GUIDE TO COMPOSITES

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DELIVERING THE FUTURE OF COMPOSITE SOLUTIONS

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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

Initsmostbasicformacompositematerialisone,whichiscomposedofatleasttwoelementsworkingtogethertoproducematerialpropertiesthataredifferenttotheprop-ertiesofthoseelementsontheirown.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,theywillberestrictedtoasmallnumberoffibreswiththeremainderexhibit-ingthematerial’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,alimitforFVFisapproximately30-40%.Withthehigherquality,moresophisticatedandpreciseprocessesusedintheaerospaceindustry,FVF’sapproaching70%canbesuccessfullyobtained.

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Thegeometryofthefibresinacompositeisalsoimportantsincefibreshavetheirhighestmechanicalpropertiesalongtheirlengths,ratherthanacrosstheirwidths.Thisleadstothehighlyanisotropicpropertiesofcomposites,where,unlikemetals, themechanicalpropertiesofthecompositearelikelytobeverydifferentwhentestedindifferentdirections.Thismeansthatitisveryimportantwhenconsideringtheuseofcompositestounderstandatthedesignstage,boththemagnitudeandthedirectionoftheappliedloads.Whencorrectlyaccountedfor,theseanisotropicpropertiescanbeveryadvantageoussinceitisonlynecessarytoputmaterialwhereloadswillbeapplied,andthusredundantmaterialisavoided.

Itisalsoimportanttonotethatwithmetalsthepropertiesofthematerialsarelargelydeterminedbythematerialsupplier,andthepersonwhofabricatesthematerialsintoafinishedstructurecandoalmostnothingtochangethose‘in-built’properties.How-ever,acompositematerialisformedatthesametimeasthestructureisitselfbeingfabricated.This is aFUNDAMENTALdistinctionof compositematerials andMUSTalwaysbeconsideredduringdesignandmanufacturingstages.

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

Figure3showsacompositeexperiencingashear load. This load is tryingtoslideadjacentlayersoffibresovereachother.Undershearloadstheresinplaysthemajorrole,transferringthestressesacrossthecomposite.Forthecompositetoperformwellundershearloadstheresinelementmustnotonlyexhibitgoodmechanicalpropertiesbutmustalsohavehighadhesiontothereinforcementfibre.Theinterlaminarshearstrength(ILSS)ofacompositeisoftenusedtoindicatethispropertyinamulti-layercomposite(‘laminate’).

Figure3–Shearloading

2.1.4Flexure

Flexuralloadsarereallyacombinationoftensile,compressionandshearloads.Whenloadedasshown,theupperfaceisputintocompression,thelowerfaceintotensionandthecentralportionofthelaminateexperiencesshear.

Figure4–Flexuralloading

2.2Stressorstrain?

Thestrengthofalaminateisusuallythoughtofintermsofhowmuchloaditcanwith-standbeforeitsufferscompletefailure.Thisultimateorbreakingstrengthisthepointatwhichtheresinexhibitscatastrophicbreakdownandthefibrereinforcementsbreak.

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

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Figure5–TypicalFRPstress/straingraph

Thestrain thata laminatecanreachbeforemicrocrackingdependsstronglyonthetoughnessandadhesivepropertiesoftheresinsystem.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.

Increased resin/fibreadhesion isgenerallyderived fromboth the resin’s chemistryanditscompatibilitywiththechemicalsurfacetreatmentsappliedtofibres.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.Themisalignmentoffibrescausesdramatic lossofmechanicalproperties,particularly incompression,due toincreasedlikelihoodofbuckling.

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,whicharebulkpropertiesoftherawmaterialsimplepropertiesofcompositematerialscanbeestimatedbasedonthecontributionofeachpartofthecomposite.Thismethodisreferredtoastheruleofmixtures(RoM).

Fora2componentcomposite:

Vf +Vm =1

where Vf =Volumefractionfibre

Vm=Volumefractionmatrix

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BasedonRoMacompositeproperty,Pc,canbeestimatedby:

Pc =Pf Vf+PmVm=PfVf+Pm(1-Vf)

wherePf&Pm=Thepropertyoffibre

Forexampleelasticmodulus,E1,paralleltofibredirectioncanbecalculatedbasedontheYoungsmodulusofeachoftheconstituentmaterials:

E1 =Ef Vf+Em 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

RoMformulaeandmodelsareuseful forcalculatingthepropertiesofsingle layersbut more complex methodologies are required to enable characterisation of multipliedlaminates.LaminatePlateTheory(LPT)isacomplexbutwellestablishedandac-ceptedapproach.WhilsttoocomplicatedtodescribeindetailhereLPTdescribesthedeformationofalaminateunderexternalloadingbasedonthepropertiesofthefibre,matrixandthe%ofeachineachaxisofloading.Itislogicalandeasilyautomatedforincorporationintoengineeringmodels.

Itisquiteobviousthatmechanicalpropertiesofaplywithinthelaminatearedepend-antonthealignmentandorientationofthefibrewithintheply.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,thelayupnotationisdescribedasshownbelow:

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.IneffectthecoreactslikethewebinanI-beam,wherethewebprovidesthelightweight‘separator’betweentheload-bearingflanges.InanI-beamtheflangescarrythemaintensileandcompres-siveloadsandsothewebcanberelativelylightweight.Corematerialsinasandwichstructurearesimilarlylowinweightcomparedtothematerialsintheskinlaminates.

Engineeringtheoryshowsthattheflexuralstiffnessofanypanelisproportionaltothecubeofitsthickness.Thepurposeofacoreinacompositelaminateisthereforetoincreasethelaminate’sstiffnessbyeffectively‘thickening’itwithalow-densitycorematerial.Thiscanprovideadramaticincreaseinstiffnessforverylittleadditionalweight.

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 long chain-likemolecules consistingofmany simple repeatingunits.Man-madepolymersaregenerallycalled‘syntheticresins’orsimply‘resins’.Polymerscanbeclassifiedundertwotypes,‘thermoplastic’and‘thermosetting’,accordingtotheeffectofheatontheirproperties.

Thermoplastics,likemetals,softenwithheatingandeventuallymelt,hardeningagainwithcooling.Thisprocessofcrossingthesofteningormeltingpointonthetempera-turescalecanberepeatedasoftenasdesiredwithoutanyappreciableeffectonthematerialpropertiesineitherstate.Typicalthermoplasticsincludenylon,polypropyleneandABS,andthesecanbereinforced,althoughusuallyonlywithshort,choppedfibressuchasglass.

Thermosettingmaterials,or‘thermosets’,areformedfromachemicalreactioninsitu,wheretheresinandhardenerorresinandcatalystaremixedandthenundergoanon-reversiblechemicalreactiontoformahard, infusibleproduct.Insomethermosets,suchasphenolicresins,volatilesubstancesareproducedasby-products(a‘conden-sation’ reaction). Other thermosetting resinssuchaspolyesterandepoxycurebymechanismsthatdonotproduceanyvolatilebyproductsandthusaremucheasiertoprocess(‘addition’reactions).

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

Although therearemanydifferent typesof resin inuse in thecomposite industry,themajorityofstructuralpartsaremadewith threemain types,namelypolyester,vinylesterandepoxy.

5.1.1 PolyesterResins

Polyesterresinsarethemostwidelyusedresinsystems,particularly inthemarineindustry.Byfarthemajorityofdinghies,yachtsandwork-boatsbuilt incompositesmakeuseofthisresinsystem.

Polyesterresinssuchastheseareofthe‘unsaturated’type.Unsaturatedpolyesterresinisathermoset,capableofbeingcuredfromaliquidorsolidstatewhensubjectto the rightconditions.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.Polyesterresinshavealimitedstor-agelifeastheywillsetor‘gel’ontheirownoveralongperiodoftime.Oftensmallquantitiesofinhibitorareaddedduringtheresinmanufacturetoslowthisgellingaction.

Foruseinmoulding,apolyesterresinrequirestheadditionofseveralancillaryprod-ucts.Theseproductsaregenerally:

■ Catalyst

■ Accelerator

■ Additives: Thixotropic Pigment Filler Chemical/fireresistance

Amanufacturermaysupplytheresininitsbasicformorwithanyoftheaboveaddi-tivesalreadyincluded.Resinscanbeformulatedtothemoulder’srequirementsreadysimplyfortheadditionofthecatalystpriortomoulding.Ashasbeenmentioned,givenenoughtimeanunsaturatedpolyesterresinwillsetbyitself.Thisrateofpolymerisationistooslowforpracticalpurposesandthereforecatalystsandacceleratorsareusedtoachievethepolymerisationoftheresinwithinapracticaltimeperiod.Catalystsareaddedtotheresinsystemshortlybeforeusetoinitiatethepolymerisationreaction.Thecatalystdoesnottakepartinthechemicalreactionbutsimplyactivatestheprocess.Anaccelerator isaddedtothecatalysedresintoenablethereactiontoproceedatworkshoptemperatureand/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)

Thepolyester resin is thensaid tobe ‘cured’. It isnowachemically resistant (andusually)hardsolid.Thecross-linkingorcuringprocessiscalled‘polymerisation’.Itisanon-reversiblechemicalreaction.The‘side-by-side’natureofthiscross-linkingofthemolecularchainstendstomeansthatpolyesterlaminatessufferfrombrittlenesswhenshockloadingsareapplied.

Greatcareisneededinthepreparationoftheresinmixpriortomoulding.Theresinandanyadditivesmustbecarefullystirredtodisperseallthecomponentsevenlybeforethecatalystisadded.Thisstirringmustbethoroughandcarefulasanyairintroducedintotheresinmixaffectsthequalityofthefinalmoulding.Thisisespeciallysowhenlaminatingwith layersof reinforcingmaterialsasairbubblescanbe formedwithintheresultantlaminatewhichcanweakenthestructure.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.Additionofcertainfillerscanalsocontributetoincreasingthefire-resistanceofthelaminate.

ABA B A BA

ABA B A BA

S S S

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5.1.2VinylesterResins

Vinylesterresinsaresimilarintheirmolecularstructuretopolyesters,butdifferprimarilyinthelocationoftheirreactivesites,thesebeingpositionedonlyattheendsofthemolecularchains.Asthewhole lengthofthemolecularchain isavailabletoabsorbshockloadingsthismakesvinylesterresinstougherandmoreresilientthanpolyes-ters.Thevinylestermoleculealsofeaturesfewerestergroups.Theseestergroupsaresusceptibletowaterdegradationbyhydrolysiswhichmeansthatvinylestersexhibitbetterresistancetowaterandmanyotherchemicalsthantheirpolyestercounterparts,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’ refers toachemicalgroupconsistingofanoxygenatombondedtotwocarbonatomsthatarealreadybondedinsomeway.Thesimplestepoxyisathree-memberringstructureknownbytheterm‘alpha-epoxy’or‘1,2-epoxy’.Theide-alisedchemicalstructureisshowninthefigurebelowandisthemosteasilyidentifiedcharacteristicofanymorecomplexepoxymolecule.

Figure19-IdealisedChemicalStructureofaSimpleEpoxy(EthyleneOxide)

Usually identifiable by their characteristic amber or brown colouring, epoxy resinshaveanumberofusefulproperties.Boththeliquidresinandthecuringagentsformlowviscosityeasilyprocessedsystems.Epoxyresinsareeasilyandquicklycuredatanytemperaturefrom5°Cto150°C,dependingonthechoiceofcuringagent.Oneofthemostadvantageouspropertiesofepoxiesistheirlowshrinkageduringcurewhichminimises fabric ‘print-through’ and internal stresses. High adhesive strength andhighmechanicalpropertiesarealsoenhancedbyhighelectricalinsulationandgoodchemical resistance.Epoxiesfindusesasadhesives,caulkingcompounds,castingcompounds,sealants,varnishesandpaints,aswellaslaminatingresinsforavarietyofindustrialapplications.

Epoxyresinsareformedfroma longchainmolecularstructuresimilartovinylesterwithreactivesitesateitherend.Intheepoxyresin,however,thesereactivesitesareformedbyepoxygroupsinsteadofestergroups.Theabsenceofestergroupsmeansthattheepoxyresinhasparticularlygoodwaterresistance.Theepoxymoleculealsocontainstworinggroupsatitscentrewhichareabletoabsorbbothmechanicalandthermalstressesbetterthanlineargroupsandthereforegivetheepoxyresinverygoodstiffness,toughnessandheatresistantproperties.

Thefigurebelowshowstheidealisedchemicalstructureofatypicalepoxy.Notetheabsenceoftheestergroupswithinthemolecularchain.

Figure20-IdealisedChemicalStructureofaTypicalEpoxy(DiglycidylEtherofBisphenol-A)

0

CH2–CH–

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Epoxiesdifferfrompolyesterresinsinthattheyarecuredbya‘hardener’ratherthanacatalyst.Thehardener,oftenanamine,isusedtocuretheepoxybyan‘additionreac-tion’wherebothmaterialstakeplaceinthechemicalreaction.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,andaretherefore

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frequentlyfoundinmanyhigh-strengthadhesives.Thisisduetotheirchemicalcom-positionandthepresenceofpolarhydroxylandethergroups.Asepoxiescurewithlowshrinkagethevarioussurfacecontactssetupbetweentheliquidresinandtheadherendsarenotdisturbedduringthecure.Theadhesivepropertiesofepoxyareespeciallyusefulintheconstructionofhoneycomb-coredlaminateswherethesmallbondingsurfaceareameansthatmaximumadhesionisrequired.

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

5.1.4.2 MechanicalProperties

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

Figure22–Comparisonofresintensilestrength&modulus

Afteracureperiodofsevendaysatroomtemperatureitcanbeseenthatatypicalepoxywillhavehigherpropertiesthanatypicalpolyesterandvinylesterforbothstrengthandstiffness.Thebeneficialeffectofapostcureat80°Cforfivehourscanalsobeseen.

Alsoofimportancetothecompositedesignerandbuilderistheamountofshrinkagethatoccursinaresinduringandfollowingitscureperiod.Shrinkageisduetotheresinmoleculesrearrangingandre-orientatingthemselvesintheliquidandsemi-gelledphase.Polyesterandvinylestersrequireconsiderablemolecularrearrangementtoreachtheircuredstateandcanshowshrinkageofupto8%.Thedifferentnatureoftheepoxyreaction,however,leadstoverylittlerearrangementandwithnovolatilebi-productsbeingevolved,typicalshrinkageofanepoxyisreducedtoaround2%.Theabsenceofshrinkageis,inpart,responsiblefortheimprovedmechanicalpropertiesofepoxiesoverpolyester,asshrinkageisassociatedwithbuilt-instressesthatcanweakenthematerial.Furthermore,shrinkagethroughthethicknessofalaminateleadsto‘print-through’ofthepatternofthereinforcingfibres,acosmeticdefectthatisdifficultandexpensivetoeliminate.

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5.1.4.3DegradationfromWaterIngress

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

Figure23-EffectofPeriodsofWaterSoakat100°ConResinInter-LaminarShearStrength

Figure23demonstratestheeffectsofwateronanepoxyandpolyesterwovenglasslaminate,whichhavebeensubjectedtoawatersoakat100°C.Thiselevatedtem-peraturesoakinggivesaccelerateddegradationpropertiesfortheimmersedlaminate.

5.1.4.4Osmosis

All laminates in a marine environment will permit very low quantities of water topassthroughtheminvapourform.Asthiswaterpassesthrough,itreactswithanyhydrolysablecomponentsinsidethelaminatetoformtinycellsofconcentratedsolu-tion.Undertheosmoticcycle,morewateristhendrawnthroughthesemi-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.Thecondensationnatureoftheircuringprocesstendstoleadtotheinclusionofmanyvoidsandsurfacedefects,andtheresinstendtobebrittleanddonothavehighmechanicalproperties.Typicalcosts:£2-4/kg.

5.1.5.2 CyanateEsters

Primarilyusedintheaerospaceindustry.Thematerial’sexcellentdielectricpropertiesmakeitverysuitableforusewithlowdielectricfibressuchasquartzforthemanufac-tureofradomes.Thematerialalsohastemperaturestabilityuptoaround200°Cwet.Typicalcosts:£40/kg.

5.1.5.3 Silicones

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

5.1.5.4 Polyurethanes

Hightoughnessmaterials,sometimeshybridisedwithotherresins,duetorelativelylow laminatemechanical properties in compression. Usesharmful isocyanatesascuringagent.Typicalcosts:£2-8/kg

5.1.5.5 Bismaleimides(BMI)

Primarilyusedinaircraftcompositeswhereoperationathighertemperatures(230°Cwet/250°Cdry)isrequired.e.g.engineinlets,highspeedaircraftflightsurfaces.Typi-calcosts:>£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-ess used. However, reinforcing fabrics with closely packed fibres will give higherFibreVolumeFractions(FVF)inalaminatethanwillthosefabricswhicharemadewithcoarserfibres,orwhichhavelargegapsbetweenthefibrebundles.Fibrediameterisanimportantfactorherewiththemoreexpensivesmallerdiameterfibresprovidinghigherfibresurfaceareas,spreadingthefibre/matrixinterfacialloads.Asageneralrule,thestiffnessandstrengthofalaminatewillincreaseinproportiontotheamountoffibrepresent.However,aboveabout60-70%FVF(dependingonthewayinwhichthefibrespacktogether)althoughtensilestiffnessmaycontinuetoincrease,thelaminate’sstrengthwillreachapeakandthenbegintodecreaseduetothelackofsufficientresintoholdthefibrestogetherproperly.

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

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5.2.1BasicPropertiesofFibresandOtherEngineeringMaterials

MaterialType TensileStr. TensileModulus TypicalDensity 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

Thesegraphsshowthestrengthsandmaximumstrainsofthedifferentcompositesat failure. Thegradientofeachgraphalso indicates thestiffness (modulus)of thecomposite;thesteeperthegradient,thehigheritsstiffness.Thegraphsalsoshowhowsomefibres,suchasaramid,displayverydifferentpropertieswhen loaded incompression,comparedwithloadingintension.

5.2.1.2LaminateImpactStrength

Figure25-ComparisonofLaminateImpactStrength

Impactdamagecanposeparticularproblemswhenusinghighstiffnessfibresinverythinlaminates.Insomestructures,wherecoresareused,laminateskinscanbelessthan0.3mmthick.Althoughotherfactorssuchasweavestyleandfibreorientationcansignificantlyaffectimpactresistance,inimpact-criticalapplications,carbonisoftenfoundincombinationwithoneoftheotherfibres.Thiscanbeintheformofahybridfabricwheremorethanonefibretypeisusedinthefabricconstruction.Thesearedescribedinmoredetaillater.

TensilePropertiesofU/DPrepregLaminate CompressivePropertiesofU/DPrepregLaminates

0

50

100

150

200

250

300

E-Glass S-GlassYarn 6781

Aramid HS Carbon

Impa

ctS

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

calC

ost

of~

300g

/m2 W

oven

Fab

ric(£

/m2 )

25

E-Glass1200 tex

S-Glass410 tex

Aramid 252 tex

HS CarbonT700-12k

IM CarbonT800-12k

Typi

calC

ost

of~

300g

/m2 U

/D(£

/m2 )

0

5

10

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).Goodtensile andcompressive strength andstiffness, goodelectrical properties andrelativelylowcost,butimpactresistancerelativelypoor.DependingonthetypeofEglassthepricerangesfromabout£1-2/kg.E-glassisthemostcommonformofreinforcingfibreusedinpolymermatrixcomposites.

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

c. R,SorT-glass–manufacturers tradenamesforequivalentfibreshavinghighertensile strengthandmodulus thanEglass,withbetterwetstrength retention.HigherILSSandwetoutpropertiesareachievedthroughsmallerfilamentdiameter.S-glassisproducedintheUSAbyOCF,R-glassinEuropebyVetrotexandT-glassbyNittoboinJapan.Developedforaerospaceanddefenceindustries,andusedinsomehardballisticarmourapplications.Thisfactor,andlowproductionvolumesmeanrelativelyhighprice.DependingonthetypeofRorSglassthepricerangesfromabout£12-20/kg.

EGlassfibreisavailableinthefollowingforms:

a. strand-acompactlyassociatedbundleoffilaments.Strandsarerarelyseencommerciallyandareusuallytwistedtogethertogiveyarns.

b. yarns - a closely associatedbundleof twistedfila-mentsorstrands.Eachfilamentdiameterinayarnisthesame,andisusuallybetween4-13µm.Yarnshavevaryingweightsdescribedbytheir‘tex’(theweightingrammesof1000linearmetres)ordenier(theweightinlbsof10,000yards),withthetypicaltexrangeusuallybeingbetween5and400.

c. rovings-alooselyassociatedbundleofuntwistedfilamentsorstrands.Eachfila-mentdiameterinarovingisthesame,andisusuallybetween13-24µm.Rovingsalsohavevaryingweightsandthetexrangeisusuallybetween300and4800.Wherefilamentsaregatheredtogetherdirectlyafter themeltingprocess, theresultantfibrebundleisknownasadirectroving.Severalstrandscanalsobebroughttogetherseparatelyaftermanufactureoftheglass,togivewhatisknownasanassembledroving.

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

Itisalsopossibletoobtainlongfibresofglassfromshortfibresbyspinningthem.Thesespunyarnfibreshavehighersurfaceareasandaremoreabletoabsorbresin,buttheyhavelowerstructuralpropertiesthantheequivalentcontinuouslydrawnfibres.

5.2.2.2AramidAramidfibreisaman-madeorganicpolymer(anaromaticpolyamide)producedbyspinningasolidfibrefromaliquidchemicalblend.Thebrightgoldenyellowfilamentsproducedcanhavearangeofproperties,butallhavehighstrengthandlowdensitygivingveryhighspecificstrength.Allgradeshavegoodresistancetoimpact,andlowermodulusgradesareusedextensivelyinballisticapplications.Compressivestrength,however,isonlysimilartothatofEglass.

Althoughmostcommonlyknownunder itsDupont tradename‘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,carbonisation and graphitisation of carbon-rich organicprecursors which are already in fibre form. The mostcommonprecursor ispolyacrylonitrile (PAN),because itgivesthebestcarbonfibreproperties,butfibrescanalsobemadefrompitchorcellulose.Variationofthegraphi-tisationprocessproduceseitherhighstrengthfibres(@~2,600°C)orhighmodulusfibres(@~3,000°C)withothertypesinbetween.Onceformed,thecarbonfibrehasasurfacetreatmentappliedtoimprovematrixbondingandchemicalsizingwhichservestoprotectitduringhandling.

Whencarbonfibrewasfirstproducedinthelatesixtiesthepriceforthebasichighstrengthgradewasabout£200/kg.By1996theannualworldwidecapacityhadin-creasedtoabout7,000tonnesandthepricefortheequivalent(highstrength)gradewas£15-40/kg.Carbonfibresareusuallygroupedaccordingtothemodulusbandinwhichtheirpropertiesfall.Thesebandsarecommonlyreferredtoas:highstrength(HS),intermediatemodulus(IM),highmodulus(HM)andultrahighmodulus(UHM).

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Thefilamentdiameterofmosttypes isabout5-7µm.Carbonfibrehasthehighestspecificstiffnessofanycommerciallyavailablefibre,veryhighstrengthinbothten-sionandcompressionandahigh resistancetocorrosion,creepandfatigue. Theirimpactstrength,however,islowerthaneitherglassoraramid,withparticularlybrittlecharacteristicsbeingexhibitedbyHMandUHMfibres.

StrengthandModulusFiguresforCommercialPAN-basedCarbonFibres:

Grade TensileModulus TensileStrength Country (GPa) (GPa) ofManufacture

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.Itslackofstiffnessusuallyprecludesitfrominclusioninacompositecomponent,butitisusefulwherelowweight,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

Averyhighsilicaversionofglasswithmuchhighermechanicalpropertiesandexcel-lentresistancetohightemperatures(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.Withcarbonandaramidfibresforuseincompositeapplications,thesurfacefinishorsizeappliedusuallyperformsbothfunctions.Thefinishisappliedtothefibreatthepointoffibremanufac-tureandthisfinishremainsonthefibrethroughouttheconversionprocessintofabric.Withglassfibrethereisachoiceofapproachinthesurfacefinishthatcanbeapplied.

GlassFibreFinishes

Glassfibrerovingsthataretobeusedindirectfibreproc-essessuchasprepregging,pultrusionandfilamentwind-ing,aretreatedwitha‘dual-function’finishatthepointoffibremanufacture.

Glass fibre yarns, however, when used for weaving aretreated in two stages. The first finish is applied at thepointoffibremanufactureatquiteahighlevelandispurelyforprotectionofthefibreagainstdamageduringhandlingandtheweavingprocessitself.Thisprotectivefinish,whichisoftenstarchbased,iscleanedoffor‘scoured’aftertheweavingprocesseitherbyheatorwithchemicals.Thescouredwovenfabricisthenseparatelytreatedwithadifferentmatrix-compatiblefinishspecificallydesignedtooptimisefibretoresininterfacialcharacteristicssuchasbondstrength,waterresistanceandopticalclarity.

CarbonFibreFinishes

Finishes,orsizes,forcarbonfibresusedinstructuralcompositesaregenerallyepoxybased,withvaryinglevelsbeinguseddependingontheenduseofthefibre.Forweav-ingthesizelevelisabout1-2%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.

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

5.3.1UnidirectionalFabrics

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

Trueunidirectionalfabricsoffertheabilitytoplacefibreinthecomponentexactlywhereitisrequired,andintheoptimumquantity(nomoreorlessthanrequired).Aswellasthis,UDfibresarestraightanduncrimped.Thisresultsinthehighestpossiblefibrepropertiesfromafabricincompositecomponentconstruction.Formechanicalprop-erties,unidirectionalfabricscanonlybeimprovedonbyprepregunidirectionaltape,wherethereisnosecondarymaterialatallholdingtheunidirectionalfibresinplace.Intheseprepregproductsonlytheresinsystemholdsthefibresinplace.

5.3.1.1UnidirectionalConstruction

Therearevariousmethodsofmaintainingtheprimaryfibresinpositioninaunidirec-tional includingweaving, stitching, andbonding.Aswithother fabrics, 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’sintegrityismaintainedbythemechanicalinterlockingofthefibres.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.Superiorwetoutanddrapeisseeninthetwillweaveover theplainweavewithonlyasmall reduction instability.Withreducedcrimp,thefabricalsohasasmoothersurfaceandslightlyhighermechanicalproperties.

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5.3.3.3 SatinSatinweavesarefundamentallytwillweavesmodifiedtoproducefewerintersectionsofwarpandweft.The‘harness’numberusedinthedesignation(typically4,5and8)isthetotalnumberoffibrescrossedandpassedunder,beforethefibrerepeatsthepattern.A‘crowsfoot’weaveisaformofsatinweavewithadifferentstaggerintherepeatpattern.Satinweavesareveryflat,havegoodwetoutandahighdegreeofdrape.Thelowcrimpgivesgoodmechanicalprop-erties.Satinweavesallowfibrestobewovenintheclosestproximityandcanproducefabricswithaclose‘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 LenoLenoweaveimprovesthestabilityin‘open’fabricswhichhavealowfibrecount.

Aformofplainweave inwhichadjacentwarpfibresaretwistedaroundconsecutiveweftfibrestoformaspiralpair,effectively‘locking’eachweftinplace.Fabricsinlenoweavearenormallyusedinconjunctionwithotherweavestylesbecauseifusedalonetheiropennesscouldnotproduceaneffectivecompositecomponent.

5.3.3.6MockLenoAversionofplainweaveinwhichoccasionalwarpfibres,atregularintervalsbutusuallyseveralfibresapart,deviatefromthealternateunder-overinterlacingandinsteadinter-laceeverytwoormorefibres.Thishappenswithsimilarfrequencyintheweftdirection,andtheoveralleffectisafabricwithincreasedthickness,roughersurface,andad-ditionalporosity.

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5.3.3.7 WovenGlassYarnFabricsvsWovenRovings

Yarn-based fabricsgenerallygivehigherstrengthsperunitweight than roving,andbeinggenerallyfiner,producefabricsatthelighterendoftheavailableweightrange.Wovenrovingsarelessexpensivetoproduceandcanwetoutmoreeffectively.How-ever,sincetheyareavailableonlyinheaviertexes,theycanonlyproducefabricsatthemediumtoheavyendoftheavailableweightrange,andarethusmoresuitableforthick,heavierlaminates.

5.3.3.8 Stitched0/90oFabrics

0/90°fabricscanalsobemadebyastitchingprocess,whicheffectivelycombinestwolayersofunidirectionalmaterialintoonefabric.Stitched0/90°fabricscanoffermechani-calperformanceincreasesofupto20%insomepropertiesoverwovenfabrics,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.Bothfibreshavelowdensitybutrelativelyhighcost.

Aramid/Glass

Thelowdensity,highimpactresistanceandtensilestrengthofaramidfibrecombineswiththegoodcompressiveandtensilestrengthofglass,coupledwithitslowercost.

Carbon/Glass

Carbonfibrecontributeshightensilecompressivestrengthandstiffnessandreducesthedensity,whileglassreducesthecost.

5.3.4MultiaxialFabricsInrecentyearsmultiaxialfabricshavebeguntofindfavourintheconstructionofcom-positecomponents.Thesefabricsconsistofoneormorelayersoflongfibresheldinplacebyasecondarynon-structuralstitchingtread.Themainfibrescanbeanyofthestructuralfibresavailableinanycombination.Thestitchingthreadisusuallypolyesterduetoitscombinationofappropriatefibreproperties(forbindingthefabrictogether)andcost.Thestitchingprocessallowsavarietyoffibreorientations,beyondthesim-ple0/90°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.

Tomake aquadraxial (four-layer:+45°, 0°, 90°, -45°) fabric by thismethod, aweftunidirectionalwouldbewovenandskewedinonedirectiontomakethe+45°layer,andintheothertomakethe-45°layer.The0°and90°layerswouldappearasasinglewovenfabric.Thesethreeelementswouldthenbestitchedtogetheronastitchingframetoproducethefinalfour-axisfabric.

5.3.4.2 SimultaneousStitch

Simultaneous stitch manufacture is carried out on special machines based on theknittingprocess,suchasthosemadebyLiba,Malimo,Mayer,etc.Eachmachinevar-iesintheprecisionwithwhichthefibresarelaiddown,particularlywithreferencetokeepingthefibresparallel.

Thesetypesofmachinehaveaframewhichsimultaneouslydrawsinfibresforeachaxis/layer, until the required layers have been assembled, and then stitches themtogether,asshowninthediagrambelow.

WeftUnidirectional

Layer1

Layer2

Skewto45oStitchtheskewed

layerstogether

-45o

+45o

±45ofabric

CourtesyLibaMaschinenfabrickGMBH

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5.3.5 OtherFabrics

5.3.5.1 ChoppedStrandMat

Choppedstrandmat(CSM)isanon-wovenmaterialwhich,asitsnameimplies,con-sistsofrandomlyorientatedchoppedstrandsofglasswhichareheldtogether-formarineapplications-byaPVAemulsionorapowderbinder.DespitethefactthatPVAimpartssuperiordrapinghandlingandwettingoutcharacteristicsusers inamarineenvironmentshouldbewaryofitsuseasitisaffectedbymoistureandcanleadtoosmosislikeblisters.

Today,choppedstrandmatisrarelyusedinhighperformancecompositecomponentsasitisimpossibletoproducealaminatewithahighfibrecontentand,bydefinition,ahighstrength-to-weightratio.

5.3.5.2 Tissues

Tissuesaremadewithcontinuousfilamentsoffibrespreaduniformlybutrandomlyover a flat surface.These are then chemically bound together with organic basedbindingagentssuchasPVA,polyester,etc.Havingrelativelylowstrengththeyarenotprimarilyusedasreinforcements,butassurfacinglayersonlaminatesinordertoprovideasmoothfinish.Tissuesareusuallymanufacturedwithareaweightsofbetween5and50g/sqm.Glasstissuesarecommonlyusedtocreateacorrosionresistantbar-rierthroughresinenrichmentatthesurface.Thesameenrichmentprocesscanalsopreventprint-throughofhighlycrimpedfabricsingelcoatsurfaces.

5.3.5.3 Braids

Braidsareproducedbyinterlacingfibresinaspiralnaturetoformatubularfabric.Thediameterofthetubeiscontrolledbythenumberoffibresinthetube’scircumference,theangleofthefibres inthespiral,thenumberof intersec-tionsoffibreperunitlengthofthetubeandthesize(tex)ofthefibresintheassembly.Theinterlacingcanvaryinstyle(plain,twill,etc.)aswith0/90°wovenfabrics.Tubediameter isnormallygivenforafibreangleof±45°butthebraidingprocessallowsthefibrestomovebetween anglesofabout25°and75°,dependingonthenumberandtexofthefibres.Thenarrowanglegivesasmalldiameterwhereasthewideranglegivesalargediameter.Thereforealongthelengthofonetube it ispossibletochangethediameterbyvariationofthefibreangle -asmallerangle(relativetozero)givingasmallerdiameterandviceversa.Braidscanbefoundinsuchcompositecomponentsasmasts,antennae,driveshaftsandothertubularstructuresthatrequiretorsionalstrength.

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5.4 Corematerialsforsandwichconstruction

5.4.1FoamCores

Foamsareoneofthemostcommonformsofcoremate-rial.Theycanbemanufacturedfromavarietyofsyntheticpolymers includingpolyvinylchloride(PVC),polystyrene(PS), polyurethane (PU), polymethyl methacrylamide(acrylic), polyetherimide (PEI) and styreneacrylonitrile(SAN). Theycanbesupplied indensities ranging fromlessthan30kg/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, theyhavesome lowermechanicalpropertiesthananequivalentdensity of cross-linked PVC, and a lower resistance to elevated temperatures andstyrene.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

Foragivendensity,polymethylmethacrylamide(acrylic)foamssuchasRohacellof-fersomeofthehighestoverallstrengthsandstiffnessesoffoamcores.Theirhighdimensionalstabilityalsomakesthemuniqueinthattheycanreadilybeusedwithconventional elevated temperature curing prepregs. However, they are expensive,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,whichhelpsin themanufactureofcurvedparts.Heat-stabilisedgradesofSAN foamscanalsobemoresimplyusedwithlow-temperaturecuringprepregs,sincetheydonothavetheinterferingchemistryinherentinthePVC’s.TypicalSANproductsincludeGurit’sCorecell®A-seriesfoams.

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5.4.1.6Otherthermoplastics

Asnewtechniquesdevelopfortheblowingoffoamsfromthermoplastics,therangeofexpandedmaterialsofthistypecontinuestoincrease.TypicalisPEIfoam,anexpandedpolyetherimide/polyethersulphone,whichcombinesoutstandingfireperformancewithhighservicetemperature.Althoughitisexpensive,thisfoamcanbeusedinstructural,thermalandfireprotectionapplicationsintheservicetemperaturerange-194°C(-320°F)to+180°C(+355°F).Itishighlysuitableforaircraftandtraininteriors,asitcanmeetsomeofthemoststringentfireresistantspecifications.

5.4.2Honeycombs

Honeycombcoresareavailableinavarietyofmaterialsforsandwichstructures.Theserangefrompaperandcardforlowstrengthandstiffness,lowloadapplications(suchas domestic internal doors) to high strength and stiffness, extremely lightweightcomponentsforaircraftstructures.Honeycombscanbeprocessedintobothflatandcurvedcompositestructures,andcanbemadetoconformtocompoundcurveswithoutexcessivemechanicalforceorheating.

Thermoplastichoneycombsareusuallyproducedbyextrusion,followedbyslicingtothickness.Otherhoneycombs(suchasthosemadeofpaperandaluminium)aremadebyamulti-stageprocess. In thesecases largethinsheetsof thematerial (usually1.2x2.4m)areprintedwithalternating,parallel,thinstripesofadhesiveandthesheetsarethenstackedinaheatedpresswhiletheadhesivecures.Inthecaseofaluminiumhoneycombthestackofsheetsisthenslicedthroughitsthickness.Theslices(knownas‘blockform’)arelatergentlystretchedandexpandedtoformthesheetofcontinu-oushexagonalcellshapes.

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

Inbothcases,byvaryingthedegreeofpullintheexpansionprocess,regularhexagon-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.

Honeycombcorescangivestiffandverylightlaminatesbutduetotheirverysmallbondingareatheyarealmostexclusivelyusedwithhigh-performanceresinsystemssuchasepoxiessothatthenecessaryadhesiontothelaminateskinscanbeachieved.

5.4.2.1 Aluminiumhoneycomb

Aluminiumhoneycombproducesoneof thehighest strength/weight ratiosof anystructuralmaterial.Therearevariousconfigurationsoftheadhesive-bondingofthealuminiumfoilwhichcanleadtoavarietyofgeometriccellshapes(usuallyhexago-nal).Propertiescanalsobecontrolledbyvaryingthefoilthicknessandcellsize.Thehoneycombisusuallysuppliedintheunexpandedblockformandisstretchedoutintoasheeton-site.

Despiteitsgoodmechanicalpropertiesandrelativelylowprice,aluminiumhoneycombhastobeusedwithcautioninsomeapplications,suchaslargemarinestructures,becauseofthepotentialcorrosionproblemsinasalt-waterenvironment.Inthissitu-ationcarealsohastobeexercisedtoensurethatthehoneycombdoesnotcomeintodirectcontactwithcarbonskinssincetheconductivitycanaggravategalvaniccorrosion.Aluminiumhoneycombalsohastheproblemthatithasno‘mechanicalmemory’.Onimpactofacoredlaminate,thehoneycombwilldeformirreversiblywhereastheFRPskins,beingresilient,willmovebacktotheiroriginalposition.Thiscanresultinanareawithanunbondedskinwithmuchreducedmechanicalproperties.

5.4.2.2 Nomexhoneycomb

NomexhoneycombismadefromNomexpaper-aformofpaperbasedonKevlar™,ratherthancellulosefibres.Theinitialpaperhoneycombisusuallydippedinaphenolicresintoproduceahoneycombcorewithhighstrengthandverygoodfireresistance.Itiswidelyusedforlightweightinteriorpanelsforaircraftinconjunctionwithphenolicresinsintheskins.Specialgradesforuseinfireretardantapplications(egpublictrans-portinteriors)canalsobemadewhichhavethehoneycombcellsfilledwithphenolicfoamforaddedbondareaandinsulation.

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

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Figure28–Comparativecorecosts

5.4.2.3 Thermoplastichoneycomb

Corematerialsmadeofotherthermoplasticsarelightinweight,offeringsomeusefulpropertiesandpossiblyalsomakingforeasierrecycling.Theirmaindisadvantageisthedifficultyofachievingagood interfacialbondbetweenthehoneycombandtheskinmaterial,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.Balsawoodcoresfirstappearedinthe1940’sinflyingboathulls,whichwerealuminiumskinnedandbalsa-coredtowithstandtherepeatedimpactoflandingonwater.Thisperformanceledthemarineindustrytobeginusingend-grainbalsaasacorematerialinFRPconstruction.

Apartfromitshighcompressiveproperties,itsadvantagesincludebeingagoodthermalinsulatorofferinggoodacousticabsorption.Thematerialwillnotdeformwhenheated

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andactsasaninsulatingandablativelayerinafire,withthecorecharringslowly,allow-ingthenon-exposedskintoremainstructurallysound.Italsoofferspositiveflotationandiseasilyworkedwithsimpletoolsandequipment.

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,otherfactors,besidesdensity,alsocomeintoplaywhenlookingattheweightofacoreinasandwichstructure.Forexample,lowdensityfoammaterials,whilecontributingverylittletotheweightofasandwichlaminate,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) Only short fibres are incorporated which severely limits the mechanical propertiesofthelaminate.

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) Limitingairbornestyreneconcentrationstolegislatedlevelsfrompolyestersandvinylestersisbecomingincreasinglyhardwithoutexpensiveextractionsystems.

iv) Resins need to be low in viscosity to be workable by hand. This generallycompromisestheirmechanical/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:Thevacuumbagreducestheamountofvolatilesemittedduringcure.

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.Theprocesspullsthematerialsthroughthediefor impregnation,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.ThisisknownasVacuumAssistedResinInjection(VARI).Onceallthefabric iswetout,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.Thefibrestackisthencov-eredwithpeelplyandaknittedtypeofnon-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.Thecatalystislargelylatentatambi-enttemperaturesgivingthematerialsseveralweeks,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/catalystlevelsandtheresincontentinthefibreareaccuratelysetbythemateri-alsmanufacturer.Highfibrecontentscanbesafelyachievedwithlowvoidcontents.

ii) Thematerialshaveexcellenthealthandsafetycharacteristicsandarecleantoworkwithandhavethepotentialforautomationandlaboursaving.

iii) Fibrecostisminimisedinunidirectionaltapessincethereisnosecondaryprocesstoconvertfibreintofabricpriortouse.

iv) Resinchemistrycanbeoptimisedformechanicalandthermalperformance,withthehighviscosityresinsbeingimpregnableduetothemanufacturingprocess.

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

vi) Potentialforautomationandlaboursaving.

MainDisadvantages:

i) Materials cost is higher for preimpregnated fabrics but for these applicationsexpensiveadvancedresinsareoftenrequired.

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

LowTemperatureCuringprepregsaremadeexactlyasconventionalautoclaveprepregsbuthaveresinchemistriesthatallowcuretobeachievedattemperaturesfrom60-120°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, canbeuseddue to the lower curetemperaturesinvolved.

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

iv) Conventionalfoamcorematerialscanbeused,providingcertainproceduresarefollowed.

v) Lowerenergycostthanautoclaveprocess.

vi) Robustprocessprovidingahighlevelofdimensiontoleranceandrepeatability.

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MainDisadvantages:

i) Materialscostisstillhigherthanfornon-preimpregnatedfabricsalthoughresincostsarelowerthanthoserequiredforaerospaceapplications.

ii) ToolingneedstobeabletowithstandhighertemperaturesthanInfusionProcesses.(typically80-140°C).

TypicalApplications:

High-performancewind-turbineblades,largeracingandcruisingyachts,rescuecraft,traincomponents.

6.10SPRINT®/SparPregTM-“OutofAutoclave”

Description

Whenusingprepregmaterialsinthicklaminates(>3mm)itbecomesdifficulttoremoveentrappedairbetweenplies,andaroundlaminatedetailslikeplyoverlaps,duringthecuringprocess.Toovercomethisproblemintraditionalprepregmaterialsanumberofwarmdebulkingstagesareintroducedduringlay-uptoremovetrappedairwhichsig-nificantlyincreasesmanufacturingtimes.InrecentyearsGurithaspatentedanumberofmodifiedprepregproductsthatenablethemanufactureofhighquality(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) Materialscost isstillhigherthanfornon-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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7.1.1 Mechanicalinterlocking

Many users of adhesives will intuitively understand the importance of mechanicalinterlocking.Often referred toas“keying”asurfaceprior tobonding,paintersandbondershaveimprovedadhesionbyusingmechanicalabrasionofsomeformformanyyears.Thebenefitsofthissurfacepreparationincluderemovalofsurfacecontaminantsandincreasingtheeffectivesurfaceareaforbondingduetosurfacetopographyandroughness.

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

7.1.2 Diffusiontheory

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

Thediffusiontheorycanbeusedtoexplaintheauto-adhesionofpolymers,andsolventweldingofamorphouspolymers,buthasnoplaceintheconsiderationofadhesionbetweenorganicandinorganicmaterialphases.

7.1.3 Electronictheory

ThebasisoftheElectronictheoryisthatiftwomaterialswithdifferentelectronbandstructuresarebroughtintointimatecontact,therewillsometransferofelectrons,lead-ingtoelectronicinteractionofthemolecules.Thereismuchcontroversysurroundingthistheoryanditisnotgenerallyacceptedastheprimereasonforadhesion.

7.1.4 Adsorptiontheory

The adsorption theory states that provided intimate contact is obtained betweensubstrate and adhesive, adhesion will occur because of intermolecular forces thatoccurbetweenatoms,moleculesandionsofthetwophases.ThereareavarietyofbondtypesthatcanoccurbuthemostcommonisthoughttobeVanderWaalsforces.

7.1.5 WeakBoundaryLayers

Theconceptofa“proper”jointisbasedoncohesivefailureoftheadhesiveorsubstrate.Anyjointthatfailsinterfaciallycanbeconsideredtobeimproper.Suchfailuresaresaidtobeduetoa“weakboundarylayer”.

The simplestWBL is thepresence of a layer of contamination (eggrease) on thesurfaceofasubstrate.HoweverWBLscanbeduetomanysourcessuchasincorrectpre-treatmentprocesses,segregationofcomponentswithintheadhesiveorsubstrate,orthedevelopmentofaweakenedlayerduetoexposuretoenvironmentalattack.

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7.2 Pre–treatmentpriortobonding

AdequatepreparationofsurfacesforbondingisCRITICALtoachievinganimmediatebond,andalso,moreimportantly,alongterm,durablebond.

Thesubjectofsurfacepreparationisamassiveone,andthemostimportantconceptto understand is that surface preparation techniques must be validated with eachsubstrateandadhesivecombinationpriortouseonanycomponent.

Techniquescaninclude,butarenotlimitedto:

Degreasing

■ Soap+water

■ Pressurewashing

■ Ultrasoniccleaning

■ Solventwipe

■ Vapourdegreasing

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

Adhesivesmustbeselectedthatarecompatiblewithbothadherendsandmeetperform-anceanddesignrequirements.Thechoiceofadhesivetypesisvastbutforstructuralapplicationstheycanbebrokendownbychemistrygroup.

7.3.1 Epoxiesandtoughenedepoxies

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.

Epoxy adhesives areoftenused for aircraftmanufacturing, for partswhich requirehighmodulus,highstiffness(forinstanceflightcontrolsurfaces:flaps,rudders),forleadingandtrailingedges,enginenacelles,bondingofsandwichpanels,stringersandstiffenersonthefuselage,etc.

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

7.3.2 Polyurethanes

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

ThesePUadhesivescanbeonecomponentcured(withmoisture)or2componentscured(atroomtemperatureorlowtemperatures).Pricesaregenerallylowerthanepoxies.

Currentapplicationsincludethebondingofpanelsformasstransportationequipment(buses),bondingofhatchesanddoorsforautomotives,constructionsandwichpanels,andFRPboats.Forexample,trams,trucksandrefrigeratedtruckbodiesarenowmadewithastructuralsteeloraluminiumframeonwhichGRPexteriorpanelsarebonded,usuallywithPU2componentsadhesives.

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7.3.3 Acrylicsandmethacrylatesstructuraladhesives

2componentsadhesives,withvariousmixingratios,fastcuringatroomtemperature(from10minutesto2hours),highshearresistance(10to30MPa),excellentadhesiontoalmostallplasticsandcomposites,highimpactstrengthandfatigueresistance(widelyusedforboatassemblywhichmustresisttoveryfrequentshocksonthewaves),goodresistancetowaterandchemicals,mediumprice.

Theseadhesivesaredevelopingfast,theymayreplacethepolyesteradhesivesandsomeepoxieswhenamoreflexiblebondisrequired.

7.3.4 Polyesters

Polyesterresinmanufacturersareofferingsomeimprovedpolyesteradhesives,whichreactaftertheadditionofacatalyst.

Theseadhesivesaremostlyusedforbondingreinforcedpolyesterpartsinboatcon-struction,becausethisindustryisaccustomedtomakereinforcedpolyesterparts.Theyarerigid,maybebrittle,buttheyhaveverygoodgapfillingproperties.

Shearresistanceisapproximately10MPa.Theymaybeusedfordecktohullassembly.Theyaregenerallylowcost.

7.3.5 Urethane-acrylates

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

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

Theyaremostlyusedforboatconstructionandbondingofautomotivebodyparts.

7.3.6 Heatstableadhesives:Bismaleimides,polyimides,cyanateesters

Theseareexpensiveadhesives,mostlydeliveredinfilms,whichresisttohightem-peratures,forinstanceservicetemperaturesof250to300°Cforpolyimides,200to250°Cforthebismaleimides,Theyaredifficulttousebecausetheyrequirecuringunderhightemperatures(300°C)andhighpressures(15bars)–thereforeautoclaveorpressprocessingisarequisite.

Usuallyonlyusedinaerospaceapplicationstobondpartswhichareresistanttohightemperatures(forinstancesomepartsofthesurfaceofmilitarysupersonicaircraftsmayreach260°Cduringsomeflightconditions).

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7.3.7 Anaerobicsandcyanoacrylates

Maybeusedtobondsmallpartsmadeofthermoplasticpolymers,formechanicalpartsandappliances.Butcyanoacrylatesmaybebrittle.Anaerobicshavegoodadhesiontomanythermoplasticsandthermosets.

7.4 Jointdesign

Inadditiontocorrectsurfacepreparationandmaterialselection,itiscriticallyimportantthatthejointiscorrectlydesignedandmanufactured.

Themainconsiderationwhendesigningbondedjointsisloadtransferpaths.Adhesivesworkwellincompressionandshearloadingbutarenotgenerallygoodintensileloadsituations.Application of peel loads should be minimised, and avoided completelywhereverpossible.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,ofhardenerinanepoxyresin.Generallypolyesterresinsproduceamoresevereexothermandafasterdevelopmentofinitialmechanicalpropertiesthanepoxiesofasimilarworkingtime.

Withbothresintypes,however,itispossibletoacceleratethecurebytheapplicationofheat,sothatthehigherthetemperaturethefasterthefinalhardeningwilloccur.Thiscanbemostusefulwhenthecurewouldotherwisetakeseveralhoursorevendaysatroomtemperature.Aquickruleofthumbfortheacceleratingeffectofheatonaresinisthata10°Cincreaseintemperaturewillroughlydoublethereactionrate.Thereforeifaresingelsinalaminatein25minutesat20°Citwillgelinabout12minutesat30°C,providingnoextraexothermoccurs.Curingatelevatedtemperatureshastheaddedadvantagethatitactuallyincreasestheendmechanicalpropertiesofthematerial,andmany resinsystemswill not reach theirultimatemechanicalpropertiesunless theresinisgiventhis‘postcure’.

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

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

9. Inspectionandtesting

Sincetherawmaterial iseffectivelyprocessedatthesametimeasthecompositecomponent,therearemanyaspectswhichrequirevalidationtoensurequalityoffinishedparts.Assumingthemanufacturingprocess,componentdesignandmaterialselectionhavebeenadequatelyandcorrectlyspecified,visualandnon-destructivetestingformthefirststageofqualitycontrol.

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9.1 Nondestructivetesting

9.1.1Visualinspection

Delaminations,misplacedplies,excessresinbleed,dryfabric,porosity,toolingmarksandsurfacedefectsareamongst themanydefectswhichcaneasilybeobservedbyvisualexamination.Inadditiontogeometricanddimensionalchecks,visualchecksofcompositequalitycanofteneasilyidentifyprocessingerrorsormaterialqualityproblems.

Sometimesdefectswillnotbevisibletothenakedeye,orwillbewithinthebulkofacomponentsomorecomplexnondestructiveexaminationisrequired:

9.1.2Taptesting

Tap-testingisacommonmethodfordeterminingvoidsanddefectsincuredlaminatesduetoit’scheap,efficientnature.Themethodinvolvestappingthesurfaceofthelami-natewithahardobject,usuallyacoin,andlisteningforachangeinthepitch/volumeofthenoiseproduced.

Advantages

■ Cheap,simpleandquicktechnique

Disadvantages

■ Methodreliesheavilyonuser

■ Uncured materialswon’t have thenecessary properties to transmit noise/with standtheforceofimpact

9.1.3Ultrasonics

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

Advantages

■ Abletodetectawiderangeofdefects

■ Cangiveinformationof3-Dlocationofdefectwithinthelaminate

■ Canbeslow-resolutiondependentonspeed

Disadvantages

■ Levelofskillrequiredtoaccuratelyinterpret

■ Unsuitableforuseonuncuredmaterial

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9.1.4ComputedTomography

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

Advantages

■ 3-Dimagegivesaccurateinformationaboutdefectsize/location

■ Highresolutionofpartachievable

Disadvantages

■ Verycostlyandtime-consuming

■ Health&safetyconsiderations

9.1.5Shearography

Inshearography,aspecimenisdeformedbyanexternallyappliedforce,andthere-sultingout-of-planedisplacementsmeasuredusinglaserspeckleinterferometry.Theexpecteddeformationpatternisknownfromtheboundaryconditionsplaceduponthespecimenbytheloadingconfiguration,withanydifferencesfromthisbeingaresultofdamageinthespecimen.

Advantages

■ Effectiveinlocatingdebondedareas

■ Givesaccurateinformationonthesizeandshapeofthedefect

Disadvantages

■ Willonlydetectdifferencesindisplacements

■ Theequipmentisexpensive

9.1.6Thermography

Inthermography,thesampleisheated,usuallyviaaflashgunorsimilar,andaninfraredcamerausedtoviewthethermalconductivityofthepart.Theresponsewillbediffer-entwheretherearedefectspresentcomparedtothoseareaswheretherearenot.

Advantages

■ Themethodisquicktoimplementandcancoverlargeareas

Disadvantages

■ Defectsatadepthgreaterthan3xtheirdiametercannotbedetected

■ Theequipmentisexpensive

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9.1.7LambWaveSensing

LambwavesensinginvolvespropagatingLambwaves(aformofelasticwavesabletopropagatethroughasolidmaterial)throughasample,andusingatransducertopickuptheirsignal.Theamplitudeofthesignalisusedtodeterminethedamagepresentinthepart.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)and thepre-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 isveryimportanttoconsiderwhattestmethodtouse,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

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

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.ydg/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.ftkg/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.ing/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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