Fastener Fatigue

Fastener fatigue is a failure mode where a bolt, screw, or stud cracks and eventually breaks after being subjected to repeated or fluctuating loads over time. Unlike an overload failure that happens all at once, fatigue damage accumulates cycle by cycle, often starting as a small crack at a high-stress location such as the first engaged thread, the thread runout, under the head, or at a notch or surface defect. Fatigue fractures can occur even when the applied loads are below the fastener’s ultimate strength, which is why fatigue is a common concern in vibrating or cyclic-duty equipment.

Fatigue risk increases when a joint experiences movement or load reversal, when the fastener is under-preloaded, or when stress concentrations are high due to poor thread engagement, misalignment, bending, or damaged threads. Corrosion can accelerate fatigue by creating pits that act as crack starters, and improper installation (over-torque, galling, or cross-threading) can also reduce fatigue life. Improving fatigue performance typically involves achieving proper bolt preload to keep the joint clamped, using the correct fastener grade and geometry, ensuring adequate thread engagement, minimizing bending and shear in the bolt, and selecting materials and coatings suitable for the environment.