4.2 MTP Specimen LCF Dwell Test
The CF dwell tests at high temperature can be used to determine the
viscous parameters, i.e., visco-plastic resistance and visco-plastic
exponent within Chaboche UVP model, based on the Cottrel’s stress
partition method1,3. As discussed in Section 3.2.3,
only CF dwell test using the MTP-3 specimen at 600 °C is carried out and
compared with the corresponding SSFS CF dwell test1,
due to the weakest geometry constraint effect. Once again, as can be
seen from Fig.11a, the cyclic response of the MTP-3 specimen duplicates
very well with the behavior of the SSFS specimen under the equivalent
loading conditions. However, a mismatch can be observed in the latter
tension-loading stage probably due to slippage between the clamps and
specimens. Obvious significant stress relaxation with a dwell period of
2.5hours is observed in the MTP-3 specimen as expected. The
corresponding measured stress evolutions versus normalized time for both
MTP and SSFS tests at high temperature are presented in Fig.11b. Very
small difference of peak stresses can be seen at maximum tension and
compression loading points. During the dwell period at the same applied
mechanical strain of 0.7%, the stress relaxation in the MTP-3 test is
almost the same as that from SSFS test. Satisfactory comparison is
obtained for influence of hold time on peak stresses. Thus, the viscous
parameters required by UVP model can also be accurately assessed for
FV566 steel based on the stress relaxation data from MTP-3 CF testing at
high temperature.