Fatigue crack growth from small to long cracks in very-high-cycle fatigue with surface and internal "fish-eye" failures for ferrite-perlitic low carbon steel SAE 8620

In recent years, the importance of knowing the materials behavior under very-high-cycle fatigue has been pointed out by many research laboratories around the world. An important failure phenomenon has been seen between 10^6 and 10^8 cycles, where the failure initiation may switch location from the specimen surface to an interior "fish-eye". The Paris-Hertzberg-McClintock crack growth rate law, da/dN=b((delta)Keff / E(square root(b))^3, will demonstrate that crack growth is not of significant portion of total life if over 107 cycles for a low carbon steel SAE 8620, with ferrite-pearlite microstructure. In this discussion, the presence of closure will be explored for a range of transition sizes from small to long crack and various load ratios. Cracks initiating at both surface and internal sites will also be considered. Finally, the stress intensity factor at final failure for internal failure will be estimated to see its impact.