2001 年 2001 巻 46 号 p. 19-25
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 105 cycles, whereas internal fracture, where cracks initiated at the internal inclusions, occurred at the high cycle region above 105 cycles. An unique curve of σa/σyc vs. Nf was obtained for the surface fracture regardless of steel and strength level. Where σa and σyc are stress amplitude and cyclic yield strength, respectively. Using a crack propagation parameter of σa3ai1/2, it was observed that fatigue life for Al-oxide, Ca-oxide and Cr-carbide inclusions in the descending order. Where ai is inclusion diameter. This suggests that the internal fatigue properties are affected by species of inclusion as well as its size.