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.