Abstract
Fatigue tests of forged low-alloy steel, KSFA110 (NK standards, σ_B=1,080MPa class), with various surface conditions were carried out to clarify fatigue property in the ultra-high-cycle regime over (10)^8 cycles of repeated stress. Two-fold S-N curves were observed and they were remarkably affected by the existence of residual stresses. Fatigue crack propagation life was calculated assuming that the crack propagation life is predominant in the fatigue life. Calculated parameters of fatigue crack propagation low for interior fracture were m=7.1 and C=7.87×(10)^<-19>. Threshold stress intensity factor range of propagation depended on the initial crack size.
