ON FATIGUE CRACK INITIATION FROM CORROSION PITS.pdf

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ON FATIGUE CRACK INITIATION FROM CORROSION PITS

IN 7075-T7351 ALUMINUM ALLOY

P.S. Pao, S.J. Gill and C.R. FengNaval Research Laboratory, Washington, DC 20375

(Received March 7, 2000)

(Accepted in revised form April 6, 2000)

Keywords: Aluminum alloys; Fatigue crack initiation; Corrosion pits

1. Introduction

High-strength, precipitation-hardened aluminum alloys, such as 7075, are used extensively in primary

wing and fuselage structures in many Navy and commercial aircraft. These commercial-grade alloys

contain numerous constituent particles of various sizes, which may have electrochemical potentials

different from those of the surrounding matrices. Because of the Navys special service environments,these aircraft are subjected to prolonged periods of salt water spray and/or salt fog. In the presence of

salt water, electrochemical reactions are possible and corrosion pits are readily formed at or around the

constituent particles in 2000- and 7000-series aluminum alloys (113). Indeed, many such corrosion pitswere observed in wing teardown analyses of Navy aircraft. These corrosion pits, once formed, act as

stress concentration sites and can facilitate crack initiation under both cyclic and sustained loading (3,4).

However, only limited studies of the effects of pre-existing pits on fatigue crack initiation in aluminum

alloys have been performed (3,4). Additionally, many of these studies used a smooth specimen

geometry and results could not be easily translated to more complex aircraft structural configurations,

such as rivet holes. Thus, quantitative characterization of the influences of corrosion pits on fatigue

crack initiation in 7000-series alloys, using a fracture mechanics approach, is highly desirable and is

essential for the development of life prediction methodology for aging aircraft.

In the present investigation, the influence of pre-existing corrosion pits, produced by prior immersion

in 3.5 wt% NaCl solution, on fatigue crack initiation in 7075-T7351 aluminum alloy was studied using

blunt-notch wedge-opening-load (WOL) type fracture mechanics specimens. Post-initiation SEMfractography was also utilized to identify the microstructural features at the fatigue crack initiation sites.

2. Materials and Experimental Procedures

63.5 mm-thick rolled plate of 7075-T7351 was used in this study. The chemical composition in weight

percent supplied by the vendor is: Al-5.7Zn-2.52Mg-1.59Cu-0.20Cr-0.05Mn-0.04Ti-0.090Si-0.17Fe.12.7 mm-thick blunt-notch WOL specimens with height H 63 mm and width W 64.8 mm were

used in the fatigue crack initiation studies (14). The fatigue specimens were oriented in the S-T

direction. The blunt-notch had a radius of 3.18 mm which resulted in a stress concentration factor Kt

3.1. These blunt-notch specimens are similar to those used for fatigue crack initiation studies on steels(15,16) and titanium alloys (17). The parameterK/, whereK is the applied stress intensity factor

range and is the notch root radius, has been shown to correlate with local notch-tip strain and is used

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to represent the mechanical driving force for crack initiation. The K for the WOL specimen can be

computed from Eq. 1 (14):

K P/BW1/ 22 a/W/1 a/W3/ 20.8072 8.858a/W 30.23a/W2

41.088a/W3 24.15a/W4 4.951a/W5 (1)

P applied load range; B specimen thickness; W specimen width; and a notch depth

measured from the load line.

The root of the blunt-notch was polished in the circumferential direction with the final step using 3

m diamond paste. To produce corrosion pits at the notch tip, specimens were immersed in 3.5 wt%

NaCl solution for 336 hrs.

Fatigue crack initiation tests were conducted at various stress intensities in ambient air at a stress

ratio R0.10 and a frequency f5 Hz on specimens with and without corrosion pits at the notch root.

Fatigue crack initiation was continuously monitored using a crack-mouth-opening-displacement(CMOD) technique and the initiation tests were stopped when the normalized crack length, a/W,

increased by 0.005. Post-initiation fracture surface morphologies were examined by SEM to identify the

microscopic features at the crack initiation sites.

To determine the corrosion pit population at the blunt notch root surfaces of the fatigue initiation

specimens, 15.9-mm-diameter cylindrical specimens were cut in the short transverse plane with surface

normal parallel to the S surface. Thus, the surfaces of these cylindrical specimens were parallel to the

blunt notch root surfaces of the WOL specimens. Simulating the preparation of pitted WOL fatigue

crack initiation specimens, cylindrical specimen surfaces were mechanically polished down to a 3 m

finish and then immersed in 3.5 wt% NaCl solution for 336 hrs. After exposure, a solution containing

phosphoric acid and chromic trioxide was used to strip off the thick layer of corrosion products before

the pitted specimens were examined by scanning electron microscope (SEM) to determine the average

corrosion pit size and density.

3. Results and Discussion

Pit Formation on S-T Surface

7000-series, commercial-grade aluminum alloys contain large numbers of constituent particles which

play an important role in corrosion pit formation. In the 7075 aluminum alloy, these constituent particles

have previously been identified as Al23CuFe4, Al2CuMg, and Si-containing particles (18).Constituent particles such as Al23

CuFe4

and Al2CuMg have different electrochemical potentials

relative to the surrounding aluminum matrix (19,20). In the presence of salt water, corrosion pits can

readily form by the dissolution of the matrix around constituent particles or the constituent particles

themselves. After prolonged exposure to salt water, these corrosion pits can grow to significant sizes.

Details of the corrosion pit formation sequence have been reported in previous investigations (1,2). Anexample of corrosion pits is shown in Fig. 1A for 7075-T7351 in the S surface after 336 hrs immersion

in 3.5 wt% NaCl solution. Because constituent particles in aluminum alloys tend to line up as stringers

parallel to the rolling direction, the corrosion pits thus formed exhibit rectangular shapes with high

aspect ratios on the surface. At higher magnification, as shown in Fig. 1B, these corrosion pits not only

grow along the rolling direction but also can coalesce with neighboring pits. The width of each pit is

often less than 10 m while the length (in the rolling direction of the plate) may vary from as small asa few m to longer than 50m. The average pit size and the two dimensional pit density in 7075-T7351

following 336 hrs immersion in 3.5 wt% NaCl solution are tabulated in Table 1. The pit depth and the

FATIGUE CRACK INITIATION392 Vol. 43, No. 5


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pit width beneath the surface as revealed in post-initiation fracture surface morphological examination,

however, could be several times larger than those measured on the surface.

Fatigue Crack Initiation Kinetics

The effects of pre-existing corrosion pits on fatigue crack initiation kinetics in 7075-T7351 are shown

in Fig. 2 which compares the fatigue crack initiation in ambient air of as-polished and polished-and-

pitted (by 336 hours pre-exposure to 3.5 wt% NaCl solution) specimens. Here, the number of fatigue

cycles required to initiate a crack is plotted against K/, the initial applied stress intensity factor

range normalized by the square root of the notch root radius. The initial stress intensity factor range,K, is calculated by Eq. 1 with the crack length equal to the notch depth measured from the load line

for both as-polished and polished-and-pitted 7075-T7351 specimens. For polished-and-pitted speci-

mens, because the average pit depth is small relative to the notch depth, the difference between the

actual initial applied stress intensity factor range and the nominal initial applied stress intensity factor

range is only about 1%. The general response of fatigue initiation life vs K/exhibited in Fig. 2 is

similar to that of S-N curves in that the fatigue initiation lives increase with decreasing K/until

a threshold, (K/)th, is reached. In the present study (K/)this arbitrarily defined as the K/below which a fatigue crack does not initiate after ten million cycles.

As shown in Fig. 2, the presence of pre-existing corrosion pits significantly reduces the fatigue crack

initiation life and threshold stress intensity. The pitted specimens shown in Fig. 2 were prepared by 336

hrs immersion in 3.5 wt% NaCl solution and should have had a notch root surface morphology similar

to that shown in Fig. 1 and pit populations at the blunt root similar to that reported in Table 1 because

both S surfaces have been exposed to 3.5 wt% NaCl solution for 336 hrs. As shown in Fig. 2, the

presence of these pits not only reduces the number of fatigue cycles required for initiation by a factor

of two to three but also, more importantly, lowers the threshold stress intensity by about fifty percent

when compared to those of specimens with a polished root surface. This is because these pre-existing

Figure 1. Corrosion pit morphology in 7075-T7351 after 336 hrs in 3.5 wt% NaCl solution.

TABLE 1

Corrosion Pit Size and Density

Alloy

Exposure Time

(hrs)

Mean Size

(m)

Density

(1/mm2)

7075-T7351 336 30 7.5

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corrosion pits at the blunt root surface act as stress concentration sites at which local stresses are

elevated to facilitate fatigue crack initiation. Post fractographic analyses, as will be discussed in detaillater, confirm that the origin of the fatigue cracking can be traced to these pits.

The fatigue crack initiation data presented in Fig. 2 were obtained from fatigue tests in a laboratory

air environment. Previous studies have established that water vapor in air can significantly increase

fatigue crack growth rates in aluminum alloys (2123). This is because water vapor can react with

freshly fractured aluminum surfaces at the crack tip region to generate hydrogen which in turn will

facilitate fatigue crack growth. The water vapor in air is anticipated to have similar detrimental effectson fatigue crack initiation in high strength aluminum alloys as well and these effects are currently under

investigation.

Post-Fatigue Fractographic Examinations

Fatigue cracks in polished blunt-notched specimens fatigued in air almost all initiated from constituent

particles that were on or near the blunt-notch root surface. An example of a fatigue crack initiating from

large constituent particles is shown in Fig. 3A for a 7075-T7351 aluminum alloy stressed at 276 MPa.

The origin of the crack can be easily traced back to these constituent particles located at the polished

blunt root surface by following the cleavage-like river lines emanating from the particle. Apparently,these large constituent particles effectively acted as stress concentrators to raise the local stresses that

facilitate fatigue crack initiation. Examination of many initiation sites in S-T oriented 7075 alloy

specimens revealed that the sizes of these constituent particles range from a few m to over 30 m.

The notch root surface of blunt-notched specimens that had been previously immersed in 3.5 wt%

NaCl solution for 336 hrs should contain many pre-existing corrosion pits much like those shown in Fig.

1. Because of the large number of these corrosion pits and the effectiveness of these pits as stress

concentration sites during fatigue in air, multiple fatigue crack initiation from these pits is oftenobserved. At higher magnifications, each of the multiple fatigue cracks can be seen to have initiated

from a pre-existing corrosion pit. An example of a fatigue crack initiating from a pre-existing corrosion

pit is shown in Fig. 3B for S-T 7075-T7351 fatigued at 276 MPa. The fatigue region can be easily

identified by its relatively flat features as compared to the dimpled fracture overload region surroundingit. The outline of the pre-existing pit, within the black dashed lines in Fig. 3B, can be distinguished by

the unique mudcake-like appearance associated with the corrosion pits. It is important to note from Fig.

Figure 2. Effect of pre-existing corrosion pits on fatigue crack initiation in S-T oriented 7075-T7351.



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3B that while the pit width at the blunt-notch root surface is only about 60 m, the depth and the interior

width of this particular pit are each over 300

m. Thus, the actual stress concentration from this pit

would be higher than that derived from the surface pit dimensions. Because of these large pre-existing

corrosion pits, fatigue crack initiation lives and fatigue crack initiation threshold stresses of pre-

corroded specimens are significantly inferior to those of polished ones.

4. Conclusions

1. The presence of pre-existing corrosion pits, produced by immersion in 3.5 wt% NaCl solution for

336 hrs, in 7075-T7351 aluminum alloy shortens the fatigue crack initiation life in air by a factor of

two to three and decreases the fatigue crack initiation threshold, (K/)th, by about 50 percent.

2. Fatigue cracks in polished blunt-notched specimens often initiated from large constituent particles

located on the notch root surface. For specimens that contained pre-existing corrosion pits, fatiguecracks always initiated from these corrosion pits.

Acknowledgments

The authors gratefully acknowledge many helpful discussions with Professor R. P. Wei of Lehigh

University and Dr. Ming Gao of Mobil Corporation. This work was supported by the Office of Naval

Research.

References

1. C.-M Liao, J. M. Olive, M. Gao, and R. P. Wei, Corrosion. 54, 451 (1998).

2. G. S. Chen, M. Gao, and R. P. Wei, Corrosion. 52, 8 (1996).

Figure 3. Fatigue crack initiating from (A) a Si-containing particle and (B) a pre-existing corrosion pit.

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3. G. S. Chen, K. C. Wan, M. Gao, R. P. Wei, and T. H. Flournoy, Mater. Sci. Eng. A219, 126 (1996).

4. K. K. Sankaran, B. Johnson, R. Perez, and K. V. Jata, in Proceedings of the 1997 Tri-Service Conference on Corrosion,

NSWC, Bethesda, MD (1998).

5. T. G. Dunford and B. E. Wilde, Field Metallography, Failure Analysis, and Metallography, p. 263, ASM International,Materials Park, OH (1987).

6. R. S. Piascik and S. A. Willard, Fat. Frac. Eng. Mater. Str. 17, 1247 (1994).

7. D. Najjar, T. Magnin, and T. J. Warner, Mater. Sci. Eng. A238, 293 (1997).

8. W. K. Johson, Br. Corros. J. 6, 200 (1972).

9. J. Zahavi, A. Zangvil, and M. Metzger, J. Electrochem. Soc. 125, 438 (1978).

10. D. G. Harlow and R. P. Wei, Eng. Fract. Mech. 59, 305 (1998).

11. K. Nisancioglu, K. Y. Davanger, and . Strandmyr, J. Electrochem. Soc. 128, 1523 (1981).

12. M. A. Alodan and W. H. Smyrl, J. Electrochem. Soc. 144, L282 (1997).

13. R. G. Buchheit, Jr., J. P. Moran, and G. E. Stoner, Corrosion. 46, 610 (1990).

14. A. Saxena and S. J. Hudak, Jr., Int. J. Fract. 14, 453 (1978).

15. J. M. Barsom and R. C. McNicol, Fracture Toughness and Slow-Stable Cracking, STP 559, p. 183, ASTM, Philadelphia,

PA (1974).

16. W. G. Clark, Jr., Fracture Toughness and Slow-Stable Cracking, STP 559, p. 205, ASTM, Philadelphia, PA (1974).17. G. R. Yoder, L. A. Cooley, and T. W. Crooker, in 23rd Structures, Structural Dynamics, and Materials Conference, Paper

No. AIAA 820660-CP, p. 132, AIAA, New York, NY (1982).

18. R. P. Wei, C. M. Liao, and M. Gao, Met. Mater. Trans. A. 29A, 1153 (1998).

19. R. G. Buchheit, J. Electrochem. Soc. 142, 3994 (1995).

20. R. G. Buchheit, R. P. Grant, P. F. Hlava, B. Mckenzie, and G. L. Zender, J. Electrochem. Soc. 144, 2621 (1997).

21. M. Gao, P. S. Pao, and R. P. Wei, Met. Trans. A. 19A, 1739 (1988).

22. R. P. Wei, P. S. Pao, R. G. Hart, T. W. Weir, and G. W. Simmons, Met. Trans. A. 11A, 151 (1980).

23. F. J. Bradshaw and C. Wheeler, Int. J. Fract. Mech. 5, 255 (1969).


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