Characteristics of crack initiation and propagation in rotating bending fatigue tests were investigated on Ti-6Al-4V alloy. Successive observations were made by the plastic replica method to determine the physical meaning of fatigue limit σw, and crack propagation behavior was studied using a small crack growth law. The main results obtained are summarized as follows.
(1) When σ is over σw, microcracks appear at α phase.
(2) The fatigue life is determined by the crack propagation period except for the stress near the fatigue limit.
(3) When σ is equal to σw, no microcracks are observed on the specimen after 107 cycling of stress under a metallurgical microscope. Therefore, the fatigue limit is determined by the limiting condition for the initiation of microcrack.
(4) The relation dl/dN∝σnl holds when σ/σ0.2>0.6, and the relation dl/dN∝ΔKm holds when σ/σ0.2<0.5.
