Fatigue tests of forged low-alloy steels, two kinds, of KSFA110 (NK standards, σB=1, 080MPa class), with various surface conditions were carried out to clarify the fatigue properties in the ultra-high-cycle regime over 10
8 cycles of repeated stress. Interior fractures occurred in the ultra-high-cycle regime, which were affected by the residual stresses. Fatigue crack propagation lives of the interior fractures were calculated assuming that the crack propagation life is predominant in the fatigue life. Parameters for the fatigue crack propagation law of the interior fracture were estimated as m=10.4 and
C=2.33 x 10
-18. Threshold stress intensity factor range of the interior fatigue crack propagation depended on the initial crack size.
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