Very high cycle fatigue behaviors of a turbine engine blade alloy at various stress ratios

Ultrasonic fatigue tests were carried out to investigate the effect of stress ratio on the very high cycle fatigue (VHCF) behaviors of titanium alloy (Ti-8Al-1Mo-1V). The fatigue strength at stress ratios of 0.1 and 0.5 were lower than the safely modified Goodman approximation, and a significant deviation was observed in the VHCF regime. Based on fractography and fracture mechanics, the fatigue failure process was congruously divided into four stages: (1) crack initiation induced by cleavage of primary α grains and its coalescence; (2) microstructure-sensitive slow crack propagation; (3) microstructure-insensitive fast crack propagation; and (4) final fatigue failure. Meanwhile, for surface crack initiation, fatigue crack propagation life only occupied less than 3% of total life in the VHCF regime, and over 95% of the fatigue crack propagation life was expended in the microstructure-sensitive crack propagation stage. The effect of stress ratio on crack propagation life was not distinct.