doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Cause of Cracked Frame

Is it the force of recoil that causes cracks in the frame where the water table meets the standing breech? I wouldn't think chamber pressure would be involved but I'm no physicist. Thanks.

This might spare some verbiage...but not likely Possibly we could restrict our comments to shotgun design, metallurgy and mechanical engineering.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=609802

The point at which the action bar meets the standing breech is the point of maximum stress under firing. The action bar must resist a bending moment because of forces acting upon the standing breech and the hinge pin (and the draw, if it's fitted to touch the circle). In an O/U this bending moment does not exist because the action walls act as a shear web.

Here is an FEA analysis that Boxall & Edmiston did:

The picture from the FEA analysis that Kip posted above looks impressive, and I'm sure at least one guy here is intensely excited by it. But that is a very incomplete bit of information, and it only illustrates what has been quite obvious to anyone with an ounce of intelligence. It is hardly newsworthy to note that the 90 degree juncture of the action bar and the breech would be a point of the action most subject to stress from firing the gun.

FEA Analysis is a computer simulation that engineers use to approximate real life situations. In the case of Kip's picture from a Boxall and Edmiston FEA Analysis, we see a CAD generated depiction of a shotgun frame, and a color scale going from 0 to 220,594,000 newtons/sq. meter of force. 220,594,000 newtons/sq. meter is the value given for the yield strength of the action being tested in this simulation.

The color depicted at the juncture of the action bar and standing breech appears to me to show around 73,531,336 newtons/sq. meter of force. If that is accurate, then the force being exerted at that point is roughly 1/3 of the yield strength, which is the point of permanent plastic deformation.

All that is missing from this picture is the most important information necessary to understand what we are attempting to interpret. There is zero force exerted at this point when the gun is just sitting. The juncture of the action bar and the standing breech would show deep blue, or 0 newtons/sq. meter of force. I'd assume the force depicted comes from firing a shotgun shell. If the shell used in the simulation is a standard recommended load, then this simulation shows the action is only around 1/3 of the way to permanent deformation. That's a large margin of safety. But this simulation could also be showing the force generated by firing a proof load, or a 3 1/2 heavy magnum duck load in a 2 3/4" chamber. We just don't know, so this impressive picture is almost useless to us.

ASSUMING this particular FEA simulation is depicting the maximum forces from firing a normal load, then it shows us why frame cracking in both vintage and modern doubles is actually quite rare. It would take considerably more force to permanently deform this particular action, let alone actually crack it.

Gun designers pretty much know this. They have known this for a very long time, and build shotgun frames that are a good compromise of strength and weight for their intended use. Double Rifles are subject to greater pressures, so they are typically designed with more meat in this area to withstand the greater strain. This is also why guys who decide to build Double Rifles on Shotgun frames have to be very careful about what make of frame they use. And from the Ithaca Flues Frame Cracking Thread the Preacher linked, along with other sources, it seems evident that in those rare instances when frames do crack, the cause is usually due to some extreme event such a excessive loads or doubling with very heavy loads. Other than that, shotgun frame cracking is pretty much a non-issue that guys with normal temperament really don't have to be concerned about.

For the purposes of this discussion, and previous discussions on double shotgun frame cracking, it might be better to see a Fatigue or LEFM FEA Analysis done on say a 20 gauge Ithaca Flues frame. Then we could see a SIMULATION of the forces and/or number of cycles required to theoretically crack a Flues frame. And if we could somehow verify the actual forces generated by a load involved in one of these rare frame cracking occurrences, we could more confidently judge if Flues frames are actually weak and risky to shoot, or the the gun was abused by firing inappropriate loads, or if that particular gun had some machining or metallurgical defect that made it weaker than most.

Meanwhile, I just looked outside, and the sky is still not falling!

This was a problem with some early breech loaders where the angle was at 90 degrees. Cracking occurred either at Proof or after prolonged use. It was soon understood that it could be cured by a radius at the point between the breech face and the action flats. I have a very early hammer gun with a sharp cut off by Turner of Reading. It's not shown any sign of failure yet although I very seldom use it. I would never think of trying to submit it to Nitro Proof. Firing the gun produces a downwards flip of the barrels which puts strain at that point and the reason some guns have top bolts. Lagopus.....

Years ago, we had a member who was documenting Ithaca Flues failures due to cracked frames. As best I can recall, most of the failures were lightweight 20ga guns through which shooters often fired modern factory ammunition. Often the cheap stuff which was "light" only in terms of shot charge. Usually bought at local big box stores because of attractive prices. They had to produce enough punch to operate autoloaders which might have been cleaned last year or something like that.

A simple experiment to prove this flexing action is to stick a piece of paper tightly across the top of the breech and fire the gun. You should find that the paper is torn. Nothing to worry about as this is quite normal and everything flexes back again. This is one of the places that guns crack open undergoing Proof firing. Very rare but the extra pressure will show this weakness. I have only had one gun fail this way which was a double 20 bore black powder proof gun undergoing nitro proof. The action gaped quite a bit but was skilfully welded up and re-submitted; this time passing o.k. Lagopus.....

http://www.boxallandedmiston.co.uk/shotgun-anatomy/stress-analysis-in-gunmaking/

100,000,000 newtons/sq. meter = 14,504 psi yield (not ultimate) strength.
Boxall & Edmiston stated in 2011 that the frame was EN series chrome moly steel

Shotshell pressure is distributed in every direction; most, likely down the (unobstructed) barrel, some contained by the chamber walls, and some directed toward the standing breech.
Recoil is directed back against the standing breech, up (depending on the axis of the bore), and (in a SxS) laterally and (maybe) down initially.

Peter Blakeley "Successful Shotgunning"

I'd like to thank the Preacher for that link he provided for the complete story about the Boxall and Edmiston FEA Stress Analysis. It confirms what I said earlier; i.e. that the picture Kip posted was very incomplete information.

However, I'm not sure how or where the Preacher came up with these statements:


The stress analysis pic clearly shows a color chart or table on the right of the frame. And at the very bottom of the scale it notes that a red color indicates the yield strength for the steel in this frame. As I noted earlier, that yield strength is 220,594,000 newtons/ sq. meter.

If this action really had a yield strength of only 100,000,000 newtons/sq. meter, or 14,505 psi, it is likely a standard SAAMI 19,800 psi average proof load would cause permanent plastic deflection, or worse. Uh... no! The action depicted in this FEA simulation is far stronger than that.

The Boxall and Edmiston link informs us this:

Please note - for this stress analysis we put far more force onto the action than would be exerted with a normal cartridge, to highlight potential weak points.

So even with their simulated proof load, the junction of the action bar and standing breech shows a green color, which is far below the actual stated yield strength.

It was also incorrect for the Preacher to state that "Shotshell pressure is distributed in every direction; most, likely down the (unobstructed) barrel, some contained by the chamber walls, and some directed toward the standing breech." Actually, at any given point in time, the pressure recorded by a transducer or crusher would be roughly equal on all points inside the barrel, the chamber, and behind the wad and shot. We've all seen pressure curves that show a very rapid rise to peak pressure, and a rapid drop as the wad and shot moves down the barrel. So the pressure and resulting force exerted upon the base of the wad would be roughly equal to an equivalent area at the breech. And this is why a shell with a larger base, as in a 12 ga. vs. a 20 ga., will exert a greater force on the standing breech if the pressures in both are equal. This is called breech thrust, and is based upon the formula; FORCE = PRESSURE x AREA

My experience with FEA Stress analysis is admittedly limited. But from having a lot of experience with other analytical tools such as FDA, ibaPDA, and ibaAnalyzer, I can confidently say any of these tools are only as useful as the ability of the user to understand and interpret the data they are looking at.

"In the case of Kip's picture from a Boxall and Edmiston FEA Analysis, we see a CAD generated depiction of a shotgun frame, and a color scale going from 0 to 220,594,000 newtons/sq. meter of force. 220,594,000 newtons/sq. meter is the value given for the yield strength of the action being tested in this simulation."

keith: yield strength of 220,600,000 newtons/ sq. meter is only 32,000 psi
The bright green area of greatest stress IF 100,000,000 newtons/ sq. meter is 14,504 psi; IF it is 130,000,000 that would be 18,855 psi
B&E did not share the max. psi of the simulated stress, but CIP 'High Performance Superior' proof 1320 BAR = 19,145 psi which is 132,000,128 newtons / sq. meter

B&E reported in 2011 that the actions were EN series chrome moly steel
The industrial standard for (non-heat treated) AISI 4140 Chrome Moly is ultimate tensile strength - 95,000 psi; yield - 60,000 psi
Yield strength of AISI 1020, which was used for double gun frames, is 51,000 psi

SO something is not right with the simulation pressures, but I've always learned best with pictures and that helps.

Yes, pressure generated by the expanding gas is exerted in every direction. The transducer is at 1" in the wall of the shotgun pressure test barrel.

BTW: you believe yourself to be mocking me with "The Preacher". I am not the one you are mocking, and you might consider the consequence for doing so. Really (Galatians 6:7)
Please call me by my name.