Fatigue stress results from which type of loading?

• A. Cyclic or repetitive loading
• B. Static loading only
• C. Temperature-only loading
• D. Random static loading

Answer: (A)

Fatigue stress arises from repetitive, cyclic loading. When a component is loaded and unloaded many times, tiny cracks tend to initiate at stress concentrators (like surface flaws or material boundaries) and slowly grow with each cycle until the material fails. Importantly, the peaks of the stress can be well below the material’s strength, yet still cause failure after enough cycles. In nuclear plant components, this cyclic loading shows up as vibrations, startup/shutdown shifts, or thermal cycling that repeatedly reverses or fluctuates the stress state. Static loading, applied once or held constant, does not produce fatigue failure in the same way, and while temperature changes can create stresses, fatigue is fundamentally about many cycles of loading rather than a single or non-repeating load.