13th International Conference on Fracture June 16–21, 2013, Beijing, China life. All the solid lines in figures are fitted by a power fitting function, f(N)=aNb. 100 1000 10000 0.5 1.0 1.5 2.0 2.5 3.0 980 o C,DZ125,L R=-1 R=0 Strain range (%) Cycles to failure (Cycles) Fig. 5 The effect of stress ratio on LCF -0.010 -0.005 0.000 0.005 0.010 0.015 -600 -400 -200 0 200 400 600 Experimental tensile stress(MPa) Experimental tensile strain DZ125,980 o C,L 1.6% R=0 R=-1 Fig. 6 The strain-stress curves LCF results at 980℃ with different strain ratio are shown in Fig. 5. It is interesting to found that little difference could be found at short life region, on the contrary, better fatigue life are obtained with strain ratio Rε=0 at long life region where obvious mean stress existed. The potential reason for the slight difference may be due to the similar area of steady-state hysteresis loop at 980℃ shown in Fig. 6, which also exhibits similar mean stress. 2.2 Effect of dwell types and dwell times on LCF 2.2.1 Dwell types LCF life with and without dwell times at 980℃ are shown in Fig. 7. Compared with continuous fatigue, all the three dwell types with the same dwell times lead to obvious life degradation. Roughly, no obvious difference of these three dwell types could be found except that the balanced dwell type gets the entirely shortest life, the bigger area of simulated hysteresis loop of balanced dwell type 30/30 in second paragraph is much more big than the other ones, which maybe the potential reasons; the tensile dwell type with reduced mean stress shows predominant fatigue property at low strain range but bad fatigue resistance at high strain range, the possible reason is the competition between mean stress related fatigue damage and creep damage. 2.2.2 Dwell times Fig. 8 shows the evolution curves of cyclic peak stress with different dwell times at 850℃. As the mentioned cyclic stress response before, cyclic hardening behavior could be observed with no dwell times, but it turns to be cyclic softening behavior with dwell times. Especially the relaxation of cyclic peak stress is also associated with dwell times. Firstly, with the increased dwell times from 0s to 60s, increased stress relaxation is obtained. However, with the increased dwell times reach up to 120s and 300s, similar relaxation curves are exhibited. Compared with the fatigue life with no dwell times, the introduction of increased dwell times leads to an obvious gradually life degradation with strain ratio equals to -1 at both 850 and 980℃ shown in Fig. 9, except for the abnormal fatigue life with 300s dwells at 980℃; however, when Rε=0, no obvious fatigue life degradation could be distinguished at 850℃ but not for 980℃. The observed dwell-times-related cyclic
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