Which fatigue stage is described as unstable?

• A. Initiation
• B. Propagation
• C. Failure
• D. All stages are unstable

Answer: (C)

In fatigue, crack development occurs in stages: initiation, where small cracks form at stress concentrators; propagation, where the crack grows with cycles; and failure, where the crack reaches a critical size and the remaining section can no longer carry the load. The unstable stage is the final failure because once the crack is large enough, the remaining material cannot support the applied stresses, causing rapid, uncontrolled fracture with little warning. Initiation and propagation involve crack growth that is relatively gradual and predictable, whereas failure is the abrupt, unstable collapse of the member.