Low- and high-cycle fatigue properties were investigated for spring steels SUP7, 9A and 12, and tool steels SKD61 and 11 whose tensile strength ranged from 1300 to 2130 MPa. Surface fracture, where cracks initiated at the specimen surface, occurred at the low cycle region below 10
5 cycles, whereas internal fracture, where cracks initiated at the internal inclusions, occurred at the high cycle region above 10
5 cycles. An unique curve of σ
a/σ
yc vs. N
f was obtained for the surface fracture regardless of steel and strength level. Where σ
a and σ
yc are stress amplitude and cyclic yield strength, respectively. Using a crack propagation parameter of σ
a3a
i1/2, it was observed that fatigue life for Al-oxide, Ca-oxide and Cr-carbide inclusions in the descending order. Where a
i is inclusion diameter. This suggests that the internal fatigue properties are affected by species of inclusion as well as its size.
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