13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- larger, following with increasing electric resistance, and the electric resistance change ratio transits from a relatively flat stage to an acceleration stage, which is called stable fatigue crack propagation stage (stage II). With increasing number of loading cycles, the fatigue propagation moves from a stable stage to an acceleration stage (stage III), and finally leads to fracture failure. In this stage, the damage change rates very large, and the corresponding electric resistance change ratios are also large, as shown in the last parts of fatigue damage evolution curves. 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.1 0.2 0.3 0.4 (a) Resistance change ratio ΔR/R Nondimensionalized fatigue cycles N/Nf σa=320MPa 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.05 0.10 0.15 0.20 0.25 (b) Resistance change ratio ΔR/R Nondimensionalized fatigue cycles N/Nf σa=400MPa 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.05 0.10 0.15 0.20 0.25 (c) Nondimensionalized fatigue cycles N/Nf Resistance change ratio ΔR/R σa=432MPa 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.03 0.06 0.09 0.12 0.15 0.18 (d) Nondimensionalized fatigue cycles N/Nf Resistance change ratio ΔR/R σa=462MPa Figure 4. Fatigue damage evolution curves at various stress amplitudes As aforementioned above, when the stress amplitude σa increases from 320MPa to 462MPa, the value of parameter 1+ β+ β1( σa) increases from 26.543 to 46.882. However, the chang rate of electric resistance change ratio is reversed with increasing stress amplitude, which indicates that when the stress amplitude becomes higher, to achieve the same degree of damage as that at lower one, the proportion of consumed fatigue life over the total fatigue life will increase. That is, in the fatigue tests, as the stress amplitude increases, the proportion of fatigue life in the stage II and stage III of crack propagation will decrease, and the total fatigue life is mainly consumed in the stage I of fatigue crack initiation and small crack propagation. Therefore, the change rate of electric resistance change ratio is also relatively slow. 4. Fatigue crack propagation behavior and fractography reverse model 4.1. Fatigue crack propagation tests The fatigue crack propagation tests for standard compact tension samples of GH4133B superalloy are performed on the CSS-280S-20 test machine. Under cyclic loading conditions (6Hz sine wave, stress ratio R=0.02, 0.1, 0.2, 0.4), the fatigue crack propagation tests are carried out. According to the different values of stress ratio, the tests are categorized as four groups, and two samples prepare
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