January 2015 

NASA/TM−2015-218672 

 

The Growth of Small Corrosion 

Fatigue Cracks in Alloy 7075 

Robert S. Piascik 

Langley Research Center, Hampton, Virginia 
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January 2015 

NASA/TM−2015-218672 

 

The Growth of Small Corrosion 

Fatigue Cracks in Alloy 7075 

Robert S. Piascik 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA STI Program / Mail Stop 148 

NASA Langley Research Center 

Hampton, VA 23681-2199 

Fax: 757-864-6500 

The use of trademarks or names of manufacturers in the report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration. 
iii 

Table of Contents 

1.0 Introduction .......................................................................................................................... 1

2.0 Experimental Procedures ..................................................................................................... 1

3.0 Experimental Results and Discussion .................................................................................. 3

3.1 Fatigue Crack Growth Rate ................................................................................................. 3

3.2 Small Crack Fractography ................................................................................................... 4

4.0 Conclusions .......................................................................................................................... 5

5.0 References ............................................................................................................................ 6

List of Figures 

Figure 1. ECT specimen. ........................................................................................................ 2 

Figure 2. Corrosion fatigue setup. .......................................................................................... 2

Figure 3. The fatigue crack growth characteristics of small cracks in air. ............................. 3 

Figure 4. The corrosion fatigue crack growth characteristics of small and long cracks in 

deaerated salt water. ................................................................................................ 4 

Figure 5. Fatigue crack/initiation. .......................................................................................... 5 

Figure 6. Surface pitting. ........................................................................................................ 5 
iv 

Nomenclature 

a Crack Length 

Ag/AgCl Silver/Silver Chloride 

da/dN Crack Growth Rate

ECT Extended Compact Tension 

L-T Longitudinal-Transverse 

NaCl Sodium Chloride 

K Crack-tip Stress Intensity Factor 

P Load 

R Stress Ratio 
Abstract 

The corrosion fatigue crack growth characteristics of small 

(>35 μm) surface and corner cracks in aluminum alloy 7075 is 

established. The early stage of crack growth is studied by 

performing in situ long focal length microscope (500×) crack 

length measurements in laboratory air and 1% sodium chloride 

(NaCl) environments. To quantify the “small crack effect” in the 

corrosive environment, the corrosion fatigue crack propagation 

behavior of small cracks is compared to long through-thethickness cracks grown under identical experimental conditions. 

In salt water, long crack constant Kmax growth rates are similar 

to small crack da/dN.

1.0 Introduction

The propagation of small fatigue cracks from surface defects (5 μm to 10 μm) constitutes a large 

percentage (50% to 90%) of the total fatigue life of structural components [1]. Thus, accurate 

prediction of small crack da/dN is required for damage tolerant based life predictions. A problem 

arises because small cracks propagate at unpredictable accelerated rates compared to long fatigue 

cracks [2]. The difference in long and small crack growth behavior is due to numerous effects 

(i.e., diminished crack-tip shielding (closure) effects at small crack sizes). Compounding the 

small crack problem are complex environmental effects. Research on high strength steels in salt 

water has shown that chemically short (< 500-μm) cracks propagate 1.5 to 500 times faster than 

long cracks subjected to the same mechanical driving force [3]. Limited data on 2000 series 

alloys exposed to deaerated salt water suggest little chemical-crack-length effect [4]. The 

purpose of this work is to study the chemical crack length effect in environmentally sensitive 

alloy 7075-T6. 

2.0 Experimental Procedures 

Constant amplitude fatigue crack growth experiments were conducted using a pin-loaded 

extended compact tension (ECT) specimen (also known as the eccentrically loaded single-edge 

cracked tension specimen) as shown in Fig. 1. The blunt notched alloy 7075-T6 sheet specimens 

were oriented in the longitudinal-transverse (L-T) direction. The growth of small surface and 

corner cracks located at the root of the polished (through 0.3-μm diamond paste) blunt notch 

were monitored in situ by using the long focal length (×500 magnification) microscope shown in 

Fig. 2. The surface crack length (a) was measured to an accuracy of approximately 3 μm in air 

and 5 μm in salt water. The acetate replica method [5] was also used to verify microscope-based 

surface dimension of small cracks at the root of the notch. Crack length measurements were 

conducted while the specimen remained at 70% of maximum load. Small surface and corner 

crack growth rates and stress intensity factors were calculated assuming uniform semicircular 

crack geometry and procedures documented elsewhere [4]. 
2 

Figure 1. ECT specimen. 

Figure 2. Corrosion fatigue setup. 

Small crack experiments in laboratory air were conducted at constant ΔP and stress ratio (R) of 

0.05 with a minimum crack length resolution of 35 μm. All corrosion fatigue tests were 

performed using the similar loading history; crack initiation was performed at constant ΔP 

(R = 0.05) and small crack growth testing was conducted at low and high R by varying Pmin at 

constant Pmax. Constant Pmax testing eliminated possible load history effects when varying ΔK. 

Reduced in situ crack length measurement resolution limited the study of corrosion fatigue 

cracks to a ≥100-μm and to high R (0.7 ≤ R ≤ 0.8) testing for the investigation of low ΔK 

environmental da/dN. 

The direct current electrical potential drop method was used to monitor the growth of throughthe-thickness cracks. The computer-controlled long crack experiments were conducted by 

y

c

D = 0.2W

d = 0.2W 1.85W

1.5W

W

P

P

X2

X

W = 38.0 mm

X = 12.7 mm

r = 3.18 mm

Radius (r)

Microscope Chamber
3 

continuously adjusting the load to maintain the programmed ΔK and R. The decreasing ΔK test 

was conducted at variable R (0.2 ≤ R ≤ 0.09) at a constant Kmax (15 MPa√m). 

A 1.5-litre O-ring sealed plastic chamber, shown in Fig. 2, was used for salt-water corrosion 

fatigue experiments. The mid-half of the ECT specimen was continuously immersed in helium 

deaerated 1% NaCl (pH = 8) solution. All aqueous NaCl experiments were conducted at constant 

electrode potential using a potentiostat, two fully immersed platinum counter electrodes, and a 

silver/silver chloride (Ag/AgCl) reference electrode. All deaerated corrosion fatigue experiments 

were conducted at a constant electrochemical potential of −830 mVSCE (approximately 

100 mVSCE cathodic of the breakaway or pitting potential). The distilled water/salt solution was 

continuously circulated through the test chamber at a rate of 30 ml/min. from a reservoir 

containing 25 liters of argon deaerated NaCl solution. A quartz window located on the side of the 

chamber and adjacent to the notch was used for in situ long focal length microscope viewing of 

the notch root surface fatigue cracks. 

3.0 Experimental Results and Discussion 

3.1 Fatigue Crack Growth Rate 

Figure 3 shows the crack growth characteristics of small surface and corner cracks in air at 

R = 0.05. Here, data are shown for a range of crack lengths, 10 μm ≤ a ≤ 1000 μm for replicabased measurements and 35 μm ≤ a ≤ 1400 μm for microscope-based measurements. Typical of 

small crack data, the scatter is presumed to be a result of crack-front/microstructure interaction 

[2]. A power law fit of the microscope and replica data (solid and dotted lines, respectively) 

show that both measurement techniques yield nearly identical small crack da/dN for alloy 7075. 

Further comparisons with literature small crack data (dashed line power law fit) for similar low 

R (R = 0) show that the ECT results correlate well with replica-based data from single edge 

notch tests [6]. Long crack growth characteristics (R = 0) are similar to small crack da/dN for 

ΔK > 3 MPa√m, but small cracks exhibit accelerated da/dN in the long crack growth threshold 

regime. 

Figure 3. The fatigue crack growth characteristics of small cracks in air.

Range of Stress Intensity Factor (MPa m1/2)

1 10

Fatigue Crack Growth Rate (mm/cycle)

10-7

10-6

10-5

10-4

10-3

Alloy 7075-T6 Laboratory Air

Microscope Data

Replica Data

Replica (NASA/CAE)

Long Crack (NASA/CAE)
4 

The results in Fig. 4 show that small cracks ranging in size from 100 μm to 1400 μm exhibit 

accelerated fatigue crack growth rates in deaerated 1% NaCl compared to laboratory air at a 

loading frequency of 5 Hz. A comparison of the salt water and air power law fit (solid and 

dashed lines, respectively) show that fatigue crack growth rates are accelerated by a factor of 

three in NaCl. The corrosion fatigue crack propagation behavior of small cracks is also compared 

to long crack da/dN shown in Fig. 4. Long crack growth rates at a constant Kmax = 15 MPa√m 

are essentially closure free, accelerated compared to R = 0.1 da/dN, and envelop the majority of 

small crack data; similarly, small and long crack behavior in deaerated salt-water suggests no 

crack length chemical effect. The increase in long crack constant Kmax and R da/dN for 

ΔK > 7 MPa√m may be a result of stress corrosion cracking effects. 

Figure 4. The corrosion fatigue crack growth characteristics of small and long cracks in 

deaerated salt water.

3.2 Small Crack Fractography 

Figure 5 shows the region of crack initiation and the typical morphology of a semicircularshaped corrosion fatigue surface crack (a = 355 μm) that was exposed to deaerated NaCl 

(−800 mV). The micrographs are oriented normal to the fatigue crack surface and the surface of 

the blunt notch is located at the bottom of both micrographs. The fatigue crack initiated at a 

corrosion pit located at the root of the blunt notch (arrow shown in Fig. 5a). Following initiation, 

the crack propagated along a transgranular semicircular-shaped crack path marked by the dashed 

lines in Fig. 5a. The high magnification micrograph in Fig. 5b shows that the fatigue crack 

initiated at a corrosion pit approximately 10 μm in depth. The pit is elongated in shape with 

microcrack-like features emanating from the region of greatest pit depth. Figure 6 shows typical 

examples pit surface morphology. The micrographs in Fig. 6 are oriented normal to the corroded 

blunt notch surface and in the highly stressed region very near the fatigue crack. Here, both 

micrographs show that surface pits, 10 μm to 20 μm in size, contain regions of microcracking. 

The pit shown in Fig. 6b exhibits a microcrack-like morphology similar to the subsurface pit 

morphology observed in Fig. 5b. 

Alloy 7075-T6, 1% NaCl, -830 mVSCE , f=5 Hz

Range of Stress Intensity Factor (MPa m1/2)

1 10

Fatigue Crack Growth Rate (mm/cycle)

10-8

10-7

10-6

10-5

10-4

10-3

10-2

Small Crack - NaCl

Small Crack - Air

NaCl - Const. Kmax = 15 MPa M1/2 

Air - R = 0.05 (NASA/CAE)

NaCl - R = 0.1

Long Crack Data
5 

Figure 5. Fatigue crack/initiation. 

Figure 6. Surface pitting.

4.0 Conclusions 

No chemical crack length effect was observed. For deaerated salt-water solution, small surface 

and corner fatigue cracks exhibited similar crack growth rates as those observed for long thoughthe-thickness cracks. Small cracks exposed to salt water exhibited a factor of three increase in 

200 μm

Initiation Site

10 μm

(a)

(b)

20 μm

10 μm (a)

(b)
6 

da/dN compared to small crack growth rates in air. A similar factor of three increase in long 

crack da/dN (R = 0.1) was observed in NaCl compared to long crack growth rates (R = 0.05) in 

air. Presumably, small crack growth rate data shown herein was truncated near a ΔK = 1 MPa√m 

due to crack length resolution. It is likely that small cracks grow at accelerated rates for 

ΔK < 1 MPa√m. 

5.0 References 

1. Newman, J. C., Jr.; Phillips, E. P.; Swain, M. H.; and Everett, R. A., Jr.: Fatigue 

Mechanics: An Assessment of a Unified Approach to Life Prediction, pp. 5-27, 

“Advances in Fatigue Lifetime Predictive Techniques,” eds. Mitchell, M. R.; and 

Landgraf, R. W., ASTM STP1122, 1992. 

2. Ritchie, R. O.: “Small Fatigue Cracks,” ed. J. Lankford, TMS-AIME, Warrendale, PA, 

1986. 

3. Gangloff, R.; and Wei, R.: in “Small Fatigue Cracks,” ed. J. Lankford, TMS-AIME, 

Warrendale, PA, 1986, pp. 239-264. 

4. Piascik, R. S.; and Willard, S. A.: “The Growth of Small Corrosion Fatigue Cracks In 

Alloy 2024,” Journal of Fatigue & Fracture of Engineering Materials & Structures, Vol. 

17, No. 11, pp. 1247-1259, 1994. 

5. Swain, M. H.: Monitoring Small-Crack Growth by the Replication Method, pp. 34-56, 

“Small-Crack Test Methods,” eds. Larsen, J. M.; and Allison, J. E., ASTM STP1149, 

1992. 

6. Newman, J. C., Jr.; Wu, X. R.; Venneri, S. L.; and Li, C. G.: Small Crack Effects in 

High-Strength Aluminum Alloys, NASA-RP-1309, 1994. 
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The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

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Piascik, Robert S.

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NASA Langley Research Center

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14. ABSTRACT

The corrosion fatigue crack growth characteristics of small (>35 m) surface and corner cracks in aluminum alloy 7075 is 

established. The early stage of crack growth is studied by performing in situ long focal length microscope (500×) crack length 

measurements in laboratory air and 1% sodium chloride (NaCl) environments. To quantify the "small crack effect" in the 

corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long 

through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant Kmax growth 

rates are similar to small crack da/dN.

15. SUBJECT TERMS

Fatigue Crack Growth; Corrosion; Fractography; Extended Compact Tension; Aluminum Alloy 

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