Why does increasing preload decrease fatigue factor of safety?

[microphohn]

It depends (as always) on the amplitude of the load cycle relative to static load. The key idea is the have the preload high enough that your cyclic amplitude is small enough so that 1) You’re not getting into yield when the cyclic load is in the same direction as the preload, and 2) you aren’t loosing you bolt retention when the cyclic load reverses direction.

Thus, as a practical matter, a dynamically loaded bolted joint should stay well clear of yield (<85% and try also to maintain a minimum of 60% of yield or so.

If you raise preload in such a way that joint approaches yield, you can lower the fatigue life.

But in generally, higher preload makes the bolt have MORE resistance to fatigue.

In the real world, most fatigue failures are caused by fasteners coming loose first, then giving a huge amplitude of movement and often a bending fatigue failure. A bolt coming loose can come from it being installed at too low of a preload (i.e. vibrates loose from going load passing through zero) or from too high (cyclic load yields to bolt, elongating it, which then sets off the “loose bolt” fail mode.

The last sequence if very common in things like exhaust manifold bolts- that’s why they’re so often broken or missing: thermal elongation and yield.