13th International Conference on Fracture June 16–21, 2013, Beijing, China concentration and produces the occurrence of PSB in subsurface [13]. Then irreversible slip bands in the subsurface makes easier the fatigue crack initiation stage. Fatigue crack at subsurface at initiation stage is form by connecting subscale defaults, such as Micron Abreast Pipes in same slip plane, inharmonious of deformation at grain boundary or links between small facets of irreversible slip bands. Fig. 9. Fatigue crack at initiation area relative to PSB in material Regarding fatigue propagation threshold and temperature elevation [14,15], it may estimate following mechanism of fatigue initiation in ferrite at VHCF regime. PSB occurs on specimen surface at beginning stage of VHCF test. With following loading cycles, more PSBs formed in surface and subsurface, evidently at location of initiation area. In means material volume in a small initiation area is damaged by PSB but not opened by a crack. PSBs and other multiplied dislocation lines make more volume at initiation area extended and weak to undertake the long range stress. Therefore pronouncedly stress concentration takes place in front of area extended. Consequently extended area grown to the edge of initiation area which correspond to the fatigue stress intensity factor threshold ∆Kth=E√b (Herzberg-Paris-McClintock law) [16]. By sufficient stress concentration, the initiation area opened suddenly and remained some area in initiation cracked without plastic deformation. As blue arrows in Fig 7, between two slip planes, crack at stairs shows no plastic deformation on surface. The temperature recording [14] of VHCF test has shown that the crack initiation stage occurs at the end of VHCF test (less than 1% of the total life). More than 99% of the total life is takes up by the micro-damage due to forming PSBs in surface and subsurface, especially initiation area. The first fatigue crack appears on the specimen surface, the entire crack initiation zone appears supervening by connecting all defaults in the bulk material with in short time at end of VHCF. 4. Conclusions According to the results described above, the following conclusions may draw for ferrite material failed in VHCF:
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