Steel alloys have a dinstinctly unique property, the Fatigue Limit. If a steel alloy is exposed to cyclic stresses below the Fatigue Limit, it actually gains in strenhth and will NEVER fracture as long as it's stressed below the Fatigue Limit. It's why back in the early days of hot rodding and Nascar, the better engine builders would hit the junkyards looking for a high mileage engine out of a station wagon. Theory was that station wagons were used for hauling kids and normally driven quite gently. As a result, the connecting rods and crankshafts were stronger and able to withstand more "abuse" without failing. This pretty much faded away in the 70's with the factories introducing race specific rods and crankshafts. However, back in the mid 60's nearly every NASCAR team was building their race engines using parts scrounged from junkyard engines.
Aluminum, Brass, or any other non ferritic alloy doesn't have this particular property. As one Materials Professor put it in class one day, you could sink a 2 foot diameter bar of 6061-T6 aluminum into a concrete pad and snap it off at the base by pushing on it with one hand. It might take a million years but eventually that bar would snap.
IMO Fatigue is part of the reason why we are constantly seeing posts about cracked frames, it's partially a function of Fatigue. However, a contributing factor is the interference fit that S&W is now using to mount the barrels. That interference fit alone adds hoop stresses to this area and it will only speed up the Fatigue Process. IMO S&W should stop using an interference fit on the alloy frame and instead change to a "close clearance" fit and use something like Locktite Red to insure the barrels won't shoot loose. That would reduce the Hoop stresses in this area and extend the life of the frame. They'll still crack eventually but who's going to care if that happens after 30 or 40 thousand rounds fired.
BTW, I was taught about the Fatigue Limit in steel alloys in 1974. From what I've seen of recent Engineering grads today, subjects like this are no longer covered. Steel has such a high density that in some circles it's become Politically Incorrect to point out that steel still has some distinct advantages over any other alloy.
