Repair of aircraft using adhesive bonding

TWI Bulletin, September/October 1988

by Laura Cook

Traditional materials used for aircraft construction include steel, aluminium and titanium. However, in recent years, advanced composite technology has progressed sufficiently to allow the use of polymer composite materials in aircraft construction.

An engineering composite material is manmade, consisting of at least two chemically distinct components with distinct interfaces, and possessing properties which either separate component could not achieve. ^[1] Common composite materials used in aircraft construction are fibre reinforced polymers such as carbon fibre composites (CFRP), glass fibre composites (GFRP), boron fibre composite (BFRP), Kevlar composite ('Kevlar' is a trade name for Dupont's aromatic polyamide fibre), and sandwich type composites. The use of composites in aircraft construction offers the following advantages:

• Lower structural weight;
• Lower fuel consumption;
• Closer aerodynamic conformity;
• Improved performance. ^[1]

The composite materials used generally have a thermosetting polymer matrix, but the advantages of thermoplastics ( e.g. they are thermoformable) are being realised and they are finding increasing use in aircraft structures ( Fig.1).

Over the past few decades, adhesive bonding has been receiving increasing attention as a method for the fabrication of metallic and composite aircraft structures. Although the use of adhesives for cloth and wood structures has a long history, adhesives were first used in the production of metallic aircraft structures in 1944, when a vinyl-phenolic liquid-powder adhesive was used to bond aluminium alloy spars to wood in the wings of the De-Havilland Hornet aircraft. ^[2] Since then, adhesive bonding has had increasing use in both civil and military aircraft; for example, it has been reported that a Boeing 747 contains approximately 370m ² of adhesive film. ^[3]

Through the use of adhesive bonding as a replacement for metal rivets in the construction of aircraft structures a significant weight saving, improved fatigue performance, and a more aerodynamic profile can be achieved. Dissimilar materials can be readily joined and the possibility of galvanic corrosion between dissimilar materials can be reduced. In general, an adhesively bonded structure is considered to be more corrosion resistant than a riveted structure because the adhesive may act as a sealant around the joint. Nevertheless, care is required in the selection and application of adhesive bonding, because if the wrong choice of surface preparation, primer or adhesive is made, serious corrosion problems may be introduced. The improved fatigue life of an adhesively bonded structure, as compared with a riveted one, is largely due to the elimination of rivet holes which create stress concentrations and can initiate fatigue cracks. ^[2] The directional strength advantages of a fibre reinforced structure can be reduced if the fibre pattern is interrupted by rivet holes.

Adhesive bonding is applied to a variety of aircraft components, including stiffeners to fuselage and wing skin panels, bonded doublers to reinforced openings for windows and doors, bonded multi-ply laminates for root ends of lower wing skins, helicopter rotor blades, and radomes. Bonded honeycomb structures are used for flaps, elevators, rudders, tailerons, vertical fins, doors, floor panels, engine cowlings and thrust reversers. ^[2]

Repair

When an aircraft part fails to withstand the load applied, the part can either be replaced or repaired. Complete replacement of parts is very expensive and often requires removal of surrounding fixtures to allow access. This work would nearly always have to be carried out under workshop conditions which means having to transport the aircraft back to its base. On the other hand, the repair of failed structures can often be carried out in the field, and it may no longer be necessary to remove surrounding parts.

The potential time and cost savings gained by using repair methods on failed structures are very significant, so a great deal of research is being carried out in this area. Adhesive bonding has been found to be an excellent repair technique for failed structures that have not necessarily been constructed using adhesives, being faster, cheaper, more reliable and more flexible than traditional repair methods such as metal riveting.

Damaged metal structures which have traditionally been repaired using metal parts with mechanical fastenings, may now be adhesively repaired with composites or metals. In some cases, some form of mechanical fastening may still be used to reinforce the adhesive bond, although this is unwise when using composites because rivet holes can interrupt the fibre pattern and reduce the strength. Composite repairs to metal parts are particularly useful where double curvatures are involved and where shaping a suitable metal patch would be very difficult. Composite parts are sometimes repaired with metal but usually with more composite.

Different parts of an aircraft are designed to bear different loads to varying degrees. Tertiary structures are non-load bearing, secondary structures are load bearing while primary structures are critically loaded. It is difficult to generalise on the degree of loading for specific aircraft components, but as an example, wings and engine cowlings are considered as primary loaded, while nose skins and certain fairings may be secondary loaded. Loading can take several forms ( e.g. static, impact or fatigue), and also several modes ( e.g. tensile, shear, peel or torsion). Components usually have to withstand a combination of several types of loading. For example, an aircraft wing may experience repetitive tensile, shear and torsion stresses.

The type and degree of loading of a structure will determine the design of the repair.

An aircraft part may fail because of any one or more of the following reasons:

1. The initial design was not adequate to withstand the service loads.
2. The service loads were increased above those for which it was originally designed.
3. Manufacturing was not carried out correctly resulting in below standard performance of the structure.
4. The structure had been subjected to accidental loading above that considered reasonable and for which it was designed.

Several different types of failure can be identified. Cracks may form in a structure due to fatigue, usually initiating from stress concentrations due for example, to rivet holes or from imperfections in the material. A joint may also fail as a result of a sudden impact loading. Holes or cracks can be the result of projectile damage or corrosion. In all these cases a patch or a new adhesively bonded joint may be applied. Delamination can occur between a honeycomb structure and its skin, or within a laminate material, in which case the structure may be rebonded. In some cases an adhesively bonded reinforcement patch or stiffener may be applied before failure to an area where recurrent failures have been known to occur in similar structures. For example, the Central Servicing Development Establishment at RAF Swanton Morley apply a wet lay-up reinforcement patch using Kevlar composite to the nose of aircraft, where birdstrike has been known to penetrate the outer metal skin and cause internal damage. These failures may be due to inadequate design of the structure, and modification before failure is more economical than redesign and replacement.

Repairs to aircraft structures may be made to varying extents ranging from those to survive for one flight only, to those made to last for as long as the aircraft. The extent of the repair depends on the conditions where it is made, equipment, materials and expertise available, and required flight length.

Field repairs may have to be made outdoors in diverse uncontrolled conditions. Available equipment, tools and materials may be limited, and there will very likely be no electric power for heat curing of the adhesive bonds and no refrigeration for the preservation of adhesives. Simple procedures for the application of the repair are therefore needed, ideally requiring no special skills and only limited previous training of the operatives. ^[4] By their nature, field repairs are usually temporary and more frequently applied to military aircraft. They are usually made to enable the aircraft to make just one journey back to its base where a permanent repair can then be carried out.

Similarly, if a damaged aircraft lands at a foreign base, a quick temporary repair can be made to enable the plane to fly back to its own base for permanent repair.

Temporary repairs can be made to last for longer than one flight. Fast, cheap repairs can be made either in the field or at the aircraft base and designed to last until the next service of the aircraft when it can be permanently repaired. The idea of this type of repair is to reduce the downtime of the aircraft between services to a minimum.

Permanent repairs are nearly always made at the aircraft base under ideal workshop conditions. All the required materials, tools and expertise are available to produce a repair that is expected to last for the remaining life of the aircraft.

Adhesives

Numerous adhesives exist, but epoxy resins, both toughened and untoughened are the most common adhesives used for aircraft repair. Others used include toughened acrylics, cyanoacrylates, polyurethanes and hot melt film adhesives. The adhesives are generally applied manually by brushing, or by spraying if liquid, or by laying if in the form of tape or film. A summary of the properties of these adhesives is shown in the Table.

Properties of adhesives used in aircraft repair [3,5]

NB: This table is meant as a guide only. The properties of individual adhesives can vary greatly within a family group.

Surface preparation

When repair is required for a damaged part, the area must be prepared prior to adhesive bonding. A badly damaged area may be removed entirely by cutting around it, and the hole so formed is subsequently repaired. For small cracks or slight damage, a patch may be applied to the part as it is. The choice depends very much on the type and extent of damage and deformation. In the case of a crack, small holes may be drilled at each end of the crack to minimise the risk of crack propagation by reducing stress concentrations. Before adhesive bonding repair takes place, the adherend surfaces must undergo further mechanical or chemical preparation.

The surface preparation of adherend materials has been found to influence directly strength and durability of adhesive bonds. Contamination by moisture, release agents, paint stripper, detergent and aviation oils and fuels can seriously reduce bond strength. The effects of various contaminants on the strength of carbon fibre composite adhesive bonds for aircraft applications have been studied. ^[6] As well as removing contaminants such as grease, surface preparation is necessary to remove poorly adhering oxide layers, and to provide a mechanical key.

Eight methods of surface preparation are commonly used for aircraft repair either individually or in combination. Abrasion can be carried out using abrasive paper or by grit blasting and is used to remove paint, contaminants or oxide layers. Abrasion provides a mechanical key on a surface and also increases surface bonding area. Paint stripper is applied by brushing to remove paint from a surface before bonding. Aircraft paint is specially formulated to withstand the extreme service conditions that can be experienced and hence a particularly aggressive paint stripper is required. This paint stripper cannot be used on composite surfaces because the methylene chloride that it contains will attack the polymer matrix and degrade the material. Solvents are used to remove grease from a surface to be bonded but care must be taken to ensure that no contaminating deposits from the solvent are left behind.

Hot air can be used to dry adherend materials to remove any traces of moisture or solvents, either in an air recirculating oven or with a vacuum bag and heater. Corrosion and wear resistant oxide films can be formed on a metal by anodic oxidation or anodising. This treatment is commonly applied to aluminium using a phosphoric anodise.

The adhesion and durability of an adhesive can be improved through the use of a primer coating. Usually a dilute solution of an adhesive in an inorganic solvent is applied to the adherend surface by brushing or spraying. The term 'priming' also covers the method of silane coupling. A silane coupling agent is generally a silicone based compound which provides a coupling medium between the adherend and the adhesive by reacting separately with both. Chemical etching is a method that is usually applied only to metals but is suitable for some plastics. The chemical prepares the adherend by removing the top layer of the surface, which is usually oxide, to reveal a better adhering surface.

Research

The aircraft industry successfully uses adhesive bonding for the permanent and temporary repair of a wide range of primary and secondary loaded structures ( Fig.2). In the UK much research is being carried out in this area in order to improve techniques and to increase our knowledge and understanding of the technology. The Royal Aircraft Establishment at Farnborough is carrying out work on mechanical testing, because there does not exist a standardised test that provides relevant strength data that can be applied to real applications. They are also performing long term durability and weathering tests. The Central Servicing Development Establishment at RAF Swanton Morley is developing adhesive repair techniques for secondary (non-critically) loaded structures. The Military Aircraft Division at British Aerospace is performing fundamental research into the behaviour of adhesives and is developing permanent repair techniques for critically loaded aircraft structures. Further work is required in the development of non-destructive testing techniques to indicate the quality of adhesive bonds.

Summary

When an aircraft component fails to withstand the load applied it can either be repaired or replaced, depending on the extent of the resultant damage. If repair is possible then this is usually the faster and more economical alternative. Adhesive bonding is being used increasingly for such repairs because of the ease of application of the techniques and the improved fatigue and structural performance produced.

Much research is being carried out in this field but more is needed to increase the span of application. The Welding Institute will become involved in such research through the generation of base data on adhesive systems, by establishing correct application procedures, and by developing suitable destructive and non-destructive testing techniques for joint evaluation and quality control.

Acknowledgements

This work was funded jointly by Research Members of The Welding Institute and the Minerals and Metals Division of the UK Department of Trade and Industry.

For their help and co-operation, the author would like to thank K B Armstrong of British Airways, Dr B B Parker and M H Stone of the Royal Aircraft Establishment, Wing Commander P Perry, Squadron Leader C Elkins and Sergeant J Noble of the Central Servicing Development Establishment at RAF Swanton Morley, P Munday, J J Quinn and P A Tutton of British Aerospace, and N S Taylor of The Welding Institute.

References