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Procedía Engineering 101 (2015) 85 - 92
Procedía Engineering
www.elsevier.com/locate/procedia
3rd International Conference on Material and Component Performance under Variable Amplitude Loading, VAL2015
Corrosion fatigue crack growth of 7475 T7351 aluminum alloy under flight
simulation loading
A. Chemina*, D. Spinellia, W. Bose Filhoa, C. Rucherta
a,*Department of Materials Engineering, Sao Carlos School of Engineering, University of Sao Paulo, Av. Joao Dagnone, 1100 Jd. Sta Angelina, zip code: 13563-120, Sao Carlos - Sao Paulo - Brasil
Abstract
Corrosion crack propagation experiments were carried out on specimens of 7475 T7351 aluminum alloy. Variable amplitude tests were performed with flight-simulated spectrum Twist (Transport Aircraft Wing Structures) and Falstaff (Fighter Aircraft Loading Standard for Fatigue) exposed to air and saline environment. The results showed that fatigue crack propagation life of specimens tested on saline environment were longer than specimens tested on air environment. The fatigue surfaces of specimens tested on saline environment examined in the SEM showed oxide and Na crystals in the wake of crack, which can promote retardation of crack propagation.
© 2015TheAuthors.PublishedbyElsevier Ltd. Thisisan open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the Czech Society for Mechanics
Keywords:Corrosion fatigue, aluminum alloy, flight load
1. Introduction
Fatigue and corrosion are recognized as degradation mechanisms that affect the integrity of components, which are built with aluminum alloy. Corrosion is a phenomenon that can start naturally and is usually associated to intermetallic particles that can transform the aluminum alloy to electrochemical cell, whereas fatigue is a phenomenon that occurs because the cyclic load and for this reason, the structure of material is deformed permanently (1,2).
The combination of these phenomena have been studied by development of new test methodologies that promotes research and knowledge, in laboratory testing, as the behavior material varies according with environment and loading
* Corresponding author. Tel.: +55-16-3373-9591; fax: +55-16-3373-9590. E-mail address:aline.albuquerque@usp.br
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the Czech Society for Mechanics
doi:10.1016/j.proeng.2015.02.012
simultaneously (3,4,5,6). The corrosion of commercial alloys for aircraft occurs because of intermetallics, such as Al7Cu2Fe presents in all 7000 series alloy (3,7). The fatigue crack growth in metals that leads to final fracture initiates from slip lines subjected to both plastic deformation and corrosion at the same time (5). When the crack is open the slip motion exposes fresh metal surface and then becomes anodic, the adjacent regions where no slip occurs become cathodic, thereby forming a local electrochemical cell (5). The material under random loading, as during a flight for example, plus corrosion effects cause the crack growth analyses to be more complex.
In 1970, two standards for flight simulation loading, TWIST and FALSTAFF by NLR (National Laboratory Research), were developed and are generate with the mean stress for TWIST and maximum stress for FALSTAFF
(8,9).
The main aim of this paper is evaluate the fatigue life of 7475 T7351 aluminum alloy under flight simulated loading and corrosive environment at the same time. The NaCl solution was chosen as one of the corrosive environments, because of the saline nature of seawater.
Nomenclature
a Crack size, mm
F Flight numbers
TWIST A Standardized load sequence for flight simulation tests on transport aircraft wing structures
FASLTAFF Fighter Aircraft Loading Standard for Fatigue
NLR National Laboratory Research
2. Experimental Detail
2.1. Material
The chemical composition of 7475 T7351 was detected by X-ray optical spectroscopy and presented on Table 1.
Table 1. Chemical composition of 7475 T7351 aluminum alloy (% wt)
Si Fe Cu Mn Mg Cr Zn Ti P V B Al
% 0.03 0.08 1.67 0.01 2.156 0.23 5.47 0.043 0.001 0.01 0.011 Base
The values of chemical composition of 7475 T7351 aluminum alloy presented on Table 1 is according with recommendations of ASM 2355 -89. Tables 2 and 3 present the tensile properties from tests conducted in the T and L directions, as recommended in ASTM E8M-00.
Table 2. Strength test of specimen in direction L (ASTM E8M-00).
O(mm) A (mm2) cr (MPa) ay (MPa) AR (%) ALa (%) E (GPa)
Mean Values 6,052 28,77 469,8 395,1 19 16,55 71
DP 0,064 0,61 13,28 13,04 3 1,17 7,7
A. Chemin et al. / Procedia Engineering 101 (2015) 85 - 92 Table 3. Strength test of specimen in direction T (ASTM E8M-00).
O(mm) A (mm2) C (MPa) Cy (MPa) AR (%) AL" (%) E (GPa)
Mean values 5,99 28,23 472,2 398,3 15 11,18 73
DP 0,052 0,61 13,28 13,04 0,8 0,46 1,9
2.2. Fatigue tests
The two simulated flight histories were generate from Genesis for Fatigue. TWIST has 4000 flights and 362665 cycles and was simulated with mean stress 80MPa, Figure 1 (a); the FALSTAF has 200 flights or 35966 points of load reverse and was simulated with maximum stress 200 MPa.
Fatigue crack growth tests used M(T) specimens in T-L direction, dimension of height, width and thickness (244x100x3mm), center notch 10mm and pre crack 1mm (2a=2mm). The tests were performed at room and saline environment of 3.5% and 5.0% NaCl on MTS dynamic test machine, Figure 1. The 3.5% NaCl was chosen as one of the environments because of the composition of seawater, and 5.0%NaCl was chosen because this composition is recommended for corrosion acceleration tests. The crack size was determined by the electrical-potential method. Table 4 shows the sequence of fatigue crack growth tests performed and shows the parameters obtained experimentally, to generate saline fog.
Table 4. Parameter of Fatigue tests under spectrum loading: a= Mean Stress and b= Maximum Stress.
Specimen Fatigue Test Environment Stress^ (MPa)
CP1 TWIST Air 80a
CP2 TWIST 3.5% NaCl 80a
CP3 TWIST 5.0% NaCl 80a
CP4 FALSTAFF Air 200b
CP5 FALSTAFF 3.5% NaCl 200b
CP6 FALSTAFF 5.0% NaCl 200b
Figure 1. Corrosion fatigue test (a) MTS machine and (b) the scheme of crack encapsulation system.
The data obtained by fatigue tests under spectrum loading were processed and the fatigue crack rate was determined by equation (1):
d2a = ai+i □ a, dF FM □ F
3. Results
The figure 2 (a) shows the crack size versus Flight number and Figure 2(b) shows fatigue crack rate versus crack size to flight TWIST exposed for each chose environment.
TWIST AIR TWIST 3.59é NaCl TWIST 5.0% NaCl FASTAFF AIR FALSTAFF 3.5% NaCl FALSTAFF 5.09É NaCl
50 60 70 80
Figure 2. Fatigue test under TWIST and FALSTAFF flight (a) 2a vs Flight numbers, (b) d2a/dF vs 2a. (enlarge these figures, the page limit is 8,
so they can be enlarged, so the legends and data can be read)
Figure 2(a) shows that for fatigue crack growth tests under TWIST flight loading and environment of 3.5% NaCl, the specimen failed under flight numbers close the test in air, on Figure 2(b) is possible to see that the fatigue crack rate of air test and 3.5%Cl were close. However, when the tests were performed on environment of 5.0%NaCl, the specimen fractured with larger flight number. For this fatigue crack growth test at 5.0%NaCl, the specimen presented a large life than tests at air and 3.5%NaCl, in other words, this environment (5.0%NaCl) decelerated the crack growth and not by hydrogen embritllement or acidification of surface, as expected (10).
The fatigue crack growth tests under FALSTAFF flight loading and saline environment shown a similar behavior, Figure 2 (a), as the tests performed under TWIST flight: the specimen tested at saline environment fractured with larger flights than air tests. The fatigue crack propagation rate revealed that the crack growth occurred with acceleration and retardation and with low rate values than tests executed at air.
The flight is a random loading that promotes a load interaction effect on crack tip as formation of plastic zone, which can decelerates the crack growth. However, the oxide and NaCl crystals formed by corrosion on material surface
can induce the crack closure (11) and so, decelerates the crack growth too. Both phenomenon compete during the crack growth process, the positive load can form plastic zone and the negative load can promotes residual stress that accelerate the growth; the corrosion products deposited on crack tip and on wake of the crack, because this competition, crack growth rate showed peaks and valleys that are the acceleration and delay. Figures 3 and 4 shows the fractured fatigue surfaces.
Figures 3 (a) and 4 (a) show the fatigue surfaces on tests under TWIST and FALSTAFF, respectively, for air. Both surfaces reveal striation that is feature of fatigue random loading. The Figure 3 (b) and Figure 4 (b) show the surface fracture at 3.5% NaCl, in these surfaces, the striation is not clear and is possible see oxide products. The Figure 3 (c) the marks made by corrosion products, the load pressed these products against the surface, and this showed the marks. Figure 4 (c) does not show the marks of corrosion products.
4. Conclusions
The results of corrosion fatigue crack growth tests showed a competition between corrosion effects and load effects as observed by graphs of fatigue crack growth rate to FALSTAFF and TWIST flight, feature by peaks and valleys of rate curves. The fatigue surfaces showed the presence of corrosion products that can promotes closure induced by oxide.
Acknowledgements
The authors thank the Brazilian aircraft manufacturer EMBRAER for supplying the material used in this research, to Professor Dr. Artur Motheo at Sâo Carlos Institute of Chemistry - IQSC/University of Sao Paulo for discussion about corrosion phenomena and CNPq (Brazil's National Council for Scientific and Technological Development) for the scholarships granted to them.
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