March 2003

NASA/TM-2003-212166

ARL-TR-2927

Simulation of Fatigue Crack Initiation at

Corrosion Pits With EDM Notches

Stephen W. Smith

Langley Research Center, Hampton, Virginia

John A. Newman

U.S. Army Research Laboratory

Vehicle Technology Directorate

Langley Research Center, Hampton, Virginia

Robert S. Piascik

Langley Research Center, Hampton, Virginia
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March 2003

NASA/TM-2003-212166

ARL-TR-2927

Simulation of Fatigue Crack Initiation at

Corrosion Pits With EDM Notches

Stephen W. Smith

Langley Research Center, Hampton, Virginia

John A. Newman

U.S. Army Research Laboratory

Vehicle Technology Directorate

Langley Research Center, Hampton, Virginia

Robert S. Piascik

Langley Research Center, Hampton, Virginia
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 1

Abstract 

Uniaxial fatigue tests were conducted to compare the fatigue life of 

laboratory produced corrosion pits, similar to those observed in the 

shuttle main landing gear wheel bolt-hole, and an electro-dischargedmachined (EDM) flaw. EDM flaws are used to simulate corrosion pits 

during shuttle wheel (dynamometer) testing. The aluminum alloy (AA 

7050) laboratory fatigue tests were conducted to simulate the local stress 

level contained in the wheel bolt-hole. Under this high local stress 

condition, the EDM notch produced a fatigue life similar to test 

specimens containing corrosion pits of similar size. Based on the 

laboratory fatigue test results, the EDM flaw (semi-circular disc shaped) 

produces a local stress state similar to corrosion pits and can be used to 

simulate a corrosion pit during the shuttle wheel dynamometer tests.

Introduction 

During a scheduled maintenance inspection of an orbiter (shuttle vehicle), visual 

inspection of forged aluminum alloy (AA 7050) main landing gear (MLG) wheels revealed small 

regions of localized corrosion (pitting) in the tie-bolt holes shown in Figure 1. The photograph 

in Figure 2 shows a typical tie-bolt hole containing corrosion pits. Inspections revealed that most 

pits were less than 0.5 mm (0.02 inch) deep, but a few pits were as deep as 1.0 mm (0.04 inch). 

A common procedure in the aerospace industry is to remove corrosion pitting by grinding. 

However, if any corrosion damage is still present after assembly, increased local stress at the pits 

may result in reduced fatigue strength of the component [1], i.e., fatigue cracking may occur 

during shuttle roll out, landing, etc. 

To determine the effect of pitting on shuttle wheel fatigue life, a series of shuttle wheel 

dynamometer fatigue tests were planned. However, there were no MLG wheels available for 

testing containing corrosion pits consistent with the “worst case” in-service damage. Therefore, 

it was necessary to produce such a flaw in each MLG wheel article prior to testing. For 

aluminum alloys, corrosion pit configuration is associated with local microstructure (constituent 

particles, precipitates, and grain boundaries). Consequently, the location of corrosion pits is 

difficult to control and a wide range of pit morphologies can result when components are 

corroded, even in a laboratory environment. Additionally, it is difficult to control the location of 

individual pits. Therefore, the generation of corrosion pits on MLG wheels may result in the 

formation of pits that are not consistent with those produced in-service; thereby, resulting in tests 
 2

that do not replicate service conditions. Because manufactured defects (electro-discharge 

machined (EDM) notches) can be fabricated more consistently than corrosion pits and there are 

limited MLG wheels available for testing, EDM notches were machined in MLG wheels to 

simulate the local stress states produced by corrosion pits. However, this approach raises the 

question of whether the fatigue behavior of MLG wheels containing EDM notches is similar to 

that of in-service components. The present investigation was undertaken to answer this question. 

The configuration of each corrosion pit affects the initiation and nucleation of fatigue 

cracks [1]. This work was performed to determine if an EDM notch will result in a similar 

reduction in fatigue strength of an AA7050 component containing corrosion pits similar to those 

observed in the MLG wheels (See Figure 3). Mechanical test specimens containing either an 

EDM notch or a corrosion pit were examined under fatigue loading at stress levels representative 

of the service loading in the bolt-hole region and the fatigue life of each specimen was 

determined. 

Test Procedure 

The shuttle MLG wheel is forged and constructed of aluminum alloy AA 7050. Because 

the forged wheel configuration is complex, it is extremely difficult to simulate the exact 

microstructural attributes (e.g., constituent particles, precipitates, and grain boundaries) that 

produce corrosion pits of the size, shape and morphology similar to corrosion pits observed in 

the fastener hole region. Therefore, axially loaded fatigue specimens were machined from 

AA7050 plate in two different orientations. (See Figure 4.) Both specimen configurations were 

produced with the loading axis parallel to the rolling direction and with a nominal gage section 

that was 25.4 mm (1.0 inch) wide and 12.7 mm (0.5 inch) thick. The defect (either an EDM 

notch or corrosion pit) was located in the center of the gage section on a 25.4 mm wide surface 

as indicated by the hatched region shown in Figure 4 (one side only). For L-S orientation 

specimens (loading axis parallel to the longitudinal direction and the major crack growth 

direction parallel to the short-transverse direction), defects were produced on a surface of the 

specimen parallel to the longitudinal and long-transverse directions. For L-T orientation 

specimens (loading axis parallel to the longitudinal direction and the major crack growth 

direction parallel to the long-transverse direction), defects were produced on a surface of the 
 3

specimen parallel to the longitudinal and short-transverse directions. Because of the grain 

structure in a rolled plate, corrosion pits produced in L-S and L-T specimen orientations result in 

a wide range of configurations and morphologies. The L-S specimen orientation exposes fewer 

grain boundaries leading to broad shallow pits that are nearly hemispherical in shape (lower local 

stress concentration factor, kt). The L-T specimen orientation, on the other hand, exposes more 

grain boundaries, constituent stringers, etc., leading to more irregular shaped and possibly 

elongated pits (higher local kt). The EDM notches used to simulate a corrosion pit stress 

concentration were semicircular with a height of approximately 0.254 mm (0.01 inch), a depth 

(a) and surface width (2c). (See Figure 5.) The depth and width of each EDM notch and 

corrosion pit is summarized in Tables 1 and 2. 

To produce corrosion pits, specimens were coated with a protective wax and a small 

“pinhole” in the wax was introduced to expose a small region in the center of the specimen gage 

section (hatched region in Figure 4). The specimens were then immersed in an aqueous solution 

containing 3-g/L sodium chloride (NaCl) and 0.11 N nitric acid (HNO3). A graphite counter 

electrode was also placed into the solution and electrically coupled to the test specimens and 

potentiostat. Three specimens were coupled together and corroded simultaneously by applying 

an anodic current of 1.0 mA for the L-S specimens and 2.5 mA for the L-T specimens. A larger 

current was applied to the L-T specimens to compensate for a lower corrosion rate observed for 

the L-T orientation compared to the L-S orientation. To produce a variety of pit sizes in the 

desired range sizes, 0.5 mm (0.02 inch) < a < 1.5 mm (0.06 inch), total exposure times were 

varied from 24 to 96 hours. 

Following the introduction of surface defects (EDM notch or corrosion pit), specimens 

were fatigue tested using a closed-loop servo-hydraulic test machine. Specimens were cyclically 

loaded (constant amplitude) at a maximum load of Pmax = 100 kN (22,500 lbs) and a load ratio, R 

= 0.05 (Pmin = 5 kN or 1,125 lbs). A maximum remote stress of σmax = 310 MPa (45 ksi), 

approximately 70% of the yield stress (σy = 452 MPa [2]), was used to simulate the service stress 

in the shuttle MLG wheel bolt-hole region.1

 Testing was performed in room-temperature 

laboratory air at a loading frequency of 4 Hz. Periodically, fatigue loading was stopped for 

approximately 1 minute (specimen was held at mean load, Pmean = 52.5 kN (11.81 kips)) while 

visual surface crack length, 2c, measurements were made. Fatigue loading continued until 

 

1

 NASA - Johnson Space Flight Center (JSC) specified the maximum remote stress level. 
 4

specimen failure occurred, i.e., the specimen fractured into two pieces. The fractured surfaces of 

each specimen were examined using a scanning electron microscope (SEM) and the EDM notch 

or corrosion pit dimensions were measured. 

Results and Discussion 

Surface crack-length versus load cycle (2c versus N) data is plotted in Figures 6a and 6b 

for L-S and L-T specimens, respectively. All fatigue cracks exhibited similar fatigue crack 

growth rate (d2c/dN) characteristics. The fatigue test results shown in Figure 6 indicate a large 

variation in the number of load cycles required to produce a detectable fatigue crack. The 

number of cycles to initiate a visually detectable surface fatigue crack, Ni, and the number of 

cycles at specimen failure, Nf, are listed in Tables 1 and 2. In all cases, specimen failure was 

caused by fatigue cracks that initiated from the gage-section flaw (corrosion pit or EDM notch) 

and propagated by sub-critical fatigue crack growth (typical crack length at fracture was 19 mm 

(0.75 inches)) followed by final facture. The crack surfaces and typical EDM flaw 

configurations for two L-S oriented specimens are shown Figure 7. The EDM flaw 

configuration shown here is also representative of those in specimens of the L-T orientation. 

Some EDM notches exhibited small irregularities; typical irregularities are noted (arrows) in 

Figure 7. These small irregularities had no detectable effect on fatigue crack shape or growth 

rate. 

The SEM micrographs shown in Figures 8 and 9 reveal the corrosion pit morphology and 

the region of fatigue crack initiation for specimens in the L-S and L-T orientations, respectively. 

The micrographs were used to estimate the corrosion pit shape (dashed lines) characterized by 

values of ai and 2ci listed in Tables 1 and 2. The corrosion pits in the L-S specimens (Figure 8) 

were nearly hemispherical in shape. The region of fatigue crack nucleation in the L-T specimens 

shown in Figures 9a, c and d contained multiple pits. The dashed line estimates an “affected pit 

region”, which is characterized by ai and 2ci as listed in Table 2. In Figures 9b and e, dashed 

lines outline a single pit; the single pit was likely formed when multiple smaller pits (similar to 

those in Figures 9a, c and d) coalesced. The pits shown in Figures 9b and e exhibit a greater 

aspect ratio (depth/width ratio) than pits in the L-S specimen orientation (Figure 8). The 

configuration of the pits in the L-T orientation is a result of greater grain boundary exposure for 
 5

pitting compared to that found in the L-S specimens. For both the L-S and L-T orientations, it is 

likely that multiple small fatigue cracks nucleated along the irregular pit surface. The small 

fatigue cracks rapidly coalesced into a single crack having a stable (nearly semi-circular in 

shape) crack front configuration. The single surface fatigue crack continued to propagate until 

unstable crack growth was followed by fracture. Figure 10 is a metallographic cross-section of 

the pit shown in Figure 8b. Here, the fracture surface was removed by polishing; the micrograph 

shows a cross-section of the corrosion pit and its morphology on the plane slightly below but 

very near the fatigue crack surface. The root of this L-S specimen pit exhibits an irregular 

surface and local intergranular attack (See Figure 10b.) similar that observed in the MLG wheel 

pits (Compare with Figure 3.). 

Plots of initial flaw depth (ai) versus the cycles to failure (Nf) for L-S and L-T orientation 

specimens are displayed in Figures 11 and 12, respectively. These data show that specimens 

containing EDM notches exhibit comparable fatigue behavior to specimens containing corrosion 

pits. Here, a similar increase in fatigue life is observed with decreasing EDM (closed symbols) 

and pit (open symbols) flaw depth. Figure 13 reveals that the fatigue properties for L-S and L-T 

orientation specimens containing EDM and pit flaws are similar. These results show that at a 

high local stress level fatigue behavior is largely insensitive to significant differences in flaw 

morphology. At the maximum remote stress of σmax = 310 MPa (45 ksi), the local stress at the 

surface flaw results in yielding at the root of the flaw. Thus, crack nucleation is so rapid at this 

high stress level that geometric differences between EDM flaws and corrosion pits are a second 

order effect. 

Concluding Remarks 

Test results showed that corrosion pits, with a variety of configurations and 

morphologies, and EDM notches produce similar fatigue behavior in both L-S and L-T 

orientation specimens of AA 7050. As a result of high local stress levels and local yielding, 

fatigue cracks rapidly nucleated at both EDM flaws and corrosion pits, leading to similar fatigue 

lives. Because crack nucleation is rapid at high local stress levels, flaw geometry and other 
 6

effects are likely to become second order.2

 At lower local stress levels, these effects may 

become first order and alter the fatigue life. Based on the results of laboratory fatigue tests 

conducted at a high stress level, representative of service loading in the bolt-hole region of MLG 

wheels, EDM notches produce a local stress state similar to that for corrosion pits. Thus, EDM 

notches can be used to simulate the effect of corrosion pits during the shuttle wheel 

dynamometer tests. 

References 

1. G.H. Bray, R.J. Bucci, E.L. Colvin, and M. Kulak. “Effects of Prior Corrosion on the S/N Fatigue Performance 

of Aluminum Sheet Alloys 2024-T3 and 2524-T3”, Effects of the Environment on the Initiation of Crack 

Growth, ASTM STP 1298, W.A. Van Der Sluys, R.S. Piascik, and R. Zawierucha, Eds., ASTM, 1997, pp. 89-

103. 

2. J. R. Davis, Editor. Aluminum and Aluminum Alloys, ASM International, Materials Park, OH, 1993, pp. 694-697. 

 

2

 Other effects may include local embrittlement caused by corrosion, local high kt produced by highly irregular 

corroded surfaces within the corrosion pit, etc. These effects become less important when the pit or EDM flaw is 

dominated by plasticity and crack nucleation and small crack growth rates becomes extremely rapid. 
 7

Table 1. Summary of flaw characteristics and fatigue data for L-S specimens. 

specimen flaw type ai (mm) 2ci (mm) Nf Ni

LS-N-1 EDM notch 1.02 2.37 10,327 1,848 

LS-N-2 EDM notch 0.84 2.32 11,349 2,011 

LS-N-3 EDM notch 0.89 2.32 10,502 3,509 

LS-N-4 EDM notch 0.52 1.43 16,946 5,014 

LS-N-5 EDM notch 0.68 1.49 13,835 3,390 

LS-N-6 EDM notch 1.47 3.17 7,881 1,005 

LS-P-1 Corrosion pit 0.85 2.63 19,698 10,010 

LS-P-2 Corrosion pit 1.24 2.39 10,852 3,504 

LS-P-3 Corrosion pit 0.68 2.64 18,761 6,206 

LS-P-4 Corrosion pit 0.29 1.29 30,472 20,009 

LS-P-5 Corrosion pit 0.91 2.59 10,383 2,009 

Table 2. Summary of flaw characteristics and fatigue data for L-T specimens. 

specimen flaw type ai (mm) 2ci (mm) Nf Ni

LT-N-1 EDM notch 0.51 1.15 17,627 6,005 

LT-N-2 EDM notch 0.79 1.72 12,853 4,523 

LT-N-3 EDM notch 0.83 1.71 12,343 2,513 

LT-N-4 EDM notch 0.96 1.95 12,021 2,516 

LT-N-5 EDM notch 1.09 2.31 11,366 3,513 

LT-N-6 EDM notch 1.31 2.87 9,352 2,011 

LT-N-7 EDM notch 1.59 3.40 7,192 1,524 

LT-P-1 Corrosion pit 0.51 1.14 30,861 24,024 

LT-P-2 Corrosion pit 0.49 0.76 16,425 5,002 

LT-P-3 Corrosion pit 0.21 1.27 32,490 19,024 

LT-P-4 Corrosion pit 0.44 1.62 24,282 18,017 

LT-P-5 Corrosion pit 1.51 2.29 10,984 7,812 
 8

Figure 1. Schematic of MLG wheel outer half. 

Figure 2. Typical MLG wheel tie-bolt pitting. 

Figure 3. Metallographic cross-section of a MLG wheel tie-bolt hole pit.

tie-bolt

Pitting 

400 μ m
 9

Figure 4. Fatigue specimen configurations for: (a) L-S specimen, (b) L-T specimen. The hatched 

region in the center of the gage section represents the location of the corrosion pit or EDM notch. 

Figure 5. A schematic of the EDM notch configuration.

12.7 mm

(0.5 inches)

101.6 mm

(4.0 inches)

25.4 mm

(1.0 inch)

38.1 mm

(1.5 inches)

38.1 mm

(1.5 inches)

radius

203.2 mm

(8.0 inches) 177.8 mm

(7.0 inches)

FRONT VIEW SIDE VIEW

38.1 mm

(1.5 inches)

19.1 mm

(0.75 inches)

38.1 mm

(1.5 inches)

38.1 mm

(1.5 inches)

25.4 mm

(1.0 inch)

radius

230.2 mm

(9.06 inches)

38.1 mm

(1.5 inches)

38.1 mm

(1.5 inches)

25.4 mm

(1.0 inch)

12.7 mm

(0.5 inches)

FRONT VIEW SIDE VIEW

(a) L-S (b) L-T 

P

ai

2ci

≈0.254 mm 
 10

(a) L-S specimens 

(b) L-T specimens 

Figure 6. Surface crack length (2c) versus load cycle count (N) data plotted for AA 7050 

(a) L-S specimens and (b) L-T specimens. Open and solid symbols are used for pitted and 

EDM notch specimens, respectively. 

N (x 1000 cycles)

0 5 10 15 20 25 30 35 40

2

c (mm)

0

5

10

15

20

25

LS-P-1

LS-P-2

LS-P-3

LS-P-4

LS-N-1

LS-N-2

LS-N-3

LS-N-4

LS-N-5

LS-N-6

LS-P-5

(a) 

N (x 1000 cycles)

0 5 10 15 20 25 30 35 40

2

c (mm)

0

5

10

15

20

25

LT-N-1

LT-N-2

LT-N-3

LT-N-4

LT-N-5

LT-N-6

LT-N-7

LT-P-1

LT-P-2

LT-P-3

LT-P-4

LT-P-5

(b)
 11

Figure 7. Typical fracture-surface SEM micrographs of EDM notched AA 7050 specimens in the L-S 

orientation: (a) LS-N-1 and (b) LS-N-2. Small irregularities were observed for some of the EDM 

notches, as indicated by arrows in the figures. 

(a) 500 μm (b) 500 μm
 12

 

 

Figure 8. Fracture-surface SEM micrographs of corrosion pitted AA 7050 specimens in the L-S 

orientation (a) LS-P-1, (b) LS-P-2, (c) LS-P-3, (d) LS-P-4 and (e) LS-P-5. 

500 μm 500 μm

500 μm 500 μm

(a)

(c) (d)

(b)

(e) 500 μm
 13

Figure 9. Fracture-surface SEM micrographs of corrosion pitted AA 7050 specimens in the L-T 

orientation (a) LT-P-1, (b) LT-P-2, (c) LT-P-3, (d) LT-P-4 and (e) LT-P-5. 

(a) 250 μm (b) 200 μm 

(c) 250 μm (d) 250 μm

(e) 500 μm 
 14

a) b) 

Figure 10. (a) Fracture surface of AA 7050 specimen LS-P-2 in the L-S orientation, polished to reveal corrosion 

pit configuration. (b) Higher magnification view of the root of the pit after the surface has been lightly etched to 

reveal evidence of intergranular cracking. 

Figure 11. Fatigue life data plotted as flaw depth, ai, versus cycles to 

failure, Nf, for AA 7050 specimens in the L-S orientation. Open and 

solid symbols are used for corrosion-pit and EDM notch specimens, 

respectively. 

Nf (x 1000 cycles)

0 10 20 30 40

ai (mm)

0.0

0.5

1.0

1.5

2.0

corrosion pit 

EDM notch 

500 μm 100 μm
 15

Figure 12. Fatigue life data plotted as flaw depth, ai, versus cycles to 

failure, Nf, for AA 7050 specimens in the L-T orientation. Open and 

solid symbols are used for corrosion-pit and EDM notch specimens, 

respectively. 

Figure 13. Fatigue life data plotted as flaw depth, ai, versus cycles to 

failure, Nf, for AA 7050 specimens in the L-S and L-T orientations. 

Open and solid symbols are used for corrosion-pit and EDM notch 

specimens, respectively.

Nf (x 1000 cycles)

0 10 20 30 40

ai (mm)

0.0

0.5

1.0

1.5

2.0

corrosion pit 

EDM notch

Nf ( x 1000 cycles)

0 10 20 30 40

ai (mm)

0.0

0.5

1.0

1.5

2.0

L-S, corrosion pit

L-S, EDM notch

L-T, corrosion pit

L-T, EDM notch
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

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4. TITLE AND SUBTITLE

Simulation of Fatigue Crack Initiation at Corrosion Pits With EDM 

Notches 

5. FUNDING NUMBERS

762-60-61-09 

6. AUTHOR(S)

Stephen W. Smith, John A. Newman, and Robert S. Piascik 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center U.S. Army Research Laboratory 

Hampton, VA 23681-2199 Vehicle Technology Directorate 

 NASA Langley Research Center 

 Hampton, VA 23681-2199

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REPORT NUMBER 

 L-18262 

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National Aeronautics and Space Administration 

Washington, DC 20546-0001 

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 U.S. Army Research Laboratory 

 Adelphi, MD 20783-1145

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NASA/TM-2003-212166 

ARL-TR-2927

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Unclassified-Unlimited 

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13. ABSTRACT (Maximum 200 words)

Uniaxial fatigue tests were conducted to compare the fatigue life of laboratory produced corrosion pits, similar 

to those observed in the shuttle main landing gear wheel bolt-hole, and an electro-discharged-machined (EDM) 

flaw. EDM flaws are used to simulate a corrosion pit during shuttle wheel (dynamometer) testing. The 

aluminum alloy (AA 7050) laboratory fatigue tests were conducted to simulate the local stress level contained in 

the wheel bolt-hole. Under this high local stress condition, the EDM notch produced a fatigue life similar to test 

specimens containing corrosion pits. Based on the laboratory fatigue test results, the EDM flaw (semi-circular 

disc shaped) produces a local stress state similar to corrosion pits and can be used to simulate a corrosion pit 

during the shuttle wheel dynamometer tests. 

14. SUBJECT TERMS

Corrosion pit, EDM notch, fatigue crack initiation, aluminum alloy 7050 

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20 

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