Requirements for increased durability of gas turbine hot section components have placed a greater degree of importance on accurate structural analysis and life prediction. Various life prediction approaches for high temperature applications were investigated. Basic models were selected and developed for simple-cycle, isothermal loading conditions. Models will be developed which address thermomechanical cycling, multiaxial conditions, cumulative loading, environmental effects, and cyclic mean stress. Verification tests of models will be conducted on an alternate material and coating system

Moreno, V.

English

NASA Technical Reports Server

N87- 11213
CREEP FATIGUE LIFE PREDICTION FOR ENGINE HOT SECTION
MATERIALS (ISOTROPIC) - TWO YEAR UPDATE l
Vito I%reno
Uni ted Technologi es Corporati on
Pratt & Whitney
INTRODUCTION
Requirements for increased durability of gas turbine hot section components have
placed a greater degree of importance on accurate structural analysis and life pre-
diction. The development of improved life prediction technology for structures oper-
ating at elevated temperatures is one of the objectives of the NASA Hot Section
Technology (HOST) program. As part of HOST, the current contract will investigate
fundamental approaches to high temperature life prediction, identify modeling strat-
egies and develop specific models for component relevant loading conditions.
This contract is a ._-year, ?-part effort (2-year base program, plus a 3-year
optional program) and includes two isotropic hot section materials and protective
coating systems. The recently completed hase program concentrated on the investiga-
tion of various life prediction approaches for high temperature applications and the
selection and development of basic models for simple-cycle, isothermal loading con-
ditions. The optional program will consider the development of models to address
thermo-mechanical cycling, multi-axial conditions, cumulative loading, environmental
effects and cyclic mean stress. Verification tests of models on an alternate ma-
terial and coating system will also be conducted.
TECHNICAL PROGRESS SUM_4ARY
Base Program Material and Testin 9
Monotonic tensile, creep, and cyclic fatigue tests were conducted on specimens
fabricated from a single heat of cast BlgOO+Hf material. Casting parameters were
selected to produce a small, uniform grain size of approximately O.Ol8 cm (0.007
in.) to 0.025 cm (O.OlO in.) throughout the test specimen gage sections to provide
an isotropic nature to the crack initiation process. The fatigue tests were conduct-
ed in an axial strain-controlled mode at temperatures between 538°C (lO00°F) and
982°C (IPOO°F). These tests investigated effects on initiation life of strain range,
strain rate, mean strain, and compressive and tensile strain dwell periods. In all
tests, crack initiation was defined as the occurrence of a 0.075 cm (0.30 in.) sur-
face crack, as determined by replication. A total of approximately 150 tests were
run in the base program.
INASA Contract NAS3-23288.
3O5
https://ntrs.nasa.gov/search.jsp?R=19870001780 2020-03-23T15:00:56+00:00Z
Screening of Candidate Life Prediction Approaches
During the base program, various life prediction approaches were reviewed to
assess their accuracy and practicality for elevated temperature life prediction.
Approaches and representative models included correlation of macroscopic parameters
(strain range, mean stress, etc.), inelastic strain (SRP), strain rate (Majumdar,
ref. l), work (Ostergren, ref. 2), damage accumulation (ductility exhaustion) and
fracture mechanics models. The following observations were made:
I. A "ductility exhaustion" format provides a workable means of incorporating the
most desirable features of the models reviewed.
(a) The ductility can be considered a loading history-dependent parameter. This
allows prediction of loading path effects, such as mean strain or overload,
without including this data in the base regression for the model constants.
Separate ductilities can be estimated for the grain and the grain boundaries
to potentially include the mode of initiation (transgranular versus inter-
granular) in the prediction.
(b) The damage (ductility exhaustion) parameter can be formulated to include
time-independent and time-dependent components. For the BlgO0 + Hf fatigue
tests conducted in this program, the two-damage components provided an
improvement in the predictive capability for a range of strain rates and
hold times as compared to a single-damage parameter formulation.
(c) The required information to determine the specific damage parameter of a
loading cycle is obtained from rapid cycle fatigue testing, the stress
response of the cycle being predicted, and the cycle period (time).
2. Less desirable features of other models limited their applicability and in-
creased the required analytical input. Some of these features include:
(a) Inelastic strain or strain rate (EpL or _PI,) is difficult to calculate
accurately and relatively small for the loadlng cycles considered in this
program.
(b) Use of a crack growth model to predict initiation requires small crack
growth data which is relatively difficult to obtain. Furthermore, the cur-
rent level of development of time-dependent inelastic crack-tip parameters
(e.g. J, C*) is such that the prediction and application to a surface-
initiated crack under complex loading is not clearly defined for an immedi-
ately useable life model.
(c) A number of the models evaluated showed good predictive capability but re-
quired various types of fatigue tests (strain rate, hold times) in the ini-
tial model data base to evaluate the necessary constants. This was consider-
ed an expensive requirement for development of a model. The applicability of
the models is also limited to those materials where the data is available or
planned.
306
From these observations, the damagemodel discussed below is considered as the
approach having the greatest predictive capability with the most practical data base
requirements. It becomesa good starting point for the additional loading cycles to
be considered in the optional program.
Fatigue Model Development and Evaluation
The proposed model assumes that fatigue cracks are initiated when a measure of
the grain cyclic capability is exhausted by the cycle damage and is expressed as:
I
GRAIN CYCLIC DAMAGE I
CAPABILITY (DUCTILITY) I DAMAGE I X CYCLES : 0 (1)
The grain cyclic ductility is determined at higher temperatures (>760 °C
(>14OO°F)) as the amount of primary creep strain that could have been .generated if
the maximum stress on the first loading cycle was held constant. At lower tempera-
tures, the cyclic ductility is determined as the amount of tensile elongation. The
cycle damage function is determined as the product of a reference damage rate, (from
fully reversed tests) and the ratios of tensile stress, stress range and period.
Assuming that the cycle damage is composed of time-independent
time-dependent components, the equation for initiation is written as:
and
P N_LD
dNR
dN = 0 (2)
Where : _p
dD/dNR -
CT
t -
R -
bI ,c
grain cyclic capability for specific test
being predicted
damage rate from fully reversed testing
stress range
maximum tensile stress
I/2 cycle period
reference condi tion
constants determined from monotonic creep tests
con dition
Here the integration reflects the fact that the stress response varies during the
cycl ing.
307
Application of equation (2) to the prediction of BI900 + Hf 871°C (1600°F) fa-
tigue tests produced the results shown in figure I. The prediction of rapid cycle
tests at various mean strains (-0.25, +0.25, +0.50%) is shown in figure IA. The
trend of longer life with decreasing mean strain is clearly predicted. The predic-
tion of slower strain rate and hold time tests is shownin figure IB. The trends and
the prediction in life relative to the fully reversed baseline data (not shown) are
also predicted.
Prediction of mean strain and strain rate effects is also demonstrated at 537°C
(lO00°F) and 982°C (180O°F) in figure 2. The model predicts the reduction in life
associated with a positive mean strain at 537°C (lO00°F) (fig. 2A). Time-dependent
damage is not included, so the trend in life is associated only with the time-
independent term. At 9820C (18DO°F) (fig. 2B) the correct trend in life at lower
strain rates is also shown.
I •
PEFERENCES
Majumdar, S.; and Maiya, P.S.: A _echanistic Model For Time Dependent Fatigue.
Jour. of l_aterials & Technology. January 1980, Vol. I02, pp. 159-167.
Ostergren, $1.J.: A General Damage Equation For Low Cycle Fatigue Life Prediction
at Elevated Temperatures. Pensselaer Polytechnic Institute. Zerox University
Microfilm, Ann Arbor, Mich. No. 76-27,203. 1976.
308
leee.
ACTUAL LIFE
(A)
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lee liltlit lllet
ACTUAL LIFE
Figure 1
(B)
Mean Strain and Rate Effects Predicted at 871°C (1600:F) - Model
Veri ficati on Tests
309
LL
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a.
m
.J
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O
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537°C (]O00°F)
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OI|IGINAL PAGE IS
OF POOR QUALITY
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Fi gure 2
1i lel tilt
(B) ACTUAL LIFE
Similar Predictive Capability Displayed at Other Temperatures
310


Creep fatigue life prediction for engine hot section materials (isotropic): Two year update

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