13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 10 0 10 1 10 2 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 Cs25-specimens R=0.1 P91, T=580°C ∆KI (MPa m 0.5 ) da/dN (mm/cyc) FCG, T=580°C, f=0.5Hz CFCG, T=580°C, HT=6min CFCG, T=580°C, HT=60min Paris for P91 from HIDA, T=565°C, f=0.1 Hz [9] 10 0 10 1 10 2 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 Cs25-specimens R=0.1 P91, T=600°C ∆KI (MPa m 0.5 ) da/dN (mm/cyc) FCG, T=600°C, f=0.5Hz CFCG, T=600°C, HT=6min CFCG, T=600°C, HT=60min Paris for 10CrMoWVNbN, T=600°C, f=0.05 Hz [8] upper scatter band of Paris for P91, T=600°C, f=0.05 Hz [7] Figure 5. Creep-fatigue crack growth rate over ∆KI for Cs25-specimens at a) T=580°C and b) T=600°C compared to literature data from [7-9] 3.4. Metallografic investigations In Fig. 6 and Fig. 7, the crack paths for selected specimens of different crack growth tests are shown. A comparison of CCG-tests and CFCG-tests with a short and long holding time of 6 min and 60 min and under fatigue loading shows the different mechanisms that play a significant role in the crack propagation process. Figure 6: a) Specimen after CCG-test at 600°C, b) detail view from a); c) Specimen after FCG-test at 600°C, d) detail view from c) A large amount of creep cavities is found on the prior austenite grain boundaries at the crack tip and along the crack path of a creep crack growth tested specimen (see Fig. 6a and b). Due to the strong oxidation of the crack surface, the crack cannot be clearly identified as inter-granular. Nevertheless, by comparing this specimen with the sample tested under pure fatigue loading (see Fig. 6 c and d), it is clear that the crack paths are different. This was already visible on the fracture surface. The surface of the specimen tested under fatigue loading was smooth, which suggests that the crack was a) b) Crack propagation direction a) c) b) d)
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