A bolt can fail by fatigue when its tensile stress varies repeatedly, even if the maximum stress stays below its static tensile strength.

Preload changes this cyclic stress history.

Preload reduces load fluctuation

While the joint remains clamped, only part of an external separating load increases bolt tension. The rest reduces compression in the joined members.

A sufficiently high and well-controlled preload can therefore keep the alternating part of the bolt stress much smaller than the external load variation might suggest.

Avoiding separation

If the external load unloads the clamped members completely, the joint separates and the bolt begins to carry a much larger fraction of further load. This greatly increases bolt stress amplitude and can accelerate fatigue damage.

Stress concentrations

Threads introduce local stress concentrations, so fatigue cracks commonly initiate near the first engaged thread or thread runout. Surface condition, residual stress and manufacturing quality also matter.

Design trade-off

More preload is not unlimitedly beneficial: the assembly preload plus service load must remain compatible with bolt strength, proof load and joint materials.

Reliable bolted-joint design balances preload, stiffness, separation margin and fatigue resistance rather than sizing the bolt only from peak external force.