Eric, thank you! I appreciate you taking the time to educate. Your explanation is perfect and I now "get it". This should be one of those interview questions like thermal expansion of a donut, as it's not intuitive, at least for me... and I worked with MechEs for decades mitigating fatigue risk!
After my last post, I tried to educate myself and found a couple useful sites online.
Wiki has this cartoon illustrating exactly what you just clarified:
ARP has more insight (
link):
"
Pre-Load
Finally, although not a design parameter, the subject of bolt installation preload must be addressed. It is a fundamental engineering concept that the force in a bolt in an ideal preloaded joint will remain equal to the preload until the externally applied force exceeds the preload. Then the force in the bolt will be equal to the external force. This means that fluctuating external forces will not cause fluctuating forces in a preloaded bolt as long as the preload exceeds the external force. The result is that fatigue failure will not occur.
In a non-ideal joint, such as in a connecting rod, the bolt will feel fluctuating stresses due to fluctuating rod distortions. These are additive to the preload, so that fatigue could result. In connecting rods, precise preloads are required because if they are too low, the external forces (the reciprocating weights) will exceed the preloads, thus causing fatigue. If they are too high, they provide a high mean stress that combines with the fluctuating stresses due to rod distortion. Again, fatigue is promoted. The objective, then, is to preload a bolt so that it just exceeds the external load, and no higher.
To sum up: both insufficient preloads and excessive preloads can lead to fatigue failures."
Great education, thanks all.
Mike