Don: Reading in a text book on Fatigue design for aluminum structures,
there is at least one design technique that assumes that there are cracks
inherent in the material to start with that are not readily detectable by a
visual inspection. The design philosophy progresses from there. This whole
area of study is explaining why the time life specs on parts can be very
meaningful. It also can allude to the excessive safety factors that can be
used in determining time life specs.
It looks like I've got a lot more studying to do in this area.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"Don W" wrote in message
...


Stuart & Kathryn Fields wrote:
Don: Be sure to share what you find. I think that I'm only scratching
the surface of some stuff. One thing I found and had never seen before
was an equation relating the increase in stress due to a crack. It
scares the hell out of me. I'm afraid to use a metal fork in my salad.
The equation basically says that the max stress is 2 times the load
divided by the area times the square root of the crack length divided by
the radius of curvature of the end of the crack!!! If the radius of
curvature was equal to the crack length, the max stress is already twice
what you would calculate using the applied load and the element cross
section. Now put a reasonably sharp crack and see what happens....as the
radius approaches 0.001 times the length of the crack......????

That is because the load is not equally spread across the part, but is
concentrated at the end of the crack.

Think about a piece of metal bar in tension with a crack halfway across
it. The cross sectional area that has already seperated cannot bear any
load at all as it has already failed. The end of the crack is taking a
lot of load because the crack pulls apart when it is under tension.

Solution: Don't use cracked parts!! (Well duh)

Don W.