The objective was to examine the room temperature fatigue and nonlinear cumulative fatigue damage behavior of the cast nickel-based superalloy, MAR M-247. The fatigue test matrix consisted of single-level, fully reversed fatigue experiments. Two series of tests were performed: one of the two baseline fatigue LCF (Low-Cycle Fatigue) life levels was used in the first loading block, and the HCF (High-Cycle Fatigue) baseline loading level was used in the second block in each series. For each series, duplicate tests were performed at each applied LCF life fraction

Halford, Gary R.

Kalluri, Sreeramesh

Mcgaw, Michael A.

English

NASA Technical Reports Server

?{93-31575
CUMULATIVE FATIGUE DAMAGE BEHAVIOR OF MAR M-247
Michael A. McGaw and Gary R. Halford
NASA Lewis Research Center
Cleveland, Ohio 44135
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Sreeramesh Kalluri
Sverdrup Technology, Inc.
Brook Park, Ohio 44142
Cumulative Fatigue Damage Analysis is the term used to describe the
assessment of fatigue lives of metallic materials under variable amplitude or
block loading. A linear life-fraction rule, known as the Palmgren-Miner Linear
Damage Rule (LDR) is the standard model currently used in engineering design for
assessing cumulative fatigue-damage. In recent years, it has been discovered
that engineering metals and alloys exhibit pronounced nonlinear cumulative
fatigue damage behavior under certain conditions of loading. Conditions for
which pronounced nonlinearity and subsequent fatigue life degradation might
exist can now be identified readily using engineering models developed at the
Lewis Research Center of NASA. The models reflect the accumulation of fatigue
damage through the nonlinear sequence of microcrack initiation, microcrack
growth, macrocrack initiation, macrocrack growth, and eventual fracture. Despite
the extreme complexity of the physical mechanisms of fatigue, relatively simple
engineering models have evolved. They are proving useful in guiding the
evaluation of the cumulative fatigue damage behavior of materials tested in
laboratory environments.
The simplest and most common laboratory test procedure for evaluating
cumulative damage behavior is the two-level loading sequence in which low-cycle
fatigue (LCF) is applied for a portion of the expected LCF life followed by
high-cycle fatigue (HCF) loading until failure. Analysis of hundreds of two-
level loading experimental results from the literature has led to the develop-
ment of several simple engineering models, including: the Double Linear Damage
Rule (DLDR), the Damage Curve Approach (DCA), and the Double Damage Curve Ap-
proach (DDCA). Each has its regime of utility, but all three give rise to essen-
tially the same numerical results. Another common attribute in these models is
the recognition that the degree of nonlinearity in damage accumulation depends
solely upon relative life levels of the extreme cycles. In other words, LCF/HCF
lives of 102/105 or 104/107 will exhibit equal degrees of nonlinearity and hence
equal degrees of fatigue life degradation.
The object of this study was to examine the room temperature fatigue and
nonlinear cumulative fatigue damage behavior of the cast nickel-base superalloy,
MAR M-247. This is the bill-of-material for the turbine housings and inlet
guide vanes of both the oxidizer and fuel turbopump designs from the Alternate
Turbopump Development (ATD) program. Pratt & Whitney Corp. (West Palm Beach
group) is performing this program under contract to NASA Marshall Space Flight
Center. Through a small cooperative agreement with NASA Lewis Research Center,
a casting of MAR M-247 was obtained from Pratt & Whitney Corp. The casting was
173
https://ntrs.nasa.gov/search.jsp?R=19930022386 2020-03-17T05:09:14+00:00Z
produced using the MICROCAST-Xprocess of the HowmetCorp. and had been HIPed
and heat treated according to engineering use specifications from the ATDpro-
gram requirements. Axial fatigue specimens possessing a half-inch uniform gage
section were machined from this casting.
The fatigue test matrix carried consisted of single-level, fully reversed
(R = -i, either load or strain control) fatigue experiments (fig. I). Duplicate
tests were performed at each of three test conditions. The mechanical test pa-
rameters were chosen to establish two lower-life LCFlevels (NIA & NIB)and one
higher-life HCFlevel (N2) for use in the two-level loading experiments. Two-
level block loading experiments (in which higher amplitude strain or load
cycling, corresponding to lower life levels, NIA and NIR) were conducted for
various life fractions, followed by load cycllng (corresponding to a higher fa-
tigue life level, Np) to failure (fig. 2). In these tests, LCF life fractions
(nlA/NIA and nlB/NIB)-Of approximately 0.1, 0.2, and 0.3 were applied and the HCF
life remaining in the second loading level was tsobserved. Twoseries of tes
were performed: one of the two baseline fatigue LCF life levels was used in the
first loading block, and the HCFbaseline loading level was used in the second
block in each series. For each series, duplicate tests were performed at each
applied LCF life fraction.
The results of the baseline fatigue characterization tests are shown in
figure 3. The lowest life tests were performed under strain control at 10 cpm,
AEt = 1.0 percent. A small amount of inelastic strain range, on the order of 0.1
percent, was observed. The balance of the tests was performed under load con-
trol, at 100 Hz, and nominally elastic behavior was observed. These results
agreed with results obtained by Pratt & Whitney (ref. 2).
The results of the two-level loading experiments are shown as cumulative
fatigue damage interaction plots (figs. 4 and 5). In figure 4, the LCF life
level, NIA corresponding to the first loading block was approximately 7000
cycles to failure. The second load level corresponded to approximately 107
cycles to failure (N) These life levels result from a consideration of the
. 2 •
average of the fatigue lives obtained in the baseline fatigue characterization.
Also shown in the figure are predicted results based on a linear damage ap-
proach, and two nonlinear cumulative damage approaches, the Double Linear Damage
Rule (ref. i) and the Damage Curve Approach (ref. I). As can be observed, for
the three duplicated test conditions employed in this series, MAR M-247 exhibits
a very strong nonlinear cumulative interaction behavior. This behavior is pre-
dicted qualitatively based on the DCA and DLDR approaches. By contrast, pre-
dictions according to the LDR approach are in error by nearly a factor of 8 on
the unconservative side with respect to the experimental results (the remaining
HCF life is overpredicted by a factor of 33).
In figure 5, the LCF life level, NIB corresponding to the first block of
loading was approximately 340,000 cycles to failure. The second loading ]eveI
was as before. The experimental results are again shown with predictions made
according to both linear and nonlinear cumulative damage approaches. Nonline:_r
cumulative fatigue damage behavior is again exhibited. Predictions by the DCA
and DLDR models again are qualitatively correct for all but two of the result_
shown in this figure. The cumulative fatigue damage results in this figure do
show a bifurcation in behavior that has not previously been observed. The re-
174
maining HCF lives are either shorter than expected, or substantially greater,
possibly indicating that the samples are from two separate populations of dif-
fering grain size, hardness, strength, etc. There is presently no physical evi-
dence to explain the behavior• The specimens are to be thoroughly examined at
MSFC using fractographic and metallographic techniques in anticipation of ex-
plaining the duality in behavior•
References
I • Manson, S.S.; Halford, G.R.: Practical Implementation of the Double Linear
Damage Rule and Damage Curve Approach• Intern• J. Fracture, vol. 17, no. 2,
Apr. 1981, pp. 169-192.
2. Heine, J.: Pratt & Whitney Aircraft, West Palm Beach group, private
communication•
Approach: Baseline Fatigue Behavior
Log _Et
N1A
N1B
LCF
Strainor 0 _Aj_V/V_Vlo d t
R =-1
N 2 } HCF
Log N f, cycles to failure
• Establish base life levels for HCF/LCF interaction study
• Two LCF life levels, one HCF life level
• Replicate tests at each condition
CD-91-52290
FigureI
175
Approach: HCF/LCF Interaction Behavior
LCF/HCF loading pattern
LCF
+ HCF
Imposed _ITime
strain 0 Remaining
life
fraction,
n2/N 2
1.0
"I "%'''%
% LDR
"%
%%%%
%%%
,I
0.1.2.3 1.0
Initial life fraction, nl/N 1
• Two-level block loading tests run
• Three applied LCF life fractions designed to bracket
maximum expected nonlinear interaction
• Duplicate tests at each condition
Figure 2
Results: Baseline Fatigue Behavior
MAR M-247 in Air at Room Temperature
CD-9t-52291
1000 --
Stress
amplitude,
ksi
100
10
103
I I I I
104 105 106 107
Cycles to failure, Nf
I
108
Figure 3 CD-91-52292
176
High
cycle
fatigue
life
fraction,
n2/N 2
Results: LCF/HCF Interaction Behavior
MAR M--247 Fatigue Interaction; N1 = 6764; N 2 -- 11 136 861
Data
DCA
LDR
DLDR
0 .2 .4 .6 .8 1.0
LOW cycle fatigue life fraction, nl/N 1
CD.91-52293
Figure 4
Results: LCF/HCF Interaction Behavior
MAR M-247 Fatigue Interaction; N1 = 342 016; N2 = 11 136 861
2.0
1.6
High
cycle
fatigue 1.2
life
fraction,
n2/N 2 .8
.4
• Data
DCA LCF/HCF loading pattern
LDR LCF
1 HCF
o.o __ • strain 0 Time
.2 .4 .6 .8 1.0
Low cycle fatigue life fraction, nl/N 1
Figure 5 CD-91-52294
177



Cumulative fatigue damage behavior of MAR M-247

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