Fatigue crack growth tests were conducted on 0.09 inch thick, 3.0 inch wide middle-crack tension specimens cut from sheets of 2024-T3 aluminum alloy. The tests were conducted using a load sequence that consisted of a single block of 2,500 cycles of constant amplitude loading followed by an overload/underload combination. The largest fatigue crack growth life occurred for the tests with the overload stress equal to 2 times the constant amplitude stress and the underload stress equal to the constant amplitude minimum stress. For the tests with compressive underloads, the fatigue crack growth life decreased with increasing compressive underload stress

Dawicke, David S.

English

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NASA Contractor Report 201668
Overload and Underload Effects on the
Fatigue Crack Growth Behavior of the
2024-T3 Aluminum Alloy
David S. Dawicke
Analytical Services & Materials, Inc., Hampton, Virginia
Contract NAS1-96014
March 1997
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23681-0001
https://ntrs.nasa.gov/search.jsp?R=19970015461 2020-06-16T02:02:57+00:00Z

Overload and Underload Effects on the Fatigue Crack Growth Behavior of the 2024-T3
Aluminum Alloy
D. S. Dawicke
Analytical Services & Materials, Inc.
ABSTRACT
Fatigue crack growth tests were conducted on 0.09 inch thick, 3.0 inch wide middle-crack
tension specimens cut from sheets of 2024-T3 aluminum alloy. The tests were conducted
using a load sequence that consisted of a single block of 2,500 cycles of constant
amplitude loading followed by an overload/underload combination. The largest fatigue
crack growth life occurred for the tests with the overload stress equal to 2 times the
constant amplitude stress and the underload stress equal to the constant amplitude
minimum stress. For the tests with compressive undedoads, the fatigue crack growth life
decreased with increasing compressive undedoad stress.
KEYWORDS
Fatigue crack growth, retardation, overload, underload
INTRODUCTION
An experimental program was conducted to evaluate the effects of periodic overloads and
underloads on the fatigue crack growth behavior of 2024-T3 aluminum alloy. Middle
crack tension (M(T)) specimens were fatigue cycled, until failure, with a load sequence
that consisted of blocks of 2,500 constant amplitude cycles followed by an
overload/underload combination. The objective of this study was to develop a database
that could be used to evaluate the ability of fatigue crack growth analysis codes to predict
load sequence effects.
EXPERIMENTALPROCEDURE
Fatiguecrackgrowth testswereconductedon 3 inch wide middlecracktension(M(T))
specimenscut from sheetsof 0.09inch thick 2024-T3aluminumalloy. The specimens
hadan initial notchthat was0.6 incheslong (2aN)and0.125incheswide (Nh),asshown
in Figure 1. The specimenswere fatigue precracked, to a crack length of 2ai =
1.00(O-0.002inches,underconstantamplitudeloadingwith a maximumstressof Smax=
10ksi anda stressratio of (R = Smin/Smax) of 0.02. After the fatigue precracking, the
specimens were cycled to failure using a load sequence that consisted of a single repeated
block. The block began with 2,500 constant amplitude cycles with a maximum stress of
Smax - 10 ksi and a stress ratio of R = 0.02. The constant amplitude loading was
followed by a single spike overload at a stress of Sol that ranged from 1.125 to 3.4Smax.
The overload was followed by a single spike underload at a stress of Sul that ranged from
0 to -3Smax. This load sequence is illustrated in Figure 2. In each test, the number of
cycles to failure was measured.
RESULTS AND DISCUSSION
A total of 41 fatigue tests were conducted. For 24 of the tests, the minimum stress (Stain)
was 0.2 ksi. Thus, the load sequence consisted of a single spike overload that repeated
every 2,500 cycles. The number of cycles to failure was recorded for each test and is
plotted in Figure 3 against the ratio of overload stress to constant amplitude stress
(Sol/Smax). The tests with constant amplitude loading (Sol/Smax -- 1.0) had an average
fatigue life of about 30,700 cycles. The addition of a 12.5% (Sol/Smax = 1.125) overload
every 2,500 cycles increased the average fatigue life to about 52,800 cycles, an increase
of about 70%. This increase in fatigue life was a result of a reduction of the effective
stress-intensity factor range due to the increased crack opening load following the spike
overload. The fatigue life continued to increase with increasing overload stress up to
Sol]Smax -- 2.0, where the corresponding fatigue life was about 2,090,000 cycles. Further
increases in the overload ratio resulted in a drop in the number of cycles to failure as a
result of damage accumulation during the spike overload cycles. At an overload stress of
SdSmax = 3.4, the test failed during the application of the first spike overload.
The single spike overload was followed by a compressive underload in the remaining 17
tests. The application of the underload following the overload resulted in a shorter
fatigue life than in the tests with just the spike overload, as shown in Figure 4. During
2
the underloadthe crack surfacesyielded in compression,reducing the tensile plastic
deformationdueto the overload,thus decreasingthe subsequentcrack openingstress
(andincreasingtheeffectivestress-intensityfactor).
CONCLUDINGREMARKS
Fatiguecrackgrowthtestswereconductedon0.09inchthick, 3.0 inch widemiddle-crack
tensionspecimenscut from sheetsof 2024-T3aluminumalloy. Thetestswereconducted
using a load sequencethat consistedof 2,500 cycles of constantamplitude loading
followed by anoverload/underloadcombination. For thetestswith theunderloadstress
equalto Sul]Smax = 0.02, the fatigue crack growth life increased for overloads in the range
of 1.0 < Sol/Smax <---2.0. At an overload of Sol/Smax = 2.0, the fatigue life was more than
60 times greater than the constant amplitude fatigue life. For overloads greater than
Sol/Sma x = 2.0, the fatigue crack growth life decreased with increasing overload stress.
For the tests with compressive underloads, the fatigue life decreased with increasing
compressive underload stress.
3
Figure 1 Middle-crack tension specimen with crack starting from notch.
4
Smax
S rain
Sul
_J 2500
Figure 2 Schematic of overload/underload spectra.
5
Cycles
to
Failure
10 _-
,o,
10 _
,,f
10 4
10 , , , , I , , , , I , , , , I , , , , I , , , , I
1 1.5 2 2.5 3 3.5
OJSma x
Figure 3 Fatigue test results for the tests conducted with repeated spike overloads
(Sul = 0.2 ksi).
6
f 0 SOJ/s max--1.5
Cycles _ " S%m.xZ°
to lOS_" O Soj_ =3.0
Failure I ___*_ .
[] <>
10 s [] 0
-3 -2.5 -2 -1.5 -1 -0.5 0
Summa x
Figure 4 Fatigue test results for the tests conducted with repeated spike overloads
followed by spike underloads.
7
APPENDIX
Table 1
Cycles to Failure for the Single Spike Overload Sequence (SjSmax = 0.02)
Sor/Smax Cycles to
Failure
1.0 26,764
1.0
1.0
24,417
34,628
1.0 32,936
1.0 37,484
1.0 28,088
1.125 53,608
1.125 52,073
1.25
1.5
101,023
397,659
1.5 373,005
1.75 1,168,164
2.0 1,930,938
2.0 2,088,070
2.25 1,750,202
2.5
2.75
865,438
330,132
3.0 97,359
3.0 120,048
3.0 147,559
3.25 47,519
3.3 7,503
3.4 2,501
3.5 2,501
8
Table2
Cyclesto Failurefor theSingleSpikeOverload/UnderloadSequence
Sol/Sm_ Sul/Smax Cycles to
Failure
1.125 0.0 53,608
1.125 0.0 52,073
1.125 -1.0
-2.0
-3.0
1.125
1.125
33,192
29,083
20,472
1.5 0.0 397,659
1.5 0.0 373,005
1.5 -0.25 345,098
1.5 -0.5 294,052
1.5 -1.0 195,075
1.5 -1.5 135,177
1.5 -2.0 85,529
1.5 -2.5 42,042
1.5 -3.0 25,753
2.0 0.0 2,088,070
2.0 0.0 1,930,938
2.0 660,264-1.0
-2.0
-3.0
2.0
2.0
160,064
30,012
3.0
3.0
0.0
0.0
3.0 0.0
3.0 -0.5
3.0 -1.0
3.0 -2.0
3.0 -3.0
120,048
97,359
147,559
52,521
20,008
12,505
12,505
9
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March 1997 Contractor Report
4. TITLEANDSUBTITLE
Overload and Underload Effects on the Fatigue Crack Growth Behavior of
the 2024-T3 Aluminum Alloy
6. AUTHOR(S)
David S. Dawicke
7, PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES)
Lockheed Martin Engineering & Sciences
Langley Program Office
Langley Research Center, Mail Stop 371
Hampton, VA 23881-0001
9. SPONSORINGI MONITORINGAGENCYNAME(S)ANDADDRESS(ES)
National Aeronautics and Space Administration
Langley Research Center
Hampton, VA 23681-0001
5. FUNDING NUMBERS
C NAS1-96014
WU 538-02-10-01
8. PERFORMING ORGANIZATION
REPORT NUMBER
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
NASA CR-201668
11. SUPPLEMENTARY NOTES
This report was prepared for Langley under contract NAS1-96014, Task DM01, by Analytical Services & Materials, Inc.,
Hampton,VA, under subcontractto Lockheed Martin Engineering& Sciences Company, Hampton,VA.
Langley Technical Monitor: James C. Newman, Jr.
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified - Unlimited
Subject Category 26
12b. DISTRIBUTION CODE
13. ABSTRACT (Maximum 200 words)
Fatigue crack growth tests were conducted on 0.09 inch thick, 3.0 inch wide middle-crack tension specimens cut
from sheets of 2024-T3 aluminum alloy. The tests were conducted using a load sequence that consisted of a
single block of 2,500 cycles of constant amplitude loading followed by an overload/underload combination. The
largest fatigue crack growth life occurred for the tests with the overload stress equal to 2 times the constant
amplitude stress and the underload stress equal to the constant amplitude minimum stress. For the tests with
compressive underloads, the fatigue crack growth life decreased with increasing compressive underload stress.
14. SUBJECTERMS
Fatigue crack growth, retardation, overload, underload
i
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Overload and Underload Effects on the Fatigue Crack Growth Behavior of the 2024-T3 Aluminum Alloy

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