Composite Repair: Prep & Bonding Rooms

Preparation Room

• It is a controlled, enclosed environment for surface preparation, preferably with an airlock.
• It must be separate from the open workspace and bonding room.
• Should be used only for adhesive bonding and nothing else.

Bonding Room

• It is a controlled, enclosed environment, separated from the preparation area, preferably with an airlock.
• Should be used for composite fabrication and adhesive application.

Storage of Materials

• Composite and adhesive bonding materials and components awaiting repair should be stored properly.
• Store resins, hardeners, catalysts, accelerators, and solvents in flame-proof cabinets when not in use.
• Keep resins, solvents, catalysts, and accelerators in original or clearly labelled containers.
• Ensure resin lids are secure when not in use, resin systems give off harmful fumes and are hygroscopic.
• Store resin systems and curing agents in cool, dry conditions.
• Don't mix resin and hardener batch/lot numbers or use expired resin systems.
• Let resin systems reach room temperature before opening.
• Store adhesives and pre-preg composite materials in cold storage (refrigeration to -18°C or lower using frost-free freezers).
• Vital to monitor storage/shelf-life limits to ensure composite repairs meet specifications.
• Recording criteria: Batch Identification, Vendor/Supplier Information, Storage Life, Time In and Out of Storage, and Recertification Information.
• Storage life may vary from manufacturer recommendations.
• Store rolls on racks supported by the central tube.
• Avoid storing rolls on-end or lying flat.
• Prevent loose fibres and off-cuts from contaminating other products.
• Oven dry fabrics at a specified temperature before use if the storage area lacks humidity control.
• Bulk honeycomb materials should be stored in sealed plastic bags within a clean environment, avoiding crushing.
• Store non-metallic core materials (Nomex, paper, or fibreglass) in humidity-controlled environments to prevent moisture absorption and dimensional changes.

Moisture Concerns

• Moisture in honeycomb or foam cores of damaged sandwich composites can cause issues.
• Freeze/thaw cycles cause expansion and contraction of moisture, which can damage cells and dis-bond facings.
• Moisture prevents adhesion of the core to a repaired or new facing.
• During hot-cure repairs, trapped moisture turns to steam, which can prevent bonding or dis-bond previously sound facings.

Machining Composites

• Machining includes drilling, cutting, sanding, or grinding.
• Composite materials behave differently than aluminum when machined.
• Different fabric types also machine differently.
• Before curing, fibreglass or carbon/graphite fabrics can be cut with conventional scissors.
• Aramid fabric needs special steel-blade scissors with serrated edges or ceramic-blade scissors.
• Serrated edges prevent fraying.
• Regular scissors separate the weave but don't cut aramid well, dulling quickly.
• Use specific scissors for specific materials.
• Pre-impregnated materials can be cut with a razor blade and utility knife.
• Resin holds fibres in place during cutting.
• Reserve sharp, defect-free cutting edges tools for specific materials for longevity.
• Air tools for cutting, trimming, drilling, and finishing composites should be rear-exhausting to prevent contamination.
• Water is typically the only accepted cutting fluid, as many fibres absorb cutting fluids, which can affect bonding.
• Aramid cutting tools should only be used on aramid to maintain cutting performance.

Drilling Composites

• Composites are more susceptible to material failures during machining than metals, making hole quality crucial.
• Using the right cutting tools can provide structurally sound holes.
• Delamination, fracture, breakout, and separation can occur during drilling.
• Delamination: Peeling away of the bottom layer.
• Fracture: Crack forming along a layer.
• Breakout: Splintering of the bottom layer.
• Separation: Gap opening between layers.
• Back material being drilled with wood to prevent these issues.
• Use the correct drill style for the specific composite.
• High-speed drills can be used; however, shortened drill life can lead to rejectable conditions such as delaminations and fibre pull-out.
• Carbide drill bits are effective on all composites and last longer than steel bits.
• Diamond dust-charged cutters work well on fibreglass and carbon but cause fuzzing on aramid.
• Aramid or Kevlar™ machining requires different tools than those for fibreglass or carbon/graphite.
• Conventional drill bits fuzz aramid due to fibre flexibility.
• Brad point bits are designed for aramid to produce clean, fuzz-free holes.
• Fibreglass or carbon drilling can be done with conventional tools, but these materials' abrasiveness reduces cutting edge quality and tool life.
• Carbide, diamond-charged, or carbide-coated tools improve results and tool life.
• Dagger or spade bits can be used to reduce fibre breakage.
• Carbide dagger bits for best results drilling and countersinking carbon.
• Uni-drills can drill and ream carbon and fibreglass, but not aramid as it causes excessive fuzzing.

Fasteners

• Countersinking fastener holes is important in composites.
• Produce countersunk holes with the correct angle, depth, and finish.
• Always use a pilot with relieved cutters for a firm seat.
• Common damage is wear around hole edges, impairing load transfer.
• Place fasteners far enough from edges to prevent wear or pull-out, considering fabric weave direction.
• Seal material around drilled holes to prevent moisture wicking, often with resin-coated inserts.
• Special composite fasteners similar to Hi-Locks are available for blind applications.
• Use composite fasteners with a large bearing area to prevent puncturing thin face sheets.
• Order fasteners in the correct diameter and length.

Sanding Composites

• Used for removing single layers of fabric.
• Avoid aluminium oxide for carbon fibres; use silicon carbide or carbide to prevent electrolytic action.
• Hand sanding removes single layers or thin paint coats, exercising caution to avoid removing too many fabric layers.
• Aim to avoid sanding into the top fabric layer when removing paint.
• Wet sanding is preferred, using approximately 240-grit sandpaper.
• Right-angle sanders are used for scarfing and step-cutting repairs.
• Rear-exhausting tools are preferable due to a lower risk of contaminating surfaces with moisture and oil.
• If sanding carbon, prevent dust from settling on aluminium surfaces to avoid electrolytic action.
• Use a dust collector or downdraft table where possible.
• Standard composite safety calls for a dust respirator when sanding.
• Expect aramid to fuzz when sanding; lighter fuzz and gloss indicate nearing the underlying layer, showing one layer removed.
• Carbon material creates a fine powder when sanded.
• Changes in weave direction indicate a new layer is being reached.

Trimming Composites

• Standard machining equipment can be used, but tooling modifications might be needed.
• All cutting surfaces should be carbide-coated.
• Diamond-edged blades work well on carbon and fibreglass.
• Carbide or diamond-cut router bits are best for routing honeycomb, carbon, or fibreglass laminates.
• Routing out damaged core requires first routing a circular or oval area of the top laminate skin using a pointed router bit.
• Adjust bit depth depending on damage penetration.
• Hole saws can cut holes but tend to tear out honeycomb core and fray aramid edges; not recommended for aramid laminates.
• Diamond-dust blades on hole saws can produce cleaner cuts in carbon/graphite.
• Roto-bores effectively cut composite materials due to peripheral cutting.
• Bandsaws with fine teeth (12-14 teeth per inch) can be used.
• Use carbide or diamond-dust blades for carbon material.
• Blades with alternating tooth patterns and wave sets should be used when cutting aramid.
• Counterbores can be used with all composite materials except aramid - may create excessive fuzzing.

Safety

• Dust masks or respirators with fine filters are needed when sanding, drilling, or trimming.
• A dust collector or downdraft table is desirable to pull fine particles from the air in conjunction with dust masks or respirators.
• Depending on the resin systems, respirators may be required.
• Eye protection is necessary when using chemicals or power tools.

Inspection Methods

• It is difficult to establish the strength and durability of a given composite repair without destroying it.
• However, there are methods of establishing that the cured resin matrix should perform as designed.
• Lap shear testing establishes whether a given resin system has fully cured.
• Barcol hardness testing establishes whether a resin matrix has achieved full strength.
• Inspect the adhesive layer for voids, dis-bonding, cracking, poor flow, and porosity.
• Visual inspection detects many defects.
• Tap hammer inspection detects large voids and dis-bonds.
• Ultrasonic inspection (Pulse-Echo) is used when tap hammer inspection is not enough.
• Voiding and dis-bonding are acceptable if within pre-established limits.
• Cracking in the adhesive bond caused by thermal shock indicates repair rejection.
• Poor adhesive flow during the cure indicates inadequate pressurization, slow heat-up, or out-of-life adhesive, all causes for repair rejection.
• Porosity, caused by rapid heat-up, incorrect pressurization, or moisture contamination, adversely affects bond durability and leads to repair rejection.

Defect Causes

• Repairs must follow approved instructions from the aircraft manufacturer or military organization.
• Instructions specify limits on size/location of repairable damage that vary by part.
• Damage outside allowable criteria requires part replacement or engineered repair by a qualified engineer, often with manufacturer cooperation.
• Defects generally arise from resin defects, finishing process damage, and lay-up defects.
• Causes include: poor surface preparation, non-observance of procedures, use of expired materials, inadequate storage/handling, lack of training, contamination, lack of process control, and inadequate quality control.
• Resin defects: Bond line too thick/thin, rubbery resin, porosity, voiding, thermal stress, cracking, crazing.
• Finishing process damage: Incorrect sanding, failure to remove lay-up materials, poor surface preparation, surface contamination, incorrect handling.
• Lay-up defects: Resin starvation/richness, warped patch, crushed core.

Protective Treatments

• After manufacturing, parts are painted to seal from moisture and for cosmetic reasons and offer protection from UV rays, heat soakage, erosion, lightning.
• The same type of paint used for metal portions of the aircraft is suitable for composites.
• Some companies, like Boeing, use Tedlar (a plastic coating) for an additional moisture barrier.
• New-generation paints are used on composites just as on aluminium parts.
• Protective treatments provide protection from: Ultraviolet (UV) rays, Heat soakage, Erosion, Lightning.

UV Protection

• UV radiation degrades resins.
• Can be reduced by special blocking additives in the liquid resin, or more usually by a special UV-blocking dark paint applied after cure.

Reducing Heat Soakage

• White or light-coloured paint finishes on fibreglass-reinforced plastics lessen temperature rise through heat soakage from the sun or high ambient temperatures.
• The top surfaces of aircraft, in particular, should be white.
• It also helps if the aircraft is kept out of the sun while it is on the ground.

Combating Erosion

• A well applied and maintained polyurethane paint provides good primary protection from erosion.
• It should be white or light-coloured to reduce temperature rise.
• Polyurethane paint is not waterproof, so it must have the right waterproofing undercoat.

Environmental Factors

• Ultraviolet (UV) light weakens epoxy and aramid fibres. Use opaque paint with a UV barrier.
• Infrared light (heat radiation) is problematic for wing and tail bottoms on hot surfaces.
• Paint components white to reflect infrared radiation.
• Moisture and aircraft fluids are detrimental.
• Protection can be provided by applying bondable Tedlar to internal faces, maintaining sealing media, and maintaining surface finishes.
• Polyurethane paints provide superior erosion protection compared to other surface finishes, however, polyurethane is not waterproof, so a suitable waterproof undercoat must be applied.

Lightning Strike Protection

• Electrical contact between metallic and composite parts is required.
• Aluminium provides conductivity for dissipation of electrical energy.
• Composites do not conduct electricity, so lightning protection must be built-in.
• Without protection, resins will evaporate, leaving bare cloth.
• Carbon composite does not conduct enough electricity alone.
• Methods for dissipating electrical charge:
• Aluminium wires woven into the top layer of composite fabric (usually fibreglass or Kevlar™).
• Fine aluminium screen laminated under the top fabric layer (sandwiched between fibreglass layers with carbon/graphite to prevent galvanic potential).
• Thin aluminium foil sheet bonded to the outer composite layer.
• Flame-sprayed aluminium or aluminized paint.
• Metal bonded to the composite to dissipate charge to another metal component or static wick.
• Using conductive adhesives when bonding is required from composites to bonding leads.
• After repairing protection methods, electrical continuity checks are vital to verify conductivity.