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.