What is fatigue failure?

• A. Fracture due to single overload
• B. Cracking due to corrosion
• C. Cracking due to repeated stress cycles below the yield strength
• D. Failure due to high temperature

Answer: (C)

Fatigue failure is the progressive cracking and eventual breaking of a material caused by repeated loading, where each individual load may be well below the level that would cause immediate yield or fracture. In a cyclic environment, tiny cracks tend to start at stress risers such as sharp corners, holes, or surface roughness. With every cycle, the crack grows a little more, and the remaining section carries less load. After enough cycles, the crack reaches a critical size and the part fractures suddenly, often with little warning. This behavior is captured on S-N curves, which show how many cycles a given stress range can withstand, and some materials have an endurance limit below which failure may not occur for practical purposes. While fatigue is driven by repeated stress, other mechanisms like corrosion cracking or high-temperature creep involve different processes, so the repeated-cycle, below-yield scenario best describes fatigue failure.