Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue（VHCF） properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz.Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation.For smooth specimens,a fine granular area（FGA） is observed near the grain boundary in the fracture surface of the specimens broken after 10~7 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime,and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens.At last,an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.