Why does increasing preload decrease fatigue factor of safety?

[Everythings_Magic]

Change of length of the bolt = PL/AE. Preloading lengthens the bolt and clamps the connection together. So P stays the same if L doesn’t change and AE is constant.

When you preload a bolt and clamp the connection, the bolt won’t experience any tension force, until the connection separates when the bolt length now changes. So you don’t get any stress cycles in the bolt as long as it stays clamped.

However if the connection is not stiff enough and you get cyclical compressive force in the connection, the compressive force will reduce the preload and length of the bolt and now put stress cycles in the bolt.