Which fatigue load combination is related to infinite load-induced fatigue life?

• A. Fatigue limit state
• B. Fatigue II
• C. Fatigue I
• D. Fracture limit state

Answer: (C)

Infinite-load fatigue life means the detail is expected to endure an unlimited number of load cycles without fatigue failure. This is represented by Fatigue I. The idea is that if the stress range is kept at or below the material’s endurance limit, fatigue damage does not accumulate to a critical crack no matter how many cycles occur. This distinguishes Fatigue I from Fatigue II, which uses a finite target life (a specified number of cycles to failure) with higher allowable stresses, and from fracture or general fatigue limit states that either focus on crack growth or on different criteria. In practice, Fatigue I applies when the goal is infinite life under repeated loading.