Fatigue failure is associated with what type of stresses?

• A. Static and constant stresses
• B. Thermal stresses only
• C. Compressive stresses only
• D. Dynamic and fluctuating stresses

Answer: (D)

Fatigue failure comes from repetitive, time-varying loads rather than a single static load. Each load cycle concentrates stress at flaws or microstructural weak points, initiating tiny cracks that propagate a little with every cycle. Over many cycles, these cracks grow until the component ultimately fractures, even if the peak stress is well below the material’s strength. What matters is the fluctuating, dynamic nature of the stress, often involving tension and compression or changing directions, that drives damage accumulation. Static and constant loading can cause failure if the load is high enough, but it does not build up damage cycle by cycle. Thermal stresses can contribute only if they produce cycling of stress; by themselves, steady thermal gradients don’t define fatigue. So the hallmark of fatigue failure is dynamic and fluctuating stresses.