Gigacycle fatigue properties of Ti–6Al–4V alloy under tensile mean stress

The mean stress effects on the gigacycle fatigue properties of three heats of Ti–6Al–4V alloy were investigated by means of ultrasonic and electromagnetic resonance fatigue tests. All heats developed internal fractures under mean stress conditions of R≥0 with negligible frequency effects. The origins of the internal fractures proved not to be inclusions, but clusters of facets. Cross-sectional observations of the internal fractured specimen revealed the facets to have formed in the α-phase in an inclined direction. The gigacycle fatigue strength of the Ti–6Al–4V alloy at R=−1 matched that of quenched and tempered steel, while that under R=0 was clearly lower. Ti–6Al–4V alloy is more prone to internal fracturing under tensile mean stress conditions, resulting in reduced fatigue strength. Its gigacycle fatigue strengths are thus below the modified Goodman line at around R=0. This trend is very dangerous, since the modified Goodman line generally gives predictions with a good margin of safety.