On the formation of GBF of high-strength steels in the very high cycle fatigue regime

Very high cycle fatigue tests were conducted for six high-strength steels by using a 20-kHz ultrasonic fatigue testing machine. Most tests were conducted on one stress amplitude until 109 cycles; however, for further examination of the formation of the granular-bright-facet (GBF) on the fracture surface, a two-step loading test for commercial 60Si2Mn specimens was adopted. The failure of almost all specimens is caused by internal non-metallic inclusion. Experimental results show that the stress intensity factor at the edge of GBF on the fracture surface, KGBF, is not a constant but proportional to (areaGBF)1/3. It means that KGBF does not correspond to the threshold value of a long crack but the threshold value of a small crack. The stress intensity factor, kH, caused by hydrogen plays an important role during formation of GBF.