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Contents

1. Introduction 4

1.1 Basiccompositetheory 4

1.2 Polymermatrixcomposites 4

2. Designingwithcomposites 6

2.1 Loading 6

2.1.1 Tension 6

2.1.2 Compression 6

2.1.3 Shear 7

2.1.4 Flexure 7

2.2 Stressorstrain? 7

2.3 Fibreorientation 10

2.4 FatigueResistance 10

3. Monolithiclaminates 10

3.1 Ruleofmixtures 10

3.2 Laminatetheory 12

3.3 Laminatenotation 13

4. Sandwichpanels 13

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5. Materialselection 14

5.1 Resinmatrix 14

5.1.1 PolyesterResins 15

5.1.2 VinylesterResins 18

5.1.3 EpoxyResins 19

5.1.4 ComparisonofResinProperties 20

5.1.5 OtherResinSystemsusedinComposites 23

5.2 Fibres 24

5.2.1 BasicPropertiesofFibresandOtherEngineeringMaterials 25

5.2.2 FibreTypes 28

5.3 Fabrictypes 33

5.3.1 UnidirectionalFabrics 33

5.3.2 0/90°Fabrics 34

5.3.3 WovenFabrics 34

5.3.4 MultiaxialFabrics 37

5.3.5 OtherFabrics 39

5.4 Corematerialsforsandwichconstruction 40

5.4.1 FoamCores 40

5.4.2 Honeycombs 42

5.4.3 Designconsiderations 45

6. Processingroute 46

6.1 SprayLay-up 47

6.2 WetLay-up/HandLay-up 48

6.3 VacuumBagging 49

6.4 FilamentWinding 50

6.5 Pultrusion 51

6.6 ResinTransferMoulding(RTM) 52

6.7 OtherInfusionProcesses-SCRIMP,RIFT,VARTMetc. 53

6.8 Autoclave 54

6.9 Oven 55

6.10 ResinFilmInfusion(RFI) 56

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7. Secondarybonding 57

7.1 Scienceofadhesion 57

7.1.1 Mechanicalinterlocking 58

7.1.2 Diffusiontheory 58

7.1.3 Adsorptiontheory 58

7.1.4 WeakBoundaryLayers 58

7.2 Pre–treatmentpriortobonding 59

7.3 Adhesiveselection 60

7.3.1 Epoxiesandtoughenedepoxies 60

7.3.2 Polyurethanes 60

7.3.3 Acrylicsandmethacrylatesstructuraladhesives 61

7.3.4 Polyesters 61

7.3.5 Urethane-acrylates 61

7.3.6 Heatstableadhesives:Bismaleimides,polyimides,cyanateesters 61

7.3.7 Anaerobicsandcyanoacrylates 62

7.4 Jointdesign 62

8. Gelation,curing&postcuring 63

9. Inspectionandtesting 63

9.1 Nondestructivetesting 64

9.1.1 Visualinspection 64

9.1.2 Taptesting 64

9.1.3 Ultrasonics 64

9.1.4 ComputedTomography 65

9.1.5 Shearography 65

9.1.6 Thermography 65

9.1.7 LambWaveSensing 66

9.1.8 Radiography 66

9.2 Destructivetesting 66

9.2.1 Mechanicaltesting 67

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GuidetoStructuralCompositesIntroduction

Tofullyappreciatetheroleandapplicationofcompositematerialstoastructure,anunderstandingisrequiredofthecomponentmaterialsthemselvesandofthewaysinwhichtheycanbeprocessed.Thisguidelooksatbasiccompositetheory,propertiesofmaterialsusedandthenthevariousprocessingtechniquescommonlyfoundfortheconversionofmaterialsintofinishedstructures.

1.1BasicCompositeTheory

In itsmostbasic formacompositematerial isone,which iscomposedofat leasttwoelementsworking together toproducematerialproperties thataredifferent tothepropertiesofthoseelementsontheirown.Inpractice,mostcompositesconsistofabulkmaterial(the‘matrix’),andareinforcementofsomekind,addedprimarilytoincreasethestrengthandstiffnessofthematrix.Thisreinforcementisusuallyinfibreform.Today,themostcommonman-madecompositescanbedividedintothreemaingroups:

PolymerMatrixComposites(PMC’s)–Thesearethemostcommonandwillbedis-cussedhere.AlsoknownasFRP-FibreReinforcedPolymers(orPlastics)-thesematerialsuseapolymer-basedresinasthematrix,andavarietyoffibressuchasglass,carbonandaramidasthereinforcement.

MetalMatrixComposites(MMC’s)-Increasinglyfoundintheautomotiveindustry,thesematerialsuseametalsuchasaluminiumasthematrix,andreinforceitwithfibres,orparticles,suchassiliconcarbide.

CeramicMatrixComposites(CMC’s)-Usedinveryhightemperatureenvironments,thesematerialsuseaceramicasthematrixandreinforceitwithshortfibres,orwhisk-erssuchasthosemadefromsiliconcarbideandboronnitride.

1.2 Polymermatrixcomposites

Resinsystemssuchasepoxiesandpolyestershavelimiteduseforthemanufactureofstructuresontheirown,sincetheirmechanicalpropertiesarenotveryhighwhencomparedto,forexample,mostmetals.However,theyhavedesirableproperties,mostnotablytheirabilitytobeeasilyformedintocomplexshapes.

Materialssuchasglass,aramidandboronhaveextremelyhightensileandcompressivestrengthbutin‘solidform’thesepropertiesarenotreadilyapparent.Thisisduetothefactthatwhenstressed,randomsurfaceflawswillcauseeachmaterialtocrackandfailwellbelowitstheoretical‘breakingpoint’.Toovercomethisproblem,thematerialisproducedinfibreform,sothat,althoughthesamenumberofrandomflawswilloccur,theywillberestrictedtoasmallnumberoffibreswiththeremainderexhibitingthematerial’stheoreticalstrength.Thereforeabundleoffibreswillreflectmoreaccuratelytheoptimumperformanceofthematerial.However,fibresalonecanonlyexhibittensilepropertiesalongthefibre’slength,inthesamewayasfibresinarope.

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Itiswhentheresinsystemsarecombinedwithreinforcingfibressuchasglass,carbonandaramid,thatexceptionalpropertiescanbeobtained.Theresinmatrixspreadstheloadappliedtothecompositebetweeneachoftheindividualfibresandalsoprotectsthefibresfromdamagecausedbyabrasionandimpact.Highstrengthsandstiffnesses,easeofmouldingcomplexshapes,highenvironmentalresistanceallcoupledwithlowdensities,maketheresultantcompositesuperiortometalsformanyapplications.

Since PMC’s combine a resin system and reinforcing fibres, the properties of theresultingcompositematerialwillcombinesomethingofthepropertiesoftheresinonitsownwiththatofthefibresontheirown.

Overall,thepropertiesofthecompositearedeterminedby:

i) Thepropertiesofthefibre

ii) Thepropertiesoftheresin

iii) Theratiooffibretoresininthecomposite(FibreVolumeFraction)

iv) Thegeometryandorientationofthefibresinthecomposite

Thefirsttwowillbedealtwithinmoredetaillater.Theratioofthefibretoresinderiveslargelyfromthemanufacturingprocessusedtocombineresinwithfibre,aswillbedescribedinthesectiononmanufacturingprocesses.However,itisalsoinfluencedbythetypeofresinsystemused,andtheforminwhichthefibresareincorporated.Ingeneral,sincethemechanicalpropertiesoffibresaremuchhigherthanthoseofresins,thehigherthefibrevolumefractionthehigherwillbethemechanicalpropertiesoftheresultantcomposite.Inpracticetherearelimitstothis,sincethefibresneedtobefullycoatedinresintobeeffective,andtherewillbeanoptimumpackingofthegenerallycircularcross-sectionfibres.Inaddition,themanufacturingprocessusedtocombinefibrewithresinleadstovaryingamountsofimperfectionsandairinclusions.Typically,withacommonhandlay-upprocessaswidelyusedintheboat-buildingindustry,alimit forFVF isapproximately30-40%. With thehigherquality,moresophisticatedandpreciseprocessesusedintheaerospaceindustry,FVF’sapproaching70%canbesuccessfullyobtained.

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Thegeometryof thefibresinacomposite isalso importantsincefibreshavetheirhighestmechanicalpropertiesalong their lengths, rather thanacross theirwidths.Thisleadstothehighlyanisotropicpropertiesofcomposites,where,unlikemetals,themechanicalpropertiesofthecompositearelikelytobeverydifferentwhentestedindifferentdirections.Thismeansthatitisveryimportantwhenconsideringtheuseofcompositestounderstandatthedesignstage,boththemagnitudeandthedirectionoftheappliedloads.Whencorrectlyaccountedfor,theseanisotropicpropertiescanbeveryadvantageoussinceitisonlynecessarytoputmaterialwhereloadswillbeapplied,andthusredundantmaterialisavoided.

Itisalsoimportanttonotethatwithmetalsthepropertiesofthematerialsarelargelydeterminedbythematerialsupplier,andthepersonwhofabricatesthematerialsintoafinishedstructurecandoalmostnothingtochangethose‘in-built’properties.How-ever,acompositematerialisformedatthesametimeasthestructureisitselfbeingfabricated.This isaFUNDAMENTALdistinctionofcompositematerialsandMUSTalwaysbeconsideredduringdesignandmanufacturingstages.

2Designingwithcomposites

2.1Loading

Therearefourmaindirectloadsthatanymaterialinastructurehastowithstand:ten-sion,compression,shearandflexure.

2.1.1TensionFig.2showsatensileloadappliedtoacomposite.Theresponseofacompositetotensileloadsisverydependentonthetensilestiffnessandstrengthpropertiesofthereinforcementfibres,sincethesearefarhigherthantheresinsystemonitsown.

Figure1–Tensileloading

2.1.2Compression

Figure2showsacompositeunderacompressiveload.Here,theadhesiveandstiff-nesspropertiesoftheresinsystemarecrucial,asitistheroleoftheresintomaintainthefibresasstraightcolumnsandtopreventthemfrombuckling.

Figure2–Compressiveloading

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2.1.3Shear

Figure3showsacompositeexperiencingashearload.Thisloadistryingtoslideadjacentlayersoffibresovereachother.Undershearloadstheresinplaysthemajorrole,transferringthestressesacrossthecomposite.Forthecompositetoperformwellundershearloadstheresinelementmustnotonlyexhibitgoodmechanicalpropertiesbutmustalsohavehighadhesiontothereinforcementfibre.Theinterlaminarshearstrength(ILSS)ofacompositeisoftenusedtoindicatethispropertyinamulti-layercomposite(‘laminate’).

Figure3–Shearloading

2.1.4Flexure

Flexuralloadsarereallyacombinationoftensile,compressionandshearloads.Whenloadedasshown,theupperfaceisputintocompression,thelowerfaceintotensionandthecentralportionofthelaminateexperiencesshear.

Figure4–Flexuralloading

2.2Stressorstrain?

Thestrengthofalaminateisusuallythoughtofintermsofhowmuchloaditcanwith-standbeforeitsufferscompletefailure.Thisultimateorbreakingstrengthisthepointitwhichtheresinexhibitscatastrophicbreakdownandthefibrereinforcementsbreak.

However,beforethisultimatestrengthisachieved,thelaminatewillreachastresslevelwheretheresinwillbegintocrackawayfromthosefibrereinforcementsnotalignedwiththeappliedload,andthesecrackswillspreadthroughtheresinmatrix.Thisisknownas‘transversemicro-cracking’and,althoughthelaminatehasnotcompletelyfailedatthispoint,thebreakdownprocesshascommenced.Consequently,engineerswhowantalong-lastingstructuremustensurethattheirlaminatesdonotexceedthispointunderregularserviceloads.

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Figure5–TypicalFRPstress/straingraph

Thestrainthatalaminatecanreachbeforemicrocrackingdependsstronglyonthetoughnessandadhesivepropertiesoftheresinsystem.Forbrittleresinsystems,suchasmostpolyesters,thispointoccursalongwaybeforelaminatefailure,andsoseverelylimitsthestrainstowhichsuchlaminatescanbesubjected.Asanexample,testshaveshownthatforapolyester/glasswovenrovinglaminate,micro-crackingtypicallyoccursatabout0.2%strainwithultimatefailurenotoccurringuntil2.0%strain.Thisequatestoausablestrengthofonly10%oftheultimatestrength.

Astheultimatestrengthofalaminateintensionisgovernedbythestrengthofthefibres,theseresinmicro-cracksdonotimmediatelyreducetheultimatepropertiesofthelaminate.However,inanenvironmentsuchaswaterormoistair,themicro-crackedlaminatewillabsorbconsiderablymorewaterthananuncrackedlaminate.Thiswillthenleadtoanincreaseinweight,moistureattackontheresinandfibresizingagents,lossofstiffnessand,withtime,aneventualdropinultimateproperties.

Increasedresin/fibreadhesion isgenerallyderivedfromboth theresin’schemistryanditscompatibilitywiththechemicalsurfacetreatmentsappliedtofibres.Herethewell-knownadhesivepropertiesofepoxyhelplaminatesachievehighermicrocrackingstrains.Resintoughnesscanbehardtomeasure,butisbroadlyindicatedbyitsultimatestraintofailure.AcomparisonbetweenvariousresinsystemsisshowninFigure6

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Figure6-TypicalResinStress/StrainCurves(Post-Curedfor5hrs@80°C

Itshouldalsobenotedthatwhenacompositeisloadedintension,forthefullme-chanicalpropertiesofthefibrecomponenttobeachieved,theresinmustbeabletodeformtoatleastthesameextentasthefibre.Figure7givesthestraintofailureforE-glass,S-glass,aramidandhigh-strengthgradecarbonfibresontheirown(i.e.notinacompositeform).Hereitcanbeseenthat,forexample,theS-glassfibre,withanelongationtobreakof5.3%,willrequirearesinwithanelongationtobreakofatleastthisvaluetoachievemaximumtensileproperties.

Figure7–Typicalstrainstofailure

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2.3 Fibreorientation

Itisrareforfibresinacompositelaminatetobeperfectlyaligned.Wovenfabricsin-troduceacrimptofibreswhichcausesmisalignmentofloadpaths.Evennonwovenfabricscansufferfromsomecrimpingaroundstitchpoints.Themisalignmentoffibrescausesdramaticlossofmechanicalproperties,particularlyincompression,duetoincreasedlikelihoodofbuckling.

Whenusingunidirectionalfabrics,tapesortows,itbecomescriticaltoensureaccuratealignmentoffibresduringcomponentmanufacturetoensureloadsaretransferredefficientlyandtomaximizetheadvantagesofutilizingcompositematerials.

2.4 FatigueResistance

Generallycompositesshowexcellentfatigueresistancewhencomparedwithmostmetals.However,sincefatiguefailuretendstoresultfromthegradualaccumulationofsmallamountsofdamage,thefatiguebehaviourofanycompositewillbeinfluencedbythetoughnessoftheresin,itsresistancetomicrocracking,andthequantityofvoidsandotherdefectswhichoccurduringmanufacture.Asaresult,epoxy-basedlaminatestendtoshowverygoodfatigueresistancewhencomparedwithbothpolyesterandvinylester,thisbeingoneofthemainreasonsfortheiruseinaircraftstructures.

3.MonolithiclaminatesMostcompositestructuresarebuiltupintousefulsectionsbystackingpliesoffibreorfabricontoeachother.Sincetheorientationofthefibrescanbevaried,thepropertiesofthelaminatearenotconsistentthroughthethicknessofthepart,andareconsideredhighlyanisotropicinall3planes.

3.1 Ruleofmixtures

Whilstdeformationofhomogeneous,isotropicmaterialscanbedescribedrelativelysimplybyuseofYoungsandShearmoduli,whicharebulkpropertiesof the rawmaterialsimplepropertiesofcompositematerialscanbeestimatedbasedon thecontributionofeachpartofthecomposite.Thismethodisreferredtoastheruleofmixtures(RoM).

Fora2componentcomposite:

Vf +Vm =1

where Vf =Volumefractionfibre

Vm=Volumefractionmatrix

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BasedonRoMacompositeproperty,Pc,canbeestimatedby:

Pc =Pf Vf+ P+ PPPmVm= P= PPPfVVf+ P+ PPPm(1-V(1-Vf)

wherePf&Pm=Thepropertyoffibre

Forexampleelasticmodulus,E1,paralleltofibredirectioncanbecalculatedbasedontheYoungsmodulusofeachoftheconstituentmaterials:

E1 =Ef Vf+EEm Vm

Thisequationiseasilyunderstoodbyconsideringtheanalogyofcalculatingthestiff-nessof2springsconnectedinparallel:

Figure8–RoMmodelforlongitudinalproperties

Fortheelasticmodulusperpendiculartofibredirectionthecalculationbecomesalit-tlemoresophisticated,dependantonthefibreshapeandfibrecontent,butaagoodestimatecanbeobtainedusingthemodeloftwospringsconnectedinseries:

Figure9–RoMmodelfortransverseproperties

Withthismodel,thetotaldeformationatapointofloadapplicationinadirectionper-pendiculartofibredirectionisthesumofthedeflectionsinthefibreandthematrix.Theresultingexpressionfortransversemodulusis:

1 Vf Vm

E2=

Ef+

Em

Unitvolume Stiffnessrepresentation

Unitvolume Stiffnessrepresentation

Matrix-Fibre-Matrix

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3.2 Laminatetheory

RoMformulaeandmodelsareusefulforcalculatingthepropertiesofsinglelayersbutmorecomplexmethodologiesarerequiredtoenablecharacterisationofmultipliedlaminates.LaminatePlateTheory(LPT) isacomplexbutwellestablishedandac-ceptedapproach.WhilsttoocomplicatedtodescribeindetailhereLPTdescribesthedeformationofalaminateunderexternalloadingbasedonthepropertiesofthefibre,matrixandthe%ofeachineachaxisofloading.Itislogicalandeasilyautomatedforincorporationintoengineeringmodels.

Itisquiteobviousthatmechanicalpropertiesofaplywithinthelaminatearedepend-antonthealignmentandorientationof thefibrewithin theply.Howthepropertiesvarydependsonthetypeoffibre.CarbonforexampleismoresensitivetoloadingdirectionthanKevlar.

Figure 10 shows how the primary mechanical properties, Youngs modulus, shearmodulusandpoisson’sratiovarywithplyangle:

Figure10–Typicaleffectofplyangleoncarbonlaminate

FurtherinformationonLPTcanbefoundinanycompositematerialtextbook.Agoodinitialtextis:

An introduction to composite materials

Derek Hull

Cambridge solid state series

ISBN 0 521 28392 2

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3.3 Laminatenotation

Consideringalaminatemanufacturedusingunidirectional(UD)plies,thelayupnota-tionisdescribedasshownbelow:

Startingatthetopsurface(0/+45/-45/90/-45/+45/0)

Figure11–Stackingsequenceexample

WhendesigningalaminateitisimportanttoconsiderthestackingsequenceofpliesandtobeawareofSYMMETRYandBALANCEofthestack.Thestackaboveisbothsymmetric(aboutthemidplane)whichhelpstoeliminateanytendencytobendorwarp,andbalancedmeaningthatthereisanequalnumberof+45°&-45°plies,whichreducesshearcoupling.

4. SandwichpanelsSingleskinlaminates,madefromglass,carbon,aramid,orotherfibersmaybestrong,buttheycanlackstiffnessduetotheirrelativelylowthickness.Traditionallythestiffnessofthesepanelshasbeenincreasedbytheadditionofmultipleframesandstiffeners,addingweightandconstructioncomplexity.

Asandwichstructureconsistsoftwohighstrengthskinsseparatedbyacorematerial.Insertingacoreintothelaminateisawayofincreasingitsthicknesswithoutincurringtheweightpenaltythatcomesfromaddingextralaminatelayers.Ineffectthecoreacts likethewebinanI-beam,wherethewebprovidesthelightweight ‘separator’betweenthe load-bearingflanges. InanI-beamtheflangescarry themain tensileandcompressiveloadsandsothewebcanberelativelylightweight.Corematerialsinasandwichstructurearesimilarlylowinweightcomparedtothematerialsintheskinlaminates.

Engineeringtheoryshowsthattheflexuralstiffnessofanypanelisproportionaltothecubeofitsthickness.Thepurposeofacoreinacompositelaminateisthereforetoincreasethelaminate’sstiffnessbyeffectively‘thickening’itwithalow-densitycorematerial. Thiscanprovideadramatic increase instiffness for very littleadditionalweight.

0o

+45o

-45o

90o

-45o

+45o

0o

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Figure12showsacoredlaminateunderabendingload.Here,thesandwichlaminatecanbelikenedtoanI-beam,inwhichthelaminateskinsactastheI-beamflange,andthecorematerialsactasthebeam’sshearweb.Inthismodeofloadingitcanbeseenthattheupperskinisputintocompression,thelowerskinintotensionandthecoreintoshear.Itthereforefollowsthatoneofthemostimportantpropertiesofacoreisitsshearstrengthandstiffness.

Figure12–Sandwichpanelloading

Inaddition,particularlywhenusing lightweight, thin laminateskins, thecoremustbecapableoftakingacompressiveloadingwithoutprematurefailure.Thishelpstopreventthethinskinsfromwrinkling,andfailinginabucklingmode.

5. Materialselection

5.1 Resinmatrix

The resins thatareused infibre reinforcedcompositesaresometimes referred toas ‘polymers’.Allpolymersexhibitan importantcommonproperty in that theyarecomposedof longchain-likemoleculesconsistingofmanysimplerepeatingunits.Man-madepolymersaregenerallycalled‘syntheticresins’orsimply‘resins’.Polymerscanbeclassifiedundertwotypes,‘thermoplastic’and‘thermosetting’,accordingtotheeffectofheatontheirproperties.

Thermoplastics,likemetals,softenwithheatingandeventuallymelt,hardeningagainwithcooling.Thisprocessofcrossingthesofteningormeltingpointonthetemperaturescalecanberepeatedasoftenasdesiredwithoutanyappreciableeffectonthemate-rialpropertiesineitherstate.Typicalthermoplasticsincludenylon,polypropyleneandABS,andthesecanbereinforced,althoughusuallyonlywithshort,choppedfibressuchasglass.

Thermosettingmaterials,or‘thermosets’,areformedfromachemicalreactioninsitu,wheretheresinandhardenerorresinandcatalystaremixedandthenundergoanon-reversiblechemicalreactiontoformahard,infusibleproduct.Insomethermo-sets,suchasphenolicresins,volatilesubstancesareproducedasby-products(a‘condensation’ reaction). Other thermosettingresinssuchaspolyesterandepoxycurebymechanismsthatdonotproduceanyvolatilebyproductsandthusaremucheasiertoprocess(‘addition’reactions).

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Oncecured,thermosetswillnotbecomeliquidagainifheated,althoughaboveacertaintemperaturetheirmechanicalpropertieswillchangesignificantly.ThistemperatureisknownastheGlassTransitionTemperature(Tg),andvarieswidelyaccordingtotheparticularresinsystemused,itsdegreeofcureandwhetheritwasmixedcorrectly.AbovetheTg,themolecularstructureofthethermosetchangesfromthatofarigidcrystallinepolymertoamoreflexible,amorphouspolymer.ThischangeisreversibleoncoolingbackbelowtheTg.AbovetheTgpropertiessuchasresinmodulus(stiffness)dropsharply,andasaresultthecompressiveandshearstrengthofthecompositedoestoo.Otherpropertiessuchaswaterresistanceandcolourstabilityalsoreducemarkedlyabovetheresin’sTg.

Althoughtherearemanydifferenttypesofresininuseinthecompositeindustry,themajorityofstructuralpartsaremadewiththreemaintypes,namelypolyester,vinylesterandepoxy.

5.1.1 PolyesterResins

Polyesterresinsarethemostwidelyusedresinsystems,particularly in themarineindustry.Byfarthemajorityofdinghies,yachtsandwork-boatsbuiltincompositesmakeuseofthisresinsystem.

Polyesterresinssuchastheseareofthe‘unsaturated’type.Unsaturatedpolyesterresinisathermoset,capableofbeingcuredfromaliquidorsolidstatewhensubjectto therightconditions.Anunsaturatedpolyesterdiffers fromasaturatedpolyestersuchasTerylene™whichcannotbecuredinthisway.Itisusual,however,torefertounsaturatedpolyesterresinsas‘polyesterresins’,orsimplyas‘polyesters’.

Inchemistrythereactionofabasewithanacidproducesasalt.Similarly,inorganicchemistrythereactionofanalcoholwithanorganicacidproducesanesterandwater.Byusingspecialalcohols,suchasaglycol,inareactionwithdi-basicacids,apolyesterandwaterwillbeproduced.Thisreaction,togetherwiththeadditionofcompoundssuchassaturateddi-basicacidsandcross-linkingmonomers,formsthebasicprocessofpolyestermanufacture.Asaresultthereisawholerangeofpolyestersmadefromdifferentacids,glycolsandmonomers,allhavingvaryingproperties.

Therearetwoprincipletypesofpolyesterresinusedasstandardlaminatingsystemsinthecompositesindustry.Orthophthalicpolyesterresinisthestandardeconomicresinusedbymanypeople.Isophthalicpolyesterresinisnowbecomingthepreferredmate-rialinindustriessuchasmarinewhereitssuperiorwaterresistanceisdesirable.

Figure 13 shows the idealised chemical structure of a typical polyester. Note thepositionsoftheestergroups(CO-O-C)andthereactivesites(C*=C*)withinthemolecularchain.

Figure13–IdealisedchemicalstructureoftypicalIsophthalicpolyester

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Mostpolyesterresinsareviscous,palecolouredliquidsconsistingofasolutionofapolyesterinamonomerwhichisusuallystyrene.Theadditionofstyreneinamountsofupto50%helpstomaketheresineasiertohandlebyreducingitsviscosity.Thestyrenealsoperformsthevitalfunctionofenablingtheresintocurefromaliquidtoasolidby‘cross-linking’themolecularchainsofthepolyester,withouttheevolutionofanyby-products.Theseresinscanthereforebemouldedwithouttheuseofpressureandarecalled‘contact’or‘lowpressure’resins.Polyesterresinshavealimitedstoragelifeastheywillsetor‘gel’ontheirownoveralongperiodoftime.Oftensmallquantitiesofinhibitorareaddedduringtheresinmanufacturetoslowthisgellingaction.

Foruseinmoulding,apolyesterresinrequirestheadditionofseveralancillaryprod-ucts.Theseproductsaregenerally:

■ Catalyst

■ Accelerator

■ Additives: Thixotropic Pigment Filler Chemical/fireresistance

Amanufacturermaysupplytheresininitsbasicformorwithanyoftheaboveaddi-tivesalreadyincluded.Resinscanbeformulatedtothemoulder’srequirementsreadysimplyfortheadditionofthecatalystpriortomoulding.Ashasbeenmentioned,givenenoughtimeanunsaturatedpolyesterresinwillsetbyitself.Thisrateofpolymerisationistooslowforpracticalpurposesandthereforecatalystsandacceleratorsareusedtoachievethepolymerisationoftheresinwithinapracticaltimeperiod.Catalystsareaddedtotheresinsystemshortlybeforeusetoinitiatethepolymerisationreaction.Thecatalystdoesnottakepartinthechemicalreactionbutsimplyactivatestheprocess.Anacceleratorisaddedtothecatalysedresintoenablethereactiontoproceedatworkshoptemperatureand/oratagreaterrate.Sinceacceleratorshavelittleinfluenceontheresinintheabsenceofacatalysttheyaresometimesaddedtotheresinbythepolyestermanufacturertocreatea‘pre-accelerated’resin.

Themolecularchainsofthepolyestercanberepresentedasfollows,where‘B’indicatesthereactivesitesinthemolecule.

Figure14-SchematicRepresentationofPolyesterResin(Uncured)

Withtheadditionofstyrene‘S‘,andinthepresenceofacatalyst,thestyrenecross-linksthepolymerchainsateachofthereactivesitestoformahighlycomplexthree-dimensionalnetworkasfollows:

ABA B ABA

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Figure15-SchematicRepresentationofPolyesterResin(Cured)

Thepolyesterresin isthensaidtobe‘cured’. It isnowachemicallyresistant(andusually)hardsolid.Thecross-linkingorcuringprocessiscalled‘polymerisation’.Itisanon-reversiblechemicalreaction.The‘side-by-side’natureofthiscross-linkingofthemolecularchainstendstomeansthatpolyesterlaminatessufferfrombrittlenesswhenshockloadingsareapplied.

Greatcareisneededinthepreparationoftheresinmixpriortomoulding.Theresinandanyadditivesmustbecarefullystirredtodisperseallthecomponentsevenlybeforethecatalystisadded.Thisstirringmustbethoroughandcarefulasanyairintroducedintotheresinmixaffectsthequalityofthefinalmoulding.Thisisespeciallysowhenlaminatingwith layersofreinforcingmaterialsasairbubblescanbeformedwithintheresultantlaminatewhichcanweakenthestructure.Itisalsoimportanttoaddtheacceleratorandcatalystincarefullymeasuredamountstocontrolthepolymerisationreactiontogivethebestmaterialproperties.Toomuchcatalystwillcausetoorapidagelationtime,whereastoolittlecatalystwillresultinunder-cure.

Colouringoftheresinmixcanbecarriedoutwithpigments.Thechoiceofasuitablepigmentmaterial,eventhoughonlyaddedatabout3%resinweight,mustbecarefullyconsideredasitiseasytoaffectthecuringreactionanddegradethefinallaminatebyuseofunsuitablepigments.

Filler materials are used extensively with polyester resins for a variety of reasonsincluding:

■ Toreducethecostofthemoulding

■ Tofacilitatethemouldingprocess

■ Toimpartspecificpropertiestothemoulding

Fillersareoftenaddedinquantitiesupto50%oftheresinweightalthoughsuchaddi-tionlevelswillaffecttheflexuralandtensilestrengthofthelaminate.Theuseoffillerscanbebeneficialinthelaminatingorcastingofthickcomponentswhereotherwiseconsiderableexothermicheatingcanoccur.Additionofcertainfillerscanalsocon-tributetoincreasingthefire-resistanceofthelaminate.

ABA B A BA

ABA B A BA

S S S

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5.1.2VinylesterResins

Vinylesterresinsaresimilarintheirmolecularstructuretopolyesters,butdifferprimarilyinthelocationoftheirreactivesites,thesebeingpositionedonlyattheendsofthemolecularchains.Asthewholelengthofthemolecularchainisavailabletoabsorbshockloadingsthismakesvinylesterresinstougherandmoreresilientthanpolyesters.Thevinylestermoleculealsofeaturesfewerestergroups.Theseestergroupsaresus-ceptibletowaterdegradationbyhydrolysiswhichmeansthatvinylestersexhibitbetterresistancetowaterandmanyotherchemicalsthantheirpolyestercounterparts,andarefrequentlyfoundinapplicationssuchaspipelinesandchemicalstoragetanks.

Thefigurebelowshowstheidealisedchemicalstructureofatypicalvinylester.Notethepositionsoftheestergroupsandthereactivesites(C*=C*)withinthemolecularchain.

Figure16-IdealisedChemicalStructureofaTypicalEpoxyBasedVinylester

Themolecularchainsofvinylester,representedbelow,canbecomparedtothesche-maticrepresentationofpolyestershownpreviouslywherethedifferenceinthelocationofthereactivesitescanbeclearlyseen:

Figure17-SchematicRepresentationofVinylesterResin(Uncured)

Withthereducednumberofestergroupsinavinylesterwhencomparedtoapolyester,theresinislesspronetodamagebyhydrolysis.Thematerialisthereforesometimesusedasabarrieror‘skin’coatforapolyesterlaminatethatistobeimmersedinwater,suchasinaboathull.Thecuredmolecularstructureofthevinylesteralsomeansthatittendstobetougherthanapolyester,althoughtoreallyachievethesepropertiestheresinusuallyneedstohaveanelevatedtemperaturepostcure.

Figure18-SchematicRepresentationofVinylesterResin(Cured)

BAA A A AB

B A A A A A B

B A A A A A B B A A A A A B

S S

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5.1.3EpoxyResins

Thelargefamilyofepoxyresinsrepresentsomeofthehighestperformanceresinsofthoseavailableatthistime.Epoxiesgenerallyout-performmostotherresintypesintermsofmechanicalpropertiesandresistancetoenvironmentaldegradation,whichleadstotheiralmostexclusiveuseinaircraftcomponents.Asalaminatingresintheirincreasedadhesivepropertiesandresistancetowaterdegradationmaketheseresinsidealforuseinapplicationssuchasboatbuilding.Hereepoxiesarewidelyusedasaprimaryconstructionmaterialforhigh-performanceboatsorasasecondaryapplicationtosheathahullorreplacewater-degradedpolyesterresinsandgelcoats.

Theterm‘epoxy’referstoachemicalgroupconsistingofanoxygenatombondedtotwocarbonatomsthatarealreadybondedinsomeway.Thesimplestepoxyisathree-memberringstructureknownbytheterm‘alpha-epoxy’or‘1,2-epoxy’.Theide-alisedchemicalstructureisshowninthefigurebelowandisthemosteasilyidentifiedcharacteristicofanymorecomplexepoxymolecule.

Figure19-IdealisedChemicalStructureofaSimpleEpoxy(EthyleneOxide)

Usually identifiableby theircharacteristicamberorbrowncolouring,epoxy resinshaveanumberofusefulproperties.Boththeliquidresinandthecuringagentsformlowviscosityeasilyprocessedsystems.Epoxyresinsareeasilyandquicklycuredatanytemperaturefrom5°Cto150°C,dependingonthechoiceofcuringagent.Oneof themostadvantageouspropertiesofepoxies is their lowshrinkageduringcurewhichminimisesfabric‘print-through’andinternalstresses.Highadhesivestrengthandhighmechanicalpropertiesarealsoenhancedbyhighelectricalinsulationandgoodchemical resistance.Epoxiesfindusesasadhesives,caulkingcompounds,castingcompounds,sealants,varnishesandpaints,aswellaslaminatingresinsforavarietyofindustrialapplications.

Epoxyresinsareformedfromalongchainmolecularstructuresimilartovinylesterwithreactivesitesateitherend.Intheepoxyresin,however,thesereactivesitesareformedbyepoxygroupsinsteadofestergroups.Theabsenceofestergroupsmeansthattheepoxyresinhasparticularlygoodwaterresistance.Theepoxymoleculealsocontainstworinggroupsatitscentrewhichareabletoabsorbbothmechanicalandthermalstressesbetter than lineargroupsand thereforegive theepoxyresinverygoodstiffness,toughnessandheatresistantproperties.

Thefigurebelowshowstheidealisedchemicalstructureofatypicalepoxy.Notetheabsenceoftheestergroupswithinthemolecularchain.

Figure20-IdealisedChemicalStructureofaTypicalEpoxy(DiglycidylEtherofBisphenol-A)

0

CH2–CH–

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Epoxiesdifferfrompolyesterresinsinthattheyarecuredbya‘hardener’ratherthanacatalyst.Thehardener,oftenanamine,isusedtocuretheepoxybyan‘additionreaction’wherebothmaterials takeplace in thechemical reaction.Thechemistryofthisreactionmeansthatthereareusuallytwoepoxysitesbindingtoeachaminesite.Thisformsacomplexthree-dimensionalmolecularstructurewhichisillustratedinFigure21:

Figure21-SchematicRepresentationofEpoxyResin(Cured3-DStructure)

Sincetheaminemolecules‘co-react’withtheepoxymoleculesinafixedratio,itisessentialthatthecorrectmixratioisobtainedbetweenresinandhardenertoensurethatacompletereactiontakesplace.Ifamineandepoxyarenotmixedinthecorrectratios,unreactedresinorhardenerwillremainwithinthematrixwhichwillaffectthefinalpropertiesaftercure.Toassistwiththeaccuratemixingoftheresinandhardener,manufacturersusuallyformulatethecomponentstogiveasimplemixratiowhichiseasilyachievedbymeasuringoutbyweightorvolume.

5.1.4ComparisonofResinProperties

Thechoiceofaresinsystemforuseinanycomponentdependsonanumberofitscharacteristics,withthefollowingprobablybeingthemostimportantformostcom-positestructures:

1 AdhesiveProperties

2 MechanicalProperties

3 DegradationFromWaterIngress

5.1.4.1 AdhesiveProperties

Ithasalreadybeendiscussedhowtheadhesivepropertiesoftheresinsystemareimportantinrealisingthefullmechanicalpropertiesofacomposite.Theadhesionoftheresinmatrixtothefibrereinforcementortoacorematerialinasandwichconstructionareimportant.Polyesterresinsgenerallyhavethelowestadhesivepropertiesofthethreesystemsdescribedhere.Vinylesterresinshowsimprovedadhesivepropertiesoverpolyesterbutepoxysystemsofferthebestperformanceofall,andarethereforefrequentlyfoundinmanyhigh-strengthadhesives.Thisisduetotheirchemicalcom-positionandthepresenceofpolarhydroxylandethergroups.Asepoxiescurewith

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lowshrinkagethevarioussurfacecontactssetupbetweentheliquidresinandtheadherendsarenotdisturbedduringthecure.Theadhesivepropertiesofepoxyareespeciallyusefulintheconstructionofhoneycomb-coredlaminateswherethesmallbondingsurfaceareameansthatmaximumadhesionisrequired.

Thestrengthofthebondbetweenresinandfibreisnotsolelydependentontheadhe-sivepropertiesoftheresinsystembutisalsoaffectedbythesurfacecoatingonthereinforcementfibres.This‘sizing’isdiscussedlaterinsection5.2.2.5.

5.1.4.2 MechanicalProperties

Twoimportantmechanicalpropertiesofanyresinsystemareitstensilestrengthandstiffness.Figure22showsresultsfortestscarriedoutoncommerciallyavailablepoly-ester,vinylesterandepoxyresinsystemscuredat20°Cand80°C.

Figure22–Comparisonofresintensilestrength&modulus

Afteracureperiodofsevendaysatroomtemperatureitcanbeseenthatatypicalepoxy will have higher properties than a typical polyester and vinylester for bothstrengthandstiffness.Thebeneficialeffectofapostcureat80°Cforfivehourscanalsobeseen.

Alsoofimportancetothecompositedesignerandbuilderistheamountofshrinkagethatoccursinaresinduringandfollowingitscureperiod.Shrinkageisduetotheresinmolecules rearrangingand re-orientating themselves in the liquidandsemi-gelledphase.Polyesterandvinylesters requireconsiderablemolecular rearrangement toreachtheircuredstateandcanshowshrinkageofupto8%.Thedifferentnatureoftheepoxyreaction,however,leadstoverylittlerearrangementandwithnovolatilebi-productsbeingevolved,typicalshrinkageofanepoxyisreducedtoaround2%.Theabsenceofshrinkageis,inpart,responsiblefortheimprovedmechanicalpropertiesofepoxiesoverpolyester,asshrinkageisassociatedwithbuilt-instressesthatcanweaken thematerial. Furthermore, shrinkage through the thicknessof a laminateleadsto‘print-through’ofthepatternofthereinforcingfibres,acosmeticdefectthatisdifficultandexpensivetoeliminate.

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5.1.4.3DegradationfromWaterIngress

Animportantpropertyofanyresin,particularlyinamarineenvironment,isitsabilitytowithstanddegradationfromwateringress.Allresinswillabsorbsomemoisture,addingtoalaminate’sweight,butwhatismoresignificantishowtheabsorbedwateraffectstheresinandresin/fibrebondinalaminate,leadingtoagradualandlong-termlossinmechanicalproperties.Bothpolyesterandvinylesterresinsarepronetowaterdegradationduetothepresenceofhydrolysableestergroupsintheirmolecularstructures.Asaresult,athinpolyesterlaminatecanbeexpectedtoretainonly65%ofitsinter-laminarshearstrengthafterimmersioninwaterforaperiodofoneyear,whereasanepoxylaminateimmersedforthesameperiodwillretainaround90%.

Figure23-EffectofPeriodsofWaterSoakat100°ConResinInter-LaminarShearStrength

Figure23demonstratestheeffectsofwateronanepoxyandpolyesterwovenglasslaminate,whichhavebeensubjectedtoawatersoakat100°C.Thiselevatedtempera-turesoakinggivesaccelerateddegradationpropertiesfortheimmersedlaminate.

5.1.4.4Osmosis

Alllaminatesinamarineenvironmentwillpermitverylowquantitiesofwatertopassthroughtheminvapourform.Asthiswaterpassesthrough,itreactswithanyhydro-lysablecomponentsinsidethelaminatetoformtinycellsofconcentratedsolution.Under the osmotic cycle, more water is then drawn through the semi-permeablemembraneofthelaminatetoattempttodilutethissolution.Thiswaterincreasesthefluidpressureinthecelltoasmuchas700psi.Eventuallythepressuredistortsorburststhelaminateorgelcoat,andcanleadtoacharacteristic‘chicken-pox’surface.Hydrolysablecomponentsinalaminatecanincludedirtanddebristhathavebecometrappedduringfabrication,butcanalsoincludetheesterlinkagesinacuredpolyester,andtoalesserextent,vinylester.

Useofresinrichlayersnexttothegelcoatareessentialwithpolyesterresinstomini-misethistypeofdegradation,butoftentheonlycureoncetheprocesshasstartedisthereplacementoftheaffectedmaterial.

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Topreventtheonsetofosmosisfromthestart,itisnecessarytousearesinwhichhasbothalowwatertransmissionrateandahighresistancetoattackbywater.Whenusedwithreinforcementswithsimilarlyresistantsurfacetreatmentandlaminatedtoaveryhighstandard,blisteringcanthenbevirtuallyeliminated.Apolymerchainhavinganepoxybackboneissubstantiallybetterthanmanyotherresinsystemsatresistingtheeffectsofwater.Suchsystemshavebeenshowntoconferexcellentchemicalandwaterresistance,lowwatertransmissionrateandverygoodmechanicalpropertiestothepolymer.

5.1.5 OtherResinSystemsusedinComposites

Besidespolyesters,vinylestersandepoxiesthereareanumberofotherspecialisedresinsystemsthatareusedwheretheiruniquepropertiesarerequired:

5.1.5.1 Phenolics

Primarilyusedwherehighfire-resistanceisrequired,phenolicsalsoretaintheirproper-tieswellatelevatedtemperatures.Forroom-temperaturecuringmaterials,corrosiveacidsareusedwhichleadstounpleasanthandling.Thecondensationnatureoftheircuringprocess tends to lead to the inclusionofmanyvoidsandsurfacedefects,andtheresinstendtobebrittleanddonothavehighmechanicalproperties.Typicalcosts:£2-4/kg.

5.1.5.2 CyanateEsters

Primarilyusedintheaerospaceindustry.Thematerial’sexcellentdielectricpropertiesmakeitverysuitableforusewithlowdielectricfibressuchasquartzforthemanu-factureofradomes.Thematerialalsohastemperaturestabilityuptoaround200°Cwet.Typicalcosts:£40/kg.

5.1.5.3 Silicones

Syntheticresinusingsiliconasthebackboneratherthanthecarbonoforganicpoly-mers.Goodfire-resistantproperties,andabletowithstandelevatedtemperatures.Hightemperaturecuresneeded.Usedinmissileapplications.Typicalcosts:>£15/kg.

5.1.5.4 Polyurethanes

Hightoughnessmaterials,sometimeshybridisedwithotherresins,duetorelativelylowlaminatemechanicalproperties incompression. Usesharmful isocyanatesascuringagent.Typicalcosts:£2-8/kg

5.1.5.5 Bismaleimides(BMI)

Primarilyusedinaircraftcompositeswhereoperationathighertemperatures(230°Cwet/250°C dry) is required. e.g. engine inlets, high speed aircraft flight surfaces.Typicalcosts:>£50/kg.

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5.1.5.6Polyimides

Usedwhereoperationathighertemperaturesthanbismaleimidescanstandisrequired(useupto250°Cwet/300°Cdry).Typicalapplicationsincludemissileandaero-enginecomponents.Extremelyexpensiveresin(>£80/kg),whichusestoxicrawmaterialsinitsmanufacture.Polyimidesalsotendtobehardtoprocessduetotheircondensationreactionemittingwaterduringcure,andarerelativelybrittlewhencured.PMR15andLaRC160aretwoofthemostcommonlyusedpolyimidesforcomposites.

5.2 Fibres

Themechanicalpropertiesofmostreinforcingfibresareconsiderablyhigherthanthoseofun-reinforcedresinsystems.Themechanicalpropertiesofthefibre/resincompositearethereforedominatedbythecontributionofthefibretothecomposite.

Thefourmainfactorsthatgovernthefibre’scontributionare:

1. Thebasicmechanicalpropertiesofthefibreitself.

2. Thesurfaceinteractionoffibreandresin(the‘interface’).

3. Theamountoffibreinthecomposite(‘FibreVolumeFraction’).

4. Theorientationofthefibresinthecomposite.

Thebasicmechanicalpropertiesofthemostcommonlyusedfibresaregiveninthefollowingtable.Thesurfaceinteractionoffibreandresiniscontrolledbythedegreeofbondingthatexistsbetweenthetwo.Thisisheavilyinfluencedbythetreatmentgiventothefibresurface,andadescriptionofthedifferentsurfacetreatmentsand‘finishes’isalsogivenhere.

Theamountoffibreinthecompositeislargelygovernedbythemanufacturingproc-essused. However, reinforcing fabricswithcloselypackedfibreswillgivehigherFibreVolumeFractions(FVF)inalaminatethanwillthosefabricswhicharemadewithcoarserfibres,orwhichhavelargegapsbetweenthefibrebundles.Fibrediameterisanimportantfactorherewiththemoreexpensivesmallerdiameterfibresprovidinghigherfibresurfaceareas,spreadingthefibre/matrixinterfacialloads.Asageneralrule,thestiffnessandstrengthofalaminatewillincreaseinproportiontotheamountoffibrepresent.However,aboveabout60-70%FVF(dependingonthewayinwhichthe fibres pack together) although tensile stiffness may continue to increase, thelaminate’sstrengthwillreachapeakandthenbegintodecreaseduetothelackofsufficientresintoholdthefibrestogetherproperly.

Finally,sincereinforcingfibresaredesignedtobeloadedalongtheirlength,andnotacrosstheirwidth,theorientationofthefibrescreateshighly‘direction-specific’prop-ertiesinthecomposite.This‘anisotropic’featureofcompositescanbeusedtogoodadvantageindesigns,withthemajorityoffibresbeingplacedalongtheorientationofthemainloadpaths.Thisminimisestheamountofparasiticmaterialthatisputinorientationswherethereislittleornoload.

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5.2.1BasicPropertiesofFibresandOtherEngineeringMaterials

Material Type Tensile Str. Tensile Modulus Typical Density Specific (MPa) (GPa) (g/cc) Modulus

Carbon HS 3500 160 - 270 1.8 90 - 150

Carbon IM 5300 270 - 325 1.8 150 - 180 Carbon HM 3500 325 - 440 1.8 180 - 240 Carbon UHM 2000 440+ 2.0 200+

Aramid LM 3600 60 1.45 40

Aramid HM 3100 120 1.45 80

Aramid UHM 3400 180 1.47 120

Glass - E glass 2400 69 2.5 27

Glass - S2 glass 3450 86 2.5 34

Glass - quartz 3700 69 2.2 31

Aluminium Alloy (7020) 400 1069 2.7 26

Titanium 950 110 4.5 24

Mild Steel (55 Grade) 450 205 7.8 26

Stainless Steel (A5-80) 800 196 7.8 25

HS Steel (17/4 H900) 1241 197 7.8 25

Table1-BasicPropertiesofFibresandOtherEngineeringMaterials

5.2.1.1LaminateMechanicalPropertiesThepropertiesofthefibresgivenaboveonlyshowspartofthepicture.Thepropertiesofthecompositewillderivefromthoseofthefibre,butalsothewayitinteractswiththeresinsystemused,theresinpropertiesitself,thevolumeoffibreinthecompositeanditsorientation.Thefollowingdiagramsshowabasiccomparisonofthemainfibretypeswhenusedinatypicalhigh-performanceunidirectionalepoxyprepreg,atthefibrevolumefractionsthatarecommonlyachievedinaerospacecomponents.

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Figure24-Fibrestrengths&strains

Thesegraphsshowthestrengthsandmaximumstrainsofthedifferentcompositesatfailure.Thegradientofeachgraphalsoindicatesthestiffness(modulus)ofthecomposite;thesteeperthegradient,thehigheritsstiffness.Thegraphsalsoshowhowsomefibres,suchasaramid,displayverydifferentpropertieswhenloadedincompression,comparedwithloadingintension.

5.2.1.2LaminateImpactStrength

Figure25-ComparisonofLaminateImpactStrength

Impactdamagecanposeparticularproblemswhenusinghighstiffnessfibresinverythinlaminates.Insomestructures,wherecoresareused,laminateskinscanbelessthan0.3mmthick.Althoughotherfactorssuchasweavestyleandfibreorientationcansignificantlyaffect impact resistance, in impact-criticalapplications,carbon isoftenfoundincombinationwithoneoftheotherfibres.Thiscanbeintheformofahybridfabricwheremorethanonefibretypeisusedinthefabricconstruction.Thesearedescribedinmoredetaillater.

TensilePropertiesofU/DPrepregLaminate CompressivePropertiesofU/DPrepregLaminates

0

50

100

150

200

250

300

E-Glass S-GlassYarn 6781

Aramid HS Carbon

Impa

ct S

tren

gth

(Ft.

lbs/

in2 )

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Figure26-Comparativefibrecost(woven)

Thefiguresabovearecalculatedonatypicalpriceofa300gwovenfabric.Mostfibrepricesareconsiderablyhigherforthesmallbundlesize(tex)usedinsuchlightweightfabrics.Whereheavierbundlesoffibrecanbeused,suchasinunidirectionalfabrics,thecostcomparisonisslightlydifferent.

Figure27–Comparativefibrecost(UD)

5.2.1.3 Fibrecost

0

5

10

15

20

15

30

35

40

45

50

E-GlassRoving

E-GlassYarn 7781

S-GlassYarn 6781

Aramid HMStyle 900

HSCarbon

IMCarbon

Typi

cal C

ost o

f ~30

0g/m

2 Wov

en F

abri

c (£

/m2 )

25

E-Glass1200 tex

S-Glass410 tex

Aramid 252 tex

HS CarbonT700-12k

IM CarbonT800-12k

Typi

cal C

ost o

f ~30

0g/m

2 U/D

(£/m

2 )

0

5

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15

20

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5.2.2 FibreTypes

5.2.2.1 Glass

Byblendingquarryproducts(sand,kaolin,limestone,colemanite)at1,600°C,liquidglassisformed.Theliquidispassedthroughmicro-finebushingsandsimultaneouslycooledtoproduceglassfibrefilamentsfrom5-24µmindiameter.Thefilamentsaredrawntogetherintoastrand(closelyassociated)orroving(looselyassociated),andcoatedwitha“size”toprovidefilamentcohesionandprotecttheglassfromabrasion.

Byvariationofthe“recipe”,differenttypesofglasscanbeproduced.Thetypesusedforstructuralreinforcementsareasfollows:

a. E-glass(electrical)-loweralkalicontentandstrongerthanAglass(alkali).Goodtensileandcompressivestrengthandstiffness,goodelectricalpropertiesandrelativelylowcost,butimpactresistancerelativelypoor.DependingonthetypeofEglassthepricerangesfromabout£1-2/kg.E-glassisthemostcommonformofreinforcingfibreusedinpolymermatrixcomposites.

b. C-glass(chemical)-bestresistancetochemicalattack.Mainlyusedintheformofsurfacetissueintheouterlayeroflaminatesusedinchemicalandwaterpipesandtanks.

c. R,SorT-glass–manufacturerstradenamesforequivalentfibreshavinghighertensilestrengthandmodulusthanEglass,withbetterwetstrengthretention.HigherILSSandwetoutpropertiesareachievedthroughsmallerfilamentdiameter.S-glassisproducedintheUSAbyOCF,R-glassinEuropebyVetrotexandT-glassbyNittoboinJapan.Developedforaerospaceanddefenceindustries,andusedinsomehardballisticarmourapplications.Thisfactor,andlowproductionvolumesmeanrelativelyhighprice.DependingonthetypeofRorSglassthepricerangesfromabout£12-20/kg.

EGlassfibreisavailableinthefollowingforms:

a. strand-acompactlyassociatedbundleoffilaments.Strandsarerarelyseencommerciallyandareusuallytwistedtogethertogiveyarns.

b. yarns-acloselyassociatedbundleoftwistedfila-mentsorstrands.Eachfilamentdiameterinayarnisthesame,andisusuallybetween4-13µm.Yarnshave varying weights described by their ‘tex’ (theweightingrammesof1000linearmetres)ordenier(theweightinlbsof10,000yards),withthetypicaltexrangeusuallybeingbetween5and400.

c. rovings-a looselyassociatedbundleofuntwistedfilamentsorstrands.Eachfilamentdiameterinarovingisthesame,andisusuallybetween13-24µm.Rov-ingsalsohavevaryingweightsandthetexrangeisusuallybetween300and4800.Wherefilamentsaregatheredtogetherdirectlyafterthemeltingprocess,theresultantfibrebundleisknownasadirectroving.Severalstrandscanalsobebroughttogetherseparatelyaftermanufactureoftheglass,togivewhatisknownasanassembledroving.

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Assembledrovingsusuallyhavesmallerfilamentdiametersthandirectrovings,givingbetterwet-outandmechanicalproperties,buttheycansufferfromcatenaryproblems(unequalstrandtension),andareusuallyhigherincostbecauseofthemoreinvolvedmanufacturingprocesses.

Itisalsopossibletoobtainlongfibresofglassfromshortfibresbyspinningthem.Thesespunyarnfibreshavehighersurfaceareasandaremoreabletoabsorbresin,buttheyhavelowerstructuralpropertiesthantheequivalentcontinuouslydrawnfibres.

5.2.2.2AramidAramidfibreisaman-madeorganicpolymer(anaromaticpolyamide)producedbyspinningasolidfibrefromaliq-uidchemicalblend. Thebrightgoldenyellowfilamentsproducedcanhavea rangeofproperties,but all havehigh strengthand lowdensitygiving veryhigh specificstrength.Allgradeshavegoodresistancetoimpact,andlowermodulusgradesareusedextensivelyinballisticap-plications.Compressivestrength,however,isonlysimilartothatofEglass.

AlthoughmostcommonlyknownunderitsDuponttradename‘Kevlar’,therearenowanumberofsuppliersofthefibre,mostnotablyAkzoNobelwith‘Twaron’.Eachsupplieroffersseveralgradesofaramidwithvariouscombinationsofmodulusandsurfacefinishtosuitvariousapplications.Aswellasthehighstrengthproperties,thefibresalsooffergoodresistancetoabrasion,andchemicalandthermaldegradation.However,thefibrecandegradeslowlywhenexposedtoultravioletlight.

Aramidfibresareusuallyavailableintheformofrovings,withtexesrangingfromabout20to800.Typicallythepriceofthehighmodulustyperangesfrom£15-to£25perkg.

5.2.2.3 Carbon

Carbon fibre is produced by the controlled oxidation,carbonisationandgraphitisationofcarbon-richorganicprecursorswhicharealreadyinfibreform.Themostcom-monprecursorispolyacrylonitrile(PAN),becauseitgivesthebestcarbonfibreproperties,butfibrescanalsobemadefrompitchorcellulose.Variationofthegraphitisationprocessproduceseitherhighstrengthfibres(@~2,600°C)orhighmodulusfibres(@~3,000°C)withothertypesinbetween.Onceformed,thecarbonfibrehasasurfacetreatmentapplied to improvematrixbondingandchemicalsizingwhichserves toprotectitduringhandling.

Whencarbonfibrewasfirstproducedinthelatesixtiesthepriceforthebasichighstrengthgradewasabout£200/kg.By1996theannualworldwidecapacityhadin-creasedtoabout7,000tonnesandthepricefortheequivalent(highstrength)gradewas£15-40/kg.Carbonfibresareusuallygroupedaccordingtothemodulusbandinwhichtheirpropertiesfall.Thesebandsarecommonlyreferredtoas:highstrength

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(HS),intermediatemodulus(IM),highmodulus(HM)andultrahighmodulus(UHM).Thefilamentdiameterofmost types isabout5-7µm.Carbonfibrehas thehighestspecificstiffnessofanycommerciallyavailablefibre,veryhighstrengthinbothten-sionandcompressionandahighresistancetocorrosion,creepandfatigue.Theirimpactstrength,however,islowerthaneitherglassoraramid,withparticularlybrittlecharacteristicsbeingexhibitedbyHMandUHMfibres.

StrengthandModulusFiguresforCommercialPAN-basedCarbonFibres:

Grade Tensile Modulus Tensile Strength Country (GPa) (GPa) of Manufacture

StandardModulus(<265GPa)(alsoknownas‘HighStrength’)

T300 230 3.53 France/JapanT700 235 5.3 JapanHTA 238 3.95 GermanyUTS 240 4.8 Japan34-700 234 4.5 Japan/USAAS4 241 4.0 USAT650-35 241 4.55 USAPanex 33 228 3.6 USA/HungaryF3C 228 3.8 USATR50S 235 4.83 JapanTR30S 234 4.41 Japan

IntermediateModulus(265-320GPa)

T800 294 5.94 France/JapanM30S 294 5.49 FranceIMS 295 4.12/5.5 JapanMR40/MR50 289 4.4/5.1 JapanIM6/IM7 303 5.1/5.3 USAIM9 310 5.3 USAT650-42 290 4.82 USAT40 290 5.65 USA

HighModulus(320-440GPa)

M40 392 2.74 JapanM40J 377 4.41 France/JapanHMA 358 3.0 JapanUMS2526 395 4.56 JapanMS40 340 4.8 JapanHR40 381 4.8 Japan

UltraHighModulus(~440GPa)

M46J 436 4.21 JapanUMS3536 435 4.5 JapanHS40 441 4.4 JapanUHMS 441 3.45 USA

Table2–PANbasedfibreproperties(informationfrommanufacturer’sdatasheets)

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5.2.2.4 OtherFibres

Thereareavarietyofotherfibreswhichcanbeusedinadvancedcompositestructuresbuttheiruseisnotwidespread.Theseinclude:

Polyester

Alowdensity,hightenacityfibrewithgoodimpactresistancebutlowmodulus.Itslackofstiffnessusuallyprecludesit frominclusioninacompositecomponent,butit isusefulwherelowweight,highimpactorabrasionresistance,andlowcostarerequired.Itismainlyusedasasurfacingmaterial,asitcanbeverysmooth,keepsweightdownandworkswellwithmostresintypes.

Polyethylene

Inrandomorientation,ultra-highmolecularweightpolyethylenemoleculesgiveverylowmechanicalproperties.However,ifdissolvedanddrawnfromsolutionintoafilamentbyaprocesscalledgel-spinning,themoleculesbecomedisentangledandalignedinthedirectionofthefilament.Themolecularalignmentpromotesveryhightensilestrengthtothefilamentandtheresultingfibre.CoupledwiththeirlowS.G.(<1.0),thesefibreshavethehighestspecificstrengthofthefibresdescribedhere.However,thefibre’stensilemodulusandultimatestrengthareonlyslightlybetterthanE-glassandlessthanthatofaramidorcarbon.Thefibrealsodemonstratesverylowcompressivestrengthinlaminateform.Thesefactors,coupledwithhighprice,andmoreimportantly,thedifficultyincreatingagoodfibre/matrixbondmeansthatpolyethylenefibresarenotoftenusedinisolationforcompositecomponents.

Quartz

Averyhighsilicaversionofglasswithmuchhighermechanicalpropertiesandexcellentresistancetohightemperatures(1,000°C+).However,themanufacturingprocessandlowvolumeproductionleadtoaveryhighprice(14µm-£74/kg,9µm-£120/kg).

Boron

Carbonormetalfibresarecoatedwithalayerofborontoimprovetheoverallfibreproperties.Theextremelyhighcostofthisfibrerestrictsitusetohightemperatureaerospaceapplicationsandinspecialisedsportingequipment.Aboron/carbonhybrid,composedofcarbonfibresinterspersedamong80-100µmboronfibres,inanepoxymatrix,canachievepropertiesgreaterthaneitherfibrealone,withflexuralstrengthandstiffnesstwicethatofHScarbonand1.4timesthatofboron,andshearstrengthexceedingthatofeitherfibre.

Ceramics

Ceramicfibres,usuallyintheformofveryshort‘whiskers’aremainlyusedinareasrequiringhightemperatureresistance.Theyaremorefrequentlyassociatedwithnon-polymermatricessuchasmetalalloys.

Natural

Attheotherendofthescaleitispossibletousefibrousplantmaterialssuchasjuteandsisalasreinforcementsin‘low-tech’applications.Intheseapplications,thefibres’lowS.G.(typically0.5-0.6)meanthatfairlyhighspecificstrengthscanbeachieved.

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5.2.2.5 FibreFinishes

Surfacefinishesarenearlyalwaysappliedtofibresbothtoallowhandlingwithmini-mumdamageandtopromotefibre/matrixinterfacialbondstrength.Withcarbonandaramidfibres foruse incompositeapplications, thesurfacefinishor sizeappliedusuallyperformsbothfunctions.Thefinishisappliedtothefibreatthepointoffibremanufactureandthisfinishremainsonthefibrethroughouttheconversionprocessintofabric.Withglassfibrethereisachoiceofapproachinthesurfacefinishthatcanbeapplied.

GlassFibreFinishes

Glass fibre rovings that are to be used in direct fibreprocessessuchasprepregging,pultrusionandfilamentwinding,aretreatedwitha‘dual-function’finishatthepointoffibremanufacture.

Glass fibre yarns, however, when used for weaving aretreatedintwostages.Thefirstfinishisappliedatthepointoffibremanufactureatquiteahighlevelandispurelyforprotection of the fibre against damage during handlingandtheweavingprocessitself.Thisprotectivefinish,whichisoftenstarchbased,iscleanedoffor‘scoured’aftertheweavingprocesseitherbyheatorwithchemicals.Thescouredwovenfabricisthenseparatelytreatedwithadifferentmatrix-compatiblefinishspecificallydesignedtooptimisefibretoresininterfacialcharacteristicssuchasbondstrength,waterresistanceandopticalclarity.

CarbonFibreFinishes

Finishes,orsizes,forcarbonfibresusedinstructuralcompositesaregenerallyepoxybased,withvaryinglevelsbeinguseddependingontheenduseofthefibre.Forweavingthesizelevelisabout1-2%byweightwhereasfortapeprepreggingorfilamentwinding(orsimilarsingle-fibreprocesses),thesizelevelisabout0.5-1%.Thechemistryandlevelofthesizeareimportantnotonlyforprotectionandmatrixcompatibilitybutalsobecausetheyeffectthedegreeofspreadofthefibre.Fibrescanalsobesuppliedunsizedbutthesewillbepronetobrokenfilamentscausedbygeneralhandling.Mostcarbonfibresuppliersoffer3-4levelsofsizeforeachgradeoffibre.

AramidFibreFinishes

Aramidfibresaretreatedwithafinishatthepointofmanufactureprimarilyformatrixcompatibility.Thisisbecausearamidfibresrequirefarlessprotectionfromdamagecausedbyfibrehandling. Themain typesoffibre treatmentarecompositefinish,rubbercompatiblefinish(beltsandtyres)andwaterprooffinish(ballisticsoftarmour).Likethecarbonfibrefinishes,therearedifferinglevelsofcompositeapplicationfinishdependingonthetypeofprocessinwhichthefibrewillbeused.

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5.3 Fabrictypes

Inpolymericcompositeterms,afabricisdefinedasamanufacturedassemblyoflongfibresofcarbon,aramidorglass,oracombinationofthese,toproduceaflatsheetofoneormorelayersoffibres.Theselayersareheldtogethereitherbymechanicalinterlockingofthefibresthemselvesorwithasecondarymaterialtobindthesefibrestogetherandholdtheminplace,givingtheassemblysufficientintegritytobehandled.

Fabrictypesarecategorisedbytheorientationofthefibresused,andbythevariousconstructionmethodsusedtoholdthefibrestogether.

Thefourmainfibreorientationcategoriesare:Unidirectional,0/90°,Multiaxial,andOther/random.Thesearedescribedbelow.

5.3.1UnidirectionalFabrics

Aunidirectional(UD)fabricisoneinwhichthemajorityoffibresruninonedirectiononly.Asmallamountoffibreorothermaterialmayruninotherdirectionswiththemainintentionbeingtoholdtheprimaryfibresinposition,althoughtheotherfibresmayalsooffersomestructuralproperties. Whilesomeweaversof0/90°fabricstermafabricwithonly75%ofitsweightinonedirectionasaunidirectional,atGurittheunidirectionaldesignationonlyapplies to those fabricswithmorethan90%ofthefibreweightinonedirection.Unidi-rectionalsusuallyhavetheirprimaryfibresinthe0°direction(alongtheroll–awarpUD)butcanalsohavethemat90°totherolllength(aweftUD).

Trueunidirectional fabricsoffer theability toplacefibre in thecomponentexactlywhereitisrequired,andintheoptimumquantity(nomoreorlessthanrequired).Aswellasthis,UDfibresarestraightanduncrimped.Thisresultsinthehighestpossiblefibrepropertiesfromafabricincompositecomponentconstruction.Formechanicalproperties,unidirectionalfabricscanonlybeimprovedonbyprepregunidirectionaltape,wherethereisnosecondarymaterialatallholdingtheunidirectionalfibresinplace.Intheseprepregproductsonlytheresinsystemholdsthefibresinplace.

5.3.1.1UnidirectionalConstruction

Therearevariousmethodsofmaintainingtheprimaryfibresinpositioninaunidirec-tional includingweaving,stitching,andbonding.Aswithotherfabrics,thesurfacequalityofaunidirectionalfabricisdeterminedbytwomainfactors:thecombinationoftexandthreadcountoftheprimaryfibreandtheamountandtypeofthesecondaryfibre.Thedrape,surfacesmoothnessandstabilityofafabricarecontrolledprimarilybytheconstructionstyle,whiletheareaweight,porosityand(toalesserdegree)wetoutaredeterminedbyselectingtheappropriatecombinationoffibretexandnumbersoffibrespercm.

Warporweftunidirectionalscanbemadebythestitchingprocess(seeinformationinthe‘Multiaxial’sectionofthispublication).However,inordertogainadequatestability,itisusuallynecessarytoaddamatortissuetothefaceofthefabric.

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Therefore,togetherwiththestitchingthreadrequiredtoassemblethefibres,thereisarelativelylargeamountofsecondary,parasiticmaterialinthistypeofUDfabric,whichtendstoreducethelaminateproperties.Furthermorethehighcostofsetupofthe0°layerofastitchinglineandtherelativelyslowspeedofproductionmeansthatthesefabricscanberelativelyexpensive.

5.3.20/90°Fabrics

Forapplicationswheremorethanonefibreorientationisrequired,afabriccombining0°and90°fibreorientationsisuseful.Themajorityofthesearewovenproducts.0/90°fabricscanbeproducedbystitchingratherthanaweavingprocessandadescriptionofthisstitchingtechnologyisgivenbelowunder‘MultiaxialFabrics’.

5.3.3WovenFabrics

Wovenfabricsareproducedbytheinterlacingofwarp(0°)fibresandweft(90°)fibresinaregularpatternorweavestyle.Thefabric’sintegrityismaintainedbythemechani-calinterlockingofthefibres.Drape(theabilityofafabrictoconformtoacomplexsurface),surfacesmoothnessandstabilityofafabricarecontrolledprimarilybytheweavestyle.Theareaweight,porosityand(toalesserdegree)wetoutaredeterminedbyselectingthecorrectcombinationoffibretexandthenumberoffibres/cm2.Thefollowingisadescriptionofsomeofthemorecommonlyfoundweavestyles:

5.3.3.1Plain

Each warp fibre passes alternately under and over eachweftfibre.Thefabricissymmetrical,withgoodstabilityandreasonableporosity.However,itisthemostdifficultoftheweavestodrape,andthehighleveloffibrecrimpimpartsrelativelylowmechanicalpropertiescomparedwiththeotherweavestyles.Withlargefibres(hightex)thisweavestylegivesexcessivecrimpandthereforeittendsnottobeusedforveryheavyfabrics.

5.3.3.2Twill

Oneormorewarpfibresalternatelyweaveoverandundertwoormoreweftfibresinaregularrepeatedmanner.Thisproducesthevisualeffectofastraightorbrokendiagonal‘rib’tothefabric.Superiorwetoutanddrapeisseeninthetwillweaveovertheplainweavewithonlyasmallreductioninstability.Withreducedcrimp,thefabricalsohasasmoothersurfaceandslightlyhighermechanicalproperties.

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5.3.3.3 Satin

Satinweavesare fundamentally twillweavesmodified toproducefewerintersectionsofwarpandweft.The‘harness’numberusedinthedesignation(typically4,5and8)isthetotalnumberoffibrescrossedandpassedunder,beforethefibrerepeatsthepattern.A‘crowsfoot’weaveisaformofsatinweavewithadifferentstaggerintherepeatpattern.Satinweavesareveryflat,havegoodwetoutandahighdegreeofdrape.Thelowcrimpgivesgoodmechanicalproperties.Satinweavesallowfibres tobewoven in theclosestproximityandcanproduce fabricswithaclose ‘tight’weave. However, thestyle’slowstabilityandasymmetryneedstobeconsidered.Theasymmetrycausesonefaceofthefabrictohavefibrerunningpredominantlyinthewarpdirectionwhiletheotherfacehasfibresrunningpredominantlyintheweftdirection.Caremustbetakeninassemblingmultiplelayersofthesefabricstoensurethatstressesarenotbuiltintothecomponentthroughthisasymmetriceffect.

5.3.3.4BasketBasketweave is fundamentally thesameasplainweaveexceptthattwoormorewarpfibresalternatelyinterlacewithtwoormoreweftfibres.Anarrangementoftwowarpscross-ingtwoweftsisdesignated2x2basket,butthearrangementoffibreneednotbesymmetrical.Thereforeitispossibletohave8x2,5x4,etc.Basketweaveisflatter,and,throughlesscrimp,strongerthanaplainweave,butlessstable.Itmustbeusedonheavyweightfabricsmadewiththick(hightex)fibrestoavoidexcessivecrimping.

5.3.3.5 Leno

Lenoweaveimprovesthestabilityin‘open’fabricswhichhavealowfibrecount.

Aformofplainweaveinwhichadjacentwarpfibresaretwistedaroundconsecutiveweftfibrestoformaspiralpair,effectively ‘locking’ eachweft inplace. Fabrics in lenoweavearenormallyusedinconjunctionwithotherweavestyles because if used alone their openness could notproduceaneffectivecompositecomponent.

5.3.3.6MockLeno

Aversionofplainweaveinwhichoccasionalwarpfibres,atregularintervalsbutusuallyseveralfibresapart,devi-atefromthealternateunder-overinterlacingandinsteadinterlace every two or more fibres. This happens withsimilarfrequencyintheweftdirection,andtheoverallef-fectisafabricwithincreasedthickness,roughersurface,andadditionalporosity.

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5.3.3.7 WovenGlassYarnFabricsvsWovenRovings

Yarn-basedfabricsgenerallygivehigherstrengthsperunitweightthanroving,andbeinggenerallyfiner,producefabricsatthelighterendoftheavailableweightrange.Wovenrovingsarelessexpensivetoproduceandcanwetoutmoreeffectively.How-ever,sincetheyareavailableonlyinheaviertexes,theycanonlyproducefabricsatthemediumtoheavyendoftheavailableweightrange,andarethusmoresuitableforthick,heavierlaminates.

5.3.3.8 Stitched0/90oFabrics

0/90°fabricscanalsobemadebyastitchingprocess,whicheffectivelycombinestwolayersofunidirectionalmaterialintoonefabric.Stitched0/90°fabricscanoffermechanical performance increases of up to 20% in some properties over wovenfabrics,duetothefollowingfactors:

(i) Parallelnon-crimpfibresbearthestrainimmediatelyuponbeingloaded.

(ii) Stresspointsfoundattheintersectionofwarpandweftfibresinwovenfabricsareeliminated.

(iii) Ahigherdensityoffibrecanbepackedintoalaminatecomparedwithawoven.Inthisrespectthefabricbehavesmorelikelayersofunidirectional.

Otherbenefitscomparedwithwovenfabricsinclude:

1. Heavyfabricscanbeeasilyproducedwithmorethan1kg/sqmoffibre.

2. Increasepackingofthefibrecanreducethequantityofresinrequired.

5.3.3.9 HybridFabrics

Thetermhybridreferstoafabricthathasmorethanonetypeofstructuralfibreinitsconstruction.Inamulti-layerlaminateifthepropertiesofmorethanonetypeoffibrearerequired,thenitwouldbepossibletoprovidethiswithtwofabrics,eachcontainingthefibretypeneeded.

However,iflowweightorextremelythinlaminatesarerequired,ahybridfabricwillallowthetwofibrestobepresentedinjustonelayeroffabricinsteadoftwo.

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Itwouldbepossibleinawovenhybridtohaveonefibrerunningintheweftdirectionandthesecondfibrerunninginthewarpdirection,butitismorecommontofindalternatingthreadsofeachfibreineachwarp/weftdirection.Althoughhybridsaremostcommonlyfoundin0/90°wovenfabrics,theprincipleisalsousedin0/90°stitched,unidirectionalandmultiaxialfabrics.Themostusualhybridcombinationsare:

Carbon/Aramid

Thehigh impactresistanceandtensilestrengthof thearamidfibrecombineswithhighthecompressiveandtensilestrengthofcarbon.Bothfibreshave lowdensitybutrelativelyhighcost.

Aramid/Glass

Thelowdensity,highimpactresistanceandtensilestrengthofaramidfibrecombineswiththegoodcompressiveandtensilestrengthofglass,coupledwithitslowercost.

Carbon/Glass

Carbonfibrecontributeshightensilecompressivestrengthandstiffnessandreducesthedensity,whileglassreducesthecost.

5.3.4MultiaxialFabrics

In recent yearsmultiaxial fabricshavebegun to find favour in theconstructionofcompositecomponents.Thesefabricsconsistofoneormorelayersoflongfibresheldinplacebyasecondarynon-structuralstitchingtread.Themainfibrescanbeanyofthestructuralfibresavailableinanycombination.Thestitchingthreadisusu-allypolyesterduetoitscombinationofappropriatefibreproperties(forbindingthefabrictogether)andcost.Thestitchingprocessallowsavarietyoffibreorientations,beyondthesimple0/90°ofwovenfabrics,tobecombinedintoonefabric.Multiaxialfabricshavethefollowingmaincharacteristics:

Advantages

Thetwokeyimprovementswithstitchedmultiaxialfabricsoverwoventypesare:

(i) Bettermechanicalproperties,primarilyfromthefactthatthefibresarealwaysstraightandnon-crimped,andthatmoreorientationsoffibreareavailablefromtheincreasednumberoflayersoffabric.

(ii) Improvedcomponentbuildspeedbasedonthefactthatfabricscanbemadethickerandwithmultiplefibreorientationssothatfewerlayersneedtobeincludedinthelaminatesequence.

Disadvantages

Polyesterfibredoesnotbondverywelltosomeresinsystemsandsothestitchingcanbeastartingpointforwickingorotherfailureinitiation.Thefabricproductionprocesscanalsobeslowandthecostofthemachineryhigh.This,togetherwiththefactthatthemoreexpensive,lowtexfibresarerequiredtogetgoodsurfacecoverageforthelowweightfabrics,meansthecostofgoodquality,stitchedfabricscanberelativelyhigh compared to wovens. Extremely heavy weight fabrics, while enabling largequantitiesoffibretobeincorporatedrapidlyintothecomponent,canalsobedifficulttoimpregnatewithresinwithoutsomeautomatedprocess.Finally,thestitchingprocess,unlesscarefullycontrolledasintheGuritfabricstyles,canbunchtogetherthefibres,particularlyinthe0°direction,creatingresin-richareasinthelaminate.

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5.3.4.1 Weave&Stitch

Withthe‘Weave&Stitch’methodthe+45°and-45°layerscanbemadebyweavingweftUnidirectionalsandthenskewingthefabric,onaspecialmachine, to45°. Awarpunidirectionaloraweftunidirectionalcanalsobeusedunskewedtomakea0°and90°layerIfboth0°and90°layersarepresentinamulti-layerstitchedfabricthenthiscanbeprovidedbyaconventional0/90°wovenfabric.Duetothefactthatheavyrovingscanbeusedtomakeeachlayertheweavingprocessisrelativelyfast,asisthesubsequentstitchingtogetherofthelayersviaasimplestitchingframe.

Tomakeaquadraxial(four-layer:+45°,0°,90°,-45°)fabricbythismethod,aweftunidirectionalwouldbewovenandskewedinonedirectiontomakethe+45°layer,andintheothertomakethe-45°layer.The0°and90°layerswouldappearasasinglewovenfabric.Thesethreeelementswouldthenbestitchedtogetheronastitchingframetoproducethefinalfour-axisfabric.

5.3.4.2 SimultaneousStitch

Simultaneousstitchmanufacture iscarriedoutonspecialmachinesbasedon theknittingprocess,suchasthosemadebyLiba,Malimo,Mayer,etc.Eachmachinevariesintheprecisionwithwhichthefibresarelaiddown,particularlywithreferencetokeepingthefibresparallel.

Thesetypesofmachinehaveaframewhichsimultaneouslydrawsinfibresforeachaxis/layer, until the required layershavebeenassembled,and thenstitches themtogether,asshowninthediagrambelow.

WeftUnidirectional

Layer1

Layer2

Skewto45oStitchtheskewed

layerstogether

-45o

+45o

±45ofabric

CourtesyLibaMaschinenfabrickGMBH

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5.3.5 OtherFabrics

5.3.5.1 ChoppedStrandMat

Chopped strand mat (CSM) is a non-woven material which, as its name implies,

consistsofrandomlyorientatedchoppedstrandsofglasswhichareheldtogether

-formarineapplications-byaPVAemulsionorapowderbinder.Despitethefact

thatPVAimpartssuperiordrapinghandlingandwettingoutcharacteristicsusersin

amarineenvironmentshouldbewaryofitsuseasitisaffectedbymoistureandcan

leadtoosmosislikeblisters.

Today,choppedstrandmatisrarelyusedinhighperformancecompositecomponents

asitisimpossibletoproducealaminatewithahighfibrecontentand,bydefinition,

ahighstrength-to-weightratio.

5.3.5.2 Tissues

Tissuesaremadewithcontinuousfilamentsoffibrespreaduniformlybutrandomly

overaflatsurface.Theseare thenchemicallybound togetherwithorganicbased

bindingagentssuchasPVA,polyester,etc.Havingrelativelylowstrengththeyare

notprimarilyusedasreinforcements,butassurfacinglayersonlaminatesinorderto

provideasmoothfinish.Tissuesareusuallymanufacturedwithareaweightsofbetween

5and50g/sqm.Glasstissuesarecommonlyusedtocreateacorrosionresistantbar-

rierthroughresinenrichmentatthesurface.Thesameenrichmentprocesscanalso

preventprint-throughofhighlycrimpedfabricsingelcoatsurfaces.

5.3.5.3 Braids

Braidsareproducedbyinterlacingfibresinaspiralna-

turetoformatubularfabric.Thediameterofthetubeis

controlledbythenumberoffibresinthetube’scircumfer-

ence,theangleofthefibresinthespiral,thenumberof

intersectionsoffibreperunitlengthofthetubeandthe

size(tex)ofthefibresintheassembly.Theinterlacingcan

varyinstyle(plain,twill,etc.)aswith0/90°wovenfabrics.

Tubediameterisnormallygivenforafibreangleof±45°

butthebraidingprocessallowsthefibrestomovebetweenanglesofabout25°and

75°,dependingonthenumberandtexofthefibres.Thenarrowanglegivesasmall

diameterwhereasthewideranglegivesalargediameter.Thereforealongthelength

ofonetubeitispossibletochangethediameterbyvariationofthefibreangle-a

smallerangle(relativetozero)givingasmallerdiameterandviceversa.Braidscan

befoundinsuchcompositecomponentsasmasts,antennae,driveshaftsandother

tubularstructuresthatrequiretorsionalstrength.

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5.4 Corematerialsforsandwichconstruction

5.4.1FoamCores

Foamsareoneofthemostcommonformsofcoremateri-al.Theycanbemanufacturedfromavarietyofsyntheticpolymersincludingpolyvinylchloride(PVC),polystyrene(PS), polyurethane (PU), polymethyl methacrylamide(acrylic),polyetherimide (PEI)andstyreneacrylonitrile(SAN).Theycanbesuppliedindensitiesrangingfromlessthan30kg/m3tomorethan300kg/m3,althoughthemostuseddensitiesforcompositestructuresrangefrom40to200kg/m3.Theyarealsoavailableinavarietyofthicknesses,typicallyfrom5mmto50mm.

5.4.1.1PVCFoam

Closed-cellpolyvinylchloride(PVC)foamsareoneofthemostcommonlyusedcorematerials for theconstructionofhighperformancesandwichstructures. AlthoughstrictlytheyareachemicalhybridofPVCandpolyurethane,theytendtobereferredtosimplyas‘PVCfoams’.

PVCfoamsofferabalancedcombinationofstaticanddynamicpropertiesandgoodresistancetowaterabsorption.Theyalsohavealargeoperatingtemperaturerangeoftypically-240°Cto+80°C(-400°Fto+180°F),andareresistanttomanychemicals.AlthoughPVCfoamsaregenerallyflammable,therearefire-retardantgradesthatcanbeusedinmanyfire-criticalapplications,suchastraincomponents.WhenusedasacoreforsandwichconstructionwithFRPskins,itsreasonableresistancetostyrenemeansthatitcanbeusedsafelywithpolyesterresinsanditisthereforepopularinmanyindustries.Itisnormallysuppliedinsheetform,eitherplain,orgrid-scoredtoalloweasyformingtoshape.

TherearetwomaintypesofPVCfoam:crosslinkedanduncrosslinkedwiththeun-crosslinkedfoamssometimesbeingreferredtoas‘linear’.Theuncrosslinkedfoams(suchasAirexR63.80)aretougherandmoreflexible,andareeasiertoheat-formaroundcurves.However,theyhavesomelowermechanicalpropertiesthananequivalentdensityofcross-linkedPVC,anda lower resistance toelevated temperaturesandstyrene.Theircross-linkedcounterpartsareharderbutmorebrittleandwillproduceastifferpanel,lesssusceptibletosofteningorcreepinginhotclimates.Typicalcross-linkedPVCproductsincludetheHerexC-seriesoffoams,DivinycellHandHPgradesandPolimexKlegecellandTermantoproducts.

AnewgenerationoftoughenedPVCfoamsisnowalsobecomingavailablewhichtradesomeofthebasicmechanicalpropertiesofthecross-linkedPVCfoamsforsomeoftheimprovedtoughnessofthelinearfoams.

OwingtothenatureofthePVC/polyurethanechemistryincross-linkedPVCfoams,thesematerialsneedtobethoroughlysealedwitharesincoatingbeforetheycanbesafelyusedwithlow-temperaturecuringprepregs.Althoughspecialheatstabilisationtreatmentsareavailableforthesefoams,thesetreatmentsareprimarilydesignedtoimprovethedimensionalstabilityofthefoam,andreducetheamountofgassingthatisgivenoffduringelevatedtemperatureprocessing.

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5.4.1.2PolystyreneFoams

Althoughpolystyrenefoamsareusedextensivelyinsailandsurfboardmanufacture,wheretheirlightweight(40kg/m3),lowcostandeasytosandcharacteristicsareofprimeimportance,theyarerarelyemployedinhighperformancecomponentconstruc-tionbecauseoftheirlowmechanicalproperties.Theycannotbeusedinconjunctionwithpolyesterresinsystemsbecausetheywillbedissolvedbythestyrenepresentintheresin.

5.4.1.3PolyurethaneFoams

Polyurethanefoamsexhibitonlymoderatemechanicalpropertiesandhaveatendencyforthefoamsurfaceattheresin/coreinterfacetodeterioratewithage,leadingtoskindelamination.Theirstructuralapplicationsarethereforenormallylimitedtothepro-ductionofformerstocreateframesorstringersforstiffeningcomponents.However,polyurethanefoamscanbeusedinlightlyloadedsandwichpanels,withthesepanelsbeingwidelyusedforthermalinsulation.Thefoamalsohasreasonableelevatedservicetemperatureproperties(150°C/300°F),andgoodacousticabsorption.Thefoamcanreadilybecutandmachinedtorequiredshapesorprofiles.

5.4.1.4 PolymethylmethacrylamideFoams

For a given density, polymethyl methacrylamide (acrylic) foams such as Rohacelloffersomeofthehighestoverallstrengthsandstiffnessesoffoamcores.Theirhighdimensionalstabilityalsomakesthemuniqueinthattheycanreadilybeusedwithconventionalelevated temperaturecuringprepregs.However, theyareexpensive,whichmeansthattheirusetendstobelimitedtoaerospacecompositepartssuchashelicopterrotorblades,andaircraftflaps.

5.4.1.5 Styreneacrylonitrile(SAN)co-polymerFoams

SANfoamsbehaveinasimilarwaytotoughenedcross-linkedPVCfoams.Theyhavemostofthestaticpropertiesofcross-linkedPVCcores,yethavemuchhigherelonga-tionsandtoughness.TheyarethereforeabletoabsorbimpactlevelsthatwouldfracturebothconventionalandeventhetoughenedPVCfoams.However,unlikethetoughenedPVC’s,whichuseplasticizerstotoughenthepolymer,thetoughnesspropertiesofSANareinherentinthepolymeritself,andsodonotchangeappreciablywithage.

SANfoamsarereplacinglinearPVCfoamsinmanyapplicationssincetheyhavemuchofthelinearPVC’stoughnessandelongation,yethaveahighertemperatureperform-anceandbetterstaticproperties.However,theyarestillthermoformable,whichhelpsinthemanufactureofcurvedparts.Heat-stabilisedgradesofSANfoamscanalsobemoresimplyusedwithlow-temperaturecuringprepregs,sincetheydonothavetheinterferingchemistryinherentinthePVC’s.TypicalSANproductsincludeGurit’sCorecell®A-seriesfoams.

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5.4.1.6Otherthermoplastics

Asnewtechniquesdevelopfortheblowingoffoamsfromthermoplastics,therangeofexpandedmaterialsof this typecontinues to increase. Typical isPEI foam,anexpandedpolyetherimide/polyethersulphone,whichcombinesoutstandingfireper-formancewithhighservicetemperature.Althoughitisexpensive,thisfoamcanbeusedinstructural,thermalandfireprotectionapplicationsintheservicetemperaturerange-194°C(-320°F)to+180°C(+355°F).Itishighlysuitableforaircraftandtraininteriors,asitcanmeetsomeofthemoststringentfireresistantspecifications.

5.4.2Honeycombs

Honeycomb cores are available in a variety of materials for sandwich structures.Theserangefrompaperandcardforlowstrengthandstiffness,lowloadapplications(suchasdomesticinternaldoors)tohighstrengthandstiffness,extremelylightweightcomponentsforaircraftstructures.Honeycombscanbeprocessedintobothflatandcurvedcompositestructures,andcanbemadetoconformtocompoundcurveswithoutexcessivemechanicalforceorheating.

Thermoplastichoneycombsareusuallyproducedbyextrusion, followedbyslicingtothickness.Otherhoneycombs(suchasthosemadeofpaperandaluminium)aremadebyamulti-stageprocess.Inthesecaseslargethinsheetsofthematerial(usu-ally1.2x2.4m)areprintedwithalternating,parallel,thinstripesofadhesiveandthesheetsarethenstackedinaheatedpresswhiletheadhesivecures.Inthecaseofaluminiumhoneycombthestackofsheetsisthenslicedthroughitsthickness.Theslices(knownas‘blockform’)arelatergentlystretchedandexpandedtoformthesheetofcontinuoushexagonalcellshapes.

Inthecaseofpaperhoneycombs,thestackofbondedpapersheetsisgentlyexpandedtoformalargeblockofhoneycomb,severalfeetthick.Heldinitsexpandedform,thisfragilepaperhoneycombblockisthendippedinatankofresin,drainedandcuredinanoven.Oncethisdippingresinhascured,theblockhassufficientstrengthtobeslicedintothefinalthicknessesrequired.

Inbothcases,byvaryingthedegreeofpullintheexpansionprocess,regularhexa-gon-shapedcellsorover-expanded(elongated)cellscanbeproduced,eachwithdifferentmechanicalandhandling/drapeproperties.Duetothisbondedmethodofconstruction,ahoneycombwillhavedifferentmechanicalpropertiesinthe0°and90°directionsofthesheet.

WhileskinsareusuallyofFRP,theymaybealmostanysheetmaterialwiththeappropri-ateproperties,includingwood,thermoplastics(egmelamine)andsheetmetals,suchasaluminiumorsteel.Thecellsofthehoneycombstructurecanalsobefilledwitharigidfoam.Thisprovidesagreaterbondareafortheskins,increasesthemechanicalpropertiesofthecorebystabilisingthecellwallsandincreasesthermalandacousticinsulationproperties.

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Propertiesofhoneycombmaterialsdependonthesize(andthereforefrequency)ofthecellsandthethicknessandstrengthofthewebmaterial.Sheetscanrangefromtypically3-50mminthicknessandpaneldimensionsaretypically1200x2400mm,althoughitispossibletoproducesheetsupto3mx3m.

Honeycombcorescangivestiffandverylightlaminatesbutduetotheirverysmallbonding area they are almost exclusively used with high-performance resin sys-temssuchasepoxiessothatthenecessaryadhesiontothelaminateskinscanbeachieved.

5.4.2.1 Aluminiumhoneycomb

Aluminium honeycomb produces one of the highest strength/weight ratios of anystructuralmaterial.Therearevariousconfigurationsoftheadhesive-bondingofthealuminiumfoilwhichcanleadtoavarietyofgeometriccellshapes(usuallyhexago-nal).Propertiescanalsobecontrolledbyvaryingthefoilthicknessandcellsize.Thehoneycombisusuallysuppliedintheunexpandedblockformandisstretchedoutintoasheeton-site.

Despiteitsgoodmechanicalpropertiesandrelativelylowprice,aluminiumhoneycombhastobeusedwithcautioninsomeapplications,suchaslargemarinestructures,be-causeofthepotentialcorrosionproblemsinasalt-waterenvironment.Inthissituationcarealsohastobeexercisedtoensurethatthehoneycombdoesnotcomeintodirectcontactwithcarbonskinssincetheconductivitycanaggravategalvaniccorrosion.Aluminiumhoneycombalsohastheproblemthatithasno‘mechanicalmemory’.Onimpactofacoredlaminate,thehoneycombwilldeformirreversiblywhereastheFRPskins,beingresilient,willmovebacktotheiroriginalposition.Thiscanresultinanareawithanunbondedskinwithmuchreducedmechanicalproperties.

5.4.2.2 Nomexhoneycomb

NomexhoneycombismadefromNomexpaper-aformofpaperbasedonKevlar™,rather than cellulose fibres. The initial paper honeycomb is usually dipped in aphenolicresintoproduceahoneycombcorewithhighstrengthandverygoodfireresistance.Itiswidelyusedforlightweightinteriorpanelsforaircraftinconjunctionwithphenolicresinsintheskins.Specialgradesforuseinfireretardantapplications(egpublictransport interiors)canalsobemadewhichhavethehoneycombcellsfilledwithphenolicfoamforaddedbondareaandinsulation.

Nomexhoneycombisbecomingincreasinglyusedinhigh-performancenon-aerospacecomponentsduetoitshighmechanicalproperties,lowdensityandgoodlong-termstability.However,ascanbeseenfromFigure28,itisconsiderablymoreexpensivethanothercorematerials.

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Figure28–Comparativecorecosts

5.4.2.3 Thermoplastichoneycomb

Corematerialsmadeofotherthermoplasticsarelightinweight,offeringsomeusefulpropertiesandpossiblyalsomakingforeasierrecycling.Theirmaindisadvantageisthedifficultyofachievingagoodinterfacialbondbetweenthehoneycombandtheskinmaterial,andtheirrelativelylowstiffness.Althoughtheyarerarelyusedinhighlyloadedstructures, theycanbeuseful insimple interiorpanels.Themostcommonpolymersusedare:

ABS-forrigidity,impactstrength,toughness,surfacehardnessanddimensionalstability

Polycarbonate-forUV-stability,excellentlighttransmission,goodheatresistance&self-extinguishingproperties

Polypropylene-forgoodchemicalresistance

Polyethylene-ageneral-purposelow-costcorematerial

5.4.2.4Wood

Woodcanbedescribedas‘nature’shoneycomb’,asithasastructurethat,onami-croscopicscale,issimilartothecellularhexagonalstructureofsynthetichoneycomb.Whenusedinasandwichstructurewiththegrainrunningperpendiculartotheplaneoftheskins,theresultingcomponentshowspropertiessimilartothosemadewithman-madehoneycombs.However,despitevariouschemicaltreatmentsbeingavailable,allwoodcoresaresusceptibletomoistureattackandwillrotifnotwellsurroundedbylaminateorresin.

5.4.2.5Balsa

Themostcommonlyusedwoodcoreisend-grainbalsa.Balsawoodcoresfirstap-pearedinthe1940’sinflyingboathulls,whichwerealuminiumskinnedandbalsa-coredtowithstandtherepeatedimpactoflandingonwater.Thisperformanceledthemarineindustrytobeginusingend-grainbalsaasacorematerialinFRPconstruction.

Apart from its high compressive properties, its advantages include being a goodthermalinsulatorofferinggoodacousticabsorption.Thematerialwillnotdeformwhen

Fibre

FRP Composite

Resin

Strain

Tens

ile S

tres

s

Fig. v

SAN80 Kg/m3

PVC75 Kg/m3

Balsa100 Kg/m3

Al. Honeycomb50 Kg/m3

NomexH/c (48kg/m3)

Comparative Prices of Core Materials

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heatedandactsasaninsulatingandablativelayerinafire,withthecorecharringslowly,allowingthenon-exposedskintoremainstructurallysound.Italsooffersposi-tiveflotationandiseasilyworkedwithsimpletoolsandequipment.

Balsacoreisavailableascontouredend-grainsheets3to50mmthickonabackingfabric,andrigidend-grainsheetsupto100mmthick.Thesesheetscanbeprovidedreadyresin-coatedforvacuum-bagging,prepregorpressure-basedmanufacturingprocessessuchasRTM.Oneofthedisadvantagesofbalsaisitshighminimumdensity,with100kg/m3beingatypicalminimum.Thisproblemisexacerbatedbythefactthatbalsacanabsorblargequantitiesofresinduringlamination,althoughpre-sealingthefoamcanreducethis.Itsuseisthereforenormallyrestrictedtoprojectswhereoptimumweightsavingisnotrequiredorinlocallyhighlystressedareas.

5.4.2.6OtherCoreMaterials

Althoughnotusuallyregardedastruesandwichcores,thereareanumberofthin,low-density‘fabric-like’materialswhichcanbeusedtoslightlylowerthedensityofasingle-skinlaminate.MaterialssuchasCoremat™andSpheretex™consistofanon-woven‘felt-like’fabricfullofdensity-reducinghollowspheres.Theyareusuallyonly1-3mminthicknessandareusedlikeanotherlayerofreinforcementinthemiddleofalaminate,beingdesignedto‘wetout’withthelaminatingresinduringconstruction.However,thehollowspheresdisplaceresinandsotheresultantmiddlelayer,althoughmuchheavierthanafoamorhoneycombcore,islowerindensitythantheequivalentthicknessofglassfibrelaminate.Beingsothintheycanalsoconformeasilyto2-Dcurvature,andsoarequickandeasytouse.

5.4.3Designconsiderations

Asmightbeexpected,allthecoresshowanincreaseinpropertieswithincreasingdensity. However,other factors,besidesdensity,alsocome intoplaywhen look-ingattheweightofacoreinasandwichstructure.Forexample,lowdensityfoammaterials,whilecontributingvery little to theweightofasandwich laminate,oftenhaveaveryopensurfacecellstructurewhichcanmeanthatalargemassofresinisabsorbedintheirbondlines.Thelowerthedensityofthefoam,thelargerarethecellsandtheworseistheproblem.Honeycombs,ontheotherhand,canbeverygoodinthisrespectsinceawellformulatedadhesivewillformasmallbondingfilletonlyaroundthecellwalls.

Finally,considerationneedstobegiventotheformacoreisusedintoensurethatitfitsthecomponentwell.Theweightsavingsthatcorescanoffercanquicklybeusedupifcoresfitbadly,leavinglargegapsthatrequirefillingwithadhesive.Scrim-backedfoamorbalsa,wherelittlesquaresofthecorearesupportedonalightweightscrimcloth,canbeusedtohelpcoresconformbettertoacurvedsurface.Contour-cutfoam,whereslotsarecutpart-waythroughthecorefromoppositesidesachievesasimilareffect.However,boththesecoresstilltendtousequitelargeamountsofadhesivesincetheslotsbetweeneachfoamsquareneedfillingwithresintoproduceagoodstructure.

Inweight-criticalcomponentstheuseoffoamcoreswhicharethermoformableshouldbeconsidered.TheseincludethelinearPVC’sandtheSANfoamswhichcanallbeheatedtoabovetheirsofteningpointsandpre-curvedtofitamouldshape.

Forhoneycombs,over-expanded formsare themostwidelyusedwhenfitting thecoretoacompoundcurve,sincewithdifferentexpansionpatternsawiderangeofconformabilitycanbeachieved.

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6. Processingroute

Takingcompositematerialsasawhole,therearemanydifferentmaterialoptionsto

choosefromintheareasofresins,fibresandcores,allwiththeirownuniquesetof

propertiessuchasstrength,stiffness,toughness,heatresistance,cost,production

rateetc..However,theendpropertiesofacompositepartproducedfromthesediffer-

entmaterialsisnotonlyafunctionoftheindividualpropertiesoftheresinmatrixand

fibre(andinsandwichstructures,thecoreaswell),butisalsoafunctionofthewayin

whichthematerialsthemselvesaredesignedintothepartandalsothewayinwhich

theyareprocessed.Thissectioncomparesafewofthecommonlyusedcomposite

productionmethodsandpresentssomeofthefactorstobeborneinmindwitheach

differentprocess,includingtheinfluenceofeachprocessonmaterialsselection.

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6.1SprayLay-up

Description

Fibreischoppedinahand-heldgunandfedintoasprayofcatalysedresindirectedatthemould.Thedepositedmaterialsarelefttocureunderstandardatmosphericconditions.

MaterialsOptions:

Resins: Primarilypolyester.

Fibres: Glassrovingonly.

Cores: None.Thesehavetobeincorporatedseparately.

MainAdvantages:

i) Widelyusedformanyyears.

ii) Lowcostwayofquicklydepositingfibreandresin.

iii) Lowcosttooling.

MainDisadvantages:

i) Laminatestendtobeveryresin-richandthereforeexcessivelyheavy.

ii) Onlyshortfibresareincorporatedwhichseverelylimitsthemechanicalpropertiesofthelaminate.

iii) Resinsneedtobelowinviscositytobesprayable.Thisgenerallycompromisestheirmechanical/thermalproperties.

iv) Thehighstyrenecontentsofspraylay-upresinsgenerallymeansthattheyhavethepotentialtobemoreharmfulandtheirlowerviscositymeansthattheyhaveanincreasedtendencytopenetrateclothingetc.

(v) Limitingairbornestyreneconcentrationstolegislatedlevelsisbecoming increasinglydifficult.

TypicalApplications:

Simpleenclosures,lightlyloadedstructuralpanels,e.g.caravanbodies,truckfairings,bathtubs,showertrays,somesmalldinghies.

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6.2WetLay-up/HandLay-up

Description

Resinsareimpregnatedbyhandintofibreswhichareintheformofwoven,knitted,stitchedorbondedfabrics.Thisisusuallyaccomplishedbyrollersorbrushes,withanincreasinguseofnip-rollertypeimpregnatorsforforcingresinintothefabricsbymeansofrotatingrollersandabathofresin.Laminatesarelefttocureunderstandardatmosphericconditions.

MaterialsOptions:

Resins: Any,e.g.epoxy,polyester,vinylester,phenolic.

Fibres: Any,althoughheavyaramidfabricscanbehardtowet-outbyhand.

Cores: Any.

MainAdvantages:

i) Widelyusedformanyyears.

ii) Simpleprinciplestoteach.

iii) Lowcosttooling,ifroom-temperaturecureresinsareused.

iv) Widechoiceofsuppliersandmaterialtypes.

v) Higherfibrecontents,andlongerfibresthanwithspraylay-up.

MainDisadvantages:

i) Resinmixing,laminateresincontents,andlaminatequalityareverydependentontheskillsoflaminators.Lowresincontentlaminatescannotusuallybeachievedwithouttheincorporationofexcessivequantitiesofvoids.

ii) Healthandsafetyconsiderationsofresins.Thelowermolecularweightsofhandlay-upresinsgenerallymeansthattheyhavethepotentialtobemoreharmfulthanhighermolecularweightproducts.Thelowerviscosityoftheresinsalsomeansthattheyhaveanincreasedtendencytopenetrateclothingetc.

iii) Limitingairbornestyreneconcentrationstolegislatedlevelsfrompolyestersandvi-nylestersisbecomingincreasinglyhardwithoutexpensiveextractionsystems.

iv) Resinsneedtobelowinviscositytobeworkablebyhand.Thisgenerallycompromisestheirmechanical/thermalpropertiesduetotheneedforhigh diluent/styrenelevels.

TypicalApplications:

Standardwind-turbineblades,productionboats,architecturalmouldings.

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6.3VacuumBagging(WetLay-up)

Description

Thisisbasicallyanextensionofthewetlay-upprocessdescribedabovewherepres-sureisappliedtothelaminateoncelaid-upinordertoimproveitsconsolidation.Thisisachievedbysealingaplasticfilmoverthewetlaid-uplaminateandontothetool.Theairunderthebagisextractedbyavacuumpumpandthusuptooneatmosphereofpressurecanbeappliedtothelaminatetoconsolidateit.

MaterialsOptions:

Resins: Primarilyepoxyandphenolic.Polyestersandvinylestersmayhave problemsduetoexcessiveextractionofstyrenefromtheresinbythe vacuumpump.

Fibres: Theconsolidationpressuresmeanthatavarietyofheavyfabrics canbewet-out.

Cores: Any.

MainAdvantages:

i) Higherfibrecontentlaminatescanusuallybeachievedthanwithstandardwetlay-uptechniques.

ii) Lowervoidcontentsareachievedthanwithwetlay-up.

iii) Betterfibrewet-outduetopressureandresinflowthroughoutstructuralfibres,withexcessintobaggingmaterials.

iv) Healthandsafety: Thevacuumbag reduces theamountofvolatilesemittedduringcure.

MainDisadvantages:

i) Theextraprocessaddscostbothinlabourandindisposablebaggingmaterials

ii) Ahigherlevelofskillisrequiredbytheoperators

iii) Mixingandcontrolofresincontentstilllargelydeterminedbyoperatorskill

iv) Althoughvacuumbagsreducevolatiles,exposureisstillhigherthaninfusionorprepregprocessingtechniques.

TypicalApplications:

Large,one-offcruisingboats,racecarcomponents,core-bondinginproductionboats.

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6.4FilamentWinding

Description

Thisprocessisprimarilyusedforhollow,generallycircularorovalsectionedcomponents,suchaspipesandtanks.Fibretowsarepassedthrougharesinbathbeforebeingwoundontoamandrelinavarietyoforientations,controlledbythefibrefeedingmechanism,andrateofrotationofthemandrel.

MaterialsOptions:

Resins: Any,e.g.epoxy,polyester,vinylester,phenolic.

Fibres: Any.Thefibresareusedstraightfromacreelandnotwovenorstitched intoafabricform.

Cores: Any,althoughcomponentsareusuallysingleskin.

MainAdvantages:

i) Thiscanbeaveryfastandthereforeeconomicmethodoflayingmaterialdown.

ii) Resincontentcanbecontrolledbymeteringtheresinontoeachfibretowthroughnipsordies.

iii) Fibrecostisminimisedsincethereisnosecondaryprocesstoconvertfibreintofabricpriortouse.

iv) Structuralpropertiesoflaminatescanbeverygoodsincestraightfibrescanbelaidinacomplexpatterntomatchtheappliedloads.

MainDisadvantages:

i) Theprocessislimitedtoconvexshapedcomponents.

ii) Fibrecannoteasilybelaidexactlyalongthelengthofacomponent.

iii) Mandrelcostsforlargecomponentscanbehigh.

iv) Theexternalsurfaceofthecomponentisunmoulded,andthereforecosmeticallyunattractive.

v) Lowviscosityresinsusuallyneedtobeusedwiththeirattendantlowermechanicalandhealthandsafetyproperties.

TypicalApplications:

Chemicalstoragetanksandpipelines,gascylinders,fire-fightersbreathingtanks.

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6.5Pultrusion

Description

Fibresarepulledfromacreelthrougharesinbathandthenonthroughaheateddie.Thediecompletestheimpregnationofthefibre,controlstheresincontentandcuresthematerialintoitsfinalshapeasitpassesthroughthedie.Thiscuredprofileisthenautomaticallycuttolength.Fabricsmayalsobeintroducedintothedietoprovidefibredirectionotherthanat0°.Althoughpultrusionisacontinuousprocess,producingaprofileofconstantcross-section,avariantknownas‘pulforming’allowsforsomevaria-tiontobeintroducedintothecross-section.Theprocesspullsthematerialsthroughthedieforimpregnation,andthenclampstheminamouldforcuring.Thismakestheprocessnon-continuous,butaccommodatingofsmallchangesincross-section.

MaterialsOptions:

Resins: Generallyepoxy,polyester,vinylesterandphenolic.

Fibres: Any.

Cores: Notgenerallyused.

MainAdvantages:

i) Thiscanbeaveryfast,andthereforeeconomic,wayofimpregnatingandcuringmaterials.

ii) Resincontentcanbeaccuratelycontrolled.

iii) Fibrecostisminimisedsincethemajorityistakenfromacreel.

iv) Structuralpropertiesoflaminatescanbeverygoodsincetheprofileshaveverystraightfibresandhighfibrevolumefractionscanbeobtained.

v) Resinimpregnationareacanbeenclosedthuslimitingvolatileemissions.

MainDisadvantages:

i) Limitedtoconstantornearconstantcross-sectioncomponents

ii) Heateddiecostscanbehigh.

TypicalApplications:

Beamsandgirdersusedinroofstructures,bridges,ladders,frameworks.

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6.6ResinTransferMoulding(RTM)

Description

Fabricsarelaidupasadrystackofmaterials.Thesefabricsaresometimespre-pressedtothemouldshape,andheldtogetherbyabinder.These‘preforms’arethenmoreeasilylaidintothemouldtool.Asecondmouldtoolisthenclampedoverthefirst,andresinisinjectedintothecavity.Vacuumcanalsobeappliedtothemouldcavitytoassistresininbeingdrawnintothefabrics.ThisisknownasVacuumAs-sistedResinInjection(VARI).Onceallthefabriciswetout,theresininletsareclosed,andthelaminateisallowedtocure.Bothinjectionandcurecantakeplaceateitherambientorelevatedtemperature.

MaterialsOptions:

Resins: Generallyepoxy,polyester,vinylesterandphenolic,althoughhigh temperatureresinssuchasbismaleimidescanbeusedatelevated processtemperatures.

Fibres: Any.Stitchedmaterialsworkwellinthisprocesssincethegapsallowrapidresintransport.Somespeciallydevelopedfabricscanassistwithresinflow.

Cores: Nothoneycombs,sincecellswouldfillwithresin,andpressuresinvolvedcancrushsomefoams.

MainAdvantages:

i) Highfibrevolumelaminatescanbeobtainedwithverylowvoidcontents.

ii) Goodhealthandsafety,andenvironmentalcontrolduetoenclosureofresin.

iii) Possiblelabourreductions.

iv) Bothsidesofthecomponenthaveamouldedsurface.

MainDisadvantages:

i) Matchedtoolingisexpensive,andheavyinordertowithstandpressures.

ii) Generallylimitedtosmallercomponents.

iii) Unimpregnatedareascanoccurresultinginveryexpensivescrapparts.

TypicalApplications:

Smallcomplexaircraftandautomotivecomponents,trainseats.

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6.7OtherInfusionProcesses-SCRIMP,RIFT,VARTMetc.

Description

FabricsarelaidupasadrystackofmaterialsasinRTM.Thefibrestackisthencoveredwithpeelplyandaknittedtypeofnon-structuralfabric.Thewholedrystackisthenvacuumbagged,andoncebagleakshavebeeneliminated,resinisallowedtoflowintothelaminate.Theresindistributionoverthewholelaminateisaidedbyresinflowingeasilythroughthenon-structuralfabric,andwettingthefabricoutfromabove.

MaterialsOptions:

Resins: Generallyepoxy,polyesterandvinylester.

Fibres: Anyconventionalfabrics.Stitchedmaterialsworkwellinthisprocess sincethegapsallowrapidresintransport.

Cores: Anyexcepthoneycombs.

MainAdvantages:

i) AsRTMabove,exceptonlyonesideofthecomponenthasamouldedfinish.

ii) Muchlowertoolingcostduetoonehalfofthetoolbeingavacuumbag,andlessstrengthbeingrequiredinthemaintool.

iii) Verylargecomponentscanbefabricatedwithhighfibrevolumefractionsandlowvoidcontents.

iv) Standardwetlay-uptoolsmaybeabletobemodifiedforthisprocess.

v) Coredstructurescanbeproducedinoneoperation.

MainDisadvantages:

i) Relativelycomplexprocesstoperformconsistentlywellonlargestructureswithoutrepair.

ii) Resinsmustbeverylowinviscosity,thuscomprisingmechanicalproperties.

iii) Unimpregnatedareascanoccurresultinginveryexpensivescrapparts.

TypicalApplications:

Semi-productionsmallyachts,trainandtruckbodypanels,windenergyblades.

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6.8Prepreg-Autoclave

Description

Fabricsandfibresarepre-impregnatedbythematerialsmanufacturer,underheatandpressureorwithsolvent,withapre-catalysedresin.Thecatalystislargelylatentatam-bienttemperaturesgivingthematerialsseveralweeks,orsometimesmonths,ofusefullifewhendefrosted.Howevertoprolongstoragelifethematerialsarestoredfrozen.Theprepregsarelaidupbyhandormachineontoamouldsurface,vacuumbaggedandthenheatedtotypically120-180°C.Thisallowstheresintoinitiallyreflowandeventuallytocure.Additionalpressureforthemouldingisusuallyprovidedbyanautoclave(effectivelyapressurisedoven)whichcanapplyupto5atmospherestothelaminate.

MaterialsOptions:

Resins: Generallyepoxy,polyester,phenolicandhightemperatureresinssuchaspolyimides,cyanateestersandbismaleimides.

Fibres: Any.Usedeitherdirectfromacreelorasanytypeoffabric.

Cores: Any,althoughspecialtypesoffoamneedtobeusedduetotheelevatedtemperaturesandpressuresinvolvedintheprocess.

MainAdvantages:

i) Resin/catalyst levels and the resin content in the fibre are accurately set by thematerialsmanufacturer.Highfibrecontentscanbesafelyachievedwithlowvoidcontents.

ii) Thematerialshaveexcellenthealthandsafetycharacteristicsandarecleantoworkwithandhavethepotentialforautomationandlaboursaving.

iii) Fibrecostisminimisedinunidirectionaltapessincethereisnosecondaryprocesstoconvertfibreintofabricpriortouse.

iv)Resinchemistrycanbeoptimisedformechanicalandthermalperformance,withthehighviscosityresinsbeingimpregnableduetothemanufacturingprocess.

v) Theextendedworking times (ofup toseveralmonthsat room temperatures)meansthatstructurallyoptimised,complexlay-upscanbereadilyachieved.

vi) Potentialforautomationandlaboursaving.

MainDisadvantages:

i) Materialscost ishigher forpreimpregnated fabricsbut for theseapplicationsexpensiveadvancedresinsareoftenrequired.

ii) Autoclavesareusuallyrequiredtocurethecomponent.Theseareexpensive,slowtooperateandlimitedinsize.

5-6 atmospherespressure

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iii) Toolingneedstobeabletowithstandtheprocesstemperaturesinvolvedandcorematerialsneedtobeabletowithstandtheprocesstemperaturesandpressures.

iv) Forthickerlaminatesprepregpliesneedtobewarm“debulked”duringthelay-upprocesstoensureremovalofairfrombetweentheplies.

TypicalApplications:

Aircraftstructuralcomponents(e.g.wingsandtailsections),F1racingcars.

6.9Prepreg-“OutofAutoclave”

Description

LowTemperatureCuringprepregsaremadeexactlyasconventionalautoclavepre-pregsbuthaveresinchemistriesthatallowcuretobeachievedattemperaturesfrom60-120°C.Forlowtemperaturecuring(60°C),theworkinglifeofthematerialmaybelimitedtoaslittleasaweek,butforhighertemperaturecatalysis(>80°C)workingtimescanbeaslongasseveralmonths.Theflowprofilesoftheresinsystemsallowfortheuseofvacuumbagpressuresalone,avoidingtheneedforautoclaves.

MaterialsOptions:

Resins: Generallyonlyepoxy.

Fibres: Any.Asforconventionalprepregs.

Cores: Any,althoughstandardPVCfoamneedsspecialcare.

MainAdvantages:

i) Alloftheadvantages((i)-(iv))associatedwiththeuseofconventionalprepregsareincorporatedinlow-temperaturecuringprepregs.

ii) Cheaper toolingmaterials,suchaswood,canbeuseddueto the lowercuretemperaturesinvolved.

iii) Largestructurescanbereadilymadesinceonlyvacuumbagpressureis required,andheatingtotheselowertemperaturescanbeachievedwith simplehot-aircirculatedovens,oftenbuiltin-situoverthecomponent.

iv) Conventionalfoamcorematerialscanbeused,providingcertainproceduresarefollowed.

v) Lowerenergycostthanautoclaveprocess.

vi) Robustprocessprovidingahighlevelofdimensiontoleranceandrepeatability.

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MainDisadvantages:

i) Materialscostisstillhigherthanfornon-preimpregnatedfabricsalthoughresincostsarelowerthanthoserequiredforaerospaceapplications.

ii) Tooling needs to be able to withstand higher temperatures than InfusionProcesses.(typically80-140°C).

TypicalApplications:

High-performance wind-turbine blades, large racing and cruising yachts, rescuecraft,traincomponents.

6.10SPRINT®/SparPregTM-“OutofAutoclave”

Description

Whenusingprepregmaterialsinthicklaminates(>3mm)itbecomesdifficulttoremoveentrappedairbetweenplies,andaroundlaminatedetailslikeplyoverlaps,duringthecuringprocess.Toovercomethisproblemintraditionalprepregmaterialsanumberofwarmdebulkingstagesareintroducedduringlay-uptoremovetrappedairwhichsignificantly increases manufacturing times. In recent years Gurit has patented anumberofmodifiedprepregproducts thatenable themanufactureofhighquality(lowvoidcontent)thicklaminatesinoneprocessingstep.SPRINT®materialsconsistofaresinfilmsandwichedbetweentwodryfibrelayers.Onceplacedinthemould,avacuumispulledtoextractalloftheairinthelaminate(monolithicand/orsandwich)beforeheatisappliedtoallowtheresintosoftenandimpregnatethedryfibrelayersandthencure.SparPregTMisamodifiedprepregthatenablesairtobeeasilyremovedfrombetweenadjacentpliesduringtheapplicationofvacuumandthesubsequentcuringprocess.

MaterialsOptions:

Resins: Mostcommonlyepoxybutotherresinsavailable.

Fibres: Any.

Cores: Most,althoughPVCfoamneedsspecialproceduresduetotheelevated temperaturesinvolvedintheprocess.

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MainAdvantages:

i) Highfibrevolumescanbeaccuratelyachievedwithlowvoidcontentsforverythicklaminates(100mm).

ii) Highresinmechanicalpropertiesduetosolidstateofinitialpolymermaterialandelevatedtemperaturecure.

iii) Enablestheuseoflowercostheavyweightmaterials(eg1600gsm)andallowsfastdepositionratesreducingcomponentmanufacturingcosts.

iv) Veryrobustandrepeatableprocessastheresincontentisaccuratelycontrolledandthecomplexityoftheinfusionprocessisverylow.

MainDisadvantages:

i) Materialscostisstillhigherthanfornon-peimpregnatedfabricsalthoughresincostsarelowerthanthoserequiredforaerospaceapplications.

ii) ToolingneedstobeabletowithstandhighertemperaturesthanInfusionProcesses(typically80-140°C).

TypicalApplications:

High-performancewind-turbineblades,largeracingandcruisingyachts,rescuecraft.

7. Secondarybonding

Sincecompositematerialsareoftenusedinweightcriticalapplications,itfollowsthatmethodsofjoiningandfasteningcomponentsshouldnotaddunnecessaryweighttoastructure.Forthisreasonbonding,orjoiningwithadhesives,ofcompositepartsiscommon.Otherreasonsforselectingadhesivebondingovermechanicalfasteningorotherjoiningmethodsinclude:

■ Cosmetic-designaesthetics

■ Technical-jointperformance,assemblycomplexity

■ Economic-costofassembly,reducedpartcount

7.1 Scienceofadhesion

Thefundamentalconceptsofadhesionarecomplexandthereare4mainmechanismsidentified:

■ MechanicalInterlocking

■ Diffusiontheory

■ Electronictheory

■ Adsorptiontheory

Afifthconcept,weakboundarylayers(WBL)hasbeenproposedwhichisamechanismwhichmayimpedetheformationofagoodbond.

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7.1.1Mechanicalinterlocking

Many users of adhesives will intuitively understand the importance of mechanicalinterlocking.Oftenreferredtoas“keying”asurfacepriortobonding,paintersandbondershave improvedadhesionbyusingmechanicalabrasionofsome form formany years. The benefits of this surface preparation include removal of surfacecontaminantsandincreasingtheeffectivesurfaceareaforbondingduetosurfacetopographyandroughness.

Many studies have been performed to correlate the relationship between surfaceroughnessandadhesionperformanceand,althoughthereisobviouslyalink,itisnotpossibletoexplainalladhesionphenomenabythismechanismalone.

7.1.2Diffusiontheory

Proposedintheearly1960’sbyVoyutskii,andbasedonaseriesofexperimentswhichrelatedadhesionbetweentwopolymerstotimeandtemperature,modelledonthebasisofadiffusionprocessusingFickslaw.Theinter-diffusionoftwopolymersrequiresadegreeofchainmobilityandmutualsolubility.

Thediffusiontheorycanbeusedtoexplaintheauto-adhesionofpolymers,andsolventweldingofamorphouspolymers,buthasnoplaceintheconsiderationofadhesionbetweenorganicandinorganicmaterialphases.

7.1.3Electronictheory

ThebasisoftheElectronictheoryisthatiftwomaterialswithdifferentelectronbandstructuresarebroughtintointimatecontact,therewillsometransferofelectrons,leadingtoelectronicinteractionofthemolecules.Thereismuchcontroversysurroundingthistheoryanditisnotgenerallyacceptedastheprimereasonforadhesion.

7.1.4Adsorptiontheory

Theadsorptiontheorystatesthatprovidedintimatecontactisobtainedbetweensub-strateandadhesive,adhesionwilloccurbecauseofintermolecularforcesthatoccurbetweenatoms,moleculesandionsofthetwophases.ThereareavarietyofbondtypesthatcanoccurbuthemostcommonisthoughttobeVanderWaalsforces.

7.1.5WeakBoundaryLayers

Theconceptofa“proper”jointisbasedoncohesivefailureoftheadhesiveorsub-strate.Anyjointthatfailsinterfaciallycanbeconsideredtobeimproper.Suchfailuresaresaidtobeduetoa“weakboundarylayer”.

ThesimplestWBL is thepresenceofa layerofcontamination (eggrease)on thesurfaceofasubstrate.HoweverWBLscanbeduetomanysourcessuchasincorrectpre-treatmentprocesses,segregationofcomponentswithintheadhesiveorsubstrate,orthedevelopmentofaweakenedlayerduetoexposuretoenvironmentalattack.

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7.2 Pre–treatmentpriortobonding

AdequatepreparationofsurfacesforbondingisCRITICALtoachievinganimmediatebond,andalso,moreimportantly,alongterm,durablebond.

Thesubjectofsurfacepreparationisamassiveone,andthemostimportantconcepttounderstand is thatsurfacepreparation techniquesmustbevalidatedwitheachsubstrateandadhesivecombinationpriortouseonanycomponent.

Techniquescaninclude,butarenotlimitedto:

Degreasing

■ Soap + waterSoap+water

■ Pressure washingPressurewashing

■ Ultrasonic cleaningUltrasoniccleaning

■ Solvent wipeSolventwipe

■ Vapour degreasingVapourdegreasing

Mechanicalabrasion

■ Handabrade

■ Handtools

■ Gritblasting

Chemicalcleaning

■ Alkalinecleaning

■ Acidetching

■ Conversioncoatings

■ Anodising(Sulphuric,ChromicorPhosphoricacid)

■ Organo-silaneadhesionpromotors

■ Proprietarypolymertreatments(egTetraEtch)

Gasphasetreatments

■ Flame

■ Coronadischarge

■ Plasma

Theidentificationofappropriatesurfacetreatmentdependsonthesubstrate,adhesiveandprocessingparametersaswellasthefinalapplicationandcriticalityofthebond.

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7.3 Adhesiveselection

Adhesives must be selected that are compatible with both adherends and meetperformanceanddesignrequirements.Thechoiceofadhesivetypesisvastbutforstructuralapplicationstheycanbebrokendownbychemistrygroup.

7.3.1Epoxiesandtoughenedepoxies

Theseformthelargestchemicalfamilyandcanbesuppliedas:

■ Singlecomponentheatcuringfilms■ Pasteadhesives,eitheronecomponentheatactivated(usuallyfrom130to

180°Cin20to60minutes)or2componentroomtemperaturecuring.

■ Syntacticorfoamingfilmadhesivesforgapfillingrequirements

Epoxyadhesivesdisplay:■ Highmechanicalresistance(25to40Mpashearresistance).

■ Excellentadhesiontomostmetals,composites,manyplastics,concrete,

glassandwood

■ Highchemicalresistance

■ Longtermdurability(someairplanepartsthatwerebondedwithepoxy

adhesivessome40yearsagoarestillingoodcondition).

■ Highrigidity(exceptthetoughenedepoxies),so,thereforepoorresistanceto peelandcleavage.

Epoxyadhesivesareoftenusedforaircraftmanufacturing, forpartswhichrequirehighmodulus,highstiffness(forinstanceflightcontrolsurfaces:flaps,rudders),forleadingandtrailingedges,enginenacelles,bondingofsandwichpanels,stringersandstiffenersonthefuselage,etc.

Theyarealsousedforwindturbineblades,bondingofindustrialandchemicalequip-ment(pipes,tanks),electronicequipmentandprintedcircuits,etc...

7.3.2Polyurethanes

TherearesemistructuralPUadhesives(shearresistancefrom6to20MPaaccordingtotheformulations)intheformofpastesthatprovideexcellentadhesiontocomposites,metals,plastics,glass,wood....Theyaremoreflexiblethanmostepoxies.

ThesePUadhesivescanbeonecomponentcured(withmoisture)or2componentscured(atroomtemperatureorlowtemperatures).Pricesaregenerallylowerthanepoxies.

Currentapplications include thebondingofpanels formass transportationequip-ment(buses),bondingofhatchesanddoorsforautomotives,constructionsandwichpanels,andFRPboats.Forexample,trams,trucksandrefrigeratedtruckbodiesarenowmadewithastructuralsteeloraluminiumframeonwhichGRPexteriorpanelsarebonded,usuallywithPU2componentsadhesives.

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7.3.3Acrylicsandmethacrylatesstructuraladhesives

2componentsadhesives,withvariousmixingratios,fastcuringatroomtemperature(from10minutesto2hours),highshearresistance(10to30MPa),excellentadhesiontoalmostallplasticsandcomposites,high impactstrengthandfatigueresistance(widely used for boat assembly which must resist to very frequent shocks on thewaves),goodresistancetowaterandchemicals,mediumprice.

Theseadhesivesaredevelopingfast,theymayreplacethepolyesteradhesivesandsomeepoxieswhenamoreflexiblebondisrequired.

7.3.4Polyesters

Polyesterresinmanufacturersareofferingsomeimprovedpolyesteradhesives,whichreactaftertheadditionofacatalyst.

Theseadhesivesaremostlyusedforbondingreinforcedpolyesterpartsinboatcon-struction,becausethis industry isaccustomedtomakereinforcedpolyesterparts.Theyarerigid,maybebrittle,buttheyhaveverygoodgapfillingproperties.

Shearresistanceisapproximately10MPa.Theymaybeusedfordecktohullassembly.Theyaregenerallylowcost.

7.3.5Urethane-acrylates

Highlyflexible,oftenstructuraladhesivesadhesives.Curebyadditionof2%catalystin1toseveralhoursatroomtemperature.Excellentadhesionwithplastics,metal,woodsubtstrates.

Reasonableshearresistance(10to15MPa),goodimpactresistanceandresistancetocrackpropagation.Theyalsohavegapfillingpropertiesupto10mm(withsomegrades)andgoodresistancetomoistureingress.

Theyaremostlyusedforboatconstructionandbondingofautomotivebodyparts.

7.3.6Heatstableadhesives:Bismaleimides,polyimides,cyanateesters

Theseareexpensiveadhesives,mostlydeliveredinfilms,whichresisttohightem-peratures,forinstanceservicetemperaturesof250to300°Cforpolyimides,200to250°Cfor thebismaleimides,Theyaredifficult tousebecausetheyrequirecuringunderhightemperatures(300°C)andhighpressures(15bars)–thereforeautoclaveorpressprocessingisarequisite.

Usuallyonlyusedinaerospaceapplicationstobondpartswhichareresistanttohightemperatures(forinstancesomepartsofthesurfaceofmilitarysupersonicaircraftsmayreach260°Cduringsomeflightconditions).

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7.3.7Anaerobicsandcyanoacrylates

Maybeusedtobondsmallpartsmadeofthermoplasticpolymers,formechanicalpartsandappliances.Butcyanoacrylatesmaybebrittle.Anaerobicshavegoodadhesiontomanythermoplasticsandthermosets.

7.4 Jointdesign

Inadditiontocorrectsurfacepreparationandmaterialselection,itiscriticallyimportantthatthejointiscorrectlydesignedandmanufactured.

Themainconsiderationwhendesigningbondedjointsisloadtransferpaths.Adhesivesworkwellincompressionandshearloadingbutarenotgenerallygoodintensileloadsituations.Applicationofpeelloadsshouldbeminimised,andavoidedcompletelywhereverpossible.Thestiffnessofsubstrates,andhowtheywilldeformunderloadingshouldalsobeconsidered.Itisnotuncommonforjointswhichappearwelldesignedtoinducehighpeelstresseswhenloaded.

SomesimpleexamplesofjointconsiderationareshowninFigure29

Figure29–Jointconfigurations

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8. Gelation,curing&postcuring

Onadditionofthecatalystorhardeneraresinwillbegintobecomemoreviscousuntilitreachesastatewhenitisnolongeraliquidandhaslostitsabilitytoflow.Thisisthe‘gelpoint’.Theresinwillcontinuetohardenafterithasgelled,until,atsometimelater,ithasobtaineditsfullhardnessandproperties.Thisreactionitselfisaccompaniedbythegenerationofexothermicheat,which,inturn,speedsthereaction.Thewholeprocessisknownasthe‘curing’oftheresin.Thespeedofcureiscontrolledbytheamountofacceleratorinapolyesterorvinylesterresinandbyvaryingthetype,notthequantity,ofhardener inanepoxy resin. Generallypolyester resinsproduceamoresevereexothermandafasterdevelopmentofinitialmechanicalpropertiesthanepoxiesofasimilarworkingtime.

Withbothresintypes,however,itispossibletoacceleratethecurebytheapplicationofheat,sothatthehigherthetemperaturethefasterthefinalhardeningwilloccur.Thiscanbemostusefulwhenthecurewouldotherwisetakeseveralhoursorevendaysatroomtemperature.Aquickruleofthumbfortheacceleratingeffectofheatonaresinisthata10°Cincreaseintemperaturewillroughlydoublethereactionrate.Thereforeifaresingelsinalaminatein25minutesat20°Citwillgelinabout12minutesat30°C,providingnoextraexothermoccurs.Curingatelevatedtemperatureshastheaddedadvantagethatitactuallyincreasestheendmechanicalpropertiesofthematerial,andmanyresinsystemswillnotreachtheirultimatemechanicalpropertiesunlesstheresinisgiventhis‘postcure’.

Thepostcureinvolvesincreasingthelaminatetemperatureaftertheinitialroomtem-peraturecure,whichincreasestheamountofcross-linkingofthemoleculesthatcantakeplace.Tosomedegreethispostcurewilloccurnaturallyatwarmroomtempera-tures,buthigherpropertiesandshorterpostcuretimeswillbeobtainedifelevatedtemperaturesareused.

Thisisparticularlytrueofthematerial’ssofteningpointorGlassTransitionTemperature(Tg),which,uptoapoint,increaseswithincreasingpostcuretemperature.

9. Inspectionandtesting

Sincetherawmaterial iseffectivelyprocessedat thesametimeasthecompositecomponent,therearemanyaspectswhichrequirevalidationtoensurequalityoffin-ishedparts.Assumingthemanufacturingprocess,componentdesignandmaterialselectionhavebeenadequatelyandcorrectlyspecified,visualandnon-destructivetestingformthefirststageofqualitycontrol.

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9.1 Nondestructivetesting

9.1.1Visualinspection

Delaminations,misplacedplies,excessresinbleed,dryfabric,porosity,toolingmarksandsurfacedefectsareamongstthemanydefectswhichcaneasilybeobservedbyvisualexamination.Inadditiontogeometricanddimensionalchecks,visualchecksofcompositequalitycanofteneasilyidentifyprocessingerrorsormaterialqualityproblems.

Sometimesdefectswillnotbevisibletothenakedeye,orwillbewithinthebulkofacomponentsomorecomplexnondestructiveexaminationisrequired:

9.1.2Taptesting

Tap-testingisacommonmethodfordeterminingvoidsanddefectsincuredlaminatesdue to it’scheap,efficientnature.Themethod involves tapping thesurfaceof thelaminatewithahardobject,usuallyacoin,andlisteningforachangeinthepitch/volumeofthenoiseproduced.

Advantages

■ Cheap, simple and quick techniqueCheap,simpleandquicktechnique

Disadvantages

■ Method relies heavily on userMethodreliesheavilyonuser

■ Uncured materials won’t have the necessary properties to transmit noise/ withUncuredmaterialswon’thavethenecessarypropertiestotransmitnoise/with standtheforceofimpact

9.1.3Ultrasonics

Ultrasonicsissimilartothetap-testmethoddescribedabove,exceptthatitdealswiththefrequencyofsoundthatisoutoftherangeofhumanhearing(ie.,greaterthan20,000Hz).Theultrasonicsignalistransmittedtotheparttobeinspectedviaatrans-ducer,andreceivedeitherbythesameunit(pulse-echomethods)orbyatransducerbehindthepart(through-thicknessmethods).Recentadvancesintechnologymeanthatair-coupledpulse-echoultrasonicunitsarenowcommerciallyavailable.

Advantages

■ Able to detect a wide range of defectsAbletodetectawiderangeofdefects

■ Can give information of 3-D location of defect within the laminateCangiveinformationof3-Dlocationofdefectwithinthelaminate

■ Can be slow- resolution dependent on speedCanbeslow-resolutiondependentonspeed

Disadvantages

■ Level of skill required to accurately interpretLevelofskillrequiredtoaccuratelyinterpret

■ Unsuitable for use on uncured materialUnsuitableforuseonuncuredmaterial

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9.1.4ComputedTomography

Computedtomography(CTscanning)passesx-raysthroughabodyofmaterial,andcollectsthemviaareceiverplacedbehindthepart.Thisisdonefrommultipleanglesaroundthebody,andtheresultstheninterpretedtobuildupa3-Dimageofthepart,showingthesizeandlocationofanydefectspresent.

Advantages

■ 3-D image gives accurate information about defect size/ location3-Dimagegivesaccurateinformationaboutdefectsize/location

■ High resolution of part achievableHighresolutionofpartachievable

Disadvantages

■ Very costly and time-consumingVerycostlyandtime-consuming

■ Health & safety considerationsHealth&safetyconsiderations

9.1.5Shearography

Inshearography,aspecimen isdeformedbyanexternallyapplied force,and theresultingout-of-planedisplacementsmeasuredusing laserspeckle interferometry.Theexpecteddeformationpattern isknown from theboundaryconditionsplaceduponthespecimenbytheloadingconfiguration,withanydifferencesfromthisbeingaresultofdamageinthespecimen.

Advantages

■ Effective in locating debonded areasEffectiveinlocatingdebondedareas

■ Gives accurate information on the size and shape of the defectGivesaccurateinformationonthesizeandshapeofthedefect

Disadvantages

■ Will only detect differences in displacementsWillonlydetectdifferencesindisplacements

■ The equipment is expensiveTheequipmentisexpensive

9.1.6Thermography

Inthermography,thesampleisheated,usuallyviaaflashgunorsimilar,andaninfraredcamerausedtoviewthethermalconductivityofthepart.Theresponsewillbedifferentwheretherearedefectspresentcomparedtothoseareaswheretherearenot.

Advantages

■ The method is quick to implement and can cover large areasThemethodisquicktoimplementandcancoverlargeareas

Disadvantages

■ Defects at a depth greater than 3x their diameter cannot be detectedDefectsatadepthgreaterthan3xtheirdiametercannotbedetected

■ The equipment is expensiveTheequipmentisexpensive

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9.1.7LambWaveSensing

LambwavesensinginvolvespropagatingLambwaves(aformofelasticwavesabletopropagatethroughasolidmaterial)throughasample,andusingatransducertopickuptheirsignal.Theamplitudeofthesignal isusedtodeterminethedamagepresent inthepart.Transducersneedtobestucktotheparttobothtransmitandreceivethewaves.

Advantages

■ Themethodcanbeusedoverlargeareasquickly

Disadvantages

■ Poordefectcharacterisation,withsomedefectsbeingundetectable

■ Waveswillnotpropagateinuncuredmaterial

9.1.8Radiography

Inradiography,X-raysarefiredatonesurfaceofthesample,andthencollectedbyareceiverontheotherside.Differentmaterialsabsorbdifferentamountsofradiationasthex-rayspassthroughthesample,andthusdefectsareseen.Toobservecracksordelaminations,itcanbenecessarytouseadye-penetrant;

Advantages

■ Simpleandquickmethodfordetection

■ Goodresolutionsystemsavailable

Disadvantages

■ X-rayequipmentisexpensive

■ Health&safetyconsiderations

9.2 Destructivetesting

Toprovidematerialallowablesduringdesignstage,andtoverifymanufacturingproc-essesandmaterialproperties,itiscommontoperformmechanicaltesting.

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9.2.1Mechanicaltesting

Thenumberoftestmethodsapplicabletocompositematerialsisalmostendless.Theappropriatetestmethodtobeappliedinanycasedependsonthematerialpropertyunderinvestigation,thematerialtypeandtheequipmentavailable.Inadditionthetestcondition(i.e.temperature,humidity)andthepre-testconditioningofsamplescandramaticallyaffectobservedresults.

Someofthecommonmethodsapplyingtolaminatetesting,andtherecommendedmethodarelistedinTable3:

Property&method Recommendedstandard

Tensileproperties

- Unidirectional

- Multiaxial

BSENISO527-5&1

BSENISO527-4&1

Compressiveproperties BSENISO14126

Flexuralproperties BSENISO14125

Interlaminarshearstrength BSENISO14130

In-planeshear BSENISO14129

Notched(hole)tensilestrength ISONWI(UKdraft)/ASTMD5766

Notched(hole)compressivestrength ISONWI(UKdraft)/ASTMD6484

Bearingproperties

- Pin

- Bolt

ISONWI(UKdraft)prEN6037

ASTMD5961

Fracturetoughness

- ModeI

- ModeII

ISO15024

ISONWIproposalfromVAMAS

Fatigueproperties ISO/CD13003

Fibre,resin&voidcontent

Glassfibre

Carbonfibre

ISO1172/ISO7822

ISO14127

Densityofplastics ISO1183

Hot/wetconditioning PrEN2823

Out-gassing ESA-PSS-01-722

Coefficientoflinearexpansion ISO11359-3

Table3–Commonlaminatemechanicaltests

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Forsandwichpanelstherearemanyothertestmethods,themostcommonofwhicharelistedbelow:

Property&method Recommendedstandard

Flexuralproperties

- 3pointbend

- 4pointbend

ASTMC393

ASTMC393

Shear ISO1922/ASTMC273

Climbingdrumpeel BSENISO5350C13/ASTMD1781

Flatwisetension BSENISO5350C6/ASTMC297

Edgewisecompression ASTMC364

Table4-commonsandwichpanelmechanicaltests

Theseareonlyafractionofthetestmethodsthatareusedtotestandcharacterisethemechanicalandphysicalpropertiesofcompositematerials.

Duringanytestorvalidationprogramme, it isvery important toconsiderwhat testmethodtouse,whattheoutputoftheresultisactuallyrevealingaboutthematerial,andwhataretheacceptablecriteria.Testingisexpensiveandisonlyaddingvalueifitisunderstood.

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EstimatingQuantitiesofFormulatedProducts

LaminatingResins

Resin/ Hardener Mix Required (g) = A x n x WF x R.C x 1.5* (1 - R.C.)

Where: A = Area of Laminate (sq.m) n = Number of plies WF = Fibre weight of each ply (g/sq.m) R.C. = Resin content by weight

Typical R.C.’s for hand layup manufacturing are: Glass - 0.46 Carbon - 0.55 Aramid - 0.61

GelcoatsandCoatings

SolventFree

Resin/ Hardener Mix Required (kg) = A x t x ρm x 1.5* 1000

SolventBased

Resin/ Hardener Mix Required (kg) = A x t x ρm x 1.5* 10 x S.C.

Where A = Area to be coated (sq.m) t = Total finished thickness required (µm) ρm = Density of cured resin/hardener matrix (g/cm3) S.C. = Solids content of mix (%)

*Assuming 50% wastage, for resin residue left in mixing pots and on tools. This wastage figure is based on our experience of a wide variety of workshops, but should be adjusted to match local working practices.

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LaminateFormulaeFibreVolumeFractionFromDensities

FVF = (ρC - ρm) (assuming zero void content) (ρF - ρm)

FibreVolumeFractionfromFibreWeightFraction

FibreWeightFractionfromFibreVolumeFraction

CuredPlyThicknessfromPlyWeight

CPT (mm) = WF

ρF x FVF x 1000

Where FVF = Fibre Volume Fraction FWF = Fibre Weight Fraction ρc = Density of Composite (g/cm3) ρm = Density of Cured Resin/ Hardener Matrix (g/cm3) ρF = Density of Fibres ( g/cm3) WF = Fibre Area Weight of each Ply (g/sqm)

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Imperial/MetricConversionTables

The bold figures in the central columns can be read as either the metric or the British measure.Thus 1 inch = 25.4 millimetres: or 1 millimetre = 0.039 inches.

°C °F

-18 0 0 32 5 41 10 50 15 59 20 68 25 77 30 86 35 95 40 104 45 113 50 122 55 131 60 140 65 149 70 158 75 167 80 176 85 185 90 194 95 203 100 212 105 221 110 230

Mil(thou)Microns

(µM) 0.039 1 25.40 0.079 2 50.80 0.118 3 76.20 0.157 4 101.60 0.197 5 127.00 0.236 6 152.40 0.276 7 177.80 0.315 8 203.20 0.354 9 228.60

Inches mm

0.039 1 25.4 0.079 2 50.8 0.118 3 76.2 0.157 4 101.6 0.197 5 127.0 0.236 6 152.4 0.276 7 177.8 0.315 8 203.2

0.354 9 228.6

Pints Litres

1.760 1 0.568 3.520 2 1.137 5.279 3 1.705 7.039 4 2.273 8.799 5 2.841 10.559 6 3.410 12.318 7 3.978 14.078 8 4.546 15.838 9 5.114

FeetMetres

3.281 1 0.305 6.562 2 0.610 9.843 3 0.914 13.123 4 1.219 16.404 5 1.524 19.685 6 1.829 22.966 7 2.134 26.247 8 2.438

29.528 9 2.743

USQuartsLitres

1.057 1 0.946 2.114 2 1.892 3.171 3 2.838 4.228 4 3.784 5.285 5 4.73 6.342 6 5.676 7.400 7 6.622 8.457 8 7.568 9.514 9 8.514

Yards Metres

1.094 1 0.914 2.187 2 1.829 3.281 3 2.743 4.374 4 3.658 5.468 5 4.572 6.562 6 5.486 7.655 7 6.401 8.749 8 7.315

9.843 9 8.230

USGal. Litres

0.264 1 3.785 0.528 2 7.570 0.792 3 11.355 1.056 4 15.140 1.32 5 18.925 1.584 6 22.710 1.848 7 26.495 2.112 8 30.280 2.376 9 34.065

Ounces Grams

0.035 1 28.350 0.071 2 56.699 0.106 3 85.048 0.141 4 113.398 0.176 5 141.748 0.212 6 170.097 0.247 7 198.446 0.282 8 226.796

0.317 9 255.146

Imp.Gal. Litres

0.220 1 4.546 0.440 2 9.092 0.660 3 13.638 0.880 4 18.184 1.100 5 22.730 1.320 6 27.277 1.540 7 31.823 1.760 8 36.369 1.980 9 40.915

Sq.ydsSq.metres

1.196 1 0.836 2.392 2 1.672 3.588 3 2.508 4.784 4 3.345 5.980 5 4.181 7.176 6 5.017 8.372 7 5.853 9.568 8 6.689 10.764 9 7.525

Miles Kilomtrs. 0.621 1 1.609 1.243 2 3.219 1.864 3 4.828 2.485 4 6.437 3.107 5 8.047 3.728 6 9.656 4.350 7 11.265 4.971 8 12.875 5.592 9 14.484

oz/sq.yd g/sq.m

0.029 1 33.9 0.059 2 67.9 0.088 3 101.8 0.118 4 135.8 0.147 5 169.7 0.177 6 203.6 0.206 7 237.6 0.236 8 271.5 0.265 9 305.5

lb/cu.ft kg/cu.m 0.062 1 16.0 0.125 2 32.1 0.187 3 48.1 0.250 4 64.1 0.312 5 80.2 0.374 6 96.2 0.437 7 112.2 0.499 8 128.2 0.561 9 144.3

Pounds Kilograms

2.205 1 0.454 4.409 2 0.907 6.614 3 1.361 8.818 4 1.814 11.023 5 2.268 13.228 6 2.722 15.432 7 3.175 17.637 8 3.629

19.842 9 4.082

USGal. Imp.Gal.

1.200 1 0.833 2.401 2 1.666 3.601 3 2.499 4.802 4 3.332 6.002 5 4.165 7.203 6 4.998 8.403 7 5.831 9.604 8 6.664 10.804 9 7.497

ksiN/mm2(MPa)

0.145 1 6.9 0.290 2 13.8 0.435 3 20.7 0.580 4 27.6 0.725 5 34.5 0.870 6 41.4 1.015 7 48.3 1.160 8 55.2 1.305 9 62.1

Msi Gpa

0.145 1 6.895 0.290 2 13.790 0.435 3 20.685 0.580 4 27.580 0.725 5 34.475 0.870 6 41.370 1.015 7 48.265 1.160 8 55.160 1.305 9 62.055

Cu.FeetCu.Metres

35.315 1 0.028 70.629 2 0.057 105.944 3 0.085 141.259 4 0.113 176.573 5 0.142 211.888 6 0.170 247.203 7 0.198 282.517 8 0.227 317.832 9 0.255

lb/cu.in g/cu.cm

0.036 1 27.7 0.029 0.8 22.1 0.031 0.85 23.5 0.033 0.9 24.9 0.034 0.95 26.3 0.038 1.05 29.1 0.040 1.1 30.4 0.042 1.15 31.8 0.043 1.2 33.2

°C °F

115 239 120 248 125 257 130 266 135 275 140 284 145 293 150 302 155 311 160 320 165 329 170 338 175 347 180 356 185 365 190 374 195 383 200 392 205 401 210 410 215 419 220 428 225 437 230 446

FluidOz. Litres

35.21 1 0.028 70.42 2 0.057 105.63 3 0.085 140.84 4 0.114 176.06 5 0.142 211.27 6 0.170 246.48 7 0.199 281.69 8 0.227 316.90 9 0.256

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