3. Results
3.1. Cyclic response of specimens subjected to TMF loading
During cyclic loading, all hysteresis loops were recorded and cyclic
hardening/ softening curves are reported. Hysteresis loops in four types
of thermomechanical cycling (IP-TMF, IPD-TMF and OP-TMF and OPD-TMF)
between 250 °C and 700 °C with total strain amplitude 0.6% shows Fig.
1. The first loop and the loop at half-life are shown. Since the dwell
was always applied at the maximum temperature for IPD-TMF cycle it
coincided with maximum tensile strain and for OPD-TFM cycle with maximum
compression strain. In the first cycle, the hysteresis loops with the
dwell do not significantly differ from those without dwell since the
amount of relaxation is small. With increasing the number of cycles
material cyclically hardens and the stress relaxation also becomes
larger. This leads to the increase of the plastic strain range and also
to the modification of the stress cycle asymmetry.
The evolution of the stress amplitude σa plotted
vs. the number of cycles N of the material subjected to the
OPD-TMF constant strain rate cycling shows Fig. 2. The cyclic response
is characterized by rapid cyclic hardening from the onset of the cyclic
straining. The pronounced cyclic hardening leads to the saturation of
the stress amplitude for all total strain amplitudes (Fig. 2a). The
domain of saturation takes more than 90% of the fatigue life. The final
very rapid decrease of the stress amplitude is connected with the
appearance of the dominant crack and the distortion of the shape of the
hysteresis loop. The plastic strain amplitude vs. the number of cycles
(Fig. 2b) decreases rapidly at the start of cycling. Later, at the
number of cycles corresponding to maximum stress amplitude, the slope of
the plot in semilogarithmic coordinates decreases. This decrease is for
the majority of the fatigue life very mild and for high strain
amplitudes plastic strain amplitude is nearly constant which corresponds
to the saturation of the stress amplitude (see Fig. 2a). The evolution
of the mean stress during OPD-TMF shows Fig. 2c. The initial slightly
positive mean stress increases during the fatigue life up to 90 MPa. The
rapid drop of the mean stress in the final stage of cycling is connected
with the appearance of macroscopic crack.