Fatigue Failure occurs at relatively low stress levels in structures subjected to fluctuating and cyclic stresses. Which statement best describes fatigue failure?

• A. It occurs under fluctuating and cyclic stresses at relatively low stress levels.
• B. It occurs only at high static loads.
• C. It is the same as yield failure.
• D. It is independent of stress amplitude.

Answer: (A)

Fatigue failure is a progressive damage process that happens when a material is subjected to repeated or fluctuating loads. Even though the peak stress in each cycle may be below the material’s yield strength, cracks can initiate at flaws on the surface and grow with every cycle until the part finally breaks. This is why fatigue occurs at relatively low stress levels under cyclic loading: the damage accumulates with the number of cycles, not from a single high-load event. It’s different from yield failure, which arises from plastic deformation under a static or slowly varying load, and it’s not independent of stress amplitude—larger fluctuations and more cycles generally shorten the life.