(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.