UPDATE:
1. No word from SAAMI
2. TEAM has finished radiography on 3 barrels but final analysis awaits the return of the senior tech from Hawaii. I hope to find a crack on the Syracuse Arms twist barrels, which can then be sectioned and examined by METL
3. Fascinating meeting this am with Adam at METL. We reviewed most of the images, both SEM and photomicrographs 20X to 100X. He's going to send a very interesting image which I'll post soon, and the other metallurgists in the office are having a grand time looking at something never before recorded (the exciting life of engineers!
)
Very short version
a. There was
no evidence of low cycle fatigueb. The blow out was from an over-pressure event with ductile (stretching and sheer) fracture, with nothing to suggest an intrinsic wall defect. Photomicrographs can not prove obstruction vs. shell overload, but certainly the bulge suggests obstruction.
c. The failure did not occur at a ribband weld line
d. One crack in the piece that was retrieved was just that; a fracture through a cast iron section. Another crack did follow the low carbon/'mild' steel - cast iron juncture in one of the rods ie. within the scroll.
e. As above, the images show mild steel - cast iron - and interfaces with some blending of the metals.
f. Adam showed me a freshly cut piece of the barrel fragment, and it looks like any other cut of metal ie no 'orange lace'.
Monty sent me a pitiful Smith Twist barrel with marked rusting in the bore. I'm going to use it for tensile strength testing, but first will have Adam examine the cut edge for evidence of the mythologic interlaminar rust.
Tensile strength testing update:
I've received contributions of 3 Twist, a Damascus-Twist, and a Crescent Armory steel barrel to section and destroy. A Parker Vulcan steel is on the way. I'd very much like to have 2-3 higher grade Crolle barrels and more fluid steel pieces.
For comparison, I thought I'd include Zircon's examination of 2 Sherman Bell blow-up barrels. Remember the barrels were subjected to sequentially higher and higher pressures.
http://www.familyfriendsfirearms.com/forum/archive/index.php/t-55364.htmlThe 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 to emanate from 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 remaining ligaments 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 microvoids present. This appearance is a dead-set giveaway to a
ductile fracture.
The homogeneous, "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 was the screw hole, again. 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.
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. The 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.
