not have the knowledge or ablility to determine if they have damaged their gun during proofing.

Unquote;

I tend to agree with the Artilleryman. That is the general theory in modelboiler and steam engine work.

But with this train of thought, improved materials, and powder consistancy, what's all this I hear about sleeving of cannon barrels?

There is a breached Napolean barrel on display in the Gettysburg information center.

The discription provided by the park service says, There were no Bronze Napolean Barrels known to have exploded dureing the Civil War.

Did they sleeve barrels during the Civil War to achieve this safety record?

Why is a blown barrel on display anyway?

Just a couple of more questions.

Kap