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Research Papers: Gas Turbines: Structures and Dynamics

Fatigue-Life Prediction Method Based on Small-Crack Theory in an Engine Material

[+] Author and Article Information
James C. Newman

 Mississippi State University, Mississippi State, MS 39762 j.c.newman.jr@ae.msstate.edu

Balkrishna S. Annigeri

 Pratt & Whitney, East Hartford, CT 06118 balkrishna.annigeri@pw.utc.com

J. Eng. Gas Turbines Power 134(3), 032501 (Dec 28, 2011) (8 pages) doi:10.1115/1.4004261 History: Received April 26, 2011; Revised May 04, 2011; Published December 28, 2011; Online December 28, 2011

Plasticity effects and crack-closure modeling of small fatigue cracks were used on a Ti-6Al-4V alloy to calculate fatigue lives under various constant-amplitude loading conditions (negative to positive stress ratios, R) on notched and un-notched specimens. Fatigue test data came from a high-cycle-fatigue study by the U.S. Air Force and a metallic materials properties handbook. A crack-closure model with a cyclic-plastic-zone-corrected effective stress-intensity factor range and equivalent-initial-flaw-sizes (EIFS) were used to calculate fatigue lives using only crack-growth-rate data. For un-notched specimens, EIFS values were 25-μm; while for notched specimens, the EIFS values ranged from 6 to 12 μm for positive stress ratios and 25-μm for R = −1 loading. Calculated fatigue lives under a wide-range of constant-amplitude loading conditions agreed fairly well with the test data from low- to high-cycle fatigue conditions.

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Figures

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Figure 1

Fatigue specimens analyzed

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Figure 2

(a) Fatigue-crack-growth-rate data using the ASTM load-reduction test method. (b) Fatigue-crack-growth-rate data using the CPCA test method. (c) ΔKeff -rate data from the CPCA test method and small-crack estimates.

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Figure 3

Crack-closure behavior for small cracks under low- and high-stress levels

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Figure 4

(a) Small- and large-crack-growth-rate data with LEFM and closure-based relations at R = 0.1. (b) Small- and large-crack-growth-rate data with LEFM and closure-based relations at R = 0.5 and −1.

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Figure 5

Measured and calculated small-crack growth in round bar under constant-amplitude loading

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Figure 6

Assumed sub-surface initiation site (fish-eye) with extremely high rates observed at free-surface crack penetrating location

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Figure 7

Measured and calculated small- and large-crack-growth rates at R = 0.1

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Figure 8

Stress-life and calculated behavior for round bar with KT  = 1

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Figure 9

Stress-life and calculated behavior for flat sheet with KT  = 1

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Figure 10

Stress-life and calculated behavior for flat sheet with KT  = 3

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Figure 11

(a) Stress-life and calculated behavior for double-edge-notch specimens with KT  = 3.06 at R = −1 and 0.1. (b) Stress-life and calculated behavior for double-edge-notch specimens with KT  = 3.06 at R = 0.5 and 0.8.

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