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.