What causes fatigue cracking in materials?

Fatigue cracking in materials is primarily caused by repeated cyclic stresses that occur over time. When a material is subjected to varying loads, even if these loads are below the material's ultimate tensile strength, microscopic defects or weaknesses can develop and propagate. This is particularly relevant in aerospace applications, where components undergo significant repeated loading during operation, such as in aircraft wings or turbine blades.

As the material experiences cycles of stress, these small defects accumulate and grow, leading to the formation of cracks. Over time and with sufficient cycles, these cracks can reach a critical length, potentially leading to catastrophic failure if not detected and mitigated. This phenomenon is distinct from other forms of failure, such as those caused by instantaneous impacts or thermal effects, where the mechanisms and stresses involved are different in nature.