INTRODUCTION Composites are combinations of two or more materials that differ in composition or form. The element that make up the composite retain their individuals identities. In other word, the individual elements do not dissolve or otherwise merge into each other. Each can be physically identified.

The term composite structure refers to: Fabric resin combinations in which the fabric is embedded in the resin but retains its identity

Example of a composite structure: Reinforced concrete: - Made from a combination of cement (concrete) and steel rods The steel rods carry the tension loads The concrete carries the compression loads

Composite Resin used can be of polyester resins or epoxies and vinyl.

Fibres such as glass fibre, carbon fibre, Kevlar fibre, and hybrid are used. These are known as advance composite material.

USAGE OF COMPOSITE ON AIRCRAFT

Weight reductions of 20% or more as compared to aluminium alloy structure High strength / weight ratio High corrosion resistance. Good fatigue resistance Good vibration resistance Good impact strength Capability of achieving a smooth surfaceaerodynamic and parasitic drag reduceAdvantages using composite as compared to metal

Properties and Construction in Composite Transverse properties are: Direction of the reinforcement fibres and ability of the resin (matrix) to transfer loads from one fibre to another. Therefore the greater the ratio of reinforced fibres to matrix, the greater the strength of the composite During manufacture or while carrying out composite repair,it must be in accordance to the structural repair manual.

CARBON FIBRESCarbon fibres are made from pure carbon. In the USA carbon fibres are called graphite fibres. The fibres are manufactured by the controlled heating of PAN fibres (eg. Courtelle, Acrilan, Orlon) in a furnace filled with an inert gas. The process operates at about 1000C to 2500C. It is a slow process and uses a large amount of energy.

A range of carbon fibres are available having either high stiffness or high strength. All of them are, stiffer than glassfibres, but only the high strength carbon fibres are stronger than glassfibres in tension. The carbon fibres are woven into cloth to make them easier to handle , cut and align and to reduce damage to the fibres during lamination. Few weave patterns are available

When sanding carbon fibre composites during repair work for example, the fine carbon fibres may find their way into electrical equipment and cause short circuits.

When machining and sanding carbon fibre composite, use a vacuum cleaner to remove all the dust being produced.

ARAMID FIBRES Aramid fibres are made from aromatic polyamide, a type of nylon. Kevlar is an aramid fibre, it has very high strength and impact resistance. Kevlar 49 is the grade used in aircraft composites

1. It has the highest tensile strength and resistance to impact of any reinforcing fibre.2. It is stiffer than glass but only about half as stiff as carbon fibres.3. It is about 40% lighter than glass fibres.

4. Laminates are up to 400% stronger than comparable glass reinforced laminates.5. Laminates are up to 20% stronger than comparable carbon reinforced laminates.6. Laminates are up to 300% stiffer than glass laminates but 40% less stiff than carbon laminates.It has a very high impact strength with the damage confined to a small area.

Hybrid ComponentsHybrids are made by the addition of some complementary material such as fiberglass or Kevlar to the basic graphite/epoxy matrix.

The added materials are used to obtain specific material characteristics such as greater fracture toughness and impact resistance, and should be considered for areas subject to foreign object damage. The addition of graphite/epoxy to fiberglass structure is used to provide additional stiffness.

POLYMER RESINS The polymer used in conjunction with the reinforcing fibres is a liquid one and usually referred to as a resin.

To make a composite the resin and fibres are mixed together in the correct proportions and the resin then allowed to harden.

THERMOSETS

Plastics which are not softened by heat are known as thermosets. Most of them are supplies as liquid resins which are hardened by chemical curing. During the curing process the polymer molecules are permanently joined together by a process known as crosslinking. Once this has happened it cannot be reversed and the polymer is fixed in a permanent shape.

There are two types of thermoset used to make composites, polyesters and epoxides.Polyesters are the most widely used thermosets in composite moulding. Most everyday fibre reinforced plastic articles are based on polyesters. However, when high performance properties are required, epoxides or epoxy resins are used. That is why they are preferred for the aerospace industry

To make a composite, the resin and fibres are mixed together in correct proportions and the resin then allowed to harden or cure.CURING SYSTEMSCuring the polymer from a thick liquid to a strong solid is a chemical process. This may be brought about by cold curing or hot curing.

COLD CURING SYSTEM In this system the laminated composites resin is allowed to cure at room temperature (210C ).

HOT CURING SYSTEM In this method, the laminates are prepared and put in an oven to activate the resin A better hot cure system uses an autoclave to remove the air and to mix the fibres with the resin The autoclave uses a vacuum to suck out the air and controlled the heaters to cure the resin.

RESIN/FIBRE RATIOAn important feature of a composite structure is the proportion of fibre to resin used, or the resin fibre ratio. If too much resin is used, the fibres will have little effect and the composite will be weak.(resin rich / resin starved )There is an optimum resin/fibre ratio to produce the best composite. The value will vary depending on the fibre and resin used. With polyesters the ratio is usually about two parts of resin to one part of fibre by weight. This is because of the inferior adhesion of the polyester resin and the fibre, and it is usual to use a slight excess of resin to make sure that the fibres are thoroughly wetted with resin.Epoxy resins, with their superior adhesion, bond very well to the fibres so the fibre resin can be higher to produce a lighter but strong composite. For example, when using glass fibre and carbon fibre with epoxy resin, equal parts by weight of resin and fibre are used. Precise resin/fibre ratios are given by the fibre and resin manufacturers.

HONEYCOMB STRUCTURESHoneycomb structural sheets may be manufactured from many materials. Glass-fiber plastic materials are often used for the outer faces and also for the honey-comb core. For aircraft structures, the core material is sometimes made of glass fiber and the face sheets are of metal. Metal honeycomb core material is often used with metal faces and is bonded to the faces with chemical adhesives. .Both aluminium honeycomb and fibre glass honeycomb material are commonly used in the construction of wing and stabilizer surfaces, bulkhead, floors, control surfaces and trim tabs

INTRODUCTIONSEALANT Sealant is a material used to form a seal between two imperfectly fitting surfaces Sealants differ from gaskets in that they are usually liquid and semi-solids.

SEALING COMPOUND Used to exclude dirt, moisture and to provide a liquid and airtight joint. Also act as a preventative to contact corrosion There are various types of sealant used on the aircraft, and for the appropriate type, reference should be made to the structural repair manual (SRM). Normally, sealant is available as one part or a two-part sealant A two-part sealant normally consists of a base and an accelerator, and the mixing ratios should be as per the instructions give in the manual. The viscosity and the maximum usable time of the sealant are critical, It can be extract from structural repair manual

SEALANTExample of sealant:(1) PR 1440 A 2 (2) PR 1440 B 2 These are fuel resistant sealant. The letter A indicates that its viscosity is lower as compared to B. The number 2 indicate the usable time for this sealant is a maximum of 2 hours (Pot Life). Curing time for type A is 72 hours and type B is 48 hours. All relevant fire and personal safety precautions must be observe when handling the sealant.

Types of sealingInterfay sealingThis is the first coating of sealant applied to structural parts before they are riveted or bonded togetherJoints should be closed up when the sealant is still wet.2.Fillet SealingAs the rivets or bolts are closed, excess sealant will be squeezed out of the joints and should be removed with a spatula.Once the joint has been made, a coating of sealant is applied to the edges of the joint; this coating is called a fillet.3. Final Brush On SealantA final brush - on coat of sealant is applied to overlap the joint and the filletThe interfay, fillet and brush on coat are part of a standard treatment to seal integral fuel tanks, and all use a similar sealant.

PROCESS For correct procedure of using sealant, reference should be made to the aircraft maintenance manual and the structural repair manual. Normally the process involves: -1. Pre cleaning 2. Preparation of sealant3. Applying the sealant

PROCESS1. Pre-Cleaning It is absolutely essential that the area to be sealed is perfectly clean and dry. Prior to sealing, old sealant should be removed. Remove all particles and degrease area using solvent: MEK (methyl ether ketone) for non-painted surfaces Ethylalkohol for painted surfaces

2. Sealant Preparation Sealants should not be applied where the ambient temperature is below 15 oC. Mixing: - Small amount of sealant mixing - Use hand Large amount of sealant mixing - Carried out by mechanical means To begin: - Stir the accelerator with a spatula until it obtains a smooth appearance Then mix both parts together using a slow stirring motion until a uniform colour is obtained without any presence of stripes. For mixing ratios refer to the structural repair manual (SRM).PROCESS

3. Applying the Sealant This should be done evenly, smoothly and avoid any air gap For interfay sealing, the sealant applied should be sufficient so that during assembly of the parts:- The sealant is squeezed out The excess sealant is removed with a spatula Sealants should also be applied under to bolt but not on to the thread as this will adversely effect the torque tightening All sealing should be done before the usable life has expiredPROCESS

Composite InspectionComposite structures require ongoing inspection intervals along with nonscheduled damage inspection and testing.When a composite structure is damaged, it must first be thoroughly inspected to determine the extent of the damage, which often extends beyond the immediate apparent defect. Proper inspection and testing methods help determine the classification of damage, which is, whether the damage is repairable or whether the part must be replaced. In addition, classifying the damage helps to determine the proper method of repair. The manufacturer's structural repair manual outlines inspection procedures, damage classification factors, and recommended repair methods

The following are several common composite inspection and testing methods:

1. VISUAL INSPECTION

Its the most frequently used inspection method in aviation. This method of inspection is generally used to detect resinrich areas, resin starvation, edge delamination, fiber breakout, cracks, blistering, and other types of surface irregularities A strong light and magnifying glass are useful tools for visual inspection. 2. TAP TEST

Its the simplest methods used to detect damage in bonded parts. The laminated part is tapped with a coin or small metallic object, such as a tap hammer, to detect delamination. The tap test is an acoustic test, one in which you listen for sound differences in the part, and is not the most accurate test method.

3. ULTRASONIC INSPECTION

Its the most common instrumental NDT method used on composites todayAn ultrasonic tester is useful for detecting internal damage such as delaminations, core crush, and other subsurface defects.

4. RADIOGRAPHY

Radiography or x-ray inspection is used to detect differences in the thickness or physical density when compared to the surrounding material of a composite.It can be used to detect surface as well as internal cracks.

Depending on the manufacturer of the aircraft, classification of damage is usually placed in one of three categories:

1. Negligible DamageMay be corrected by a simple procedure with no flight restrictions

2. Repairable Damage Is a damage to the skin, bond, or core that cannot be repaired without placing restrictions on the aircraft or structure.

3. Non-repairable DamageA composite structure that is damaged beyond limits must be replaced.COMPOSITE REPAIR

The exact procedures for repair of various laminated composite structures depends partly on the type of damage incurred.

The damage can range from a relatively simple surface scratch, to damage completely through all internal plies and core honeycomb material.

There are four basic types of composite repairs:1. Bolted metal or cured composite patches2. Bonded metal or cured composite patches3. Resin injection4. Laminating new repair plies to the damageTYPES OF REPAIRS

Bolted & Bonded metal or cured composite patchesBolted and bonded surface patches are not usually recommended due to the fact that these types of patches do not restore the strength characteristics of the original structure. Resin injectionResin injection repairs are used to fill holes or voids. They are accomplished by injecting resin into the hole of a damaged area using a needle and syringe. This type of repair is usually done on nonstructural parts.

Laminating new repair plies to the damageThe most desirable type of permanent repair to composite structure is to laminate new repair plies in the damaged area. This type of repair involves removing the damaged plies, and laminating on new ones.

After installing the first layer of film, apply heat with a heat gun to soften the film. When performing a repair to a composite structure, an adhesive film is used many times to help bond laminate patches to the existing part.Figure:Remove the plastic backing and place the repair plies over the adhesive layer.Finally, apply a final layer of adhesive film over the repair plies to glue the patch in place.

Wood

Wooden Aircraft The first airplane built by Wright brothers was from WOOD WOOD was used on early aircraft because of its availability and relatively high strength to weight ratio. The cost of the additional hand labor needed for wood construction and maintenance had cause wood aircraft to become almost entirely superseded by those of all metal construction.

WOODWood is not as strong as steel or aluminum, but;the construction can be so designed that the necessary strength is achieved with corresponding savings in weight.Wood a/c structures are light,low cost,high strength,minimum special equipment for maintenanceMany designers prefer to use wooden spars in acrobatic aircraft because;wood will better withstand the bending loads imposed during aerobatics. Unlike metal, wood does not weaken from fatigue.Many light light aircrafts are made of wood for their primary & secondarystructures ( spars,ribs, control surfaces )

Wood and adhesive materials used in aircraft repair should meet aircraft quality standards and be purchased from reputable distributors to ensure such quality. ~2 types of wood :> Hardwood and Softwood ( not the hardness but the cellular structure)~Softwood - having needlelike or scale like leaves(evergreen or the conifers)- higher strength/weight ratio- desirable for a/c.-Sitka spruce,Douglas fir ,Port Oxford- Sitka spruce used as a reference because of its:- uniformity,strength and excellent shock resistance.TYPE OF WOOD

SITKA SPRUCE

Hardwood

leaves are broad deciduos heavier and stronger Mahogony , Birch & White Ash Used for support blocks , tip bows etc

PLYWOOD

Composed of a number of plies (layers) of wood veneer assembled with the grain layer at an angle of 45 o to 90 o to the adjacent layer

The layers of wood are bonded with synthetic resin glue( thermosetting ) assembled in a large heated hydraulic press

better quality than commercial type (Mahogany& Birch) Mahogany better gluing surface

Advantages:

- resist cracking- even strength- co-efficient of expansion is negligible used to make L.E coverings and surface panels

LAMINATED WOOD

Composed of a number of solid woods bonded together with the grains running in the same directions

more rigid than a piece of solid wood

resist warpage better , more uniform strength, resist shape change

commonly used for severely bent structures-tip bows,formers and bulkheads

CONSTRUCTION METHOD OF WOODEN AIRFRAME STRUCTUREWoodworking is a skill that is easily learned by the novice who usually has a basic knowledge of wood construction and some of the necessary tools.FigureStrong, rigid, light weight truss or framework wooden structures have been in use since the 1920s and are probably the easiest structural type to build.

Wood is used in fabricating spars, building ribs, floorboards, instrument panels, wing tip bows, longerons and stringers, leading edges, etc.

Wood is easily formed into shapes making it the obvious choice for wing tip bows, leading edges, and wing walkwaysFigure information:The easiest wing to build is rectangular with a constant aerofoil section, constant thickness and constant chord: commonly known as a plank wing.

Two solid wood spars are used as shown in the figure.

Types Of Defects in Wood MaterialFollowing are several examples of wood defects:1. Checks

A lengthwise separation or crack of the wood that extends along the wood grain. It develops during drying and is commonly caused by differences in radial and tangential shrinkage or because of uneven shrinkage of the tissues in adjacent portions of the wood.

2. Shakes

A separation or crack along the grain, the greater part of which may occur at the common boundary of two rings or within growth rings

3. Heartwood

The inner core of a woody stem or log, extending from the pith to the sap, which is usually darker in color. This part of the wood contains dead cells that no longer participate in the life processes of the tree

4. Knot

That portion of a branch or limb that is embedded in the wood of a tree trunk, or that has been surrounded by subsequent stem growth.Figure: Several wood defects

The adhesive used in aircraft structural repair plays a critical role in the overall finished strength of the structure.

The maintenance technician must only use those types of adhesives that meet the performance requirements necessary for use in aircraft structures.

Not every type of glue is appropriate for use in all aircraft repair situations.

Because of its importance, use each type of glue in strict accordance with the aircraft and adhesive manufacturer's instructions.AIRCRAFT ADHESIVES / GLUES

A strong joint is obtained when there is a complete contact of glue and wood surfaces over the entire joint area , unbroken by foreign particles and air bubbles.

Two assembly methods:Open assembly glue applied to both surfaces and allow time before putting it together. This thickens the glue and the set up time of the glue is faster.Do not exposed more than 20 minutes. ( Recommended)Closed assembly The parts are assembled immediately after application of glue

Gluing Pressure ( held by clamps,nails,weight )or screws~ to squeeze glue into thin film~ to force air from joint~ to produce intimate contact~Softwood 125 150 psi and hardwood 150 200 psi..

1. Casein Glue

Most older airplanes were glued with casein glue, which was a powdered glue made from milk. Casein glue deteriorates over the years after it is exposed to moisture in the air and to wide variations in temperature. Many of the more modern adhesives are incompatible with casein glue. Types of AdhesivesPlastic resin glue This type of glue usually comes in a powdered form. Mix it with water and apply it to one side of the joint. Apply a hardener to the other side of the joint, clamp the two sides together and the adhesive will begin to set. Plastic resin glue rapidly deteriorates in hot, moist and under cyclic stresses, making it obsolete for all aircraft structural repairs.

Resorcinol glue

Its twopart synthetic resin glue consisting of a resin and a hardener and is the most waterresistant of the glues used. The glue is ready for use as soon as the appropriate amount of hardener and resin has been thoroughly mixed. Resorcinol adhesive are one of the most common types of glue used in aircraft wood structure repair.

Phenolformaldehyde glue

Its the most commonly used in the manufacturing of aircraftgrade plywood.This glue requires high curing temperatures and pressures making it impractical for use in the field.

Epoxy resins

This are twopart synthetic resins that generally consist of a resin and a hardener mixed together in specific quantities. Epoxies have excellent working properties.

INSPECTION OF WOOD STRUCTURESMost wood damage is caused by conditions such as moisture, temperature, and sunlight.

Because wood is an organic material, it is subject to mildew and rot unless protected from moisture.

Keep wood airplanes in well ventilated hangars and take special care to ensure that all of the drain and ventilation holes remain open.

If a ventilation hole becomes obstructed, changes in air temperature will cause moisture to condense inside the structure, which will cause the wood to deteriorate.

When inspecting a wood structure aircraft, move it into a dry, wellventilated hangar.

One of the first steps is to check the moisture content of the wood using a moisture meter.

If the moisture content is high, dry the wood structures before inspecting further.

Wooden structures of the aircraft need to be dry to be able to effectively determine the condition of the bonded joints. INSPECTION METHODS

The following are several inspection methods and associated equipment employed for inspecting wooden structures.

1. MOISTURE METERING

Use to determine the moisture content of wood structure. Wood that is too wet or too dry may compromise the strength and integrity of the structure. (15 %)A moisture meter reads the moisture content through a probe that is inserted into a wooden member. (Marconi moisture meter)

2. TAPPING

The wood structure may be inspected for structural integrity by tapping the suspect area with a light plastic hammer or screwdriver handle. Tapping should produce a sharp, solid noise from a solid piece of wood. If the wood area sounds hollow or feels soft, inspect further.

3. PROBING

If soft, hollow wood is found during the tap test, probe the suspect area with a sharp metal tool to determine whether the wood is solid.

Ideally, the wood structure should feel firm and solid when probed.

4. PRYING

Use prying to determine whether a bonded joint shows signs of separation.

Light prying is sufficient to check the integrity of a joint.

If there is any movement between the wood members of the joint, a failure of the bond is confirmed.

5. SMELLING

Smell is a good indicator of musty or moldy areas. When removing the inspection panels, be aware of any odors that may indicate damage to the wood structure. Odor is an essential indicator of possible wood deterioration. Musty and moldy odors reveal the existence of moisture and possible wood rot.

6. VISUAL INSPECTION

Visual inspection techniques are used to determine any visible signs of damage. Both internal and external visual examinations are imperative to a complete inspection of the wood structure.

The basic criterion for any aircraft repair is that the repaired structure must not only be as strong as the original structure, but the rigidity of the structure and the aerodynamic shape must also be equivalent.

Materials used for the repair of a wooden structure should be the same as the original unless they have become obsolete.

If substitutions are made, they must produce a repair that meets the basic requirements of the manufacturer and the authority.REPAIR OF WOODEN STRUCTURE

Aircraft that incorporate plywood skins normally carry a large amount of stress from the flight load.

Therefore, repairs to plywood skins are made in strict accordance with the recommendations of the aircraft manufacturer.

There are several types of plywood patches repairSplayed PatchSurface Patch (External patch)Plug PatchScarfed PatchPlywood Skin Repair

Splayed PatchSmall holes in thin plywood skin may be repaired by a splayed patch.

This type of patch is used if the skin is less than or equal to 1/10 inch thick and the hole can be cleaned out to a diameter of less than 15 thickness (15T)

See figure in the next slide.

Figure: Splayed patch

Fabrics

There are still many requirements for fabric covered certificated a/c

it is cheap,easier to install and repair,light and durable.

Organic and inorganic fibers are used

Organics cotton and linen ( affected by UV rays and microorganisms

Inorganic fibers Polyster( Dacron) and fiberglass ( Razorback)

Polyster fabrics are most durable and strong ( Sits Poly Fiber and Ceconite)

Dopes is defined as a colloidal solution of cellulose acetate butyrate or cellulose nitrate. ( Butyrate is more fire resistance)

For sealing ,tautening,and protecting airplane fabric coverings.

Supplied in clear coating or pigmented(aluminium oxide)

A/C nitrate and butyrate dopes shrink as they dry will make the the organic and fibreglass covering.

For polyster materials the non-tautening dopes are used

Early airplanes were constructed using organic fabrics, most builders did not use any special processes to increase the strength of the material.

These covering were not airtight and tended to loosen and wrinkle with change in humidity.

In order to keep humid air from flowing through the fabric, builders began applying a rubberized or varnish finish to the covering surface.

Cellulose nitrate material consisting of cotton fibres dissolved in nitric acid was used to form a clear dope that could be worked into the fabric to produce a stronger and more durable finish.

To improve durability, manufacturers applied colored enamel over doped fabric to block the sunlight.

Further improvements led manufacturers to blend fine aluminum powder directly into the dope before it was applied to the fabric.

This technique caused the dope to take on a silver color that blocked the sunlight and also tended to reflect heat away from the fabric.

Fabric Covering ProductsFabric covering materials that are used on aircraft must meet the minimum standard.

Be aware of old dopes or solvents.

Some materials have a recommended shelf life, which if not used in a certain amount of time may not perform to specification.

Using products that are substandard or too old can not only affect safety, but may also be very costly in time.

Inspection Method For FabricInspection and repair of fabric should be performed as routine procedures during the life of a fabric covering.Inspect the interior of the fabric using appropriate mirrors and flash lights.Pay special attention to the material around control cable inlets, inspection rings and drain grommets, looking for areas of physical wear and general deterioration.Make sure that no light penetrate the fabric.

Fabric InspectionThe testing of the fabric is used to determine if the fabric has sufficient strength to assure safe operation of the a/c.- The strength of the fabric is based on its tensile strength Use the Seyboth or Maul punch tester . The tester penetrates the fabric and the strength is indicated on the scale.- e.g Min strength for grade A cotton is 56 psi

Figure: - Example of grommetsGrommets are used for reinforcing holes in textiles for drainage ,lacingor inspection

Repair of fabric coveringThe type of repairs needed to restore the fabric to its original strength vary depending on the damage and type of fabric.

Much of the damage that requires a patch results from accidental physical damage to the fabric, often referred to as hangar rush.

Many of these repairs will be on fabric covering that was applied using new adhesives.

Common example is to repair L-shaped tears in the fabric.

*TEARS -Usually repaired by sewing and doping on the fabric patch

-V tears ,the sewing should start at the apex

Doped on RepairFor a/c that have never exceed 150 mph, a doped on patch can be used

*Sewn in patchA/C speed has exceeded 150 mph and damage does not exceed 16 in in

Doped on panel repairWhen damage has exceeds 16 ins

Sewn in Panel repair used when insufficient tautness when using doped on panelrepair

*Fabric Rejuvenation-restoring dope coating ( hardened dopes and crack has occurs) but fabric is still in good condition

Repair L-Shaped Tears 12

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