13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 3. Results and Discussion 3.1. Crack initiation under creep-fatigue loading The results of creep-fatigue crack growth tests as a function of stress intensity factor and initiation time are shown in Fig. 2 a compared to the creep crack growth tests and in Fig. 2 b compared to fatigue crack growth tests. In addition, the literature data from [7] are shown there. An effect of holding time (HT) on the creep crack initiation time for the technical initiation criterion (∆ai=0.5 mm) is observed, whereby the influence of HT of 6 min for both test temperatures is greatest. For lower loaded sample at 600°C (HT=6 min), the influence of the holding period on the initiation time is significantly lower than for the higher loaded specimen. This indicates the time-dependence during the holding period and the creep process seems to be dominant. If the holding time is one hour, the difference between the results of creep crack growth test and creep-fatigue crack growth test is small, especially at 600°C. The differences between the two specimens with short and long holding time at 580°C are not significant. This means that a reduction in time is expected due to cyclic stress for crack initiation. Literature data from [7] show no influence of the holding time on crack initiation time, so that these results can be described independently of the holding time by the creep curve in the investigated area (see Fig. 2 a). Fig. 2 b shows the dependency of the cyclic stress intensity factor as a function of the number of cycles to crack initiation at ∆ai = 0.5 mm. It can be seen that the creep-fatigue specimens with short holding times are closer to the experiments under pure cyclic loading. 10 0 10 1 10 2 10 3 10 4 10 5 1 2 4 6 8 10 20 40 60 80 100 CCG mean curve P91, T=600°C from [7] CCG; T=580°C CCG, T=600°C CFCG; T=580°C, HT=6 min CFCG; T=580°C, HT=60 min CFCG; T=600°C, HT=6 min CFCG, T=600°C, HT=60 min KI0 (MPa m 0.5 ) t i (h) Cs25-specimens a 0 /W = 0.55 - 0.58 ∆ai = 0.5 mm P91, T=580°C & 600°C CFCG; T=600°C, HT=6 min [7] CFCG, T=600°C, HT=60 min [7] 10 1 10 2 10 3 10 4 10 5 1 2 4 6 8 10 20 40 60 80 100 FCG, T=600°C, f=0,05 Hz from [7] CFCG; T=600°C, HT=6 min [7] CFCG, T=600°C, HT=60 min [7] ∆KI0 (MPa m 0.5 ) Ni (cycles) Cs25-specimens a 0 /W = 0.52 - 0.58 ∆Ni = 0.5 mm P91, T=580°C & 600°C FCG, T=580°C FCG, T=600°C CFCG, T=580°C, HT=6min CFCG, T=580°C, HT=60min CFCG, T=600°C, HT=6min CFCG, T=600°C, HT=60min Figure 2. a) Stress intensity factor KI0 over time for Cs25-specimens (T=580°C & 600°C) under creep-fatigue loading compared to P91-steel mean curve for creep at 600°C from [7]; b) Cyclic stress intensity factor ∆KI0 over cycles for Cs25-specimens (T=580°C & 600°C) under creep-fatigue loading compared to the results under fatigue loading at 580°C and 600°C In Fig. 3 the parameter C* is shown depending on the time to the crack initiation (determined at ∆a=0.5 mm) under creep-fatigue condition. As expected, the creep-fatigue samples with longer holding periods are closer to the curve of creep crack growth tests. a) b)
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