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Linhas areas InteligentesCOMPOSITE FUNDAMENTALS -2

REPAIR BASIC-2

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TOWS, ROVINGS AND YARNS (CONTINUATION)

For the greater protection the yarns are coated with asize which normally consists of a film-forming mediumand a lubricant, this size protects the yarn from mechanicaldamage during the weaving process of the yarn. There aretwo basic types of size: textile size and plastic size. Thetextile size consists of starch and oils which inhibits the

adhesion between the yarn and the resin. The compositionof textile sizes is normally such that the fabric can bedesized thermally or with appropriate liquids.

Plastic sizes also contain a coupling agent (finish). Thisreduces the protective effect of the textile size and theworkability of the yarn, it promotes bonding between the

yarn and resin formulation.

For maximum adhesion and strength fabrics are treatedwith a coupling agent specially adapted to each type ofmatrix material.

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TYPES OF YARN

Fabrics made from plastic sized yarns can be usedimmediately after weaving, without desizing or anyadditional treatment.

Finished fabrics (treated with a coupling agent) are softerand smoother. In the laminating process they can be

impregnated more thoroughly and more quickly. Also thefinished fabrics offer better drapability.

The following types of yarn are in current use:

SINGLES YARNSingles yarn is manufactured from a single spun yarn.

Usually, a single yarn is given a slight twist (protectivetwist) of about 10 -40 turns per meter.

PLIED YARNPlied yarn comprises two or more singles or assembledyarns which are twisted together (about 100-200 turns).

ASSEMBLED YARNAssembled yarn comprises two or more threads which arewound together but not twisted.

ROVINGRoving consists of one or more thrads which are

assembled in parallel into a strand without twisting.

STAPLE FIBERS YARNSStaple fibers yarns consist of fibers are bound togetherby twisting. Normal fibers length range from a fewcentimeters for synthetic materials to severaldecimeters for glass fibers.

TEXTURISED YARNSTexturised yarns are those which have undergone amechanical operation designed to increase their volumeand give them bulk.

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TAPES UNIDIRECTIONAL

Woven unidirectional tapes have the tows or rovings in thewarp lengthwise) direction held together by widely spacedweft threads, generally of a very fine yarn of polyester orglass fibers. Unidirectional tape gives high strength in onedirection, but it is weak in the weft direction.

Non woven tapes consist of tows or rovings laid side-by-side without gaps. To hold the filament in position they areimpregnated with a resin, which is partially cured to make itthick and viscous. To prevent the resin sticking to otherlayers of tape it is covered with release paper. A tape isnormally limited to a maximum width of about 300 mm.

WOVEN FABRICSNumerous weaves have been developed by the textileindustry for fashion fabrics but only three basic weaves arecommonly used for the advanced reinforcing fibers: plain (square) twill

satin.

PLAIN WEAVEIn plain weave each warp and weft thread passes over oneend or pick and under the next. (An end is the warpthread; a pick is a weft thread). The plain weave consistsof a basic weave pattern. The fabric is easy to handle due

to good dimensional stability and minimal fraying when cut.

TWILL WEAVEIn twill weave each end and pick floats over two,sometimes four, crossing threads. a pattern of diagonallines is produced on the face of the cloth. The twillweave give greater mechanical strength and stiffness ofthe laminate due to limited thread deflection. The

deflection of the thread is called the crimp. Thesefabrics are more pliable and thus better suited toshaped parts than plain weaves.

SATIN WEAVEIn satin weave each pick floats over all but one of theends. This gives a smooth surface and even higher

tensile strength. The satin weave has even less threaddeflection than the twill weave. The drapability is verygood and thus suited for shaped parts with very smallradii.

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CHOPPED STRAND MATIn a chopped strand mat the rovings are chopped into

short length, say 5 cm, and bonded together in randomdirections to form a mat. You may have seen it in car repairkits. These mats are available in a range of weights, forinstance one square meter of a mat may weight between250 g and 600 g depending on the depth of fibers used.The mat gives equal properties in all directions, and it is

easy to use. It is not as strong as woven cloths, becausethe fibers are short.

SURFACING MATWhen a good surface finish is needed, this mat is used. Itis similar to chopped strand mat, but it is very fine, and atypical mat would be 0.3 mm thick.

MIXED FABRICMixed fabrics consist of two different types of fibers. Onetype of material will be used for the warp and the other kindof material will be used for the weft. This means, forexample, that a less expensive glass yarn can be used inthe weft of an unidirectional carbon fibers fabric, if therigidity of the more expensive carbon fibers is not essentialin this direction.

HYBRID FABRICSIn hybrid fabrics, both the warp and the weft consist ofdifferent types of fibers. By combining different fibers it is

possible to produce a fabric with the best characteristics ofeach. For example, the impact strength of aramid fiberscan be combined with the rigidity of carbon fibers or thecompressive strength of glass fibers.

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GLASS FIBERS

A number of different glass compositions are used toprovide particular properties:

A-glass, the material used for windows, is now obsoletefor fibers but was cheap and had good acid resistance.

The need for special electrical properties for radomeapplications led to the development of E-glass which isnow the variety produced on the largest scale.

C-glass was developed for its chemical resistance; D-glass is an improved electrical grade for modernradomes

S- or R-glass, the American and European version, forhigher strength and modulus E-glass for reinforcement of plastics.

Glass fibers are produced from molten glass by drawing itthrough bushings having, normally 102 or 204 holes, athigh speed thus stretching the filament reducing thediameter. The filaments cool very rapidly by radiation andconvection before being treated with a size which preventsthe filaments from abrading each other as they cometogether to form a strand. This is wound into a packagewhich is dried and baked.

Depending on the intended use for the glass fibers, aplastic or a textile size may be used.

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PROPERTIES OF GLASS FIBERS

Mechanical propertiesGlass fibers have great strength combined with lowthermal expansion and low specific gravity.

Electrical propertiesDue to high specific resistance and the high dielectric

strength, glass ideal for insulation of electrical conductors.

Thermal propertiesGlass fibers are incombustible. However, if fabrics arefinished with organic materials, the fire performancechanges. Glass fabrics have a high residual strength aftersubjection to high temperatures.

Chemical properties

Glass is resistant to oil, grease and solvents and also toacids and alkaline with pH values of between 3 and 9.Acids remove certain atoms from the glass surface whichcauses brittleness.Alkalis gradually erode the surface of the glass.

ToolsGlass fibers fabrics can be cut with standard tools.

Storage

Glass fibers fabrics should be stored in a cool and dryplace to prevent the absorption of humidity.

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CARBON FIBERS

Carbon fibers are made commercially from two precursors:PAN (polyacrylonitrile) and pitch (from either oil or coal).

PAN BASED CARBON FIBERSPAN (polyacrylonitrile) precursor must be spun under cleanconditions from acrylonitrile polymerized with a co-

monomer and dissolved in a solvent to form a dope. Thismust be free from air and particulate matter. It is pumpedthrough a spinnerette into a coagulating bath whichremoves the solvent leaving solid PAN filaments. Thespinnerette may have up to 320,000 holes in it to producea tow of continuous filaments. This tow is stretched whilehot, washed, dried and either wound on to spools or, for

the larger 160,000 and 320,000 filament tows, crimped andlaid into boxes.

Carbon fibers for technical applications are manufacturedby thermal treatment of the PAN precursor, in threetemperature stages;

oxidation at 200 - 300C carbonization at 800 - 1500C graphitization at 2000 - 3000C

The last two processes will be carried out in an inert gasatmosphere. Dependant on the method of treatment

(temperature & time), the carbon fibers are produced withvarying mechanical properties.

Carbon fibers are classified a

HT - fibers (High Tenacity):These are very strong due to their high elongation atbreak.

IM - fibers (Intermediate Modulus):These have a higher E - Modulus and a considerableelongation at break.

HM - fibers (High Modulus):These have a very high E - Modulus with low elongationat break.

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PITCH BASED CARBON FIBERS

Pitch from oil or coal is the second important precursor forcarbon fibers.

It was originally hoped that the use of a by-product materialavailable cheaply would result in the cost of carbon fibersbeing reduced considerably.

The present position is that The cost of cleaning andconverting the pitch into a suitable form for themanufacture of fibers offsets the low raw material cost.

However, pitch-based fibers can have different properties;they can achieve higher tensile modulus values and have a

higher electrical conductivity, for example.

Until recently, pitched-based fibers were weaker than PAN-based at the same modulus but this is changing.

For aircraft, where weight is critical, the importantcomparison is between specific strength and specificmodulus.

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LIGHTNING PROTECTIVE METHODS FOR CARBONCOMPOSITE MATERIAL

WIRE MASHWire mash is used as a lightning strike protective methodfor conductive carbon composite material, especially onengine fairings/ cowlings.

WIRE MASH PROPERTIES

Aluminium wire diameter up to 0.008 inch (0:203 mm)located on 0.125 inch (3.175 mm) centers. Aluminum incompatible with carbon composites Fibreglass scrim cloth may be sandwiched between meshand carbon to prevent corrosion

Protects carbon composites from particle erosion Weight penalty Difficult to install on contoured surfaces Increases erosion resistance Must be used on outermost surface layer only Can be painted

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PROPERTIES OF CARBON FIBERS

Thermal propertiesCarbon fibers has a very low coefficient of thermalexpansion, which gives carbon fibers composite materialsa very good dimensional stability. Carbon fibers are notcombustible. Without oxygen, they are stable at about3000C and in the presence of oxygen, oxidation occurs atabout 400C with a loss of strength.

Electrical propertiesNon - magnetic, pervious to X - rays, good electricalconductivity.

Mechanical properties

The strength of carbon fibers exceed those of most metalsand other fibers materials. The low density (1.76 g/cm3)produces excellent specific strength characteristics. Theelongation is fully elastic. Resistance to fatigue stress isexcellent. Carbon fibers are the ideal material for high techapplications.

Chemical propertiesCarbon fibers are chemically very inert (nonreactive). Thefibers are high resistance to most acids, alkalis andsolvents. Carbon fibers absorb virtually no water.

Tools

Carbon fibers fabrics can be cut with standard tools. Forworking on cured parts, laminates hard steel and diamondtipped tools are required.

StorageCarbon fibers fabrics should be stored in a cool and dryplace.

Carbon fibers are coated with a plastic size which isideal for epoxy resins and which permits textileprocessing.

They are processed both with and without a protectivetwist, depending on the application and processingmethod.

Carbon fibers are chemically inert and therefore present

no danger to health. However, the size applied to theyarns may lead to skin irritation. Protective clothingshould therefore be worn as a precaution.

Unlike asbestos, the size and structure of fibrous finedust is not critical.

Due to the good electrical conductivity of the carbonfibers, the carbon fibers and carbon fibers dust shouldnot come in contact with electrical equipment.

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ARAMID FIBERS

The term ARAMID is the abbreviation of an ARomaticpolyAMiDe.

Aramid is manufactured by spinning from a polymersolution and subsequent surface treatment. In addition tothat, high modulus fibers are mechanically stretched.

The use of Aramid fibers makes sense where weightsaving is at the premium. In addition, aramid fibers areused in plastics subject to impact and abrasion (e.g. forprotecting leading edges of aircraft control surfaces againsthail impact, etc.)

There are two varieties of aramid material:

META-ARAMIDMeta aramid paper, best known by its trade name Nomex(Dupont), is widely converted into a honeycomb corematerial for sandwich structures for aircraft flooring panels,galleys and pallets. Also, Meta aramid, due to its moretextile character, is used in fire resistant fabrics.

PARA-ARAMIDPara aramid fibers are available from severalmanufacturers, the best known being Duponts Keviarrange. Dupont were the inventors of the para-aramid

materials.

The grade of para-aramid fibers of interest forcomposites is that with the highest modulus. The lowmodulus grade is used for ropes, protective clothing andas an alternative to asbestos. A third grade is used forreinforcing tyres and conveyor belts.

It is often advantageous to use a mixture of carbon andaramid fibers in a composite; the carbon provides a highmodulus and compressive strength while the aramidprovides toughness and reduced density.

The toughness of aramid fibers makes the material verydifficult to cut or break. Special tools have been

developed for cutting aramid composites and fiberscleanly.

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PROPERTIES OF ARAMID FIBERS

Mechanical PropertiesThe high tensile strength, the high E modulus and the lowdensity of Aramid, produce excellent specificcharacteristics. Good vibration damping properties, high energy absorption, toughness and low material fatigue.

Electrical propertiesAramids are good insulators and have good dielectricconstants, so that they can be used in both in printedcircuit boards and for cable insulation.

Thermal propertiesAramid will burn, but extinguish themselves once the flameis removed. The fibers do not melt and exhibit good flameretarding behaviour. Aramid fibers have a low coefficient ofthermal expansion and low thermal conductivity.

Chemical propertiesAramids are resistant to solvents, fuels, lubricants, saltwater etc., but they are attacked by some strong acids andakalis. They are also resistant to attack from fungi andbacteria.

ToolsAramid fibers fabrics must be cut with special tools (e.g.fine toothed scissors), as normal tools will soon becomeblunt and cause serious fraying of the cut edgesAramid fibers laminates require special tools whichhave special cutting geometries as compared with metalprocessing tools.

StorageAramid fabrics should be stored in a cool and dry place,especially aramid fabrics due to their high moistureabsorption of up to 7%.

Only aramid fibers fabrics with a moisture content ofless than 4% should be used in laminates. Aramid fibersfabrics with a higher moisture content can be dried at120C.

The mechanical properties of aramid are impaired byexposure to ultraviolet light, which also can causecolour changes.

Aramid fibers fabrics therefore should be protected fromlight during storage. Once in the laminate, the fibers areprotected by the resin.

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PREPREG

A major group of laminates is formed using a dry resinprocess known as preimpregnation. Pre-impregnation(prepreg) is a process whereby fibers cloth is wet with aresinous formulation. The resin is partially cured to a B-stage, forming the dry prepreg.

PREPREG PRODUCTION

Glass cloth is dipped in a solution of formulated resin. Thewet cloth is past through an oven where solvents areevaporated and the resin is partially cured. The B-stagedproduct, or the prepreg, is wound on to a take-up roll andstored.

AEROSPACE PREPREG

Prepreg from woven or unidirectional reinforcement iscommonly used to manufacture structural composites inthe airframe industry. Application in which prepreg is used,as opposed to hand lay-up or other machine-typeoperations, are usually high value, high performance enduses. The manufacturing cost and quality controlnecessary to make a consistent product restricts thenumber of applications where prepreg is the chosenmethod to produce a laminate.

The structural prepregs are made with a one-componentepoxy! hardener formulation. Usually, the resins chosenare of the high performance, multi- functional type.

Prepreg formulation must take into considerationviscosity, cure rate and stability of the one-componentresin system.

For aircraft applications, the finished prepreg must havethe proper tack and drape properties. Prepreg withproper tack will stick to it- self or a vertical surface afterapplying a small amount of pressure.

Proper drape implies the capability of the prepreg toconform to a multiple contoured mold without bridging.

Prepregs are formulated with latent epoxy hardiness,

they are usually shipped under refrigeration since only asmall advancement of the cure reaction can alter tackand drape qualities.

The prepreg is typically supplied on rolls.

Structural parts, such as ailerons or stabilizers onaircraft, are made by unrolling and cutting apredetermined length of prepreg and laying up the partlayer by layer until the required laminate thickness isreached. Often, successive plies have differentorientations to insure strength in the proper directions.

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AEROSPACE PREPREG (CONTINUATION)

Prepregs have to be cured in an enclosed vacuum bag.This removes a large amount of entrapped air betweenlayers of the prepreg. Next, the part is placed in anautoclave where heat and pressure are applied to furtherreduce voids and complete the cure of the formulatedepoxy system.

An alternative to an autoclave is a hot-bonder. A hot-bonder is only sufficient for small repairs.

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CORE MATERIALS

Where weight is critical as in aircraft parts, it is preferableto use a sandwich construction where the outside surface,in which the highest tensile and compressive stressesoccur, are held apart by a core of much lighter material.

This material could be for example:

Honeycomb plastic foam balsa wood thixotropic filled resin

Bonded honeycomb sandwich construction has been a

basic structure concept in the aerospace industry for thelast thirty years. Virtually every aircraft flying todaydepends upon the integrity and reliability offered by thisstructural approach. The capability of the concept has beenproven and is now widely accepted.

As a result of this history of success, a growing interest has

developed in the use of honeycomb sandwich for a broadrange of commercial applications. For the designer,honeycomb is a structural material whose uniquecharacteristics can be used to create new products, andsolve design problems. Honeycomb is a series ofhexagonal cells, nested together to form panels similar in

appearance to a cross-sectional slice of a beehive.

Honeycomb, in its expanded form, is 90 to 99 percentopen space. The basic geometry of honeycombprovides six primary characteristics. Each characteristiccan be combined with the qualities inherent in thematerial selected to form the honeycomb.

Because honeycomb can be produced from almost anymaterial available in continuous web or roll form, theextent to which honeycombs characteristics can beused to advantage is unlimited.

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SIX BASIC CHARACTERISTICS OF HONEYCOMB:

Highest strength-to-weight ratio as a sandwich core

Highest stiffness-to-weight ratio as a sandwich core.

Predictable and uni-form crushing strength under

compression.

Thermal, acoustical, and fluid directionalizing versatility.

Excellent sandwich fatique resistance.

Extremely high ratio of exposed surface area to total

volume

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PROPERTIES OF HONEYCOMB

Honeycomb is best known for its structural applications.Many of its properties also can be used to great advantagein non-structural applications.

Structural applications

When a honeycomb core is rigidly attached between twofaces of a sandwich panel, the resulting structure iscapable of the highest strength-to- weight and rigidity-to-weight ratios presently obtainable by ordinary designmethods.

Missiles, spacecraft and aircraft rely heavily upon the

unique structural properties of honeycomb in airframes aswell as ground support equipment.

These structural properties also are used in a great varietyof commercial applications ranging from production mastertools and subway and railway car components to exteriorcurtain walls for high-rise buildings.

Product designer find sandwich structures to be a newdesign approach, rather than just a new structural material.The flexibility and versatility of sandwich structures cansolve many design problems.

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PROPERTIES OF HONEYCOMB IN STRUCTURALAPPLICATIONS

ThermalHoneycomb is available for a wide range of temperatureexposures. Aluminium honeycomb, made from alloy 2024,has been used for service temperatures up to 420F.Stainless steel and nickel alloy honeycombs are useful ineven higher temperature environments.

Fibersglass reinforced phenolic honeycomb has excellentthermal stability and has been used from sub zerotemperatures up to 400F service.

Polyimides resins extend the upper limit to about 500F

with short time exposures up to 600F. Good mechanicalproperties result in extensive use of non-metallichoneycomb in aircraft and missile structures operatingwithin such temperature ranges.

New products under development, such as ceramics, canextend this upper range.

ElectricalElectrical and mechanical properties of glassfibersreinforced plastic honeycomb have led to standard use ofthe material in both airborne and ground radomes. These

structures must withstand high loads which requireconsiderable core strength and they must serve as efficientradar windows without attenuating or distortingtransmissions

Fatigue resistanceBonded honeycomb sandwich has replaced rivetedstructures which were subject to high-energy, sonicvibration in jet aircraft. Stress concentration isminimized when loads are distributed evenly in this typeof bonded structure, and, as a result, operating life isincreased by several orders of magnitude.

While Fatigue resistance is relative to operatingconditions and design, all studies show honeycombstructures resist Fatigue loading to extents which are farsuperior to alternative design methods.

RigidityAs with Fatigue resistance, rigidity is a design sensitivecharacteristic. However, the nature of a honeycombsandwich allows design of low deflection structures atminimum weight.

Where smooth reflective surfaces of extremely lowdeflection and high accuracy are required, honeycombsandwich is a widely used design approach.

Applications range from mammoth ground support radarequipment through solar energy concentrators to small,

specialized reflectors for spacecraft.

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PROPERTIES OF HONEYCOMB IN STRUCTURALAPPLICATIONS (CONTINUATION)

Environmental StabilityBecause honeycomb core exhibits all of the physicalproperties of the material from which it made, the choice ofmaterial dictates its performance in structural and non-structural exposure.

A protective coating is applied to aluminium honeycomb togive excellent corrosion resistance. The effectiveness ofthe coating has been documented by extensive testingunder condition of high humidity and salt spray.

Several types of paper, films and fibrous sheet materials

are available in honeycomb form which are resistant to awide range of chemicals including mild acid and causticsolutions. In addition, graphite and some aramid coresexhibit excellent coefficients of thermal expansion.

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NON-STRUCTURAL APPLICATION

Energy absorptionAn extremely effective mechanical-energy absorber,honeycomb may be used to control forces exerted ondecelerating objects.

Materials such as sponge, solid rubber, foams, cork andpaper wadding generally exhibit spring characteristics with

the attendant rebound problem.

Aluminium, aramid and stainless steel honeycomb,however, have the unique property of failing at a constantload while completely dissipating energy otherwisereleased in rebound.

The threshold at which compressive failure begins can beeliminated by prestressing honeycomb core to produceslight initial compressive failure. Exposed to furtherloading, the pre-stressed core carries the crushing load ata near-linear rate.

DirectionalizingThe parallel cell orientation of expanded honeycomb allowsit to directionalize air and fluid flow. Honeycombs slight celledge exposure minimizes pressure loss when liquids orgasses pass through it. Typical directionalizing applicationsinclude grilles, registers and wind-tunnel straightening

vanes where honeycomb, properly placed, encourageshigher velocities while reducing turbulence.

Heat exchangeThe extremely high ratio of surface area to unit volumeprovided by honeycomb offers many possibilities in heatexchange applications ranging from air conditioners tolarge industrial cooling towers. Materials available forheat exchangers include aluminium and stainless steelalloys.

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HONEYCOMB FABRICATION

Honeycomb is produced from various aluminium alloys:glass fabric/resin composite: aramid paper (Nomex) /phenolic resin and cellulose papers with resinimpregnation.Two manufacturing processes are used. One is theexpansion process and the other is the corrugationprocess.

Expansion processThe expansion process is the most commonly used forlight-weight honeycomb. It involves stacking sheets ofaluminium foil on top of each other, with lines of adhesivewhich have been printed onto it and curing the adhesive.

This block is then expanded so that the aluminium foilbetween the bonded strips will form cells, which, dependingon the amount of expansion, can be hexagonal orrectangular.

Corrugation process

The corrugation process is used for thick materials for highdensity cores. Sheets of aluminium foil are corrugatedbetween rolls, adhesive is applied to the tops of thecorrugation and the sheets are stacked, cured andexpanded.

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HEXAGONAL COREThe standard hexagonal honeycomb core is the basic and

most common cellular honeycomb configuration. Currentlyavailable in all metallic and nonmetallic combinations.

OX-COREThe OX configuration is a hexagonal honeycomb corethat has been over- expanded in the W direction,providing a rectangular cell configuration which facilitates

curving or forming in the L direction. The OX processincreases W shear properties and decreases L shearproperties when compared to hexagonal honeycomb.

FLEX COREFlex core is an extremely flexible configuration that allows

the core to be formed to complex curvatures. Curvatures ofvery tight radii are easily formed. Flex core seems toprovide higher shear strength than comparable hexagonalcore of equivalent density. Flex core can be manufacturedin most of the materials from which hexagonal honeycombis made.

TUBE COREThe tube core configuration provides a uniquely designedenergy absorption system when the space enveloperequires a thin wall column or small diameter cylinder. Thedesign eliminates the loss of crush strength that occurs atthe unsupported edges of conventional honeycomb. Tube

core is constructed of alternate sheets of flat aluminium foiland corrugated aluminium foil wrapped around a mandreland adhesively bonded.

CORROSION-RESISTANT HONEYCOMBCorrosion resistant honeycomb is available in a low-

cost commercial grade and highly developed family ofmilitary specification grades.

Commercial grade honeycomb, offers productdesigners the advantages of an all-metal honeycombwith long service life and excellent resistant to moisture.Military grade aluminium honeycomb - 5052,5056 and

2024 alloys- is available in a wide range of cell sizesand foil gauges. These can be combined to assure thespecific mechanical properties required to fabricateprecision-engineered structural components.

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FIBERSGLASS REINFORCED COREFibersglass reinforced core is produced from impregnated

woven glass cloth post-dipped in a resin. It combines lowdielectric constant and loss tangent properties with highstrength, properties that account for it use in aircraftradomes.These honeycomb materials have found wide use asstructural cores because of their excellent mechanicalproperties When subjected to elevated temperatures or

cyclic heat exposure.The versatility of glass- reinforced honeycomb is enhancedby its availability with either conventional and elevatedtemperatures-resistant resins, such as phenolics andpolyimides.Reinforced plastic honeycomb is also furnished in over-

expanded and Flex-core form, which allows greaterflexibility in forming curved parts.

ARAMID FIBERS COREAramid fibers core produced from an aramid fibers sheet,exhibits excellent toughness and chemical and temperatureresistance.

A variety of resin treatments for this honeycomb isavailable. Aramid-type honeycomb is strong, resilient,resists impact and will not burn. It is quite flexible and canbe easily handled, formed and machined.Because of this characteristics, it has found wideacceptance in sandwich panels for aircraft interiors and

flooring, radomes helicopter blades, wing to body fairings,antennae and ship partition. For optimum flexibility, thiscore type also is available in over- expanded and flex coreconfigurations.

WR II SHELTER CORE ( Hexcel trademark)It is a highly water-resistant core, produced from special

chemicals and polymers to impart anti-water migrationcharacteristics and excellent mechanical properties.Shelter core can be bonded with basic adhesivesystems to any facing material to create low cost Ihigh-strength panels.Originally developed for constructing air-transportablemilitary shelters, WR II has also been used in

fabricating electronic equipment enclosures, light utilitybuildings and cargo containers.

SPECIALIZED CORESThey have been developed to bring the outstandingproperties of Keviar to honeycomb cores, especially its

thermal stability.When used in conjunction with graphite facing, theresulting sandwich structure remains extremely stableunder cyclic heat and cold exposure.The core material has been used successfully inradome and antennae fabrication for space satellites.Graphite honeycomb has been manufactured from

advanced composite fabrics and tapes.

COMPOSITE FUNDAMENTALS 2

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FABRICATION PROCESSES

The fabrication of laminated fibrous composites involvesmany distinct operations. Some fabrication methods likebraiding and pultrusion are only suitable for limitedapplications. Cost is an important factor to considerwhenever various types of technology are relativelyevaluated. A few of the many questions that should beasked when selecting a fabrication method are:

Does the method require a large part count to beprofitable (e.g., compression molding) ? Does the method require large investments in equipmentor tools (e.g., compression molding)? Are there extreme limitations to the part configurations

that can be fabricated by the method (e.g., pultrusion,filament winding) ? Is the method labor intensive (e.g., autoclave, vacuumbag molding)? Is the selection of materials limited by the process (e.g.,injection molding)? Does the method require highly trained technicians to

operate the equipment (e.g., autoclave molding)? Are the finished parts consistent in dimension and quality(e.g., compression molding) ?

The described fabrication methods are:

Vacuum Bag Processing Autoclave Processing Honeycomb Sandwich Fabrication Expansion Tool Molding Compression Molding

Filament Winding Pultrusion Braiding Vacuum Bag Processing

COMPOSITE FUNDAMENTALS 2

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HEXAGONAL COREThe standard hexagonal honeycomb core is the basic and

most common cellular honeycomb configuration. Currentlyavailable in all metallic and nonmetallic combinations.

OX-COREThe OX configuration is a hexagonal honeycomb corethat has been over- expanded in the W direction,providing a rectangular cell configuration which facilitates

curving or forming in the L direction. The OX processincreases W shear properties and decreases L shearproperties when compared to hexagonal honeycomb.

FLEX COREFlex core is an extremely flexible configuration that allows

the core to be formed to complex curvatures. Curvatures ofvery tight radii are easily formed. Flex core seems toprovide higher shear strength than comparable hexagonalcore of equivalent density. Flex core can be manufacturedin most of the materials from which hexagonal honeycombis made.

TUBE COREThe tube core configuration provides a uniquely designedenergy absorption system when the space enveloperequires a thin wall column or small diameter cylinder. Thedesign eliminates the loss of crush strength that occurs atthe unsupported edges of conventional honeycomb. Tubecore is constructed of alternate sheets of flat aluminium foiland corrugated aluminium foil wrapped around a mandreland adhesively bonded.

CORROSION-RESISTANT HONEYCOMBCorrosion resistant honeycomb is available in a low-

cost commercial grade and highly developed family ofmilitary specification grades.

Commercial grade honeycomb, offers productdesigners the advantages of an all-metal honeycombwith long service life and excellent resistant to moisture.Military grade aluminium honeycomb - 5052,5056 and

2024 alloys- is available in a wide range of cell sizesand foil gauges. These can be combined to assure thespecific mechanical properties required to fabricateprecision-engineered structural components.

COMPOSITE FUNDAMENTALS -2

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VACUUM BAG PROCESSING

Vacuum bag processing utilizes a flexible film or rubberbag that covers the part lay-up.The bag permits the evacuation of air to apply atmosphericpressure. The use of vacuum bag pressure alone as aconsolidation technique is widespread, being second onlyto autoclave processing.The primary limitation of vacuum bag processing is the

limited pressure that can be applied.Autoclave processing has the flexibility of applying muchhigher pressures that are required to consolidate manysophisticated engineering materials.

The bag that is used in vacuum bag processing achieves

two objectives:

It provides a means for removing volatiles during cure in aconvection heated oven; It provides a means for the application of a pressure ofone atmosphere, which is adequate for some materials.

When individual plies of prepreg material are hand-formedto the lay-up tool, a certain amount of voids exist betweenlayers. By applying a flexible membrane over the tool, andsealing this material to the tool, a vacuum can be drawn onthe plies, resulting in a pressure of up to 15 psi on the lay-up. The requirements for proper bagging are:

The bag must be impervious to air passage The bag must uniformly apply the cure pressure

The bag (and the tooling surface) must not leak underoven conditions. A good, high capacity vacuum path must be providedto evacuate air from between the bag and the tool.

Two types of bagging methods are presently in use.The most common method uses a disposable bag

made of nylon or Kapton polyimide - film The othermethod involves the use of silicone rubber bags that arereusable. The advantages and disadvantages of the twobagging systems are presented below:

A reusable bag is usually molded to the particular part

configuration resulting in less labor time required to bagthe part.

A disposable bag requires extensive hand labor toremove wrinkles and prevent the bridging effects thatcan cause areas of the lay-up to contain voids.

A reusable bag requires an exterior framework toclamp the bag in place. This requires a more complextool than is required for the disposal bag method.

Disposable bags are more susceptible to pin holeleaks and edge sealing problems than the reinforcedreusable bags.

Li h I liCOMPOSITE FUNDAMENTALS -2

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Linhas areas InteligentesCOMPOSITE FUNDAMENTALS 2

VACUUM BAG PROCESSING (CONTINUATION)

The use of the vacuum bag is unchanged irrespective ofwhether autoclave or vacuum bag processing is used.

The only major difference between the two processes isthe amount of pressure applied.

The vacuum bag process achieves a maximum pressure

of only 15 psi.

The autoclave process can achieve pressures in excess of100 psi.

The achievable temperatures are dependent on the

capabilities of the curing oven.


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AUTOCLAVE PROCESSING

Autoclave processing utilizes a pressure chamber to applyheat and pressure to the composite lay-up during theconsolidation/cure cycle Autoclave curing is the mostcommon method of fabrication utilized in the aerospaceindustry, to produce composite structural parts. Theautoclave process is an economical method for thefabrication of extremely high quality parts, and

accommodates a large variety of part configurations.The primary disadvantage of autoclave processing is thehigh initial acquisition cost of an autoclave and the highrecurring operating costs. The advantage is the use ofsimple, one surface tooling to produce parts with complexconfiguration and very large sizes.

The type of prepreg material and the part configurationgovern the pressure and temperature reqiurements. Epoxymatrix composites typically use autoclave cure cycles thatinvolve 85-1 00 psi pressures and 350 F temperatures. Theautoclave is generally provided with automaticprogrammable controllers which monitor and maintain the

required heat-up and cool-down cycles. The temperatureincreases in a stair-step fashion. The temperature dwellpoints allow volatiles to be removed from the matrix prior togelation, and flow condition the prepreg material for thefinal cure. The vacuum applied to the bay surrounding thepart lay-up is also controlled. The vacuum is discontinuedafter the initial temperature increase to prevent excessresin flow.

The autoclaves used in aircraft part fabrication aregenerally heated by convection. Natural gas or propaneis burned, and a heat exchanger is used to provide heatinternally in the autoclave. Temperatures up to 600 Fare possible using this technique. The size of theautoclave is highly variable. Many manufacturers haveautoclaves in the range of 12 to 15 feet in diameter and

45 to 50 feet in length. The pressure capacity of theseautoclaves is generally limited to 200 psi.


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as a eas te ge tes

AUTOCLAVE PROCESSING

Autoclave processing utilizes a pressure chamber to applyheat and pressure to the composite lay-up during theconsolidation/cure cycle Autoclave curing is the mostcommon method of fabrication utilized in the aerospaceindustry, to produce composite structural parts. Theautoclave process is an economical method for thefabrication of extremely high quality parts, and

accommodates a large variety of part configurations.The primary disadvantage of autoclave processing is thehigh initial acquisition cost of an autoclave and the highrecurring operating costs. The advantage is the use ofsimple, one surface tooling to produce parts with complexconfiguration and very large sizes.

The type of prepreg material and the part configurationgovern the pressure and temperature reqiurements. Epoxymatrix composites typically use autoclave cure cycles thatinvolve 85-1 00 psi pressures and 350 F temperatures. Theautoclave is generally provided with automaticprogrammable controllers which monitor and maintain the

required heat-up and cool-down cycles. The temperatureincreases in a stair-step fashion. The temperature dwellpoints allow volatiles to be removed from the matrix prior togelation, and flow condition the prepreg material for thefinal cure. The vacuum applied to the bay surrounding thepart lay-up is also controlled. The vacuum is discontinuedafter the initial temperature increase to prevent excessresin flow.

The autoclaves used in aircraft part fabrication aregenerally heated by convection. Natural gas or propaneis burned, and a heat exchanger is used to provide heatinternally in the autoclave. Temperatures up to 600 Fare possible using this technique. The size of theautoclave is highly variable. Many manufacturers haveautoclaves in the range of 12 to 15 feet in diameter and

45 to 50 feet in length. The pressure capacity of theseautoclaves is generally limited to 200 psi.


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FABRICATION PROCEDURES

HONEYCOMB SANDWICH FABRICATIONThe use of honeycomb core materials is quite common incomposite aircraft structures and will continue to be used innew and varied applications. The primary advantage that ahoneycomb core offers is an increase in the stiffness of astructure without appreciably increasing its weight. Theprimary disadvantage in the use of honeycomb

constructions is the added control that is required in thefabrication process. Special techniques are required tomachine, form, handle, bond and cure honeycombsandwich panels. An initial step in manufacturinghoneycomb sandwich aircraft parts requires cutting thecore material to the required shape. Because honeycombcore cells can be easily collapsed by side pressure loads(normal to cell walls), the core must be stabilized in someway to prevent distortion while machining. Polyethyleneglycol is a commonly used material for stabilizing metalliccores to prevent movement during machining.Polyethylene glycol is a wax like substance under normalambient conditions with a low melting temperature. During

the preparation of polyethylene glycol, a fluorescein dye isadded to facilitate inspection of the core. Moltenpolyethylene glycol is poured into the core and allowed tosolidify, prior to the machining operation. After machining iscompleted, a hot water rinse melts out the polyethyleneglycol. If any of the polyethylene glycol remains in the core,the dye will be detected by a black light. After all traces ofpolyethylene glycol are removed, the core is placed in anoven to dry.

A fiberglass core is generally stabilized for machining byusing double- backed tape to attach the core to the millfixture. Foam can also be used to stabilize a fiberglasscore for machining. Precautions must be exercisedwhen handling the core after machining. The corematerial can easily be stretched out of shape by roughhandling or when carried with support on only one side.

The cutting tools used to cut the honeycomb core arehighly specialized and individual manufacturers willoften develop their own blades. The aluminum corecutting process is more of a slicing action than thegrinding action in conventional machining, preventingdistortion of the core.

The fabrication of honeycomb sandwich compositepanels is accomplished in three different ways:

one-step cure, two-step cure, three-step cure.


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HONEYCOMB SANDWICH FABRICATION (CONT)

The one-step cure process employs adhesive typeprepregs with controlled flow characteristics, and permitscuring the inner skin, core and outer skin, all at once, toform the sandwich. Prepregs with controlled flow propertiesprevent the loss of matrix into the open cells.

The use of a matrix that has a low viscosity during cure

would result in a resin starved condition on the skin that ison the top of the assembly during cure. The controlled flowmatrix systems stay in a gelatinous or thixotropic stateduring cure and are not pulled down by gravity into thecore.

The two-step cure process cures one skin (either outer orinner) to the core in one operation, and then cures theopposite skin to the core in a subsequent operation. Thistechnique enables the manufacturer to use matrix systemsthat are not of the controlled flow type.

However, a film adhesive is required between the skin and

the core to assure proper load transfer in the finishedstructure.The three-step cure process precures one skin, say theouter skin, at pressures as high as 100 psi, cocures theinner skin with the core separately, and finally bonds theprecured outer skin with the cocured core/inner skin. Avariation on the two- and three-step methods involvesmachining the core after bonding. The skin is cured withthe core and the core is then machined using the skin asthe holding fixture.


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PRECURED MATERIAL

A precured skin has good properties because it is cured atpressures as high as 100 psi. However, if a skin is cocuredwith the core, the cure pressure is generally limited tobelow 50 psi to prevent core crushing, and consequentlythe skin properties are inferior to those of a precured skin.The one-step cure process is generally a 270 F cureprocess and is widely used for secondary structures such

as commercial aircraft fairings, interiors, radomes, andother applications that do not exceed 180 F serviceenvironments. Multiple-step cure processes are associatedwith structural applications requiring 350 F cure andservice temperatures up to 250 F.

A precured detail process cures the outer and inner skinsseparately, at pressures as high as 100 psi, and thenbonds the precured skins to the core using an adhesive. Itmay be viewed as a two-step cure process. The precuredskins, however demand an accurate core fit. The one-,two- and three-step processes are more forgiving of thecore dimensions since the skins are cured with the core

and take on the core shape during cure.

If the sandwich construction will have fasteners passingthrough it, additional steps are taken during fabrication.The core material at the fastener location is replaced by ahard member to prevent crushing of the core at fastenerlocations. The hard member is generally a solid laminate,an epoxy potting compound, an aluminum bar or a titaniumbar. Figure 2-9 shows the mismatch tolerances for hardmembers in core-containing assemblies.

A foam type adhesive is used:

in an area where a core butts against a hard member; in panels where a honeycomb core butts againstanother honeycomb core, as in panels that utilizeseveral pieces of core material; in applications where the core material ends in a

narrow wedge, to help stabilize the core duringfabrication


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EXPANSION TOOL MOLDING

Expansion tool molding utilizes rubber inserts in a metal orepoxy tool that expands when heated to provide themolding pressure. The application of expansion tooling islimited to part configurations with deep channels where therubber tooling can be utilized. The primary advantage ofexpansion tooling is its ability to fabricate parts without anautoclave.


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COMPRESSION MOLDING (MATCHED DIE MOLDING)

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COMPRESSION MOLDING (MATCHED DIE MOLDING)

The compression (matched die) molding process utilizeslarge tonnage presses to compress the prepreg materialbetween two matched steel dies.

The present use of compression molding on continuouslength fiber composites for aircraft structural parts islimited.

However, there are selective applications in the aircraftindustry for compression molding. The largest current useof this process is in the fabrication of secondary structuralparts that are made of discontinuous length fiber sheetmolding compounds (SMC).

And, as new controlled flow resin systems, specificallydesigned for compression molding, are developed, the useof the process will increase.

The primary advantage in using the compression moldingfabrication process is the ability of producing large

numbers of parts rapidly with little, if any, dimensionalvariations from part to part.

An important factor to be considered before deciding to usecompression molding, is the high tooling cost and the needfor large heated presses.


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FILAMENT WINDING

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FILAMENT WINDING

Filament winding is a mechanically automated process forcreating parts of relatively simple geometry by wrapping amale tool with filaments impregnated with matrix. Theprimary aircraft applications for filament winding consist ofwaste storage tanks and other fluid gas containmentvessels. Wide use is made of filament winding in thehelicopter industry to produce drive shafts, tail booms and

rotor blades.

The filament winding process is labeled dry if it utilizesprepreg material and wet if it uses fibers passed through aresin bath. The process is very similar regardless ofwhether it is wet or dry. The fiber material can be of anycontinuous length material such as aramid, glass orgraphite. The picture shows a typical dry filament windingprocess where a continuous fiber roving passes through ashuttle which rotates, and the roving is wrapped around arevolving or stationary mandrel.

Two basic types of filament winding are in use:

the polar or planar method, the high helical pattern winding.

The polar or planar method of winding utilizes a fixedmandrel and a shuttle that revolves around the longitudinalaxis of the part to deposit longitudinal winding patterns.

The mandrel advances one reinforcement band widthfor each wrap of the mandrel. In the high helical patternwinding, the mandrel rotates while the shuttle traversesback and forth.As the shuttle approaches the ends of the longitudinalaxis of the mandrel, the shuttle head rotates to ensurethat the reinforcement is laying flat on the highly

contoured ends of the mandrel and reverses direction atthe same time.Hoop or circumferential winding is accomplished byrotating the mandrel while advancing the shuttle onefiber band width per cylinder revolution.The result is a fiber pattern deposited at 90 to thelongitudinal axis of the mandrel.Depending on the design of the part, and the stressrequirements, there may be any combination of hoop,longitudinal and helical wraps.Quite often, when external attachments (flanges) are tobe attached to the filament wound part, hoop wraps areused to wrap the flanges directly onto the structure.


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PULTRUSION

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PULTRUSION

Pultrusion is a mechanically automated process used tocreate shapes by pulling rovings through a shaped andheated die. The use of pultruded parts in aircraft is limitedto specialized applications. The advantage in using thepultrusion method is the ability to manufacture the finishedparts at an extremely rapid rate. Practical applications arelimited to constant cross section profiles.

Pultrusion is used to manufacture constant cross sectionshapes, generally of the I-beam, box or tube variety. Theprocess utilizes preimpregnated rovings or rovings that arepulled through a resin bath to impregnate the fibers. Theuse of preimpregnated fibers eliminates the resin bath. Therovings go through a heated die that represents the crosssection of the finished part. Curing is accomplished byheating the die and/or microwave curing. The process iscontinuous and can be used to manufacture extremely longsections.

The major limitation of pultrusion for aircraft use the

constant cross section requirement. A variation onpultrusion is a process called pulmolding or pulforming. Inthis process preimpregnated fibers are heated by radio anare drawn into a curved heated die. The curved die rotateson a table causing final forming to take place by closing acorotating male die. The limitation of this pocess for aircraftuse is the size capacity of the equipment. The greatest

advantage of pultrusion and pulforming is the ability toproduce consistent parts at very low cost, and in a shortperiod of time.


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BRAIDING

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The braiding process involves the weaving of fibers(graphite, steel, glass, etc) into shape by repeatedlycrossing them back and forth over a mandrel.The use of braiding in the aircraft industry is generallyrestricted to nonstructural applications.Traditionally, the braiding process has been utilizedextensively for the covering of electrical wires and fuel

lines. Recently, actual structures have been fabricted bybraiding.The primary advantage that braiding offers is a rapid,automated method for forming an interwoven structure oran overlap of an existing part.The method is a product of textile technology, and usuallyutilizes equipment adapted from the textile industry


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