"Plastic deformation"

[Victor3]

 A thread down below, where the use of wet paper wadding is being discussed got me thinking...

 I wonder how the safety (as related to pressure) of projectile/wad materials that are sometimes used in cannons/mortars can be determined?

 The ideal condition in any cannon or mortar would be that there is NO possibility that a projectile and/or wad would be able to expand upon firing to the point where it is constrained in the bore tightly enough (or long enough) for a dangerous level of pressure to build.

 In particular, I'm thinking of what might happen to a golf ball in a long cannon barrel with a 'standard' charge and 'correct' windage (if those parameters have been defined conclusively somewhere).

 Some may remember that I had Dom make my soda can Dictator mortar with a reduced diameter bore sleeve (1.72" diameter) that could be used to fire golf balls (1.68"). Upon firing with the unrestrained golf ball sleeve, the sleeve was ejected from the barrel. I believe that this was due to deformation of the golf ball diameter upon firing, causing it to grab onto the sleeve (Note that Dom made the bbl sleeve to my design - I thought it would stay in place on its own. I plan to add a retaining feature in the future)....



 Hefty blank charges with an alum foil wad rammed on top do not cause the sleeve to move.

 So... What of a golf ball in a cannon-length tube with the same windage as my mortar? Is there cause for concern that the ball may stick in the barrel and cause a dangerous pressure spike?

 What about other combinations of wads/projectiles?

 It seems that there have been several projectile/wad combinations discussed here that might have a potential to deform and cause higher than average pressures beyond what a wad of wet newspaper might:

 1. A canvas patched ball in a large mortar.
 2. A soda can full of ice or Plaster of Paris.
 3. A wad of aluminum foil between ball and charge in a 1" cannon.

 On a related note, I have a personal experience to share...

 Many years ago I shot a long (~three calibers) 3" diameter solid Delrin (plastic) rod out of my 3.125" bore Coehorn mortar using 2 oz of Fg. It was on a sled type base with 2" thick pine side rails. The bbl was set at ~45 degrees. I had no idea that such a projectile might be dangerous at the time.

 The shot was very loud and the plastic rod, which I figured would go a few hundred yards, went out of sight. In about two seconds it was just a speck in the sky, off into the desert past what I could estimate in distance. The barrel swung back about 90 degrees from where it was at firing and slammed into the rear cross-member, and one side rail was split top to bottom. This had never happened even when firing 3" lead balls.

 I believe that the plastic rod deformed upon firing and sealed the bore until enough pressure built up to force it out of the bore. Fortunately, the barrel was heavy-walled 4130 and not cast iron.

 Anyway, be aware of the safety of what you place ON TOP of the powder, not just the charge itself.

[Cat Whisperer]

Early on in my mortar firing experiences, I filled the beer can with water and started with LIGHT charges.  It was obvious by increasing the charge that the pressure from below was transmitted to the insides and in all directions.  At some point of increase the sides of the can would scuff the walls and a little more the can would split.

Not something I do now, but it was a good learning exercse.

In perspective, the charges I use now for sand or cement filled cans are MUCH greater. 

Which is also to say anything that is 'plastic' used with 'normal' charges could be deformed readily.

[trotterlg]

If you take the area of the edge of the sleeve, thickness of the sleeve times Pi (3.14) times the diameter you get about .1 square inch, multiply that by the pressure in the bore (I chose 1,000 psi) and you will see you have about 100 pounds pushing the sleeve out.  Larry

[Victor3]

If you take the area of the edge of the sleeve, thickness of the sleeve times Pi (3.14) times the diameter you get about .1 square inch, multiply that by the pressure in the bore (I chose 1,000 psi) and you will see you have about 100 pounds pushing the sleeve out.  Larry

 I don't understand how you came up with the area  Can you expand on that?

 The area of the end of the golf ball sleeve (2.63" OD x 1.72" ID) for my mortar is 3.1".

 Regardless, I don't see how only the area of the end of the sleeve alone comes into play here; the area (5.43) calculated using the OD of the sleeve would be more useful if the ball expanding into the bore is indeed the cause of its ejection. In that case (Using your 1000 psi figure), the sleeve/ball combo would see a peak of 5430 psi.

[Victor3]

CW quote:

"Which is also to say anything that is 'plastic' used with 'normal' charges could be deformed readily."


 The above is what I'm trying to get across - many items that we normally use for projectiles may have a potential to deform, grab to the bore and become a 'plug' under less than ideal conditions related to plasticity, pressure, length of bbl, etc.

 One thing that I've never seen discussed here is the unfortunate metallurgical property of aluminum to pressure-weld itself to copper alloys under conditions similar to what happens in a cannon barrel...

[trotterlg]

Just because the sleeve is hollow doesn't mean the charge doesn't act on it.  The area that the pressure can push on is the area of the sleeve thickness, in this case it is .020 inches times 5.3 inches long (circruference of the sleeve times the thickness), so it is pushed out of the barrel just like any other projectile.    Larry

[seacoastartillery]

     Larry,  we suggest you double check your dimensions.  Without correct dimensions you can't multiply the correct factors.  We get 3.1 square inches like Victor got.  Actually it's 3.1089 sq. in. rounded to 3.1 sq. in. 

     You are correct, of course, that the end of the sleeve will be acted upon, even without a golf ball stuck in it, but probably without enough force to eject it.  Hence no ejection during blank firing.

Regards,

Mike and Tracy

[trotterlg]

I get the length of the sleeve if split and flattened out about 5.3 inches, times .020 gets me about .1 sq inch.  Larry

[Double D]

Funny, I get .10 inch area also...but I don't do math, I do artihmetic.

The sleeve is 1.72 inches in diameter and the end has an area of  2.32 sq. inches.

The bore is 1.68 inches in diameter and has an area of  2.22 sq. inches.

Since this is an open ended sleeve and the 1.68 inch diameter/2.22 sq in. of the bore is not present in the sleeve that leaves area of the bottom of the sleeve at .10.  2.32 - 2.22 = .10 at least that's what comes up with my arithmetic...   

Now if the bottom of the sleeve isn't flat  and is cone shaped the area would be slightly greater.

Now as far as this sleeve staying in the gun, the only way I can see it working is to have both a chamber sleeve and a bore sleeve as one solid unit.  I would use a vent liner as a retention device for safety purposes.

My guess is that the unit after being fired a few times would be very difficult to remove.

[GGaskill]

The sleeve is 1.72 inches in diameter and the end has an area of  2.32 sq. inches.

This is the flaw.  The sleeve is a close fit to beer can diameter, 2.65" or so.  So the area of the sleeve is 3.14159 x 2.65 x 2.65 / 4 = 5.515 sq in - 2.32 sq in (for the area of a 1.72" hole) = 3.19 sq in for the sleeve.

Now considering that the sleeve got tossed only 8 to 10 feet from the mortar, it can't have had much pressure applied to it.

[kappullen]

 

I do not comprehend the use of an open ended sleeve anyway!

I thought sleeves were supposed to be  plugged,

and, welded with full penetration to be effective.

The epoxie, or whatever you restrain the sleeve with, will shatter with a few shots anyway??

Check out fiberglass cars involved in accidents.

With an open ended sleeve, you  blow the breach off the gun,

as seen in period Civil War photos of Parrot guns.

The pressure will be contained between the sleeve and the barrel, if only instantanously,

causing a worse condition than the barrel than without a sleeve.

That sharp corner where the sleeve fits produces a stress point, as well as

burning embers collecting in any crack, or joint.   

This open ended sleeve business is nonsense to me, especially in a steel barrel.

But, when someone makes a "safety rule",  safe, or not, we follow like sheep.

That's my $.02, go ahead and flame me.

Kap

 

[Double D]

The sleeve is 1.72 inches in diameter and the end has an area of  2.32 sq. inches.

This is the flaw.  The sleeve is a close fit to beer can diameter, 2.65" or so.  So the area of the sleeve is 3.14159 x 2.65 x 2.65 / 4 = 5.515 sq in - 2.32 sq in (for the area of a 1.72" hole) = 3.19 sq in for the sleeve.

Now considering that the sleeve got tossed only 8 to 10 feet from the mortar, it can't have had much pressure applied to it.

True enough George but he says the diameter of the sleeve is 1.72 not 2.65.

 
 
   Some may remember that I had Dom make my soda can Dictator mortar with a reduced diameter bore sleeve (1.72" diameter) that could be used to fire golf balls (1.68").   

[Double D]

I do not comprehend the use of an open ended sleeve anyway!

I thought sleeves were supposed to be  plugged,

and, welded with full penetration to be effective.

The epoxie, or whatever you restrain the sleeve with, will shatter with a few shots anyway??

Check out fiberglass cars involved in accidents.

With an open ended sleeve, you  blow the breach off the gun,

as seen in period Civil War photos of Parrot guns.

The pressure will be contained between the sleeve and the barrel, if only instantanously,

causing a worse condition than the barrel than without a sleeve.

That sharp corner where the sleeve fits produces a stress point, as well as

burning embers collecting in any crack, or joint.   

This open ended sleeve business is nonsense to me, especially in a steel barrel.

But, when someone makes a "safety rule",  safe, or not, we follow like sheep.

That's my $.02, go ahead and flame me.

Kap

 

Spot on Kap!!!  There is a reason that  N-SSA require cap screws in the muzzle of a relined gun.

[GGaskill]

The area of the end of the golf ball sleeve (2.63" OD x 1.72" ID) for my mortar is 3.1".

The sleeve is to allow the use of golf balls as shot in a beer can bored mortar.  It can be removed to use beer cans.

[Victor3]

The area of the end of the golf ball sleeve (2.63" OD x 1.72" ID) for my mortar is 3.1".

The sleeve is to allow the use of golf balls as shot in a beer can bored mortar.  It can be removed to use beer cans.

 Exactly.

 I should have been more clear about the diameters to avoid confusion. Here's 2000 words worth of pictures...





 Back to the ejection of the sleeve - I believe that upon firing, the golf ball was probably deformed under the initial pressure of firing just long enough to stick for a fraction of a second and yank the sleeve out of the barrel before returning to its spherical shape and going on its way.

 I only mentioned the sleeve-ejection in order to demonstrate what appears may be evidence of what happens to a golf ball as it is fired (plastic deformation), in the hope of stimulating thought concerning the safety of common projectiles/wads that may become a bore plug in certain situations.

 A golf ball is 'rubbery', has little surface area in contact with the bore (even when deformed) and returns to its normal shape when pressure drops. This might not be the case with some potential projectiles and wads that are stuffed down the bore of a cannon or mortar (like a large wad of wet paper).

 Again, I'm thinking of safety here - I was unaware that my plastic rod might be an unsafe projectile, and I would hope that others would think about what might happen to a given projectile under the forces it will see in a cannon barrel.

[Double D]

1.72 didn't seem right but that's what was there. 

As far a plastic deformation goes its probably a contributor.  I'll bet that insert is rejected, at least partialy if not completely by even a blank round as the gas gets between the liner and fixed bore.

[blhof]

National Geographic had a series of high speed pictures that included a golf ball being struck by a club.  The club impact flattened the ball into almost a half sphere before it left the club face and slowly regained its round shape over about 10 pictures.  If a club can deform it that much; certainly a b/p charge could do the same, or at least enough to cause trouble.

[GGaskill]

There is a substantial difference between a golf ball hit with a flat faced club and one launched by high pressure gas.  The club initially impacts a very small area which puts a high pressure on the ball which in turn distorts to allow a greater area to contact the club face. 

The gas pressure, on the other hand, is applied to fully one half of the ball's surface which imparts acceleration to much more of the ball simultaneously.  While there is undoubtedly some distortion due to inertial effects, it has to be much less than that from being hit with a club.

[Squire Robin]

Are we doing obturation -EEK-

The back of the ball tries to catch up with the front.

If you load a round bullet in to a Whitworth barrel it shoots a hexagonal hole in the target. If you load a round ball it shoots a round hole. If you recover the ball you will be no wiser, you could argue it either way.

I thing balls go egg shaped because most of the inertial mass for pushing against is down the centre line of the bore, there's nothing at the rim.

OTOH golf balls are much more elastic than plastic so there would e no clues left behind

Maybe