What are the three steps to fatigue failure?

• A. Crack initiation, crack propagation, final failure
• B. Elastic deformation, yielding, fracture
• C. Creep, fatigue, fracture
• D. Proportional limit, yield, fracture

Answer: (A)

Under cyclic loading, fatigue failure happens in three stages: crack initiation, crack propagation, and final fracture. In the initiation stage, tiny cracks form at sites of stress concentration such as surface roughness, defects, or inclusions due to repeated stress cycles. In the propagation stage, those cracks grow with each load cycle, with growth rate depending on the stress range, material, environment, and microstructure. Finally, when the crack reaches a critical size, the remaining ligament can no longer sustain the load and rapid, final fracture occurs. The other options describe monotonic or time-dependent deformation processes, which aren’t the cyclic crack-growth sequence that defines fatigue.