4. Discussion
4.1. Mechanical response
The effect of dwells on the cyclic stress-strain response in the
thermomechanical cycling of Sanicro 25 steel can be best demonstrated by
comparing the present results with previous results performed without
the dwell. The change of the shape of the loop and the tendency of the
change in the stress amplitude are illustrated by Fig. 1. The
introduction of the dwell both in TMF-IPD and TMF-OPD cycling results in
stress and also plastic strain relaxation at the highest temperature in
a cycle and plastic strain amplitude thus increases. Moreover mean
stress increases in OPD-TMF cycling and decreases in IPD-TMF cycling25, while it was approximately zero in cycling without
dwells.
The effect of the dwells on the cyclic hardening/softening curves is
apparent from the plot of the stress amplitude vs. the number of cycles
for cycling with dwells and without in Fig. 16 for total strain
amplitude 6x10-3. Introduction of dwell always leads
to more rapid cyclic hardening; however, the peak stress amplitude
during the test is smaller than in constant strain rate cycling.
Pronounced saturation is found in TMF-OPD cycling. Both in IP-TMF
cycling and in IPD-TMF cycling, the early initiation and growth of the
crack prevent stress amplitude saturation since specimens are fractured.
Therefore the stress amplitudes in half-life which are used for the
construction of the cyclic stress-strain curves in IP-TMF and IPD-TMF
cycling are much lower than in OP-TMF and OPD TMF cycling. The cyclic
stress-strain curves are shown in Fig. 3. The introduction of the dwell
led to the decrease of the cyclic-stress strain curves but the drop is
much more important in OPD-TMF than in IPD-TMF cycling. It is in
agreement with lower dislocation density in specimens cycled with dwells
in a cycle (Fig. 4). It also shows that the dislocation structure
produced during the cycle is reduced more effectively in OPD-TMF cycle
than in IPD-TMF cycle.
Introduction of the dwells results in the increase of the plastic strain
amplitude in a cycle and in appreciable cycle asymmetry. In spite of
negative mean stress built up in IPD-TMF cycling fatigue life decreases
due to dwells in both types of cycling.