Neil, I'm glad you mentioned fatigue life, maybe we can kick this around
a little. I recently read a tech article by Kevin Cameron(probably the
best mc tech writer active today) in the journal of the Aircraft Engine
Historical Society on stress and fatigue, and he mentioned that ferrous
parts that operate somewhere in the 25-60% of their yield stress range
may have an INFINITE fatigue life. Maybe that's why, to cite a 'homely'
example, a stock Flathead conrod, which seems so skinny, may after 200K
miles of ordinary service where it's stroking along almost all the time
in the 1500-2500 rpm range, be perfectly sound and unstressed. So Joe
Rodder can come along, grab this old rod and the rest of them, mag 'em,
they pass, put in new small end bushes and re-size the big ends, and
stick them in his hot Flattie. And they may be actually just as good as
brand new unused ones. Not that that means as strong as Cunninghams or
Fergusons, mind you!
The trick here though, is to know what the stress level is for iron and
steel parts that keeps them in the 'infinite fatigue life' range.
Generally, we don't. Bill
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