On the Growth of Cracks from Small Naturally Occurring Material Discontinuities

2017-01-09T01:05:38Z (GMT) by Dinaz 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.