The quantiﬁcation of fatigue
crack initiators in aluminium
alloy 7050-T7451 using
Simon A. Barter
Air Vehicles Division, Defence Science and Technology Organisation,
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
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.
Keywords Fatigue cracks, Aerospace engineering, Fatigue, Aluminium alloy, Alloys,
Structural analysis, Aircraft
Paper type Research paper
Modern high-performance military aircraft is designed for economic service lives
usually between 4,000 and 8,000 ﬂight hours. To save weight and improve aircraft
performance, high stresses are allowed in their structures. Consequently, the airframe
is typically life limited by fatigue and critical cracks will usually begin early in the
aircraft’s life. For the high-strength aluminium alloys (AAs) that are most often used
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International Journal of Structural
Vol. 2 No. 3, 2011
q Emerald Group Publishing Limited