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.