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Primoz has pretty much hit the nail on the head here. The radial/axial load ratings of bearings quoted by the manufacturers are based on continuous 24hr operation over a lifetime, even the static load rating. Generally in rotating machinery design, the output for a calculation is the expected lifetime of a bearing based on operating loads and conditions - depending on the application it's always nice when the output comes out as "inifinte"...however, in my experience (designing heavy duty winch drums and selecting gearboxes for them) bearing failure seems to be much more to do with how they're mounted than...

If you can get your hands on a broken dh frame or fork- cut it up and measure wall thickness etc. back out the strength that way. could probably measure fork tube ID/OD without damaging it. that would at least give a ballpark number I ran 1.5ODx .056 wall 4130 downtube on a dh bike. Never broke or cracked it.