After some checking, the value is 50% of the ultimate tensile strength, not 60% of the yield strength as I previously stated. I do concede that corrosion can contribute to fatigue failure at lower stress levels.
From
Machine Design: An Integrated Approach 2nd Edition Robert Norton, page 332:
Quote:
ENDURANCE LIMIT Note that the fatigue strength S falls steadily and linearly (on log-log coordinates) as a function of N until reaching a knee at about 10^6 to 10^7 cycles. This knee defines an endurance limit Se(primed) for the material, which is a stress level below which it can be cycled infinitely without failure. At the lower bound of the scatter band beyond the knee, an approximate endurance limit can be defined.
for steels: Se(primed) (approximately equals sign) 0.5Sut Sut<200ksi
|
There follow a few notes that not all steels exhibit this, but most do, and that aluminum and some other alloys do not. There is also mention that endurance limits for steels with higher UTS don't generally exceet 100 kpsi.
In another chapter it mentions that if a car or truck body fails, it normally does so in low cycle fatigue coming from the infrequent large loadings on the material, rather than the very frequent small loadings.
This is why I've never seen a fatigue failure in a body component on any vehicle, regardless of mileage, that stayed on pavement its entire life, and why I've seen tons of these failures on things that are off-roaded hard.
Thanks for making me get one of my old college textbooks out. I appreciate the mental exercise
