High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

regime up to 10 7 cycles of loading. The fracture surfaces were observed by fleld emission scanning electron microscopy (FESEM). It was found that the size of inclusion has signiflcant efiect on the fatigue behavior. For AISI 4340 steel in which the inclusion size is smaller than 5.5 „m, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 „m, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 „m. In the case of internal inclusion-induced fractures at cycles beyond about 1£10 6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the flsh-eye and a granular bright facet (GBF) was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1£10 6 . Neither flsh-eye nor GBF was observed for Aermet 100 steel in the present study.