Fig. 4 The variation of endurance life with different (a) unloading rates and (b) durations under valley stress.
The elongation of all fractured specimens is analysed in Fig. 5. Cyclic creep loading leads to a slight drop in ductility as compared with static creep, but the difference in ductility under cyclic creep seems to have little influence on endurance life. Research results have shown that the fracture mode transforms from brittle type to ductile one and gradually approaches to static creep failure with the increase of duration under peak stress.11 A quite long peak stress duration of 60 min has been imposed under all cyclic creep tests in present work, which means the fracture mode is most likely to be ductile, like static creep.