On Feb 28, 6:09*pm, wrote:


Steel is different. It has an elastic limit, and as long as that
loimit is not exceeded, no cumulative stress occurs.



Not completely true. If one does not exceed 50% of the elastic limit
in steel, almost no cumulative stress occurs. From
http://www.epi-eng.com/mechanical_en..._in_metals.htm

"It is a simplistic rule of thumb that, for steels having a UTS less
than 160,000 psi, the endurance limit for the material will be
approximately 45 to 50% of the UTS if the surface of the test
specimen is smooth and polished.

That relationship is shown by the line titled "50%". A very small
number of special case materials can maintain that approximate 50%
relationship above the 160,000 psi level.

However, the EL of most steels begins to fall away from the 50% line
above a UTS of about 160,000 psi, as shown by the line titled
"Polished"."

Dan