Propensities of crack interior initiation and early growth for very-high-cycle fatigue of high strength steels

• A characteristic dimension corresponds to threshold size of crack growth for VHCF.
• The value of ΔKFGA is between 4 and 6 MPa m1/2 for high strength steels.
• Crack initiation life of FGA is larger than 99% for Nf beyond 5 × 107 cycles.
• Cracking rate of FGA is between 10−12 and 10−13 m/cycle for Nf from 107 to 4 × 108.

Fatigue tests of a high carbon chromium steel were performed using rotating bending and ultrasonic axial cycling. Fatigue crack initiated at specimen interior for very-high-cycle fatigue (VHCF) with fish-eye pattern embracing fine-granular-area (FGA) originated from inclusion. The fatigue life from FGA to fish-eye and from fish-eye to the critical crack size was respectively calculated, so as to estimate the fatigue life contributed by FGA. The crack extension rate within FGA was also estimated. Our results demonstrated that the formation of FGA is responsible for a majority part of total fatigue life.