How can repeated loading cause fatigue failure even when a part does not break after one load?

In materials testing, the phenomenon where a part fails under repeated loading despite surviving a single application of stress is known as fatigue. This occurs because fatigue is a progressive process of structural degradation rather than a single-event failure.12

The failure process typically unfolds in three distinct phases:

• Crack Initiation: Even if a load is insufficient to cause immediate rupture, repeated stress cycles cause microscopic damage within the material, such as atomic slip. Over time, this cumulative damage manifests as a minuscule crack, often on the surface.12
• Crack Propagation: With continued cyclic loading, these cracks gradually grow and spread inward. During this phase, the damage is often invisible to the naked eye, even as the structural integrity of the part is slowly compromised.1
• Final Fracture: Eventually, the crack reaches a critical size where the remaining, unaffected portion of the material is no longer strong enough to support the applied load. At this point, the part undergoes a sudden and complete fracture.1

Because fatigue is cumulative, engineers must account for the number of stress cycles a component will endure throughout its service life.12