This program is designed to investigate fundamental damage processes, identify modeling strategies, and develop practical models which can be used to guide the early design and development of new engines and to increase the durability of existing engines. A review is given of the base program, completed in 1984, which included the comparison and evaluation of several popular high-temperature life prediction approaches as applied to continuously cycled isothermal specimen tests. The option program, of which one year is completed, is designed to develop models which can account for complex cycles and loadings, such as thermomechanical cycling, cumulative damage, multiaxial stress/strain rates, and environmental effects

Nelson, Richard S.

Schoendorf, John F.

English

NASA Technical Reports Server

N88-11173
CREEP FATIGUE LIFE PREDICTION FOR ENGINE HOT SECTIO_
MATERIALS (ISOTROPIC) - THIRD YEAR PROGRESS REVIEW"
Richard S. Nelson and John F. Schoendorf
United Technologies Corporation
Pratt & Whitney
INTRODUCTION
As gas turbine technology continues to advance, the need for advanced life
prediction methods for hot section components is becoming more and more evident. The
complex local strain and temperature histories at critical locations must be
accurately interpreted to account for the effects of various damage mechanisms such
as fatigue, creep, and oxidation and their possible interactions. As part of the
overall NASA HOST effort, this program is designed to investigate these fundamenta]
damage processes, identify modeling strategies, and develop practical models which
can be used to guide the early design and development of new engines and to increase
the durability of existing engines.
This contract is a 5-year effort comprising a 2-year base program and a 3-year
option program, involving two different isotropic materials as well as two
protective coating systems. The base program, which was completed during 1984,
included comparison and evaluation of several popular high-temperature life
prediction approaches as applied to continuously cycled isothermal specimen tests.
The option program, of which one year has been completed, is designed to develop
models which can account for complex cycles and loadings, such as thermomechanical
cycling, cumulative damage, multiaxial stress/strain states, and environmental
effects.
REVIEW OF PROGRESS DURING BASE PROGRAM
Approximately 150 tensile, creep, and fatigue tests were conducted during the
base program using specimens made from a single heat of cast BI900+Hf material. The
fatigue tests were conducted in an axial strain-controlled mode at temperatures
between 538°C (IO00°F) and 982°C (1800°F). Initiation life was considered to be more
significant than separation life; this was defined to be the cycle at which a 0.75 cm
(0.030 in.) surface crack had developed, as determined by replication of selected
specimens at each test condition. These tests investigated the effects on initiation
of strain range, strain rate, mean strain, and compressive and tensile dwell periods.
The base program also included a review of many different life prediction
methodologies, such as correlation of macroscopic parameters (strain range, mean
stress), strain range partitioning (ref. l), rate-sensitive models (Majumdar, ref.
2), work-based models (Ostergren, ref. 3), damage accumulation (ductility
exhaustion), and fracture mechanics approaches. Both desirable and undesirable
features of each of these were identified and used to guide the selection of the
approach having the best combination of accurate predictive capability and practical
specimen data requirements. The damage model which was finally chosen and developed
is based on the ductility exhaustion concept and is known as the Cyclic Damage
Accumulation model. Application of this model to the data generated in the base
program testing
I NASA Contract NAS3-23288
321
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showed very good correlation. A full description of all testing and model
development conducted under the base program is contained in the second annual
report (ref. 4).
THERMOMECHANICAL MODEL DEVELOPMENT
A significant task under the option program is the development of a damage model
which is valid under conditions of thermemechanical fatigue (TI_). A total of 21TMF
specimen tests has been completed so far, covering variables such as strain range,
temperature range, mean strain, cycle type, and hold times. A schematic comparison
of some of the strain-temperature cycles used for these tests is shown in figure I.
It can be seen that the "dogleg" tests are identical to the base program isothermal
hold tests, except that the temperature was not held constant during the strain
holds. In general, the results of these specimen tests have demonstrated that TMF
damage cannot always be predicted in the same manner used for isothermal tests; the
model chosen must be sensitive to accumulation of damage from several different
sources throughout the cycle.
The isothermal CDA model developed during the base program was applied to a
selected group of the TMF specimen tests, using a rate of fatigue damage
accumulation consistent with the highest temperature seen by the specimen during the
test. The resulting predictions for the out-of-phase tests are shown in figure 2;
note that the trend of the data is correctly predicted, but that the predictions are
conservative. The same method was applied to the data from the in-phase and dogleg
tests in this group, and figure 3 shows that, for these tests, the isothermal damage
method does not predict the trends in the observed specimen initiation lives. Work
is now continuing on a differential form of the CDA model in which the damage rate
is variable and can be integrated around any arbitrary TMF cycle.
MULTIAXIAL STRESS STATE MODEL
Multiaxia] fatigue tests are now being conducted using tubular specimens
subjected to torsional loading in combination with uniaxial tension loading.
Considerable effort was expended on development of the specimen, test rig and
computer software codes. Collet-type grips were incorporated into the test rig to
reduce the fixturing complexity and allow relaxed tolerances on the specimen grip
ends which resulted in lower cost specimens.
A literature survey was conducted to identify viable approaches to predicting
multiaxial effects in hot section components. Evaluation of the approaches was based
on the following criteria: a demonstrated capability to predict non-proportional
loading effects, use of two or more parameters to characterize multiaxial fatigue
behavior, requirement for a minimal amount of multiaxial test data, and
compatibility with the Cyclic Damage Accumulation model. The plastic work (ref. 5)
and critical plane (ref. 6) theories were identified as having the greatest
potential.
CUMULATIVE LOADING MODEL
During the tests conducted as part of this task, specimens have been exposed to
mixed loading conditions to provide information regarding the interaction of
different types of damage processes. A total of 44 such tests have been completed,
including block tests (one set of conditions for the first block of cycles, followed
322
by a second set of conditions for the remainder), sequenced tests (alternating
blocks of two different sets of conditions), and interrupted tests (fatigue cycling
interrupted by periods of temperature exposure, either with or without load). The
results of these tests show that some conditions obey a linear damage rule, while
certain other conditions show a strong non-linear interaction. For example, figure 4
shows an interaction diagram for the block tests run with various combinations of
R-ratios, and it can be seen that running a specimen at R=O for only a fraction of
the initiation life at that condition will dramatically affect the life remaining
under subsequent cycling at R=-l.
As a result of these tests, it has become clear that the CDA model must be
allowed to track different cyclic ductility capabilities in each portion of a test.
Also, it was observed during certain tests (such as the temperature block tests)
that the damage accumulation is non-linear. These changes have been incorporated
into the CDA equation through the use of the concept of ductility fraction, which is
defined as the fraction of the available ductility which has been consumed:
Ductility Fraction, fc=
Ductility Exhausted
Available Ductility
The equation used to calculate this quantity may be written as follows:
i dD] ) F(_ T,A_) dN
_N Ref
(1)
where:
m
Ep
dD/dN R
grain cyclic capability for specific test condition being predicted
damage rate from fully reversed testing
_ stress range
_T maximum tensile stress
t I/2 cycle period
R reference condition
licl constants determined from monotonic creep tests
It can be seen that when f_ reaches 1, N will equal the predicted initiation
life, Ni. Note also that the current initial ductility is now inside the integral,
which permits the algorithm to switch from one loading condition to another which
has a different initial ductility.
323
We may also rewrite equation I to perform the integration over fE instead of N,
and at the same time introduce a non-linear function G(N/Ni) which has the property
that its integral over the interval from 0 to I is always l, no matter what values
are chosen for its constants:
Ni =
_D] Ref) F (_T 'A°1 G dfe
(2)
Several forms of the function G(N/Ni) have been evaluated, including linear and
power law functions• Figure 5 shows the effect of non-linear damage accumulation on
the CDA life predictions for the block loading tests, and it can be seen that in
almost every case the non-linear life prediction is more accurate.
FUTURE TASKS
One of the important tasks to be completed in the near future is the
investigation of the effects of environment on the fatigue process• Current plans
call for screening tests in two types of atmospheres, inert (argon) and aggressive
(high pressure oxygen), followed by further detailed tests in the environment which
shows the greater effect. Other tasks include development of a life model for
coated materials under creep-fatigue conditions and further investigation of the
effects of mean stress on fatigue. The final task under the contract will be the
application of the fully developed life prediction models to an alternative
material and coating system in order to verify their applicability to alloys other
than BIgOO+Hf.
I •
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.
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REFERENCES
Manson, S.S.; Halford, G.R.; and Hirshberg, M.H.: Creep-Fatigue Analysis by
Strain Range Partitioning. Symposium on Design for E]evated Temperature
Environment, ASME, 1971, pp. 12-28.
Majumdar, S.; and Maiya, P.S.: A Mechanistic Model for Time Dependent Fatigue.
Jour. of Materials & Technology, Vol. I02, January 1980, pp.159-167.
Ostergren, W.J.: A Damage Function and Associated Failure Equations for
Prediction of Hold Time and Frequency Effects in Elevated Temperature, Low
Cycle Fatigue. Journal of Testing and Evaluation, Vol. 4, No._5, September 1976.
Horeno, V.; Nissley, D.M.; Lin, L.S.: Creep Fatigue Life Prediction for Engine
Hot Section Materials (Isotropic), Second Annual Report. NASA CR-174844,
December ]984.
Garud, Y.S•: A New Approach to the Evaluation of Fatigue Under Multiaxial
toadings. ASME Journal of Engineering Materials and Technology, Vo1. 103, April
1981, pp. 118-125.
Kanazawa, K.; Miller, K.J.; and Brown, N.W.: Low-Cycle Fatigue Under
Out-of-Phase Loading Conditions. ASME Journal of Engineering Materials and
Technology, Vol. 99, No. 3, July 1977, pp. 222-228.
324
Ooo,e° °ll -T
In-phase and
out-of-phase
E
Figure 1 TMF Loading Cycles for Initial CDA Evaluation
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0
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i
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k
n
1000
100
100
o Out-of-phase TMF -- 1000-1600°F
o Out-of-phase TMF -- 800-1700°F
I I I i litl
1000 10,000
Actual life (cycles)
Figure 2 Prediction of Out-of-Phase TMF by CDA Model
325
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1000 10,000
Actual life (cycles)
Figure 3 Prediction of In-Phase and Dogleg THF by CDA Model
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327


Creep fatigue life prediction for engine hot section materials (isotropic): Third year progress review

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