This study aimed to clarify the implication of the stress amplitude of the no failure state at N = 107 cycles for commercially-pure titanium and the influence of the strain age hardening of commercially-pure titanium on the fatigue characteristics. To this end, rotating bending fatigue tests on smooth and notched specimens made of commercially-pure titanium were performed. After removing the oxygen solution layer of the specimen, a proportional relationship was noted between the maximum stress at the notch root and logarithm of the number of cycles to the fatigue crack initiation. In addition, it was observed that if only one type of fatigue crack initiates from the notch root and internal fracture does not occur, the fatigue limit is the stress amplitude at which a fatigue crack does not initiate at N = 107 cycles. After 2 × 108 cycles, the strain age hardening does not considerably influence the fatigue characteristics of commercially-pure titanium.
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