Fatigue is defined as?

• A. cracking due to thermal cycling
• B. wear under static load
• C. cracks caused by repetitive impact
• D. the initiation and propagation of cracks due to repeated normal tensile stress

Answer: (D)

Fatigue is the progressive damage that occurs when a material is subjected to repeated loading, starting with the initiation of a crack at a flaw or stress concentration and followed by crack growth with each load cycle. The key idea is that the cracks develop and propagate under cyclic, tensile-type stresses, even if individual loads are below the material’s static strength. This repeated normal tensile stress drives both the initiation and the propagation of cracks, leading to eventual failure after many cycles. In contrast, phenomena like cracking from thermal cycling or wear under static load involve different failure mechanisms, and repetitive impacts describe damage from another loading mode.