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Review of F-111 structural materials

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Abstract

The RAAF is now the sole operator of the F-111 and current plans for the fleet will keep the aircraft in service until 2020. The F-111 is a structurally complex aircraft, and its swing-wing geometry in particular requires materials of ultra high strength to handle expected loadings. In particular, the D6ac steel used in most of the critical components in the aircraft was subjected to rigorous research efforts in the early 1970s to better characterise material performance in fatigue. This report summarises many of these efforts to characterise the main alloys in the airframe, namely: D6ac steel and aluminium alloys 2024-T851, 7079-T651, and 7075-T6. The major goal is to study the available data for these critical F-111 materials, evaluate the completeness of the existing data sets and make recommendations for research efforts necessary to ensure that the F-111 fleet is operated as safely and economically as possible until retired. Particular attention is payed to the fact that the RAAF now uses JP-8 fuel rather than the original JP-4 fuel. Short crack behaviour from corrosion damage will likely be a concern for the F-111, particularly in the D6ac steel. Stress corrosion cracking is likely to continue to be the biggest problem for the 7xxx-series aluminium alloy components and will have to be monitored carefully.

Executive Summary

Safety by inspection has been the key to protecting the F-111 fleet since the early 1970s. The ageing of the F-111 fleet in conjunction with a significant increase in planned retirement date has presented a new challenge to the RAAF, namely: maintaining the level of safety from catastrophic failure while still being able to economically operate the fleet. Such a goal requires an in-depth understanding of the structure, materials, and possible failure modes. For instance, structural optimisation aimed to increase inspection intervals for critical structure (which will reduce the maintenance burden) should only be practiced if time-dependant failure modes, such as corrosion, will not become the new life-limiting scenario for that location. With this example in mind, it was imperative to review the literature and assess the state-of-knowledge associated with the F-111 structural materials and make recommendations as to what new information the RAAF may need to continue to meet safety and economic goals. This report gives an overview of the more prominent structural materials in the F-111 aircraft. The most important materials are D6ac steel and 2024- T851 aluminium, in that all the fracture critical components are made from one of these two materials-mostly D6ac. Alloy 7079- T651 has also been included as it is widely used in bulkheads and is very susceptible to stress corrosion cracking. One of the main objectives was to look at available literature data and compare it with the data used by Lockheed to perform the F-111 durability and damage tolerance analysis (DADTA). In some cases, literature data was so sparse that it was essentially limited to the same data sources used by Lockheed. The following observations were made from the review of the D6ac steel data: .The long crack propagation data in the literature seems to agree well with that used by Lockheed. .Crack nucleation studies are woefully deficient, mainly corrosion influences on fatigue. DSTO/ AMRL has important programs in place to address this lack of useful data. .Fatigue crack growth from corrosion pits is the area of primary concern because pitting is the most threatening form of corrosion to D6ac. .Discrepancies between material models and laboratory behaviour of D6ac coupons indicate possible problems with either the near-threshold crack growth data, the validity of the stress intensity solution for small crack sizes, or both. It may be necessary to revisit the material models and threshold crack growth data for D6ac if corrosion is to be accurately incorporated into life predictions. .Particularly damaging to D6ac is the possibility of pitting leading to SCC before transitioning to fatigue or corrosion fatigue. Service examples of this scenario have been uncovered, and the unpredictable nature of SCC makes the situation potentially dangerous. .In the F-lll, stress corrosion cracks have been found perpendicular to the primary load axis, an orientation where interaction with fatigue is of significant risk. Locations where this could occur should be treated very carefully as inspection intervals in such areas could be rendered unconservative. .Fatigue data for D6ac steel covers a variety of chemical environments including laboratory air, humid air, and JP-4 fuel. The F-lll now uses JP-8 fuel, which has different composition and additives, so it may be worth looking at crack propagation, SCC and threshold behaviour in this new chemical environment. The same could be said for aluminium alloy 2024- T851, the wing skin material. No major concerns were raised about available fatigue data for the aluminium alloys found in the F-lll. Aluminium alloys are much more widespread in the aircraft industry than D6ac steel; unfortunately, the aluminium alloys used in the F-lll are an exception. .Alloy 7079- T651 is avoided in new aircraft. The alloy is no longer made, and as such is no longer included in most references for material property and selection. .Alloy 2024 in the T851 temper used on the F-lll is relatively uncommon. Not much literature data was uncovered on this material, but what was found seems to be sufficient for managing the F-lll. .Aluminium alloy 2024- T851 has greatly increased stress corrosion and corrosion fatigue performance. However, the artificially aged variant is very susceptible to corrosion damage. Because of this, it is also vulnerable to fatigue originating from this type of damage. .Programs at DSTO / AMRL and around the world are focused on finding ways to model corrosion damage as an engineering parameter for life prediction. DSTO / AMRL has several programs looking at different types of corrosion damage in various aircraft aluminium alloys. This should provide enough information without starting anything new specifically for the F-lll. .The same concerns for SCC in D6ac apply to the aluminium alloys, particularly the 7xxx- T6xx materials. .Lockheed data in the fatigue crack growth threshold, as compared with literature data elsewhere, shows that the Lockheed values are conservative. By better understanding the behaviour of the critical materials in the F-lll, the safety of the fleet will be maintained or even improved. The informed decisions surrounding dealing with failure modes such as corrosion will allow the F-lll to be managed more economically. The ultimate benefit for the RAAF will be reduced aircraft down time and increased availability.

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