Don't know were to put this post, seems to me Metal and how it stresses ought to be in Gunsmith or perhaps Loading.  It's sort of the same thing case necks go through when fired repeatedly, on a grand scale

Some time ago I posted on the MOL Comfort, Container ship that broke in half Indian Ocean.  Lot of speculation why a fairly new Japanese owned and built ship would break in half.

Latest idea is she flexed amidships, steel worked more than it should every voyage. Ship like her runs 24 hours a Day 350 + days a year with heavy loads.  Steel flexes and becomes work hardened then brittle then breaks.

They put a camera in one of her sister ships, how much she works is amazing to watch. You are looking at a 1100 foot x 110 foot structure with probably 50,000 or 60,000 tons dead weight in 18 foot seas.


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Wonder if the same effect is present in weak rifle actions used with very heavy loads. Could this be why low number Springfield 03's failed, soft steel working with high pressure loads going brittle ?

Boats

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Go past the psi strength of the metal and there will be an accident

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(You need to Login or Register to view media files and links) On the graph, note the 1040 yield strength

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I once overloaded a 45 Colt using grams instead of grains and it only rung the barrel ...
4.5 Grams = 69.4456128 Grains
Sounded like a 105 howitzer going off though

Yield strength is ok as an indication of gross strength, but for flexing and work hardening you should look at tests like CHARPY notch test.  It gives you a good indication of the metal's resistance to cracking. 

Compare 1018 to 1024 to 10++ and you will see the strength goes up but the resistance to cracking drops.

Steels with a significant manganese content are well known for work hardening. Plastic deformation makes them gain strength to a considerable degree. They're used for caterpillar tracks, mud pump sleeves for oil well drilling, and railcar wheels. They hot forge well, but can be a cast iron beyotch to machine because of this property. (I dare you to ask me how I know...)   I have never heard of this type of steel being used in shipbuilding, but it might be for parts that take a lot of wear and impact.  Not very useful as a steel for making guns.

Rail cars fatigue stress.  Loadings are tracked for each car, and the cars are discarded when they reach about the 85th percent of failure loading cycles. (Not sure if that number is exactly correct, it's somewhere in that range).   I think this is tracked by miles rolled.  Minor uneveness in the track bounces the load slightly, and the supporting car flexes.  Each time is a load cycle.  The reason I know this is I did some considerable research at one time for a client that was interested in utilizing used flatcars for bridges.  We could load rate the bridges, but we were worried about the remaining fatigue life cycles.  After making a conservative estimate of the remaining fatigue life, and then doing a life cycle cost analysis, we decided that newly constructed concrete bridges were more cost effective, so we abandoned the railroad car concept.   However, they make good bridges for residences or ranches and such, where the number of loading cycles they see will be limited.   

The number of cycles that a rifle goes through in it's life is far below the number that it takes to fatigue crack steel, at least if you don't push it too close to the yield strength.  As noted before, the fatigue cycles to failure decreases sharply as the yield strength is approached.  Another good reason to keep loadings down to less than 1/3 of the yield strength of the steel.  I cringe every time I see someone calculate that a ____ (fill in the blank) action is strong enough for XXX cartridge, because the loads they're going to use are only 90% of the strength of the steel.  Really should have a factor of safety of at least 3, and at the minimum, 2.

A good way to visualize this is breaking wire.  If you flex wire sharply and quickly, it gets very hot, and will fail in less than 100 cycles.  I've utilized this many times to cut a piece of steel wire when i didn't have a pair of cutters handy.  Bend it back and forwards in the same place, and soon it cracks and breaks.

dave

sort of like bending a wire back and forth till it brakes, the more bend the less times it takes to brake. On the 03 Springfield, they did not get a heat treat oven till after the 800,000 S/N. They used the color of the metal before quenching. The actions heated to red during the daylight hours were much hotter than those heated to red at night so some were very brittle and some  safe.

I think the key here is how hard the metal is, and how much it moves. The more metal moves, the more it work hardens, so as with bending a wire or sheet metal quite a bit has more effect on work hardening.
I've had quite a bit of experience with sheet metal on cars being hammered and work hardened, and having to anneal the metal to keep it pliable. I'm not sure barrels and actions move enough to change molecular structure that much. I'm sure there's some extremely small changes, just not sure how much, or if it could ever create a failure.