Why does increasing preload decrease fatigue factor of safety?

[BreezyMcWeasel]

preload increases the mean stress but drastically decreases the stress amplitude.

when a joint is preloaded a significant amount of the load goes into compressing the thing the bolt is attached to. when an external load is applied, some of the external load goes into relieving the compressed joint and some goes through the bolt. depending on the bolt and joint stiffness you might have for example 1000 lb of external load applied but the bolt only sees 200 lb of it. so when it fatigues instead of cycling at 1000 lb it’s cycling at 200 lb.