5. Discussions and Future Work
In this work, a special clamping feature was designed and manufactured, which is capable to produce a rigid displacement transfer and ensure excellent alignment for the MTP specimen in order to minimize the detrimental effects of bending-torsional on the high temperature LCF testing results. To demonstrate the testing technique, good comparisons were achieved regarding the tensile testing between the non-standard MTP and conventional specimens at 20 °C and 600 °C, respectively.
Since the very small-sized specimen used in this work, the uniform uniaxial strain deformation is hard to directly be measured from the parallel gauge length region. Thus, scaling factor, which can be adopt to realize the correct transferability and correlation of LCF testing data between conventional standardized and non-standard MTP specimens, is proposed based on the reference strain approach. The UVP FE model coupled with the identified high temperature LCF material properties for FV566 is employed to determine the scaling factor under the required mechanical deformation. The equivalent energy principal is utilized to evaluate the geometry constraint effect that non-standard MTP specimen has. It was found that the scaling factor is more dependent on the geometry, for example, the ratio ofl0 /d0 in this study, which also in turn can be utilized to evaluate the geometry constrain effect.
The feasibility of the non-standard MTP specimen LCF testing technique developed has been further checked via high temperature fully reversed saw-tooth test and CF dwell-type LCF tests for FV566 turbine rotor steel at 600 °C. Good agreement between MTP and SSFS tests is not only achieved for description of the first cycle, but also for the cyclic softening. The experimental results also indicate that fatigue life of MTP specimen is very sensitive to the geometry constraint effect. The thin-plate specimen with lower scaling factor can provide the closest fatigue life comparing with that of SSFS test, for example, MTP-3 saw-tooth test in this work. Additionally, the MTP LCF test results have demonstrated that the methodology developed can be used to represent the high temperature cyclic response of SSFS test, which can be further adopt to identify the elastic, kinematic and viscous parameters required by Chaboche type high temperature UVP model.
High temperature LCF testing approach for MTP specimen have been discussed in this work, but many issues remaining are still required to be sorted out with more experimental testing. For example, the reasons for lower fatigue life in the MTP specimen are not apparent. Further investigations should be carried out to continuously improve the testing rigs, such as the new design for the clamp fixtures to reduce slippage, new design of MTP specimen with higher ratio ofl0 /d0 to more accurately capture the total fatigue life of SSFS specimen and obtain more accurate isotropic parameters at high temperature. The surface roughness conditions are particularly critical for the MTP specimen because the effect of surface finishing on miniature LCF life is expected to be more significant than on SSFS specimen. Moreover, more metallographic investigations are needed to identify the cause of the failure mechanism and to improve results at high temperature as well. For example, MTP specimen at high temperature is prone to have oxidation damage, leading to premature fatigue crack initiation and the reduction of fatigue life. Although the possible trends for the size effect have been introduced in the refs65,66, available data are still rather scare, especially for size effect at high temperature. More investigation should be carried out in this scope as well in the future.