Originally Posted by Airburst

I was told by my materials lecturer (I'm a first-year mechanical engineering student) that a lot of structures aren't designed to always keep the stresses below the fatigue limit, because they'd be quite a bit heavier if they were. I'm not sure traditional steel frames and forks are "designed" in that sense anyway, but if I was designing a fork that I was going to be riding, I might put up with a bit of excess weight in order to keep the stress below the fatigue limit for a decent proportion of the time. I don't know if that's actually how manufacturers do it, though.