13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- crack growth can be recognize. For a holdtime of 60 min it is obviously that the behaviour is time dependent. At 600°C and both holding times, it is apparent that the rule describes the crack growth behaviour of creep-fatigue specimens quite well. Fig. 9e shows the dependence on the crack growth rate at 600°C on the holding time for a constant value of ∆K=27.2 MPam0.5. In this case, the cycle/time dependent transition for this material can be expected at holding times greater than 0.1 minutes. 10 1 10 2 10-5 10-4 10-3 10-2 10-1 10 0 P91, T=580°C ∆KI (MPa m 0.5 ) da/dN (mm/cyc) Cs25-specimens R=0.1, f=0.5 Hz Fatigue portion Creep portion, HT=6 min Experiment CFCG, HT=6 min Predicted CFCG, HT=6 min 10 1 10 2 10-5 10-4 10-3 10-2 10-1 10 0 P91, T=600°C ∆KI (MPa m 0.5 ) da/dN (mm/cyc) Cs25-specimens R=0.1, f=0.5 Hz Fatigue portion Creep portion, HT=6 min Experiment CFCG, HT=6 min Predicted CFCG, HT=6 min 10 1 10 2 10-5 10-4 10-3 10-2 10-1 10 0 P91, T=580°C ∆KI (MPa m 0.5 ) da/dN (mm/cyc) Cs25-specimens R=0.1, f=0.5 Hz Fatigue portion Creep portion, HT=60 min Experiment CFCG, HT=60 min Predicted CFCG, HT=60 min 10 1 10 2 10-5 10-4 10-3 10-2 10-1 10 0 P91, T=600°C ∆KI (MPa m 0.5 ) da/dN (mm/cyc) Cs25-specimens R=0.1, f=0.5 Hz Fatigue portion Creep portion, HT=60 min Experiment CFCG, HT=60 min Predicted CFCG, HT=60 min e) 10-5 10-4 10-3 10-2 10-1 10 0 10 1 10 2 10-4 10-3 10-2 10-1 P91, T=600°C HT (min) da/dN (mm/cyc) Cs25-specimens ∆K=27,2MPam 0,5 Experiment Predicted Figure 9. Creep-fatigue crack growth rate over ∆KI for Cs25-specimens at a)&b)T=580°C, c)&d) T=600°C, e) Creep-fatigue crack growth rate over holding time for ∆KI =27.2 MPam0.5 at 600°C 4. Summary The results of investigations on P91-steel show, that the creep crack initiation behaviour can be described by the parameter C* and by the stress intensity factor KI. Due to the high creep ductility of P91 the C*-parameter is preferable. An effect of holding time (HT) on the creep crack initiation time is observed, whereby the influence of 6 min HT for both test temperatures is greatest. If the holding time is 60 min the difference between the creep crack growth test and creep-fatigue crack growth test is small, especially at 600°C. At 580°C the influence of short and long holding time on the crack initiation time for two creep-fatigue experiments is comparable. This means that a reduction in time is expected due to the cyclic stress for the crack initiation in the investigated range. The crack propagation behaviour of P91-steel under creep conditions can be described both by the parameter C* and the stress intensity factor KI. The Paris-law describes fatigue crack propagation. An influence of the temperature on crack propagation under fatigue loading can be observed, crack growth rate increases with increasing temperature. By comparing CFCG- and CCG-experimental a) b) c) d)
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