13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- etched. Constant amplitude fatigue tests using triangular waveform with a load ratio of 0.1were performed on a loading stage equiped in the SEM in vacuum environment (9×10-4−9×10-3 Pa). The frequency was as low as 0.03 Hz. The various values of ΔKI/ΔKII could be achieved by changing the external loading angle α. As shown in Fig. 1b, the line of application of force F extends at an angle α to the longitudinal axis of the specimen. Depending on the size of angle α, mixed mode loadings occur at 0< α<90°. It is obvious that pure mode I loading corresponds to the α equaling to zero, whereas the α at 90° stands for the pure mode II loading. After the crack extended for a given length (less than 10 μm), the experiments were intermitted to measure crack length, record the cyclic numbers and characterize the crack morphology as well. For various loading angles, the initial mixed mode fatigue crack growth parameters, which was calculated by the Eqs. (1) - (3) developed by Richard [17], are listed in Table.1. In Eq. (3), the material parameter α1 is set to 1.155. I 2 0.26 2.65 cos 1 / 1 0.55 0.08 a F a W a K WB a W a a W a W a π α + ⋅ − = ⋅ ⋅ − ⎛ ⎞ + ⋅ − ⋅ ⎜ ⎟ − ⎝ − ⎠ (1) II 2 0.23 1.40 sin 1 / 1 0.67 2.08 a F a W a K WB a W a a W a W a π α − + ⋅ − = ⋅ ⋅ − ⎛ ⎞ − ⋅ + ⋅ ⎜ ⎟ − ⎝ − ⎠ (2) ( )2 2 I eff I 1 II 1 4 2 2 K K K Kα Δ = + + (3) Table 1. Initial mixed mode FCG parameters in the FQTZ of the HAZ α a0/W ΔKI (MPa⋅m1/2) ΔKII (MPa⋅m1/2) ΔKI/ΔKeff 0 0.5451 35.27 0 1 30° 0.5449 30.46 7.95 0.92 60° 0.5476 17.75 13.82 0.65 3. Results 3.1. a-N relationship Fig. 2 shows the changes of crack extension as a function of cyclic numbers. It is observed that the crack extends slowly first and then become faster at all of the loading angles. However, with the decreasing of ΔKI/ΔKeff, which meant the reduced contribution of mode I loading, the crack extension was also decreased. Nevertheless, due to the proximity of the ΔKI/ΔKeff values at α of zero and 30°, the corresponding crack extension values were also close and even converged to each other eventually. This indicated that the load mixity dependence of crack growth behavior located mainly in the initial crack growth process. 3.2. Fatigue crack growth rate da/dN Fig. 3 indicates the FCG rate da/dN at various loading angles. It is worth noting that the da/dN at α of zero and 30° are similar to each other but larger than those of the α of 60°. It is also worth noting that the evolution of da/dN for pure mode I loading fluctuates with the crack length, i.e., the delay and acceleration phases alternates. This reflected the influence of local microstructures. Whereas for
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