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On the Growth of Cracks from Small Naturally Occurring Material Discontinuities
thesis
posted on 2017-01-09, 01:05authored byDinaz Zenobia Tamboli
The objective of this
thesis is to analyse data available in the open literature and conduct
experimental test programs to develop a robust and consistent mechanistic model
that can be used to predict the growth of fatigue cracks in high performance
combat aircraft. Military aircraft worldwide are being operated close to or
beyond their design lives. This, in part, is driven by fiscal pressures and
supported by vast improvements in structural integrity management programs
which include advanced non-destructive inspection techniques, new fatigue
damage assessment tools and radical repair techniques. The risk of failure due
to fatigue, however, remains a key concern for most aircraft operators. This is
because of differences in the fatigue prediction tools used in the design stage
and the problems encountered in service. In particular, the short crack anomaly
appears to be a major reason for this difference. To this end, this thesis
seeks to develop a model which unifies the growth of both long and short cracks
because service experience has shown that for any damage tolerant analysis, the
overall fatigue life should be an amalgam of both long and short fatigue
cracks. Moreover, it is now well known that the majority of the fatigue life of
aircraft structures is spent in the short crack regime which makes it even more
imperative to address this issue.
Solving the short crack anomaly, however, comes with its own
set of challenges. There is a large scatter associated with the growth of short
cracks. This scatter has been observed in cracks growing in specimens made from
the same material to exactly the same specifications under constant amplitude
loading, variable amplitude loading, representative flight spectra and civil
transport spectra. For ongoing risk management of the aging fleet it is
imperative that the model used in any damage tolerance analysis be able to
capture this scatter. Therefore, this thesis also seeks to investigate the
robustness of the chosen mechanistic model in capturing the variability in
crack growth under constant amplitude loading, variable amplitude loading, a
standard fighter aircraft spectrum (FALSTAFF) and a civil transport spectrum
(miniTWIST).
Since fatigue life prediction requires an amalgam of short
and long crack growth curves, it is important not only to address the
variability in short cracks but also those observed in long cracks under
variable amplitude loading and representative flight spectra. Therefore, this
thesis also seeks to address this challenge by using experimental data
available in open literature and the USAF Characteristic K approach to capture
the variability in long cracks. For all the issues detailed above, it was found
that using the Hartman-Schijve crack growth equation, the short crack anomaly
and the variability observed in both long and short cracks were captured fairly
well.
Problems with aircraft are not limited to fatigue cracking
alone. Corrosion plays an equally dominant role in challenges associated with
the life of aircraft in service. While most corrosion related studies thus far
have focussed predominantly on the interaction of corrosion and fatigue, a new
hypothesis has been proposed more recently. Some studies have shown that
corrosion is a ground-based phenomenon while fatigue cracking occurs in flight
and these two phenomena are decoupled. To investigate this hypothesis, an
experimental test program was carried out and is detailed in this thesis. The
resulting crack growth curve was measured via quantitative fractography and
analysed by the Hartman-Schijve equation. The experimental data revealed that
corrosion on ground does not appear to have a significant influence on fatigue
cracking in flight. It should be noted, however, that this hypothesis applies
only to combat aircraft which fly at high altitudes where the air is cold and
dry. For maritime aircraft, which spend extended periods of time flying close
to the sea in a humid environment, this may not hold.
This decoupling of fatigue and environment is only applicable
to pitting corrosion. There are many other forms of corrosion like exfoliation,
intergranular, stress corrosion cracking etc. that still play a major role in
compromising the life of aircraft. It is therefore important to address these
when dealing with the problems plaguing aging aircraft. A secondary focus was
thus given to using Supersonic Particle Deposition (SPD) to seal fastener holes
in lap joints, thereby preventing further environmental ingress. To check the
durability of SPD doublers on service aircraft, 12 SPD doublers were applied to
an ex-service F/A-18 centre barrel and tested to more than 3 times its average
lifetime. Non-destructive inspection (NDI) of these doublers was performed at
regular intervals and no disbonding or delamination was observed even after the
centre barrel had been fatigued for over 15,000 flight hours.
It is hoped that the work detailed in this thesis contributes
to the overall fatigue management practices of combat aircraft.