A Review of Thermo-Mechanical Fatigue ... - TMF-Workshop

A Review of Thermo-Mechanical
Fatigue behaviour in
Polycrystalline Nickel Superalloys
Dr M. Whittaker, Dr R. Lancaster
12 th May 2011
Rolls-Royce University Technology Centre in Materials

Thermo-mechanical fatigue at Swansea UTP
Induction heated, forced air cooled
tension/torsion (100kN/400Nm) ESH
servo-hydraulic test machine.
Controlled by IMPAC pyrometry
Previous investigations on IN718
Focus on Nickel Superalloys for disc
applications
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Rationale for investigation of PX Ni superalloys
Increased turbine entry temperatures (TET) in conjunction with thinner
disc rims means that TMF at disc rims requires consideration.
Development of lifing methodologies under TMF loading is critical for safe
operation of gas turbine engines.
Extrapolation of isothermal fatigue (IF) results shows limited success.
Loading conditions within gas turbine lead to complex loading regimes –
Need to investigate the effect of phase angle.
Complex interaction of phase angle, peak temperature, strain range,
mean stress and environmental/creep damage.
Rolls-Royce University Technology Centre in Materials

RR1000 test material (Coarse grain)
50μm
However, the TMF properties of
the alloy have not yet been widely
examined.
RR1000 is a heavily γ’ stabilised
nickel alloy which is expected to
be widely used in high
temperature applications in the
coming years.
20μm
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Isothermal Fatigue
• Specimen subjected to an alternating mechanical load under a
stabilised temperature
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Thermo-Mechanical Fatigue
• Specimen simultaneously subjected to alternating mechanical AND
thermal loads
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In-Phase Test Out-of-Phase Test

Investigation of phase angle effects
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• Tests keep a
constant strain
range of 1%
• R ratio and mean
strain vary from test
to test
• Investigate the
effects of direction
around the cycle

A model for qualitative TMF prediction
Rolls-Royce University Technology Centre in Materials
For intermediate temperatures
the model seems to provide an
useful qualitative approach for
rationalizing TMF data.
Model is essentially mean stress
dependent, in that at these
temperatures (300-700⁰C) ⁰ fatigue
life is inversely proportional to
mean stress.
Crossover occurs near slightly
above yield strain, influenced by
the effect of reverse yielding.
At higher temperatures the influence of environment/creep seems to
reduce the fatigue life of IP specimens.

Mean stress effect on fatigue life in RR1000
Stress (MPa)
Cycles
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Max Stress stress 4 IP IP
Min Stress stress 4 IP IP
Max Stress stress 5 OOP
Min stress OOP
Min Stress 5 OOP

Reviewing existing PX Ni superalloy data
Evans et al (2005) – IN718, 300-680⁰C
Rolls-Royce University Technology Centre in Materials
Yield in IN718 occurs at
approximately 0.4% strain.
Large amount of plasticity in
these tests and reverse yielding.
IP softens on tensile portion
and OOP softens on
compressive portion
Resultant tensile mean stress
in OOP (-∞) tests, compressive
mean stress in IP (R=0) tests.
Therefore reduced fatigue
lives in OOP tests.

Effects at increased peak temperatures
Marchionni et al (2007) – Nimonic 90,
300-680⁰C
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Initially difficult to rationalize
• Testing conducted under R = -1
loading conditions
• Would potentially expect to see
a crossover in IP/OOP results.
• However the expected shorter
response for OOP tests at higher
strains does not occur.
• Early cracking and life
dominated by fatigue crack
propagation.

Stress response of Nimonic 90 (IP and OOP tests)
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• Based on mean stresses
it would be expected that
the IP tests should show a
longer fatigue life.
• Fatigue crack
propagation must
dominate the fatigue life
• Infers that crack
propagation at 350-400MPa
at 850C is faster than at
700MPa at 400C.

Effect of peak temperature on IP/OOP ordering
Beck et al (1997) – IN792
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• According to the lines
drawn, IP loading gives
longer fatigue lives at lower
temperatures.
• Based on mean stress
arguments this fits with
earlier results.
• At higher temperatures IP
fatigue lives shorter than
OOP – most likely
dominated by crack
propagation again.
• Variability in results
however, other
interpretations could be
made.

Characterisation of TMF effects in RR1000
Investigate the behaviour of the alloy using a constant strain
range (1%), but differing phase angles.
Cycle temperature of 300-700⁰C.
Eventually examine the effect of changes in strain range.
Also vary peak temperature to see what effect it has on the
ordering of tests.
Generation of high quality stress-strain data to enable
development of modelling procedures.
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Stress (MPa)
In-Phase test
Mechanical Strain (%)
Loop 1
Loop 2
Loop 3
Loop 4
Loop 5
Loop 10
Loop 100
Loop 500
Typical of IP tests, stress
relaxation due to creep at peak
strain/temperature.
Characterised by intergranular
cracking.
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50μm
Stress (MPa)
Max Stress
Min Stress
Number of Fatigue Cycles

Stress (MPa)
Out-of phase (180⁰) test
Loop 1
Loop 2
Loop 3
Loop 4
Loop 5
Loop 10
Loop 100
Loop 1000
Strain (%)
Transgranular failure as peak strain
is applied at 300⁰C. Increasing mean
stress throughout the test at creep
occurs to increase the minimum
stress on the compressive part of the
cycle
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Stress (MPa)
50μm
Cycles
Max Stress
Min Stress

Generation of test data
Loop 1
Loop 2
Loop 3
Loop 5
Loop 10
Stress (MPa)
Mechanical Strain (%)
Clockwise 45⁰ Anticlockwise 135⁰
Anticlockwise 90⁰
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Loop 1
Loop 2
Loop 3
Loop 5
Loop 10
Loop 100
Loop 1000
High quality data allowing for
test to test data comparison.
Clear evidence of differences
appearing in mean stress is
evident.

Comparison with isothermal data
Strain range (%)
Cycles to failure
Traditional isothermal
approaches do not seem to yield
a relationship.
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Stabilsed stress range (MPa)
Comparison with isothermal data
reveals the detrimental effect of
TMF, but also the spread in
results that can occur as a
function of phase angle.
Cycles to failure

Effect of phase angle on TMF life
Also clear is that this relationship is
strongly dependent on mean
stress, as described by the model
proposed earlier.
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It can be seen that TMF lives increase
with increasing phase angle.
Although scatter in the data needs to
be considered, this relationship
seems to be well defined by this data.

Conclusions
TMF facility development is a difficult process and small errors may lead
to significant issues in valid test data.
Based on results in the open literature, the ordering of IP and OOP tests
is strongly affected by the mean stress achieved during the test.
Other influences, such as early crack development may lead to
differences in the expected response.
A crossover would be expected in the IP and OOP results as sufficient
plastic strain is generated the significantly vary the mean stress.
Phase angle is shown to have a significant effect on TMF life, with a
power law relationship existing between the two.
Ongoing work is anticipated to reveal further insights about the effects of
strain range, peak temperature, environmental interaction etc...
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Thank you for your attention
Any questions?
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