doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Signs of barrel repair

There is the situation of not raising the dent but honing out the protrusion in the bore. That would be a "worst case" attempt at "dent removal" imo. I have not attempted to turn mandrels to raise dents by the tippytap method but did return one muzzle of a "Sterlingworth Co." 12 gauge to round by inserting a lathe dead center in bore (chucking taper down and tapping away. Chuck et al, while we're at it, is there a particular steel which lends itself to turning these plugs? I'd really like to try this as I worry a bit about orphaned frames when I send barrels out to Mike Orlen. Right now I've got a Sauer 66 (Euro model) o/u with two dents in the lower barrel near muzzle (one of them an appreciable crease dent. Yes, there are a couple of others on other guns; I hope no more!

jack

Buzz,
Typical stress strain charts show elongation, often in percentage. If you had such data for your steel or felt you had such data applicable for your steel, you could measure the bulge and calculated the elongation to a comparable value and make your assessment.

The issue is really about getting applicable stress/strain data. Different alloys and hardness of steels will have different amounts of elongation.

That's what some of us were trying to do when we had a member going by "Zircon", or similar handle a few yrs ago, who was intending to do these tests on a variety of barrel samples he collected from members. I volunteered to make the test fixture and he was going to use it in a tensile test machine where he worked. It's been a few or more years since we last heard from him. The value of the data he was going to generate would be very valuable to making assessments of strength. As you also just discovered, it would be helpful for this question as well.

Jack,
My preference is to use a leaded steel like 12L14 or similar steels designed for screw machines. These steels are very easy to machine and leave a very smooth finish. Common cold rolled steel would be a second choice, but can leave a rough finish if certain techniques are not used. Very high surface speeds and some cutting lubricant will help with machining common coldrolled steel. As you read in Shotgun Technicana, brass is also an alternative, but I prefer the plug to be a little harder than brass.

Thanks Chuck for your as usual wealth of knowledge on these scientific issues and for your patience in explaining the complicated technical topics to those of us more ignorant in understandable laymen terms.

The metal (at the repair site) would be at least as strong (as the original metal), BUT the thickness of the metal (almost certainly) would not be as thick as the original. Thus, even though the metal was stronger, the place of the repair would be weaker because there was less metal there.



Following the stress-strain diagram starting from zero, the part of the diagram going from zero to the yield point is the area where the metal behaves "elastically", i.e., the metal deforms under the stress to a degree described by the curve but, when the stress is removed, the metal returns to its original shape/size. The stress-strain relationship in the elastic range is usually linear. Ideally, the gunbarrel will never exceed the yield point (leave the elastic range) ever.

Once the stress applied to the metal reaches the yield point, the metal will move from elastic deformation into plastic deformation. Any stress in excess of the yield point will result in a permanent "plastic" deformation of the metal. A bulge, a bend, a dent - these are all plastic deformation. Plastic deformation results in permanent deformation and thus damage to the metal. While, as described above, the stress and deformation can be relieved by annealing, the shape has been permanently changed. Banging out the dent or bulge can reshape the metal to the original, desired shape, but it does not restore the strength.

There is a point on the stress-strain diagram immediately following the elastic limit where the amount of stress needed to create a certain amount of strain (movement) decreases. Thus, the little dip. Following that point, the amount of stress needed to create a certain amount of strain (movement) increases. That increase is work hardening. The stress-strain relationship in the plastic range is usually non-linear.

Plastic deformation and work hardening will continue until the metal reaches the plastic limit, where it fractures. Which is what we're worried about.

Bravo Dave and Chuck. You guys have explained this complicated process where even a bonehead like Buzz can have a general understanding. Thanks.

Dave,
Thanks much for explaining further.

While I think I understand why you stated " Banging out the dent or bulge can reshape the metal to the original, desired shape, but it does not restore the strength.", I would temper that statement with a more so we don't alarm people unnecessarily about dents and their removal/raising.

As can be seen in the diagram, any deformation beyond the dip (yield point) results in higher stress required to further deform it. In the example, this higher stress is around 20-22% or so. But all deformation (aka elongation) beyond the yield point results in "necking" (thinning) of the test coupon, yet it still took 20-22% more force before necking reduced the cross section enough to have a negative effect on the strength of the coupon.

This shows that even though the metal continued to get thinner as it stretched, it still got stronger up to the crest of the curve where the reduction in cross section (thinning) was so much that it affected load carrying capability of the metal. If we knew what amount of stretching has occurred in the metal, we can use a chart like this to evaluate whether we are near a fracture. We can also measure wall thickness and compare to see if the barrel is weaker due to thinning or if the net effect is insignificant.

Also, that portion of the diagram from the yield point to the failure is the basis of why hammer forging barrels makes them stronger. As you can see it also requires close control of the process.

Unfortunately, I think there are a few problems with trying to ascertain how close a barrel is to fracture is by measuring deformation.

One dilemma is we don�t know what the barrels stress/strain curve would look like. A generic curve may not be so useful because some strain hardening and residual stresses will have occurred during original manufacture. This could be remedied by testing (lots) of barrels.

It is even more convoluted after the repair. The fix will leave some residual stresses in the fixed bulge or dent. Working the metal probably also imparts some unknown directional properties.

The biggest puzzel is using the stress/strain relationship to predict fracture. This is an interesting idea but, I can�t think of any applications which use Poisson�s ratio outside of the elastic range. Once the yield point is reached, things get unpredictable. I'm not sure predicting fracture is a correct application of the theory.

It is relatively easy to predict at what stress something will yield. Predicting what happens outside the elastic range is like trying to predict how flames dance in a fire.

Ryan,
I appreciate you point. However, as we all know, everything from the seat brackets in your car to the barrels on a Remington 700 are cold formed. I have a friend making a huge living cold forming parts for automotive.

I just don't want to leave the readers here with the notion that a dent raised will result in a weaker condition and less safe or capable. There's just no evidence nor practical data to suggest that. An arguement could be made for an increased strength.

Why would the thickness at the site of a removed bulge be thinner than the original thickness? If the metal is ever so slowly put back into place by peening over a snug fitting plug mandrel wouldn't the metal flow back to where it was originally? If not, where would it go?

I'm just a layman trying to better understand this. I have removed most of a bulge by light peening, and dents as well. I probably won't mess with bulges on a double anymore, dents are no big deal, IMO.

Stan