Effects of inclusion size and location on very-high-cycle fatigue behavior for high strength steels

Fatigue tests with rotary bending (52.5 Hz) and ultrasonic cycling (20 kHz) methods were performed, respectively on two high strength steels so as to investigate the effects of inclusion size and location on the fatigue behavior from low cycle fatigue to very-high-cycle fatigue (VHCF) regimes. Under rotary bending test, four fracture patterns were observed with respect to different locations of inclusion on fracture surface. The mean crack growth rate within FGA (fine granular area of crack origin) was estimated to give the value with the magnitude of 10–13 m/cycle for the specimens with the fracture pattern of “FGA+surface fisheye”. Under ultrasonic cycling test, the size effect of inclusions on VHCF was investigated with two groups of specimens. One is with the inclusion size between 20 and 60 μm and the other is with the inclusion size between 10 and 30 μm. The results show that large inclusions lowered the proportion of net FGA at given stress levels and led to the shortening of VHCF life. The degradation of VHCF strength caused by the increase of inclusion size is ascribed to the decrease of the critical stress of FGA formation for large inclusions.