Fatigue tests via rotary bending were performed on the specimens with and without induced surface defects for a structural steel of medium carbon content to investigate the effect of surface defects on fatigue behavior in high-cycle fatigue and very-high-cycle fatigue (VHCF) regimes. The S-N data showed that induced surface defects substantially degraded the related fatigue strength. For the specimens without surface defects failed in VHCF regime, crack initiated from the interior of specimens with inclusions or matrix inhomogeneities as crack origin and the initiation regions were of different extents of rough surface. The observations on the profile samples cut from crack initiation regions revealed that the region was a layer of nanograins for the case of inclusion as crack origin, and was without grain refinement for the case of grain boundary as crack origin. For the specimens with induced surface defects, crack initiated from surface defects and the initiation region was without grain refinement. The characteristics of crack initiation were carefully examined and the effect of surface defects on fatigue strength degradation was analyzed by available models.
The authors would like to appreciate the financial supports from the National Basic Research Program of China (2012CB937500) and from the National Natural Science Foundation of China (11572325).