Lloyd, my understanding is that there is a small zone or region (for lack of a better term) between the elastic limit and the yield point that is known as the Elastoplastic region. At this point, steel and other ductile materials will still exhibit some tendency to return to normal dimensions, but because atomic bonds are also being broken, there is some permanent plastic deformation.
Then beyond this point, once the yield point is reached, the plastic deformation is the dominant factor, and it will take less strain to cause permanent stretch or deformation, until the steel finally breaks or fails.
When I worked for General Motors, before the bankruptcy, when money was no object, they sent us to all kinds of in-depth training classes. The several day class covering fastener torque and correct tightening of fasteners was taught by a Metallurgical Engineer from a company that produced threaded fasteners for industry. It was very interesting, because he actually put on many graphic demonstrations where we could see the effect of correct torque, and also see that bolts which had been subjected to a point just beyond the elastic limit were permanently weakened. We broke a lot of bolts, while watching exactly the rotational force or torque that weakened and eventually broke them. He told us that at that point, the steel atomic structure begins to react more like taffy. Of course, he showed us many graphs, SEM photos, etc. showing the structural before and after changes, but the difference was that he actually understood and was qualified to explain them, without giving us erroneous information.
About a week after the class, I watched one co-worker who had also been schooled in the same class tighten a 5/8" bolt with all his might. This guy was "strong like bull" with wrists bigger than my ankles. Even without a torque wrench, I'm certain he was at or beyond specs. He then proceeded to use a big hammer to pound on the wrench to give the nut over another full turn. I tried to remind him about over-tightening, (my reaction was something like, "holy shit man... WTF are you doing?"), but he said our instructor didn't know what he was talking about. Over the years, I watched as this guy spent a lot of time with drills and E-Z- Outs trying to remove broken bolts that he had snapped during installation. His entire set of T-Handle Allen Wrenches looked like licorice twists. Sometimes training and education is simply a waste of time and money. Some folks understand things, and some never will.
Earlier in this thread, wall thickness dimensions were given as .044" at points just before and just after the burst, and also at .038" at the thinnest point of the burst. Those are fairly substantial wall thicknesses at that point of the barrel, especially compared to very many of our light double barrel bird guns. And the Winchester Nickel Steel had established a reputation for being much stronger and able to handle stiff charges of Nitro Powders. But it burst anyway... proving Ted's point about avoiding barrel obstructions at all costs.
It might be tempting to think that the barrels were honed or struck thin at the burst point, but the thinner dimension was most likely the result of rapid thinning as the barrel wall stretched in the milliseconds prior to letting loose. Once beyond the elastic limit and the yield point, the steel started behaving more like a Bazooka Joe Bubblegum bubble prior to popping. The wall got thinner and weaker as the extremely rapid deformation happened. There is an excellent chance that the obstruction started moving, but the rapid pressure build occurred way too fast to prevent disaster. The .044" or even .038" wall thickness that would have been more than adequate much closer to the chamber under normal circumstances, was no match for the sudden extreme pressure rise behind the obstruction... even with some of the best barrel steel available at the time.
BTW Lloyd, I haven't forgotten about those pics of Rockland Furnace and Freedom Falls. Will try to get them posted soon.
