Fig. 2 shows the curves of mean strain (em) vs. cyclic time with
different dwell times and different maximum tensile stresses. For
comparison, the curves of conventional fatigue and creep at
rmax = 527.4 MPa and 468.8 MPa are also presented. From Fig. 2,
the mean strains at different maximum tensile stresses exhibit
three different growth stages with the increase of cyclic time, i.e.
a decelerating growth stage (primary stage), a steady-state growth
stage (secondary stage) and an accelerating growth stage (third
stage). The variation of mean strains shows characteristics similar
to that of the typical creep, indicating that the creep deformation
has a significant contribution to the creep-fatigue deformation.
At rmax = 527.4 MPa, see Fig. 2a, the curves of creep-fatigue with
varying dwell times from Dt = 10–60 s all appear between the conventional
fatigue and creep ones. Moreover, with the increase of
dwell time the mean strain increases, and the fracture time
decreases at the same time. As the maximum tensile stress
decreases to 468.8 MPa and 439.5 MPa, the variations of mean
strain and fracture time with the dwell time exhibit the same trend
with that at rmax = 527 MPa, see Fig. 2b and c. For the dwell time of
60 s, the creep-fatigue curves (numbered as 6) almost coincide
with the creep curves (numbered as 7) during primary and secondary
stages, indicating that the creep deformation has become
the main deformation mode in this case.