13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- In order to clarify the creep-fatigue interaction, the da/dN vs. f data of the present alloy at four specified ΔK in log-log scale is plotted in Figure 2(a). For pure fatigue loading, according to Eq. (2), the crack growth rate showing no influence by f, therefore, ln(da/dN) vs. ln(f) curves should be parallel to the abscissa f at specified ΔK. And for pure creep crack growth, according to Eq. (3), the da/dN is proportional to th at certain stress level, hence, the gradients for ln(da/dN) vs. ln(f) curves should be -1. Thereby, the solid lines in Figure 2(a), calculated by Eq. (1), denote the simple linear superposition of the fatigue and creep components. The experimental data at four specified ΔK are also provided in the figure by various symbols. It can clearly be seen from that, first, the th effect on da/dN is nonlinear in the log-log scale. With the th increases, the effect is initially increased and then decreased, especially at low ΔK stage. Second, the experimental data in the medium ΔK and th are much greater than the solid lines. Comparisons between the fitted and experimental crack growth rates are presented in Figure 2(b). The results show good agreements at all four dwell times, ignoring slight deviation in the quite low ΔK and ln(f) stage. Thereby, interaction intensity factor η, as expressed in Eq.(5), can depict this interaction satisfactory. Comparisons between the fitted and experimental crack growth rates are presented in Figure 2(b). The results show good agreements at 0s, 90s, 450s and 1500s. Furthermore, to verify the predictive capability of the model, additional crack growth rate for th = 25s is computed by Eq. (4) and are compared with the experimental data, also shown in Figure 2(b). Excellent correlation between the predicted and observed growth rates was obtained. It appears that the model can provide accurate descriptions of the influence of th and ΔK on the crack growth behaviour at 750°C.
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