13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Figure 6. Creep crack growth rate as a function of 1/T (log scale). The slope giving the activation energy is indicated. 5. Conclusions 1. This study has confirmed that Inco 718 alloy exhibits a strong dependence to hold time, tm , effect when tested at elevated temperature (500 °C – 650 °C) in air environment with fatigue cycles including long hold times (< 1 h). These results are in good agreement with other results published in the literature. The tm dependence is related to a combination of creep strain accumulation ahead of the crack tip and detrimental effect of oxidation. 2. A simple linear damage accumulation law has been proposed to account for this hold time effect. This law includes three terms: (i) a pure fatigue component, (ii) an oxidation component (Regime A), and (iii) a pure creep component (Regime B). 3. The oxidation component (da/dN)ox leads to an acceleration in fatigue crack growth rate proportional to (tm) α , where α is close to 0.25. This component prevails for hold times, t m lower than a critical transition time, ti , which is a decreasing function of temperature. 4. The pure creep component (da/dt)cr gives rise to a crack growth rate proportional to hold time, tm. It is better to determine this component using cyclic tests with hold times, similar to those of the present study, than using fatigue precracked specimens submitted to a steady load. 5. An attempt has been made to model the time dependent regimes A and B. It has been shown that Regime A can be interpreted as resulting either from the kinetics of oxygen diffusion or from creep damage accumulation. Both explanations are more complementary than contradictory. An estimate of the transition time ti between regimes A and B has been given. Regime B which occurs for very long hold times has been interpreted as a pure creep crack growth. It has been shown that the crack growth rate in regime B is an increasing function of hold time with apparent
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