Project 3 Environmental Durability of Adhesive Bonds Report No 9 Forensic Studies of Adhesive Joints. Part 2 - Bonded Aircraft Structure. A Beevers September 1995 PART 2 - Bonded Aircraft Structure Contents 1. INTRODUCTION..........................................................................................1 2. HISTORY OF THE BONDED SAMPLES.....................................................2 3. HISTORICAL DEVELOPMENT OF REDUX ADHESIVES...........................5 3.1 Origins.................................................................................................................................... 5 3.2 Aero Research And De Havilland ......................................................................................... 5 3.3 Other Users ............................................................................................................................ 6 3.4 Redux Film ............................................................................................................................. 6 3.5 Comparison Of Redux Variants ............................................................................................ 7 4. MANUFACTURING PROCEDURES............................................................9 4.1 Surface Treatment ................................................................................................................. 9 4.2 Adhesive Application ........................................................................................................... 10 4.3 Curing Procedure................................................................................................................. 10 4.4 Inspection And Testing ........................................................................................................ 11 5. MANUFACTURERS’ PERFORMANCE DATA..........................................13 6. EXPERIMENTAL ASSESSMENT..............................................................15 6.1 Supply Of Test Materials..................................................................................................... 15 6.2 Preparation Of Test Pieces .................................................................................................. 16 6.3 Experimental Test Schedule ................................................................................................ 16 7. RESULTS ..................................................................................................18 7.1 Lap Shear Tests ................................................................................................................... 18 7.2 Wedge Tests ......................................................................................................................... 19 7.3 Lap Shear Durability Tests.................................................................................................. 22 7.4 Non-Destructive Tests (NDT)............................................................................................... 24 7.5 Material Characterisation And Analysis ............................................................................ 24 ii 8. DISCUSSION .............................................................................................32 9. CONCLUSIONS.........................................................................................33 10. REFERENCES.........................................................................................33 APPENDIX 1 ABSTRACTS FROM PROCESS SEPCIFICATION APPENDIX 2 MANUFACTURERS’ PERFORMANCE DATA iii PART 2 - Bonded Aircraft Structure. 1. INTRODUCTION This report is based on the findings of an extensive analytical study of adhesive bonded joints recovered from a “scrapped” Nimrod/Comet 4C aircraft which was built in 1963. The project was primarily intended to establish the condition of old bonded joints and to document the history of their manufacture and subsequent service. The investigation forms part of a large-scale forensic study of bonded joints from a wide range of different application areas which in turn is aimed to supplement a programme on measurement of durability. The approach consisted of a range of experimental strength tests, ultrasonic ND inspection, microanalysis of the joints and the fracture surfaces of test pieces, and further accelerated ageing of old joints. The adhesive material which forms the basis of the study is one of the Redux group of adhesives, ‘Redux’ being a generic term describing the CIBA phenol- formaldehyde/polyvinyl-formal structural adhesive systems. Archival and documentary data was obtained from an engineer who worked on the development and application of Redux material during the period of the construction of the Comet. Redux adhesives have continued to be used in the aerospace industry, and their environmental performance has been assessed in long-term projects carried out by Royal Aircraft Establishment. Summary data from this RAE investigation has been provided by DRA Farnborough. The forensic study was given an additional focus by the interests of BAe in assessing the potential future long-term life of Comet/Nimrod airframe structures. BAe provided the original Comet bonded structure material for this analysis and also procured recent Redux 775 bonded joints from a current manufacturing programme for comparison. BAe also supplied comprehensive data on manufacturing process specifications, test standards and related documentation. The accumulated evidence presented in this report is therefore derived from many different sources, and acknowledgements are extended to the following providers: A Bush AIM Aviation (ex CIBA) R A Harborne British Aerospace Defence Ltd (Farnborough) N Blackford British Aerospace Defence Ltd (Farnborough) P Higgins British Aerospace Regional Aircraft (Woodford) B Parker DRA Farnborough J G Moggeridge National NDT Centre, AEA Technology, Harwell. 1 2. HISTORY OF THE BONDED SAMPLES The Nimrod airframe is a military development of the De Havilland Comet passenger airliner which dates back to the early 1960s. This was the first transatlantic pressurised fuselage jet airline to enter commercial service. Accommodation was for between 71- 101 passengers and was originally powered by four “buried” Rolls-Royce Avon turbo- jets. The Nimrod aircraft utilised much of the original Comet airframe including the fuselage, wing and empennage. Installed in the cabin in place of the passenger seating is the necessary detection and navigation equipment, together with its operators, to perform the surveillance role. Added also to the airframe is a fuselage long pannier for transportation of stores, an air-to-air refuelling boom, a dorsal fin, finlets and numerous aerial installations. The Nimrod fleet, based at Kinloss in Scotland, performs maritime patrol and search and rescue duties generally over the North Atlantic. The bonded joint samples analysed in this study were obtained from Nimrod MR Mk2 tail number XV 147. This airframe was converted from a new build Comet 4C to Nimrod MR Mk2 specification achieving its first flight in 1967. It was one of the two first development Nimrods and was primarily used for mission systems trials. These trials included typical maritime patrol sorties involving cruising at altitude to the patrol station, descending to low altitude over the ocean and performing the search pattern manoeuvres for extended periods. On completion of the operation the aircraft returns to base at cruise altitude. Consequently the airframe experiences ordinary flight loads from the transit phases together with higher “g” loading associated with low level manoeuvres and the gust loading from turbulence. The airframe is also exposed to significant amounts of sea spray under these conditions. However, since XV 147 was a development aircraft and did not see operational service, its exact service history is unknown. After the development phase this airframe discontinued service and was kept for about 10 years at DRA Farnborough. It is believed that none of the usual long-term storage measures were conducted and the airframe fell into disrepair, as condensation collected. The level of corrosion is believed to be significantly worse than the in-service fleet. XV 147 was scrapped in 1994 with only approximately 15 to 20% of service life used. The extent of use of adhesive bonding in the Comet aircraft is shown in Figure 1. The samples supplied to Oxford Brookes University are from the top wing skin between ribs 6 and 7, about a third of the way outboard. The under-side of the skin is stiffened by spanwise stringers which are attached by Redux adhesive bonding as shown in Figure 2. The thickness of the materials and dimensions of the stringers vary slightly between samples depending on the wing position, thinner sections generally being associated with areas closer to the wing tips. The top skin is primarily subjected to plain compression and the bonded joints would see a combination of shear and tensile peel forces. These are restricted by the presence of fasteners at intervals of approximately 300mm along the length of the flange. The skin forms part of the integral fuel tank and is hence subjected to some fuel pressure loading and attack from fuel. However the whole of the inside surface was coated with a flexible sealant skin which was apparently deposited from a solvent carrier at the time of manufacture. This would have provided some protection of the bonded joints. Also the external surfaces were primed and painted, and there was no evidence of corrosion or bare metal