抄録
Low cycle fatigue crack initiation test of notched 0.4 compact tension specimen(notch tip radius of 1mm) was carried out at 800℃ in air and in vacuum under fully reversed tension-compression loading. Microcracks at the notch root surface were observed using a scanning electron microscope by interrupting the fatigue test. The relation between number of cycles to crack initiation N_i (defined as the number of cycles to 0.5mm long crack on the notch root surface) and apparent stress intensity factor K_a is linear against log-log plot. N_i in air is about ten times smaller than in vacuum. It was found that the crack in air initiates at the oxidized lamellar, while the crack in vacuum initiates independent upon the microstructure. The very short fatigue crack initiation life in air is due to the fast oxidation and microcrack initiation at the lamellar(Ti_3Al).
