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TECHNICAL PAPERS

A Mathematical Model for Competing Failure Modes in Strain Cycle Fatigue

[+] Author and Article Information
Gerald T. Cashman

 GE Aviation, 1 Neumann Way M’D G-50, Cincinnati, OH 45215jerry.cashman@ge.com

J. Eng. Mater. Technol 129(2), 293-303 (Nov 20, 2006) (11 pages) doi:10.1115/1.2712468 History: Received August 02, 2005; Revised November 20, 2006

Elevated temperature data for powder metallurgy alloy René 95 generated in vacuum are presented to demonstrate that the life differences observed between surface and internally initiated failures are due to an environmental effect. The transition in behavior from a mode at low stress dominated by internal initiations to a surface dominated mode at high stress is quantitatively described in terms of both a weakest-link model and a local strain relationship. A fatigue failure mechanism is provided that explains that the natural selection of initiation site is based upon the concept that the site displaying the highest local cyclic plastic strain is the location where fatigue initiates.

Copyright © 2007 by American Society of Mechanical Engineers
Topics: Fatigue , Stress , Failure
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References

Figures

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Figure 1

Linear fatigue response

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Figure 2

Curvilinear fatigue response

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Figure 3

Strain controlled low cycle fatigue data for René 95 at 399°C(750°F)

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Figure 4

Strain controlled low cycle fatigue data for René 95 at 538°C(1000°F)

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Figure 5

Microstructure of forged René 95 (500×)

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Figure 6

Low magnification SEM fractograph of a typical surface fatigue initiation from a ceramic inclusion

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Figure 7

High magnification SEM fractograph of Fig. 6 surface initiation site

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Figure 8

Low magnification SEM fractograph of a typical internal fatigue initiation site

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Figure 9

High magnification SEM fractograph (backscatter) of Fig. 8 internal initiation site

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Figure 10

Probability of surface initiation as a function of pseudostress amplitude for René 95 at 399°C(750°F) and 538°C(1000°F)

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Figure 11

Sketch of specimen used for all LCF data

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Figure 12

LCF data generated from donated forging plotted with base line data

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Figure 13

Electropolished LCF specimens plotted with base line data

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Figure 14

Vacuum LCF results plotted with base line data

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Figure 15

Weibull probability plot for the distribution of inclusion areas measured at the fatigue initiation sites for surface failures

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Figure 16

Weibull probability plot for the distribution of inclusion areas measured at the fatigue initiation sites for internal failures

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Figure 17

Normal probability plot for the c∕a ratios observed in the LCF specimens

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Figure 18

Simplified approximation to the grid used in the weakest-link analysis

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Figure 19

Results for the weakest-link model at 399°C(750°F) and 599°C(1000°F) versus the corresponding logistic models

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Figure 20

βk as a function of 2σa∕σty for the probability of surface initiation at 399°C(750°F) and 538°C(1000°F)

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Figure 21

Early fatigue cracking occurring from a sharp corner on an inclusion. Corner is perpendicular to the applied stress (see Ref. 28).

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Figure 22

Later fatigue cracking occurring from a sharp corner on an inclusion. Corner is perpendicular to the applied stress (see Ref. 28).

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Figure 23

Probability of surface initiation as a function of pseudostress amplitude comparing the results of the local strain analyses versus the logistic models

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Figure 24

Ratio of the fatigue notch sensitivity factor (Kf) to the stress concentration factor as a function of pseudostress amplitude

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