During fatigue failure, which stage is considered stable?

• A. Initiation
• B. Failure
• C. Unstable growth
• D. Propagation

Answer: (D)

Fatigue life after crack initiation includes a steady growth phase and a final rapid fracture. The steady or stable portion is the propagation stage, where the crack advances gradually with each load cycle under cyclic loading, in a predictable manner until it reaches a critical size. Once that size is reached, the crack grows rapidly, leading to failure in an unstable manner. So the stable stage during fatigue failure is the propagation phase. Initiation is the formation of the crack and is not the stable growth phase, and failure is the abrupt end of the crack growth.