1,Composite embedded bonded aluminium structures
Composites can be used for structural repair, restoration, or reinforcement of aluminium, steel and titanium components. The bonded composite doublers have the ability to slow or stop fatigue crack propagation, replace structural areas lost due to corrosion abrasion, and structurally enhance small and negatively compensated areas.
Boron epoxy, Glare® and carbon epoxy materials are used as composite patches to repair damaged metal wing skins, fuselage sections, floor beams and bulkheads. As a crack extension inhibitor, the rigid bonded composite material limits the cracked area, reduces the total stress in the metal, and provides an alternate load path around the crack. As structural reinforcement or doped fillers, high modulus fibre composites offer negligible aerodynamic drag and controllable performance.
Surface preparation is important to achieve bond strength. Aluminium skins were prepared using sandblasted silane and phosphoric acid anodising. Thin film adhesives using 250℉ (121℃) curing are commonly used to bond doublers to metal structures. Critical areas of the installation process include good thermal cure control, having and maintaining an anhydrous bonding surface, and chemically and physically prepared bonding surfaces.
Secondary bonded pre-cured reinforcements and in-situ cured reinforcements have been applied to a wide variety of structural geometries ranging from fuselage frames to door cutouts to blade reinforcements. Vacuum bags are used to apply bonding and curing pressure between doublers and metal surfaces.
2,Repair of fibreglass moulding mat
Fibreglass moulded mats consist of short fibres and are much weaker than other composite products that use continuous fibres. Fibreglass moulded mats are not used in structural repair applications but can be used in non-structural applications. Fibreglass moulded mats are usually used in conjunction with fibreglass fabrics. The moulded mat is impregnated with resin and acts like a wet lay-up layer of fibreglass fabric. The advantages of moulded mats are their lower cost and ease of use.
3,Repair of radome
Aircraft radomes, which act as electronic windows to radar, are usually made of a non-conductive honeycomb sandwich structure with only three or four layers of glass fibre. They have a thin outer shell so that they do not block radar signals. The thin construction, combined with their location in front of the aircraft, makes radomes vulnerable to damage from hail, bird strikes and lightning strikes. Low impact damage can lead to peeling and delamination.
Usually, water is found in the radome structure as a result of impact damage or erosion. The moisture collects in the core material and begins a freeze-thaw cycle each time the aircraft flies. This eventually damages the honeycomb material and causes soft spots in the radome itself. Damage to the radome needs to be repaired quickly to avoid further damage and blockage of the radar signal. Trapped water or moisture can create shadows on the radar image and severely degrade radar performance. To detect water in the radome, available non-destructive testing techniques include x-ray radiography, infrared thermography, and radome moisture meters, which measure RF power loss due to the presence of water. Repairs to radomes are similar to repairs to other cellular structures, but technicians need to be aware that repairs may affect radar performance. Repairing a severely damaged radome requires a special tool. This is shown in Figure 68.
Figure 68: Tools of radome repair
Transmittance testing after radome repair is a guarantee that radar signals are transmitted properly through the radome. The radome has lightning protection strips attached to the outside of the radome to dissipate the energy of a lightning strike. It is important that these lightning strips are in good condition to avoid damage to the radome structure. Typical lightning strip failures found during inspections are high resistance due to shorted lightning strips or attached hardware and stripping of the lightning strips from the surface of the radome. This is shown in Figure 69.
Figure 69: Lightning strips on radome
4,Repair by external bonding
Damaged composite structures can be repaired with an external patch. External patches can be repaired with prepregs, wet lay-ups or pre-cured patches. External patches are usually stepped to reduce stress concentrations at the edges of the patch. The disadvantage of an external patch is that the eccentricity of the load results in peeling stresses and protrusion of the patch in the air stream. The advantage of external patching is that it is easier to accomplish than stamped-around-the-corner repairs.
5,Repair by external bonding with prepreg bonding
The repair methods for carbon fibre, glass fibre and Kevlar® are similar. Glass fibre is sometimes used to repair Kevlar® materials. The main steps in repairing damage with external bonding are investigation and mapping of the damage, removal of the damage, lay-up of the repair layer, vacuum bag encapsulation, curing and surface coating.
Step 1: Investigate and locate the damage
Determine the damage using a tap test or ultrasonic test.
Step 2: Remove Damage
Trim the damaged area into a smooth circle or oval. Use cut or sandpaper to roughen the substrate surface at least 1' larger than the patch. Clean the surface with an approved solvent and a dry soft cloth.
Step 3: Lay down the repair layer
Use the SRM to determine the number, size, and orientation of the repair layers. The material and orientation of the repair layers must be the same as the orientation of the primary substructure. The repair can be stepped to minimise peel stress at the edges.
Step 4: Vacuum Encapsulation
A layer of film adhesive is placed over the damaged area and the repair layup is placed on top of the repair. Vacuum bag encapsulation material is placed on top of the repair (see Prepreg Layup and Control Encapsulation) and a vacuum is applied.
Step 5: Curing the repair
When the part can be removed from the aircraft, the prepreg patch can be cured by placing a preheated blanket inside a vacuum bag, oven or autoclave. Most prepregs and film adhesives cure at 250℉ (121℃) or 350℉ (176.67℃). Check the SRM for the correct repair cycle.
Step 6: Apply Surface Coating
Remove the vacuum bag from the repair after the repair has been made, inspect the repair, and remove the patch if the repair is not satisfactory. Lightly sand the repair with sandpaper and apply a protective coating.
To be continued
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