13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 10 0 10 1 10 2 10 3 10 4 10 5 10-5 10-4 10-3 10-2 10-1 10 0 10 1 CCG; T=600°C [7] CFCG; T=600°C, HT=6 min [7] CFCG, T=600°C, HT=60 min [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 C* (N/(mmh)) t i (h) Cs25-specimens a 0 /W = 0.55 - 0.58 ∆ai = 0.5 mm P91, T=580°C & 600°C Figure 3. C* parameter over time for Cs25-specimens (T=580°C & 600°C) under creep-fatigue loading compared to P91-steel from [7] 3.2. Crack propagation under creep-fatigue loading The comparison of CFCG- and CCG-experimental results shows (see Fig. 4 a and Fig. 4 b) that the specimens with a holding time of 6 min and 60 min have similar crack growth rates as samples without holding time, regardless of which of the two parameters describing the crack behaviour is used. No influence of holding time on crack initiation as described in the last section is observed for crack propagation. Thus, the cyclic loading has only an influence on the crack behaviour in the initial phase. In the holding time period the creep loading is dominant. Metallographic studies confirmed this observation so far (see next section). 10 0 10 1 10 2 10-6 10-5 10-4 10-3 10-2 10-1 10 0 10 1 CFCG, T=600°C, HT=6min [7] CFCG, T=600°C, HT=60min [7] CCG, T=580°C CFCG, T=580°C, HT=6min CFCG, T=580°C, HT=60min P91, T=580°C & 600°C KI (MPa m 0.5 ) da/dt (mm/h) Cs25-specimens 0.5mm < ∆a < 3.0 mm CCG, T=600°C CFCG, T=600°C, HT=6min CFCG, T=600°C, HT=60min 10-4 10-3 10-2 10-1 10 0 10 1 10-5 10-4 10-3 10-2 10-1 10 0 CFCG, T=580°C, HT=6min CFCG, T=580°C, HT=60min CFCG, T=600°C, HT=6min CFCG, T=600°C, HT=60min CCG, T=580°C CCG, T=600°C CCG, P91, T=600°C from [7] CCG upper scatter band for 10CrMoWVNbN, T=600°C [8] CCG scatter band for 9Cr piping steels,CT25, T=538°C&593°C [9] C* 2 (N/mmh) da/dt (mm/h) P91, T=580°C & 600°C Cs25-specimens 0.5mm < ∆a < 3.0 mm Figure 4. Creep-fatigue crack growth rate over a) stress intensity factor KI and b) parameter C* for Cs25-specimens (T=580°C & 600°C) compared to literature data from [7-9] In Fig.5 the results of fatigue crack growth tests at 580°C and 600°C are reported. The influence of temperature on the fatigue crack growth behaviour can be seen. The fastest crack growth rates are observed at 600°C. The results obtained in this work are similar to literature data from [9] for P91 at 565°C and a frequency f of 0.1 Hz. One sample at 600°C is slightly above the Paris-law curve, which can be explained by the different test temperature. It is also evident that results of CFCG-tests at 580°C and 600°C cannot be described by the Paris-law. The cyclic crack-growth rate increases if a hold time is imposed at maximum load. As the hold time increases, a change from cycle dependent to time dependent behaviour can be observed. a) b)
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