Hot section components of gas turbine engines are subject to severe thermomechanical loads during each mission cycle. Inelastic deformation can be induced in localized regions leading to eventual fatigue cracking. Assessment of durability requires reasonably accurate calculation of the structural response at the critical location for crack initiation. In recent years nonlinear finite element computer codes have become available for calculating inelastic structural response under cyclic loading. NASA-Lewis sponsored the development of unified constitutive material models and their implementation in nonlinear finite element computer codes for the structural analysis of hot section components. These unified models were evaluated with regard to their effect on the life prediction of a hot section component. The component considered was a gas turbine engine combustor liner. A typical engine mission cycle was used for the thermal and structural analyses. The analyses were performed on a CRAY computer using the MARC finite element code. The results were compared with laboratory test results, in terms of crack initiation lives

Thompson, R. L.

Tong, M. T.

English

NASA Technical Reports Server

N88-2 384
LIFE ASSESSMENT OF COMBUSTOR LINER USING UNIFIED
CONSTITUTIVE MODELS
M.T. Tong* and R.L. Thompson
Structural Mechanics Branch
NASA Lewis Research Center
ABSTRACT
Hot section components of gas-turbine engines are subject to severe thermo-
mechanical loads during each mission cycle. Inelastic deformation can be
induced in localized regions leading to eventual fatigue cracking. Assessment
of durability requires reasonably accurate calculation of the structural
response at the critical location for crack initiation.
In recent years nonlinear finite-element computer codes have become available
for calculating inelastic structural response under cyclic loading. Most of
these, in keeping with an accepted practice in the elevated-temperature design
community, partition nonlinear elevated-temperature material behavior into
rate-dependent creep and rate-independent plasticity components. However, ana-
lytical studies of hot-section components such as turbine blades (McKnight,
1981) and combustor liners (Moreno, 1981) have demonstrated that the classical
creep and plasticity methods do not always predict the cyclic response of the
structure accurately because of the lack of the interaction between plasticity
and creep behavior. Experimental results have shown that the interaction of
elevated temperature is very significant and cannot be ignored (Corum, 1977;
Kujawski et al., 1979; Pugh and Robinson, 1978; and Senseny et al., 1978).
Under the Hot Section Technology Project (HOST), NASA Lewis Research Center
sponsored the development of unified constitutive material models and their
implementation in nonlinear finite-element computer codes for the structural
analyses of hot-section components (Ramaswamy et al., 1984 and 1985; and
Lindholm et al., 1984 and 1985). These unified constitutive models account
for the interaction between the time-dependent and time-independent material
behavior. In eliminating the overly simplified assumptions of the classical
material model, unified models can more realistically represent the behavior
of materials under cyclic loading high-temperature environments. The purpose
of this study was to evaluate these unified models with regard to their effect
on the life prediction of a hot-section component. The component under consid-
eration was a Pratt & Whitney gas-turbine-engine combustor liner. A typical
*Sverdrup Technology Inc., Lewis Research Center Group. Work performed
on-site at the NASA Lewis Research Center under contract NAS3-24105.
2-15
81_EGEDING PAG_ BLANK NOT FILMF/)
https://ntrs.nasa.gov/search.jsp?R=19880013000 2020-03-20T06:33:17+00:00Z
engine mission cycle was used for the thermal and structural analyses. The
analyses were performed at Lewis Research Center on a CRAY-XMP computer using
the MARC finite-element code. Unified constitutive models of Bodner and Walker
were used for the analyses. The results were compared with laboratory test
results, in terms of crack initiation lives.
2-16
OVERV I EW
HARDENING EFFECT OF CREEP ON PLASTICITY
Life assessment of engine hot-section components requires a thorough knowledge
of the thermal environment, accurate material characterization, calibrated
failure data, and reasonably accurate calculation of the structural response
at the critical location for crack initiation. In recent years, nonlinear
finite-element computer codes have become available for calculating inelastic
structural response under cyclic loading. Most of these computer codes are
based on the classical inelastic methods. They partition nonlinear, elevated-
temperature material behavior into rate-dependent creep and rate-independent
plasticity components. In other words, inelastic strains are decoupled and
formulated independently for the creep and plasticity portions, as illustrated
in the left figure. However, analytical studies of hot-section components such
as turbine blades (McKnight, 1981) and combustor liners (Moreno, 1981) have
demonstrated that the classical creep and plasticity methods do not always pre-
dict the cyclic response of the structure accurately because of the lack of the
interaction between the plasticity and creep behavior. Experimental results
have shown that the interaction at high temperature is significant and cannot
be ignored (Corum, 1977; Kujawski et al., 1979; Pugh and Robinson, 1978; and
Senseny et al., 1978). The interaction is illustrated in the figure, wherein
two loading paths are indicated by o-a-b and o-a-c-d. In the second loading
path the applied stress is held constant between a and c, which results in
creep deformation. As a consequence of the accumulated creep strain, c-d may
appear to be stiffer than a-b for a creep hardening alloy. Creep softening
alloys would exhibit a less stiff response.
INELASTIC CREEP PLASTIC
-- +STRAIN STRAIN STRAIN
f ..... J¢
E
CD-88-32709
2-17
ANNULARCOMBUSTORLINER
Under the Hot-Section Technology Project (HOST), NASALewis sponsored the
development of unified constitutive material models and their implementation
in nonlinear, finite-element computer codes for the structural analyses of
hot-section components. These unified constitutive models account for the
interaction between the time-dependent and time-independent material behavior.
In eliminating the simplified assumptions of the classical material model, uni-
fied models should more realistically represent the behavior of materials
under cyclic loading and high-temperature environments. In an application of
the unified material constitutive models, life assessment of a Pratt & W_itney
gas turbine engine annular combustor liner (as shownbelow) was conducted.
Unified constitutive models of Walker and Bodner were used. The results were
comparedwith laboratory test results, in terms of crack initiation lives.
CD-88-32710
2-18
POSTER PRESENTATION
CLASSICAL MATERIAL CONSTITUTIVE MODEL - DECOUPLED STRAINS
Conventionally, the most widely used constitutive description of metals at
high-temperature employs the time-independent classical theory of plasticity
to characterize short-term deformation, while relying upon time-dependent clas-
sical theory of creep to characterize long-term deformation. The total strain,
or strain increment, then, consists of four additive contributions - elastic,
plastic, creep, and thermal components. In other words, constitutive equations
are decoupled and formulated independently for the elastic-plastic, creep, and
thermal portions. Although the classical theories of plasticity have been used
quite extensively to characterize the behavior of metallic structures at room
temperature, it does not necessarily justify their applicability to other load-
ing conditions such as high-temperature environment. Analytical studies of
engine hot section components such as turbine blades and combustor liners have
demonstrated that the classical theories do not always predict the cyclic
response of the structure accurately. For high-temperature applications the
most severe shortcoming of classical theories is that the interaction between
creep and plasticity is not adequately taken into account. Experimental
results have shown that the interaction at elevated temperature is significant
and cannot be ignored.
INELASTICSTRAIN = CREEPSTRAIN + PLASTIC STRAIN
CLASSICALUNCOUPLED
CD-88-32711
2-19
CREEP-PLASTICITYINTERACTION
Two types of creep-plasticity interactions are generally observed. First,
accumulated creep strain has a hardening effect on subsequent plastic response.
This phenomenonis shownin the left figure, wherein two loading paths are
indicated by o-a-b and o-a-c-d. In the second loading path the applied stress
is held constant between a and c, which results in creep deformation. As a
consequenceof the accumulated creep strain, c-d appears to be stiffer than
a-b. Second, recent history of plastic strain has significant influence on
subsequent time-dependent behaviors. In the right figure, a-b and c-d repre-
sent creep strains corresponding to creep tests performed at loading and
unloading branches, respectively, but at the same stress level. While the
former undergoes a significant amount of creep deformation, the latter shows
virtually no creep. The same kind of phenomenon is also observed when relaxa-
tion tests are conducted as designed by e-f and g-h in the right figure.
a G
db
..... JC
-- E e
CD-88-32712
2-20
UNIFIED MATERIAL CONSTITUTIVE MODELS
In light of the previous discussion, it is apparent that effort must be made
to improve the prediction of inelastic behavior of metals at high temperatures.
The new theories toward characterization of material behavior at high tempera-
tures are known as unified theories in the sense that plastic and creep strains
are represented and treated by a single kinetic equation and a discrete set of
internal variables. They are also known as viscoplastic theories because they
are capable of modeling both rate-dependent and rate-independent behaviors.
(9= E(4- _1_ _TH) & = hot_I- rot
I( = hk I_Zl- rk
WHERE
h,_, hk STRAIN HARDENING FUNCTIONS
rot, rk
or, K
(TH
RECOVERYFUNCTIONS
STATEVARIABLES
INELASTICSTRAIN
THERMALSTRAIN
o STRESS
CD-88-32713
2-21
COMBUSTORLINERANDITS FINITE ELEMENTMODEL
In an application of the unified material constitutive models, life assessment
of a Pratt & Whitney gas-turbine-engine annular combustor liner was conducted.
The liner is air-cooled and is made of Hastelloy-X alloy. The finite-element
model has 546 elements and 1274 nodes. Eight-node solid elements were used.
Because of symmetry only a section of the liner was modeled.
CD-88-32714
2-22
THERMAL ANALYSIS OF ANNULAR COMBUSTOR LINER
Thermal analysis was first conducted to determine the temperature distribution
of the combustor liner. These temperatures were later used for the structural
analysis of the liner. A typical engine mission cycle was used for the analy-
sis. Temperature histories at the combustor liner critical locations were also
determined.
AT MAXIMUM POWER
TEMPER-
ATURE,
°F
1650
1200
COOLSIDE HOT SIDE
750
CD-88-32715
2-23
COMPARISON WITH TEST RESULTS
Subsequent nonlinear structural analysis of the combustor liner was performed.
The analysis was conducted with the unified constitutive models of Walker and
Bodner (Lindholm et al., 198_ and 1985). Both models were implemented into the
MARC finite-element code. Stress and strain ranges at the liner critical loca-
tions were determined.
The total strain and inelastic strain ranges were used for the life assessment
of the combustor liner, based on the experimental results by Jablonski (1978).
In conjunction with the analyses, laboratory tests were performed on the com-
bustor liner. Comparisons were made of crack inititation lives. Good agree-
ment between predicted and measured life was obtained.
ANALYTICAL
METHOD
UNIFIED (WALKER)
UNIFIED (BODNER)
MECHANICALSTRAIN RANGE,
o/o
TOTAL
0.587
.580
INELASTIC
0.315
.270
OBSERVED LIFE = 1603 CYCLES
PREDICTEDLIFE,
CYCLES
1000 TO 1500
1000 TO 1500
CD-88-32716
o O/
_--__4
REFERENCES
Corum, J.M., 1977, "Evaluation of Inelastic Analysis Methods," Transactions of
the 4th SMIRT Conference, San Francisco, Vol. L, Paper L.
Jablonski, D.A., 1978, Fatigue Behavior of Hastelloy-X Elevated Temperatures
in Air, Vacuum, and Oxygen Environments. Ph.D. Thesis, University of Con-
necticut.
Kujawski, D., Kallianpur, V., and Krempl, E., 1979, "Uniaxial Creep, Cyclic
Creep and Relaxation of AISI Type 304 Stainless Steel at Room Temperature,
An Experimental Study," RPI Report CS 79-4.
Lindholm, U.S., Chan, K.S., Bodner, S.R., Weber, R.M., Walker, K.P., and
Cassenti, B.N., 1984, "Constitutive Modeling for Isotroplc Materials,"
NASA CR-174718.
Lindholm, U.S., Chan, K.S., Bodner, S.R., Weber R.M., Walker, K.P., and
Cassenti, B.N., 1985, "Constitutive Modeling for Isotropic Materials,"
NASA CR-174980, 1985.
McKnight, R.L., Laflen, J.H., and Spamer, G.T., 1981, "Turbine Blade Tip Dura-
bility Analysis," NASA CR-165268.
Moreno, V., 1981, "Combustor Liner Durability Analysis," NASA CR-165250.
Pugh, C.E., and Robinson, D.N., 1978, "Some Trends in Constitutive Equation
Model Development for High-Temperature Behavior of Fast-Reactor Structural
Alloys," Journal of Nuclear Engineering and Design, Vol. 48, pp. 269-276.
Ramaswamy, V.G., Van Stone, R.H., Dame, L.T., and Laflen, J.H., 1984, "Consti-
tutive Modeling for Isotropic Materials," NASA CR-17485.
Ramaswamy, V.G., Van Stone, R.H., Dame, L.T., and Laflen, J.H., 1985, "Consti-
tutive Modeling for Isotropic Materials, NASA CR-175004.
Senseny, P.E., Dully, J., and Hawley, R.H., 1978, "Experiments on Strain Rate
History and Temperature Effect During the Plastic Deformation of Closed-
Packed Metals," ASME Journal of Applied Mechanics, Vol. 45, pp. 60-66.
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Life assessment of combustor liner using unified constitutive models

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