The quantification of fatigue crack initiators in aluminium alloy 7050‐T7451 using quantitative fractography

The quantification of fatigue crack initiators in aluminium alloy 7050‐T7451 using quantitative... Purpose – This paper aims to present some aspects associated with the life prediction of structures with fatigue cracks growing from small natural discontinuities in aluminium alloy (AA)7050‐T7451 for a surface condition that is present in F/A‐18 A/B aircraft critical structure. Design/methodology/approach – Fatigue results are presented for thick section AA7050 plate coupons loaded with a representative fighter aircraft wing root bending moment loading spectrum. Detailed quantitative fractography (QF) was used to gain a deeper understanding of issues relevant to an improved fatigue life predictive capacity for this material by using the QF results to investigate the “effectiveness” of the fatigue initiating discontinuities. Findings – Estimates of the “effectiveness” of the fatigue initiating discontinuities as quasi pre‐existing fatigue cracks (“equivalent pre‐crack size” (EPS) here) were made with the aid of a simple crack growth model. This model, based on experience, was found to be valid for the applied spectrum and stress levels used. These stress levels were chosen to represent those that may be found in highly stressed locations of fighter aircraft; and as such would usually lead to the limiting fatigue life of such a structure. Research limitations/implications – The method has been extended to other crack growth situations and is being used to build a database large enough to determine the best probability distribution of the “effectiveness” of the fatigue initiating discontinuities for not only the surface condition reported here but several other surface conditions typical of aircraft metallic structure. Practical implications – The EPS of the discontinuities from which the cracks grew were used to investigate distributions that may be used in a risk‐based assessment using deterministic crack growth measurements from such discontinuities. Some of the problems that remain to be resolved in such an analysis, prior to its use in a risk‐based assessment are discussed. Originality/value – This work improves the understanding of the interaction of small fatigue cracks generated by representative loading spectra with the small discontinuities from which they grow and shows that the fatigue process is remarkably consistent down to very small sizes. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Structural Integrity Emerald Publishing

The quantification of fatigue crack initiators in aluminium alloy 7050‐T7451 using quantitative fractography

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Publisher
Emerald Publishing
Copyright
Copyright © 2011 Emerald Group Publishing Limited. All rights reserved.
ISSN
1757-9864
DOI
10.1108/17579861111162860
Publisher site
See Article on Publisher Site

Abstract

Purpose – This paper aims to present some aspects associated with the life prediction of structures with fatigue cracks growing from small natural discontinuities in aluminium alloy (AA)7050‐T7451 for a surface condition that is present in F/A‐18 A/B aircraft critical structure. Design/methodology/approach – Fatigue results are presented for thick section AA7050 plate coupons loaded with a representative fighter aircraft wing root bending moment loading spectrum. Detailed quantitative fractography (QF) was used to gain a deeper understanding of issues relevant to an improved fatigue life predictive capacity for this material by using the QF results to investigate the “effectiveness” of the fatigue initiating discontinuities. Findings – Estimates of the “effectiveness” of the fatigue initiating discontinuities as quasi pre‐existing fatigue cracks (“equivalent pre‐crack size” (EPS) here) were made with the aid of a simple crack growth model. This model, based on experience, was found to be valid for the applied spectrum and stress levels used. These stress levels were chosen to represent those that may be found in highly stressed locations of fighter aircraft; and as such would usually lead to the limiting fatigue life of such a structure. Research limitations/implications – The method has been extended to other crack growth situations and is being used to build a database large enough to determine the best probability distribution of the “effectiveness” of the fatigue initiating discontinuities for not only the surface condition reported here but several other surface conditions typical of aircraft metallic structure. Practical implications – The EPS of the discontinuities from which the cracks grew were used to investigate distributions that may be used in a risk‐based assessment using deterministic crack growth measurements from such discontinuities. Some of the problems that remain to be resolved in such an analysis, prior to its use in a risk‐based assessment are discussed. Originality/value – This work improves the understanding of the interaction of small fatigue cracks generated by representative loading spectra with the small discontinuities from which they grow and shows that the fatigue process is remarkably consistent down to very small sizes.

Journal

International Journal of Structural IntegrityEmerald Publishing

Published: Aug 30, 2011

Keywords: Fatigue cracks; Aerospace engineering; Fatigue; Aluminium alloy; Alloys; Structural analysis; Aircraft

References

  • The growth of small fatigue cracks in 7075‐T6 aluminum
    Lankford, J.
  • A comparison of crack growth behaviour in several full‐scale airframe fatigue tests
    Molent, L.; Barter, S.A.
  • Damage tolerance demonstration testing for the Australian F/A‐18
    Molent, L.; Barter, S.A.; White, P.; Dixon, B.
  • Evaluation of spectrum fatigue crack growth using variable amplitude data
    Molent, L.; McDonald, M.; Barter, S.A.; Jones, R.
  • Interpreting fatigue test results using a probabilistic fracture approach
    White, P.; Molent, L.; Barter, S.A.

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