13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- key factor influencing fatigue crack initiation between surface inclusion and interior inclusion. Fig. 5. Typical examples of fracture surface morphology for fatigue crack initiation. (a) interior grains boundary initiation for RL150, (b) interior grains boundary initiation with FGA for VHCF of RL150, (c) fisheye pattern with FGA for VHCF of RL180, and (d) competition between surface inclusion initiation and interior inclusion initiation. Effects of microstructures on VHCF crack initiation are shown in Table 2. On one hand, VHCF crack tends to originate from interior grain boundary at low tempering temperature for RL150, while interior inclusion is prone to be the crack origin when the tempering temperature is up to 180℃ and 300℃. On the other hand, values of stress intensity factor of FGA: ΔKFGA fluctuate around a constant for each of specimen group. ΔKFGA reaches its highest value for RL180, and the lowest value for RL300. Retained austenite content is responsible for the change of ΔKFGA, for the threshold value of crack growth ΔKth varies with the volume fraction of retained austenite[14]. Table 2. Statistical Comparison of VHCF crack initiation for the three specimen groups ΔKFGA (MPa*m0.5) ΔKInc (MPa*m0.5) Inclusion Grain boundary Mean value Standard deviation Mean value Standard deviation RL150 11% (1/9) 89% (8/9) 5.29 0.1 2.54 0.28 RL180 100% (9/9) 0 (0/9) 5.52 0.58 4.27 0.39 RL300 73% (8/11) 27% (3/11) 4.31 0.36 3.83 0.51 5. Microstructure sensitivity analysis As we know, FGA consumes more than 90 percent of VHCF life[13], thus it is reasonable to take the fatigue life of FGA formation as the whole fatigue life. Based on the conclusion that FGA is finished when the plastic zone size rp of the microcrack exceeds the low boundary width lm of the martensite[17], this paper considers that the martensite is the key factor for the FGA formation for 100μm 20μm 150#2#, 1115MPa, 1.33×108 150#12#, 970MPa, 1.81×108 (b) 50μm 29μm (d) (a) 180#3#, 808MPa, 1.79×107 180#12#, 875MPa, 1.62×106 (c) 11μm
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