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.