What is a typical fatigue behavior of a ductile material under cyclic loading?

• A. Fatigue life increases with repeated loading
• B. Cracks initiate at defect-free regions
• C. Fatigue life decreases with repeated loading; cracks initiate at defects and propagate until failure
• D. Fatigue only occurs in brittle materials

Answer: (C)

Under cyclic loading, a ductile metal typically accumulates damage until a crack starts at a defect or surface flaw where stress concentration is highest. Once a crack has formed, it grows with each load cycle, often in a gradual, stable way, until it reaches a critical size and rapid fracture occurs. Because crack growth progresses with every cycle, the remaining life shrinks as cycles accumulate, so the overall fatigue life decreases with repeated loading. This makes the description that fatigue life decreases, with cracks initiating at defects and propagating to failure, the best fit. Initiation at defect-free regions is unlikely since there aren’t strong stress concentrations to nucleate a crack, and fatigue is not exclusive to brittle materials.