The picture from the FEA analysis that Kip posted above looks impressive, and I'm sure at least one guy here is intensely excited by it. But that is a very incomplete bit of information, and it only illustrates what has been quite obvious to anyone with an ounce of intelligence. It is hardly newsworthy to note that the 90 degree juncture of the action bar and the breech would be a point of the action most subject to stress from firing the gun.
FEA Analysis is a computer simulation that engineers use to approximate real life situations. In the case of Kip's picture from a Boxall and Edmiston FEA Analysis, we see a CAD generated depiction of a shotgun frame, and a color scale going from 0 to 220,594,000 newtons/sq. meter of force. 220,594,000 newtons/sq. meter is the value given for the yield strength of the action being tested in this simulation.
The color depicted at the juncture of the action bar and standing breech appears to me to show around 73,531,336 newtons/sq. meter of force. If that is accurate, then the force being exerted at that point is roughly 1/3 of the yield strength, which is the point of permanent plastic deformation.
All that is missing from this picture is the most important information necessary to understand what we are attempting to interpret. There is zero force exerted at this point when the gun is just sitting. The juncture of the action bar and the standing breech would show deep blue, or 0 newtons/sq. meter of force. I'd assume the force depicted comes from firing a shotgun shell. If the shell used in the simulation is a standard recommended load, then this simulation shows the action is only around 1/3 of the way to permanent deformation. That's a large margin of safety. But this simulation could also be showing the force generated by firing a proof load, or a 3 1/2 heavy magnum duck load in a 2 3/4" chamber. We just don't know, so this impressive picture is almost useless to us.
ASSUMING this particular FEA simulation is depicting the maximum forces from firing a normal load, then it shows us why frame cracking in both vintage and modern doubles is actually quite rare. It would take considerably more force to permanently deform this particular action, let alone actually crack it.
Gun designers pretty much know this. They have known this for a very long time, and build shotgun frames that are a good compromise of strength and weight for their intended use. Double Rifles are subject to greater pressures, so they are typically designed with more meat in this area to withstand the greater strain. This is also why guys who decide to build Double Rifles on Shotgun frames have to be very careful about what make of frame they use. And from the Ithaca Flues Frame Cracking Thread the Preacher linked, along with other sources, it seems evident that in those rare instances when frames do crack, the cause is usually due to some extreme event such a excessive loads or doubling with very heavy loads. Other than that, shotgun frame cracking is pretty much a non-issue that guys with normal temperament really don't have to be concerned about.
For the purposes of this discussion, and previous discussions on double shotgun frame cracking, it might be better to see a Fatigue or LEFM FEA Analysis done on say a 20 gauge Ithaca Flues frame. Then we could see a SIMULATION of the forces and/or number of cycles required to theoretically crack a Flues frame. And if we could somehow verify the actual forces generated by a load involved in one of these rare frame cracking occurrences, we could more confidently judge if Flues frames are actually weak and risky to shoot, or the the gun was abused by firing inappropriate loads, or if that particular gun had some machining or metallurgical defect that made it weaker than most.
Meanwhile, I just looked outside, and the sky is still not falling!
