Glossary

A progressive mode of failure in a fastener or its material caused by repeated or fluctuating (cyclic) stresses, even when those stresses are below the material's static strength limits. Over time, these dynamic loads lead to the initiation and growth of microscopic cracks, gradually weakening the fastener. This can result in sudden, unexpected fracture without significant prior deformation, making fatigue a critical concern for fasteners exposed to vibration or alternating loads.

The maximum level of cyclic or fluctuating stress that a fastener can withstand for a specified number of cycles without experiencing fatigue failure. This property quantifies a fastener's resistance to the progressive damage caused by repeated loads. It is a critical design consideration for fasteners in applications subjected to vibration, repeated loading/unloading, or other dynamic conditions.

Fatigue-crack initiators are the small imperfections, features, or stress concentrations in a material where fatigue cracks are most likely to begin under repeated cyclic loading. They act as the “starting points” for cracks that can grow progressively with each load cycle until the material eventually fails.

These initiators can come from many different sources. On the surface, rough machining marks, scratches, dents, or corrosion pits can all create localized stress concentrations that trigger cracks. In the material itself, inclusions, voids, microstructural inhomogeneities, or welding defects can serve the same role. Even intentional design features, such as sharp corners, keyways, or threaded regions, are common fatigue-crack initiators because they naturally concentrate stress.

Once a fatigue crack initiator is present, cyclic stresses focus at that location. Even if the overall applied stress is below the material’s yield strength, repeated loading and unloading gradually weaken the area around the defect. Over time, microscopic cracks form at the initiator site, eventually propagating and leading to visible cracks and final fracture.

Controlling fatigue-crack initiators is a major goal in engineering and design. Processes like polishing, shot peening, surface coatings, and proper fillet radii are used to reduce stress concentrations. High-quality material selection, careful manufacturing, and preventive maintenance are equally critical in minimizing the sites where fatigue cracks can nucleate and threaten the reliability of a component.