(a) (b)
Figure 12. Effect of loading condition on stress triaxiality field at maximum load (P = Pmax ): (a) at Δa = 0mm and (b) at Δa = 2mm
The effect of the creep-fatigue loading condition on the stress triaxiality is shown in Fig. 12. At initiation (Δa = 0 mm), not much difference can be seen between three test cases, as shown in Fig. 12(a). However, at growth (Δa = 2 mm), the stress triaxiality of the tension-tension loading case (specimen 1) is the highest, and that of the tension-compression loading with the short hold time (specimen 5) is the lowest, as shown in Fig. 12(b). The effect of the hold time on the stress triaxiality is not so significant.
Takahashi [28] has reported from experimental data of Grade 91 steel that the predicted crack growth rates using theC* parameter were overly conservative for the tension-compression creep-fatigue case. This may be because that the low stress triaxiality induced by crack blunting under tension-compression loading condition can increase resistance for crack growth, compared to the cases of the tension-tension loading and pure creep condition. This will be discussed further in the next sub-section.