Fatigue refers to material failure that occurs at a stress level below the elastic limit due to repetitive loading.

• A. It occurs at the elastic limit
• B. It is due to repetitive loading
• C. It occurs due to a single overload
• D. It is unrelated to loading

Answer: (B)

Fatigue is a failure mechanism driven by cycles of stress, not a single loading event. Even when each peak stress is below the elastic limit and the material doesn’t yield or deform permanently on any one cycle, tiny flaws or surface irregularities can act as crack initiation sites. With repetitive loading, these cracks slowly grow with each cycle until the remaining cross-section can no longer support the load, leading to sudden fracture. That’s why the defining aspect is the repetitive loading over time, rather than a one-time overload or any event at or above the elastic limit. A single overload can cause immediate yielding or brittle fracture, and fatigue is inherently linked to many loading cycles.