As known, fatigue crack initiation mainly starts at surface defects at high stress or strain amplitudes, but shifts to pre-existing subsurface defects such as inclusions or pores at low stress amplitudes or in the very high cycle fatigue regime [
]. Recently, it was reported that fatigue crack initiation could occur in non-defect existing areas and form a subsurface non-defect fatigue crack origin (SNDFCO) [
]. Figure 1 shows a summary of these three phenomena.