As most are well aware, there was a call on the DoubleGunBBS and PGCA sites in 2005 "Contribute Junk To Advance Barrel Strength Knowledge" with the hope of obtaining vintage barrel samples for composition and strength testing.
This was posted by ‘Zircon’ in Jan. 2007. Unfortunately, the testing effort apparently ended thereafter.
http://www.familyfriendsfirearms.com...p/t-55364.html
I am doing a failure analysis of a pair of Parker barrels - one set damascus, and the other set homogenous Vulcan “fluid steel”. These barrels were used in the study by Sherman Bell and Tom Armbrust, published in Double Gun Journal. They subjected each barrel to increasingly heavier loads and they both failed at about 30,000 psi. Modern ammo gets up perhaps to 12,000 psi. Most folks that shoot these old gals use shells loaded to the 7,500 psi range.
During the failure analysis I noticed that the fracture length for the Vulcan barrels was substantially longer than for the damascus barrels. A close examination of the fracture surface showed progressive, low cycle fatigue marks on the damascus barrel. The crack advanced slightly with each increasingly higher pressured load. On the Vulcan barrels, both sides failed by a brittle fracture mechanism. By this, I mean the barrels let go in one fell swoop. Even though both sets of barrels failed at 30,000 psi, the behavior of the damascus barrels was superior to the Vulcan barrels, owing to the fact that the Vulcan barrels failed in a brittle fracture mode. A ductile fracture trumps a brittle fracture every time.
One of the (myths) with damascus is that it will fail at the welds where the original rods were forge-welded together. When I looked at this particular set of damascus barrels using a metallographically prepared sample, and up to 1,000X optical magnification, I saw NO EVIDENCE of weld joint failure, slag in the weld joints, porosity in the weld joints, etc. I have about 30 old barrels in this study, homogenous, damascus, and twist included. I am a practicing metallurgist who holds an M.S. degree, and am qualified to state the observations of barrel integrity made in this posting.
On the two old Parker barrels, there is a screw hole that comes up from the bottom and pins the extractors in place. Both barrels failed at that hole, because it takes a (segment) out of the side of the chamber and is the thinnest portion of the chambered area.
The damascus barrel let go by a mechanism known as low cycle fatigue. Each succeeding round had higher and higher pressure. After several rounds, a crack started at the extractor screw hole. Each successive round caused the crack to open up just a bit further, until finally the overpressure could not be contained and the (barrel) failed in a ductile fashion. Ductile failures in steel look like a taffy pull at about 1500 to 3000X magnification using scanning electron microscopy. There is a cup and cone appearance with a lot of micro-voids present. This appearance is a dead-set giveaway to a ductile fracture.
The "fluid steel" barrel failed by brittle rupture. The fracture surface is more or less smooth, but has some "rivulets" in it that point back towards the initiation point, which again was the screw hole. The fracture surface was about 3X as long as for the damascus barrels. In other words, the same 30,000 psi final internal load created a lot more fracture surface in the homogenous barrel than in the damascus barrel. This indicates that, for an equivalent-length fracture, it took less energy to open up the homogeneous barrel than for the damascus barrel. The words in the Sherman Bell article were that the Vulcan barrel failed much more abruptly and (violently) than the damascus barrel.
So the verbal description of the failures during firing and the visual observations of the fracture surfaces are in accord with each other. Both barrels ripped lengthwise for some distance and then the rupture terminated in a circumferential crack. In the case of the damascus barrels the crack spiraled around with the weld pattern, but it was not on a weld, rather it was on one of the in-between areas. After the damascus pattern is formed by wrapping rods together and forging them into a strip (the "skelp") which is wrapped around a mandrel, spiral paper-tube fashion, and is forge welded together. These spiral welds remained tight and the parent metal is what failed. This may seem pretty amazing, but in many, many instances the actual steel welded structure is stronger than parent metal.
Many microphotographs, chemical analysis of the steel, etc. (will) make up the (anticipated) article. I'll also be looking at several other barrel ruptures and measuring the strength of the various barrel steels in the "hoop" direction as the barrel will always fail in hoop tension with a lengthwise crack. Any internally pressurized cylinder has 2X the force in the hoop direction as in the longitudinal (axial) dimension, so it's no wonder why barrels all seem to blow out with lengthwise cracks.
I've spoken to another gentleman who abandoned his testing effort related to illness 4-5 years ago, and have no knowledge of the fate of his barrel samples
06-22-2013, 11:10 AM
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