The effect of frequency on the giga-cycle fatigue properties of a Ti-6Al-4V alloy

The effect of frequency on giga-cycle fatigue properties was investigated in smooth and 0.3 mm-hole-notched specimens at three heats (Heats A, B, and C) for a 900 MPa-class Ti-6Al-4V alloy. Fatigue tests were performed at frequencies of 120 Hz, 600 Hz, and 20 kHz using electromagnetic resonance, high-speed servohydraulic, and ultrasonic fatigue testing machines, respectively. Heats A and B developed internal fractures, and in these cases, frequency effects were negligible. On the other hand, Heat C developed only surface fractures. In this case, high-frequency tests showed a higher fatigue strength, indicating frequency effects were not negligible. The tests using the notched specimens showed almost no frequency effects regardless of the heat. The frequency effects observed in the cases of surface fracture were believed to be related to a delay in local plastic deformation in response to high-frequency loading, since temperature increases in these specimens were successfully suppressed. The delay in the plastic deformation was observed to be reduced in the notched specimens because of stress concentration and limitation in the plastic deformation zone. In turn, the significant conclusion of this research is that high-frequency tests can be applied not only to internal fractures but also to notch problems, but are not applicable to surface fractures of smooth specimens.