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Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

Fatigue and Crack Growth in 7050-T7451 Aluminum Alloy Under Constant- and Variable-Amplitude Loading

[+] Author and Article Information
James C. Newman,

e-mail: j.c.newman.jr@ae.msstate.edu

Justin W. Shaw

Mississippi State University,
Mississippi State, MS, 39762

Balkrishna S. Annigeri

e-mail: balkrishna.annigeri@pw.utc.com

Brett M. Ziegler

Pratt & Whitney,
East Hartford, CT, 06118

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received August 12, 2012; final manuscript received August 13, 2012; published online January 8, 2013. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 135(2), 022101 (Jan 08, 2013) (9 pages) Paper No: GTP-12-1328; doi: 10.1115/1.4007755 History: Received August 12, 2012; Revised August 13, 2012

The 7050 aluminum alloy is used in many aerospace structural applications. Previous studies have identified that fatigue cracks develop very rough crack-surface profiles, which cause very high crack-closure levels due to a combination of plasticity, roughness and debris. Previously, tests were conducted on compact, C(T), specimens to generate crack-growth-rate data from threshold to near fracture over a wide range in stress ratios (R). New threshold testing methods, based on compression precracking, were used to generate the data in the near-threshold regime. The plasticity-induced crack-closure model, FASTRAN, was used to correlate the data over a wide range in stress ratios and crack-growth rates from threshold to near fracture. To account for the very high crack-closure levels, a very low constraint factor, like plane-stress conditions, had to be used in the model. In addition, the crack-opening loads were measured during these tests using a local strain-gauge method to generate another ΔKeff-rate curve. These two curves differed only in the near-threshold regime. Herein, fatigue-crack-growth tests were conducted on C(T) specimens under spike overloads and simulated aircraft spectrum loading. Fatigue tests were also conducted on single-edge-notch bend (SEN(B)), specimens over a wide range in loading conditions (constant amplitude and three aircraft spectra). All specimens were machined from a single forged block of 7050-T7451. However, no residual stresses were measured in both the SEN(B) and C(T) specimens. Two European standard spectra were used, but modified to have only tension-tension loading. The purpose of this paper was to evaluate the two different effective stress-intensity factor curves for making crack-growth and fatigue-life predictions. Small-crack theory was used to make fatigue-life predictions using inclusion-particle sizes from the literature. Fatigue predictions on the SEN(B) specimens agreed fairly well (±30%) using a 12-micrometer semicircular initial flaw located at the semicircular-edge notch under all loading conditions, except the model was unconservative (factor of three) on one of the severe aircraft spectra (Mini-TWIST+, Level 1). For the C(T) specimens subjected to single-spike overloads, the life-prediction code also produced much more retardation than observed in the tests. However, the predicted crack-length-against-cycles under the Mini-Falstaff+ spectrum were only about 15% longer than the tests. The discrepancy under the single-spike overloads and the severe aircraft spectra was suspected to be caused by the low constraint factor and/or crack paths meandering around overload plastic zones. Ideally, a roughness-induced crack-closure model; in addition to the plasticity model, would be needed to obtain more reasonable results.

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Figures

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Fig. 1

(a) Compact, C(T), specimen tested and analyzed. (b) Single-edge-notch bend, SEN(B), specimen tested and analyzed.

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Fig. 2

Part of Mini-Falstaff+ load spectrum

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Fig. 3

Part of the Mini-TWIST+ load spectrum

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Fig. 4

Fatigue-crack-growth-rate data on the 7050-T7451 aluminum alloy [30]

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Fig. 5

Correlation of fatigue-crack-growth-rate data using a crack-closure model [30]

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Fig. 6

(a) Load-against-backface-strain on a C(T) specimen tested at low R [30]. (b) Elber's method of determining crack-opening loads [30].

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Fig. 7

Crack-opening loads on typical low and high-R test cases [30]

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Fig. 8

Fatigue-crack-growth-rate data using measured crack-opening loads [30]

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Fig. 9

Load-against-reduced strain after overload of 2.6 Pmax

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Fig. 10

(a) Crack-length-against-cycle results for a repeated spike overload and prediction. (b) Suspected crack path after high overload to grow around plastic zone.

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Fig. 11

Measured and predicted results under Mini-Falstaff+ loading on C(T) coupons

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Fig. 12

Cumulative-distribution function for inclusion-particle depths on 7050-T7451

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Fig. 13

S-N behavior under constant-amplitude (CA) loading for two rate curves

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Fig. 14

S-N behavior under CA loading for various initial discontinuity sizes

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Fig. 15

S-N behavior under Mini-Falstaff+ loading for the two rate curves

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Fig. 16

S-N behavior under Mini-TWIST+ (Level 1) loading for the two rate curves

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Fig. 17

S-N behavior under Mini-TWIST+ (Level 3) loading for the two rate curves

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