There is a difference between brittle fracture and yielding, that is often not brought up during the "how strong is this?" discussion. I figured out the loading at yield for a ford axle some time ago, i don't remember the numbers, but it was some number that will never be approached during normal driving. Brittle fracture doesn't seem to be a problem for these things, as I am not aware of a single axle that has been found with a crack in it.