一 : epoxy adhesive

journalofmaterialsprocessingtechnology205(2008)

article

Articlehistory:

infoabstract

Theobjectiveofthisstudywastodevelopinformationonthein?uenceofadhesivethicknessandaluminum?llercontentonthemechanicalperformanceofaluminumjointsbondedbyaluminumpowder?lledepoxy.Theadhesivestrengthofthejointswasdeterminedbyuti-lizingthesingle-lapsheartest.Thein?uenceofadhesivethicknessandaluminum?llercontentonstressdistributionwithintheadhesivewasalsoanalyzedby?niteelementmethod(FEM).BothFEManalysisandtheexperimentalinvestigationshowthatingen-eraladhesionstrengthdecreasesasthethicknessoftheadhesiveincreases.Itisobserved

Received21May2007Receivedinrevisedform26October2007

Accepted14November2007

Keywords:AdhesiveEpoxyMetal?llerAluminumJointsThickness

fromthepredictions(FEMsimulations)thatthestresslevelincreasesattheadhesive–metalsubstrateinterfaceasthealuminum?llercontentintheadhesiveincreases.Experimentalresultsshowthatepoxyadhesiveretainsitsstrengthuptothe50wt%aluminum?llercon-tent.Thejointsfailincohesivemode(failurewithintheadhesive)duetothehighstresslevelsgeneratedintheadhesive,whichindicatesthattheadhesiontothemetalsurfaceisstrongerthanthatoftheinteriorpartoftheadhesive.

?2007ElsevierB.V.Allrightsreserved.

1.Introduction

Epoxiesarewidelyusedashigh-performancestructuraladhe-sives,especiallyinautomotiveandaircraftmanufacture.Epoxyresinsareattractiveformetal-bondingadhesivesys-temsbecauseoftheirabilitytocurewithoutproducingvolatileby-productsandtheirlowshrinkageuponcuring(lessthan0.5%)(TaiandSzklarska-Smialowska,1993).Epoxiesareabletobondwelltoavarietyoftreatedoruntreatedmetalsurfaces(Mohan,1990).Inaircraftmanufacture,thereisagreatneedforevenlystressed,smoothbondingofthinaluminumsheetandhoneycombmaterials.Epoxyadhesiveshaveagoodaf?n-ityforaluminumalloysurfaces,andtheoxidelayersproducedduringsurfacepreparation(Chasseretal.,1993).

Tobeabletoobtainastrongandstablebondbetweenthemetalandtheadhesive,thenaturalsurfaceoxideshouldberemovedandreplacedwithanew,continuous,solid,corrosionresistantoxidelayer.Theremovalcanbedonemechani-callyand/orchemically.Mechanical(abrasive)cleaningalsoincreasesthesurfaceroughnessand,consequently,thebondstrengthbymechanicalinterlockingandbytheincreasednumberofchemicalbondsonthelargersurfacearea.Vari-ouschemicaltreatments,themostcommonbeingacidetches,havebeendevelopedtomodifytheoxide,torenderitmorereceptivetobonding(KozmaandOlefjord,1987a,b;Brewis,1985;HoggandJanardhana,1993;Kinloch,1983).

However,ofteninindustrialpractice,althoughthebene-?tsofsurfacepreparationarewidelyknown,itisnotdone

Correspondingauthor.Fax:+9744852491

E-mailaddress:ramazank@qu.edu.qa(R.Kahraman).

0924-0136/$–seefrontmatter?2007ElsevierB.V.Allrightsreserved.doi:10.1016/j.jmatprotec.2007.11.121

?

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journalofmaterialsprocessingtechnology205(2008)183–189

properlyforeconomicandsafetyreasons.Propersurfacepreparationtakesextraproductiontime,tendstobelaborintensiveandusuallyinvolvesuseofhazardousmaterials.Hence,anindustrialstructuraladhesivemustbesuitableforuseonlessthanidealsurfaces.

Inavarietyofindustrialapplicationsepoxyadhesivesarerequiredtohaveanenhancedthermalconductivity.Thenor-malmethodforchangingthisphysicalpropertyistoaddtotheepoxya?llerofhigherconductivitythanthecontinuousphase(TaiandSzklarska-Smialowska,1993;HermansenandTunick,1989;TomlinsonandStapley,1977;LeeandNeville,1967;Kingery,1960;NieberleinandSteverding,1977;Gaynesetal.,1997;Hahnetal.,1998;Subramanianetal.,1998;Nikkeshietal.,1998).Bytheincorporationof?llersintotheadhesive,theresincontent(andthusthecost)isalsoreduced.

Achievingimprovedthermalconductivityisdependenton?llerselectionandloadinglevel.Fillertype,size,shapeandvolumefractiondeterminetheadhesivethermalcon-ductance.Theadhesivethermalconductivityincreaseswithincreasingvolumefractionofthe?ller.Theoretically,thethermalconductivityofthe?llerisnotanimportantvari-ableexceptwhenitiswithinafactorof10ofthethermalconductivityofthepolymericmatrix(adhesive).Mostmetal?llershavethermalconductivitiesgreaterthan10timesthematrixthermalconductivity(HermansenandTunick,1989).

Aluminapowderisoneofthecommonlyused?llersforimprovingthethermalconductivityofadhesives,inpartic-ularinsulationadhesives.Aluminumandsilverpowdersor?akesareusedtoimprovethethermalandelectricalconduc-tivitiesforadhesivesintendedtobeanelectricalorthermalpath(HermansenandTunick,1989;KangandPurushothaman,1998;Luetal.,1999).The?llerlevelmustbesuf?cientlyhightoachievepoint-to-pointcontactbeforeelectricalconductiv-ityisattained(HermansenandTunick,1989).However,toohigh?llercontentmightcauseadegradationinmechani-calpropertiesoftheadhesive(Nikkeshietal.,1998).Therearealsoseveralcommerciallyavailableepoxyadhesivesrein-forcedwithothermetal?llerssuchasaluminumpowder.

Whiletheimprovementonthermalpropertiesofadhesivesbyadditionofmetal?llersisobvious(HermansenandTunick,1989;TomlinsonandStapley,1977;LeeandNeville,1967;Kingery,1960;NieberleinandSteverding,1977),theirin?uenceonthemechanicalpropertiesoftheadhesivejointsarenotclear.Theresultsofthisstudyareexpectedtoshedlightonthisaspect.Theobjectivewastoinvestigatethein?uenceofthe?llercontentandtheadhesivethicknessonthemechan-icalperformanceofaluminumsingle-lapjointsbondedwithaluminumpowder?lledepoxyadhesive.

61阅读提醒您本文地址：

Thetraditionalevaluationofadhesivejointsbystrengthmeasurementswasutilizedinthestudy.Theadhesivestrengthwasdeterminedbyutilizingthesingle-lapsheartest(Mohan,1990;Arnold,1989;TsaiandMorton,1994;AnnualBookofASTMStandards,1992).Thesingle-lapjointcon?g-uration(Fig.1)iswidelyusedintheaerospace,automotiveandwoodandplasticindustries(HermansenandTunick,1989;Arnold,1989;Stringer,1985;DeWildeetal.,1995;JialanellaandShaffer,1993).Single-lapspecimensareeconomical,practicalandeasytomake.Theyalsoalloweasycontrol(andmeasure-ment)ofthebond

thickness.

Fig.1–Single-lapshearjointcon?guration.

Thevariationofadhesionstrengthwithadhesivethick-nessisexaminedinthispaper.Theeffectofbondthicknessonadhesiveperformanceforvariousotherjointshasbeenreportedbyfewinvestigators(Stringer,1985;Foulkesetal.,1970;Hylands,1984;Beevers,1986).However,thereisalackofstudyonthevariationofadhesioncharacteristicswithadhe-sivethicknessforsingle-lapshearjoints.Consequently,inthepresentstudy,theeffectsofmetal?lleradditionandtheadhe-sivethicknessonstressdistributionwithintheadhesivejointsareanalyzedbythe?niteelementmethod.Theexperimentsarealsocarriedouttovalidateandcomparethetheoreticalpredictionsoftheadhesionstrengthofadhesivebondsatdif-ferentthicknesses.

2.

Experimental

2.1.

Materials

Theepoxyadhesiveusedinthisinvestigationisageneral-purpose,two-partepoxy(Fusor309)obtainedfromLordCorporation.Theadhesiveispreparedbymixingequalvol-umesoftheresinandhardenerparts.Themixedadhesivecuresfullyin24–48hatroomtemperaturewithhandlingstrengthinabout8h.

Thealuminumpowderusedfor?llingtheepoxyadhesivewasobtainedfromAlliedBritanniaLimited.Thealuminumparticles(?ller)werespherical/roundishwithsizesmallerthan50?mindiameter.

Thealuminumsheetsusedasadherentsinmakingalu-minumjointswerecutfromlocallyobtainedaluminumplates.

2.2.Adhesivejointpreparation

Thefollowingprocedureswereusedforcleaningthealu-minumsheets(80mm×25.4mm)beforeadhesivelyjoiningthem(KozmaandOlefjord,1987a;Prakashetal.,1987;Semerdjiev,1970).

1.Degreasebydippingshortlyintrichloroethyleneandiso-propylalcohol,separately.2.Washwithwater.

3.Roughensurfacesbyabradercleaning(mechanicalclean-ing)by400gritsiliconcarbidegrindingpaper.

4.Degreasebydippingintrichloroethyleneandisopropylalcohol(30mineach).

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Fig.2–Finiteelementmodelofthesystem.

5.Immersefor2–4hinasolutionofH2SO4,sodiumdichro-mateanddistilledwaterinproportion:22.5,7.5and70byweight,respectively(chromic–sulphuricetchingprocess).6.Washwithdistilledwater.

7.Drywithcleanpaperortissueandkeepinadesiccatoruntiluse.Thesingle-lapshearadhesivejointswerethenpreparedbybondingsurfacecleaned/treatedaluminumsheetstogetherwithneatepoxyadhesiveandthenafteradditionofaluminumpowderatseveralfractions(10,25and50wt%).Equalvolumesoftheepoxyresinandthehardenerweremixedandthenalu-minumpowderwasaddedintotheadhesivemixingagain.Adesigned?xturewasusedtoassembletheadhesivejoints.Ithadtwo?xedendplatesandamovableonebetweenthem.Themetalsheetswerebondedtogetherbetweenthemovableplateandthefarendplate.The?xtureincludedamicrometerusedforcontrollingtheadhesivethickness.Theactualadhe-sivethicknessofthecuredjointwasmeasuredbyaDigimaticCaliper.

2.3.Mechanicalcharacterizationoftheadhesivejoints

Theadhesivejointswereassembledandtestedasspeci?edinASTMD1002(AnnualBookofASTMStandards,1992).Thejointcon?gurationisshowninFig.1whiletheappliedloadsareshowninFig.2.Endtabsofthesamethicknessandmate-rialoftheadherendswereusedtoplacetheloadaxisinthesameplaneoftheoverlaparea.TheequipmentusedformechanicalcharacterizationwasanInstron5567mechani-caltestingsystem.Jointstrengths,reportedinunitsofshearstress,werecalculatedasfollows:jointstrength=

failureloadadhesivelaparea

(1)

3.Stressanalysisby?niteelementmethod

Thein?uenceofmetal?llercontentandtheadhesivethick-nessonstressdistributionwithintheadhesivejointswasanalyzedby?niteelementmethod(FEM).Dueitsversatilityandcomputationalpower,theFEMiscommonlyusedintheanalysisofmodernengineeringandscienti?csystems.Inthisstudy,theFEMisusedtomodelandanalyzetheadhesivejointthroughthewell-known?niteelementpackageANSYS.Thesystemconsistingofthejointandtwo-bondedaluminumspecimensissubjectedtotension.TheresultingshearandvonMisesstressesarecomputedforvariousadhesivecompo-sitionsandthicknessestoseetheireffectsonthemechanicalstrengthofthejoint.Thesestressesarechosen,becauseit

is

Fig.3–Theelasticbeamelement.

expectedthattheshearstressisthemodeoffailurefortheadhesivejointandthevonMisesstressisameasureonthestateofequivalentstress.

61阅读提醒您本文地址：

The?niteelementmodelofthesystemisshowninFig.2.Thetwoaluminumspecimensseeninthe?gurearemodeledusingtheBEAM3elementinANSYS(ANSYS,2005).DepictedinFig.3,thiselasticbeamelementallowsin-planehorizontalandverticaldisplacements(uandv)androtationabouttheout-of-planeaxis(?)atthetwoendnodesof1and2.AsshowninFig.3,thegrippedportionoftheleftspecimenis?xedandtherightspecimenispulledwithaforce(F)of100Nresemblingtheactualtestingconditions.Intheactualtest,thisforceisalignedwiththecentralaxisofthejointandhenceabendingmomentofM=Ft/2isappliedtoaccountfortheeccentricityoftheforceatthe?niteelementmodel(wheretistheadhesivethickness).

Thetwodimensional(2D)quadrilateralelementwithmid-sidenodesandthicknessoption,PLANE82,wasusedformodelingtheadhesivebondgivingin-planedisplacementsattheeightnodes.Thiselementisahigherversionofthefour-nodedelementinANSYSandisbettersuitedfortheapplicationswherethebendingmomentexists(ANSYS,2005).Theadhesivebondismadeofepoxyandaluminum?llerwhoseweightpercentagesarechangedtogetherwiththebondthickness(t)toobservetheireffectsonthemechanicalstrengthofthejoint.ThematerialpropertiesforthealuminumandepoxyarelistedinTable1.Themodulusofelasticity(E)andPoisson’sratio(??)fortheadhesiveareassumedasE=caEa+ceEe(2)??=ca??a+ce??e

(3)

whereEaandEearethemoduliofelasticity,and??aand??earethePoisson’sratiosforthealuminumandepoxy,respectively.Thecoef?cientscaandceintheaboveequationsdenotethefractionscorrespondingtothealuminum?llerandepoxy.

Table1–Materialproperties

Aluminum

Epoxy

Modulusofelasticity(E,GPa)68.951.0858Poisson’sratio(?)

0.333

0.38

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Fig.4–Aplotofadhesivejointstrengthvs.adhesivethicknessforaluminumjointsbondedwithneatepoxywithno?ller.

4.

Resultsanddiscussion

4.1.

Mechanicalcharacterization

AdhesivejointstrengthversusadhesivethicknessisplottedinFig.4foraluminumjointsbondedwithneatepoxywithno?llercontent.Adhesivethicknessesinvestigatedrangedfrom0.03mmto1.3mm.Asseenintheplotthereisageneraltrendofdecreaseinsingle-lapadhesivejointshearstrengthwithadhesivethickness.Increaseofadhesivethicknessfrom0.03mmto1.3mmresultedadecreaseofabout35–40%inadhesivejointshearstrength.Howevertheeffectwasnotsig-ni?cantforadhesivethicknessesuptoabout0.7mm.

Inlightoftheaboveresult,thesmallestadhesivethickness(0.03–0.05mm)wasusedfortheexperimentsoftherestoftheinvestigation.

Aplotofadhesivejointstrengthversusamountofalu-minumpowderusedintheadhesiveispresentedinFig.5.Additionofaluminumparticlesasmuchas50wt%didnotcauseasigni?cantdecreaseinadhesivejointstrength.

It

Fig.5–Aplotofadhesivejointstrengthvs.adhesive?ller

content.

Fig.6–Fracturesurfacesofanaluminumjointbondedbyepoxywithnometal

?ller.

Fig.7–Fracturesurfacesofanaluminumjointbondedbyepoxywith25wt%aluminum?ller.

shouldbenotedthat50wt%Al?llercontentintheadhesivecorrespondstoabout44.45%involumebecauseofdensitydifferencebetweenaluminumparticlesandepoxyadhesive.

Fracturedspecimenswerevisuallyexaminedtodeterminethefailuremode.Thejointsfailedalmostcompletelyincohe-sivefailuremode.Cohesivefailureoccurswhentheadhesiveseparatesfromitself(failurewithinadhesive).AdhesivelayersleftonbothjointsurfacesareclearlyshowninFigs.6and7forneatepoxyandepoxywith25wt%aluminum?llercontent,asexamples.

4.2.FEManalysis

TheshearandvonMisesstressesforvariousbondthick-nessesandvariousadhesivecompositionswereanalyzed.Accordingtothestresscontourswhicharenotshownhere,themaximumshearandvonMisesstressesoccurattheadhesive–metalsubstrateinterface.Thisisespeciallytruefortheshearstresses,whichalmostvanishtowardthemiddleoftheadhesive.ThesametrendalsoexistsforthevonMises

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Fig.8–Maximumshearstressvs.adhesivethicknessforneatepoxy.

stresses,whichattainmaximumattheedgesanddecreaseawayfromtheedges.

Inordertoinvestigatetheeffectofbondthicknessonthebondstrength,themaximumshearandvonMisesstressesareplottedagainstthebondthicknessforneatepoxyadhesiveinFigs.8and9.Asseeninthese?guresshearstressesintheadhesiveincrease(decreasingthebondstrength)asthebondthicknessincreases.AsforthemaximumvonMisesstresses,theyshowslightlydifferentormixedbehavior(Fig.9),butthegeneraltrendissimilartothetrendfortheshearstresses.Inotherwords,thevonMisesstressesarealsoincreasedasthethicknessofthebondisincreased.ItshouldbenotedthattheshearstresscontributiontothevonMisesstressissigni?cantinthebondregionclosetothealuminumplate.This,inturn,resultsinpossiblefailureofbondinginthisregion.

61阅读提醒您本文地址：

Fig.10showstheplotsof?niteelementpredictionsandexperimentalresultsfortheadhesivejointstrengthofneatepoxyatdifferentadhesivethicknesses.Itcan

be

Fig.9–MaximumvonMisesstressvs.adhesivethicknessforneat

epoxy.

Fig.10–Adhesivejointstrengthvs.adhesivethicknessforneat

epoxy.

Fig.11–Variationofmaximumshearstresswith

aluminumcontentforadhesivethicknessof0.05mm.

observedthatthe?niteelementpredictionsagreewellwiththeexperimentalresults.Somediscrepanciesbetweenbothresultsarebecauseof(i)theexperimentalerror,whichisestimatedas7%(basedontheexperimentalrepeats)and(ii)theassumptionofthehomogeneousadhesiveprop-ertiesacrossthebondingsectioninthe?niteelementsimulations.

Stressanalysiswasalsodoneforvariousadhesivecompo-sitions.Themassaverageofmechanicalpropertieswasusedinsimulationstoaccountforthealuminumpowderconcen-trationintheadhesivecomposition.Figs.11and12showtheplotsofthemaximumshearandvonMisesstressesversusadhesivecompositionsforthebondthicknessof0.05mm.Itisevidentthatthesestressesgethigherastheweightpercent-ageofaluminumcontentintheadhesivebondincreases.Inotherwords,thealuminumcontentintheadhesiveadverselyaffectstheadhesionstrengthofthejoint.However,asthebondthicknessincreasesthiseffectdiminishesandtherateofincreaseinstressestendstodecreaseathighbondthick-

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journalhomepage:www.61k.comlocate/jmatprote

c

In?uenceofadhesivethicknessand?llercontentonthemechanicalperformanceofaluminumsingle-lapjointsbondedwithaluminumpowder?lledepoxyadhesive

RamazanKahramana,?,MehmetSunarb,BekirYilbasb

ab

ChemicalEngineeringDepartment,QatarUniversity,DohaQatar

DepartmentofMechanicalEngineering,KingFahdUniversityofPetroleum&Minerals,Dhahran31261,SaudiArabia

article

Articlehistory:

infoabstract

Theobjectiveofthisstudywastodevelopinformationonthein?uenceofadhesivethicknessandaluminum?llercontentonthemechanicalperformanceofaluminumjointsbondedbyaluminumpowder?lledepoxy.Theadhesivestrengthofthejointswasdeterminedbyuti-lizingthesingle-lapsheartest.Thein?uenceofadhesivethicknessandaluminum?llercontentonstressdistributionwithintheadhesivewasalsoanalyzedby?niteelementmethod(FEM).BothFEManalysisandtheexperimentalinvestigationshowthatingen-eraladhesionstrengthdecreasesasthethicknessoftheadhesiveincreases.Itisobserved

Received21May2007Receivedinrevisedform26October2007

Accepted14November2007

Keywords:AdhesiveEpoxyMetal?llerAluminumJointsThickness

fromthepredictions(FEMsimulations)thatthestresslevelincreasesattheadhesive–metalsubstrateinterfaceasthealuminum?llercontentintheadhesiveincreases.Experimentalresultsshowthatepoxyadhesiveretainsitsstrengthuptothe50wt%aluminum?llercon-tent.Thejointsfailincohesivemode(failurewithintheadhesive)duetothehighstresslevelsgeneratedintheadhesive,whichindicatesthattheadhesiontothemetalsurfaceisstrongerthanthatoftheinteriorpartoftheadhesive.

?2007ElsevierB.V.Allrightsreserved.

1.Introduction

Epoxiesarewidelyusedashigh-performancestructuraladhe-sives,especiallyinautomotiveandaircraftmanufacture.Epoxyresinsareattractiveformetal-bondingadhesivesys-temsbecauseoftheirabilitytocurewithoutproducingvolatileby-productsandtheirlowshrinkageuponcuring(lessthan0.5%)(TaiandSzklarska-Smialowska,1993).Epoxiesareabletobondwelltoavarietyoftreatedoruntreatedmetalsurfaces(Mohan,1990).Inaircraftmanufacture,thereisagreatneedforevenlystressed,smoothbondingofthinaluminumsheetandhoneycombmaterials.Epoxyadhesiveshaveagoodaf?n-ityforaluminumalloysurfaces,andtheoxidelayersproducedduringsurfacepreparation(Chasseretal.,1993).

Tobeabletoobtainastrongandstablebondbetweenthemetalandtheadhesive,thenaturalsurfaceoxideshouldberemovedandreplacedwithanew,continuous,solid,corrosionresistantoxidelayer.Theremovalcanbedonemechani-callyand/orchemically.Mechanical(abrasive)cleaningalsoincreasesthesurfaceroughnessand,consequently,thebondstrengthbymechanicalinterlockingandbytheincreasednumberofchemicalbondsonthelargersurfacearea.Vari-ouschemicaltreatments,themostcommonbeingacidetches,havebeendevelopedtomodifytheoxide,torenderitmorereceptivetobonding(KozmaandOlefjord,1987a,b;Brewis,1985;HoggandJanardhana,1993;Kinloch,1983).

However,ofteninindustrialpractice,althoughthebene-?tsofsurfacepreparationarewidelyknown,itisnotdone

Correspondingauthor.Fax:+9744852491

E-mailaddress:ramazank@qu.edu.qa(R.Kahraman).

0924-0136/$–seefrontmatter?2007ElsevierB.V.Allrightsreserved.doi:10.1016/j.jmatprotec.2007.11.121

?

184

journalofmaterialsprocessingtechnology205(2008)183–189

properlyforeconomicandsafetyreasons.Propersurfacepreparationtakesextraproductiontime,tendstobelaborintensiveandusuallyinvolvesuseofhazardousmaterials.Hence,anindustrialstructuraladhesivemustbesuitableforuseonlessthanidealsurfaces.

Inavarietyofindustrialapplicationsepoxyadhesivesarerequiredtohaveanenhancedthermalconductivity.Thenor-malmethodforchangingthisphysicalpropertyistoaddtotheepoxya?llerofhigherconductivitythanthecontinuousphase(TaiandSzklarska-Smialowska,1993;HermansenandTunick,1989;TomlinsonandStapley,1977;LeeandNeville,1967;Kingery,1960;NieberleinandSteverding,1977;Gaynesetal.,1997;Hahnetal.,1998;Subramanianetal.,1998;Nikkeshietal.,1998).Bytheincorporationof?llersintotheadhesive,theresincontent(andthusthecost)isalsoreduced.

Achievingimprovedthermalconductivityisdependenton?llerselectionandloadinglevel.Fillertype,size,shapeandvolumefractiondeterminetheadhesivethermalcon-ductance.Theadhesivethermalconductivityincreaseswithincreasingvolumefractionofthe?ller.Theoretically,thethermalconductivityofthe?llerisnotanimportantvari-ableexceptwhenitiswithinafactorof10ofthethermalconductivityofthepolymericmatrix(adhesive).Mostmetal?llershavethermalconductivitiesgreaterthan10timesthematrixthermalconductivity(HermansenandTunick,1989).

Aluminapowderisoneofthecommonlyused?llersforimprovingthethermalconductivityofadhesives,inpartic-ularinsulationadhesives.Aluminumandsilverpowdersor?akesareusedtoimprovethethermalandelectricalconduc-tivitiesforadhesivesintendedtobeanelectricalorthermalpath(HermansenandTunick,1989;KangandPurushothaman,1998;Luetal.,1999).The?llerlevelmustbesuf?cientlyhightoachievepoint-to-pointcontactbeforeelectricalconductiv-ityisattained(HermansenandTunick,1989).However,toohigh?llercontentmightcauseadegradationinmechani-calpropertiesoftheadhesive(Nikkeshietal.,1998).Therearealsoseveralcommerciallyavailableepoxyadhesivesrein-forcedwithothermetal?llerssuchasaluminumpowder.

Whiletheimprovementonthermalpropertiesofadhesivesbyadditionofmetal?llersisobvious(HermansenandTunick,1989;TomlinsonandStapley,1977;LeeandNeville,1967;Kingery,1960;NieberleinandSteverding,1977),theirin?uenceonthemechanicalpropertiesoftheadhesivejointsarenotclear.Theresultsofthisstudyareexpectedtoshedlightonthisaspect.Theobjectivewastoinvestigatethein?uenceofthe?llercontentandtheadhesivethicknessonthemechan-icalperformanceofaluminumsingle-lapjointsbondedwithaluminumpowder?lledepoxyadhesive.

Thetraditionalevaluationofadhesivejointsbystrengthmeasurementswasutilizedinthestudy.Theadhesivestrengthwasdeterminedbyutilizingthesingle-lapsheartest(Mohan,1990;Arnold,1989;TsaiandMorton,1994;AnnualBookofASTMStandards,1992).Thesingle-lapjointcon?g-uration(Fig.1)iswidelyusedintheaerospace,automotiveandwoodandplasticindustries(HermansenandTunick,1989;Arnold,1989;Stringer,1985;DeWildeetal.,1995;JialanellaandShaffer,1993).Single-lapspecimensareeconomical,practicalandeasytomake.Theyalsoalloweasycontrol(andmeasure-ment)ofthebond

thickness.

Fig.1–Single-lapshearjointcon?guration.

Thevariationofadhesionstrengthwithadhesivethick-nessisexaminedinthispaper.Theeffectofbondthicknessonadhesiveperformanceforvariousotherjointshasbeenreportedbyfewinvestigators(Stringer,1985;Foulkesetal.,1970;Hylands,1984;Beevers,1986).However,thereisalackofstudyonthevariationofadhesioncharacteristicswithadhe-sivethicknessforsingle-lapshearjoints.Consequently,inthepresentstudy,theeffectsofmetal?lleradditionandtheadhe-sivethicknessonstressdistributionwithintheadhesivejointsareanalyzedbythe?niteelementmethod.Theexperimentsarealsocarriedouttovalidateandcomparethetheoreticalpredictionsoftheadhesionstrengthofadhesivebondsatdif-ferentthicknesses.

2.

Experimental

2.1.

Materials

Theepoxyadhesiveusedinthisinvestigationisageneral-purpose,two-partepoxy(Fusor309)obtainedfromLordCorporation.Theadhesiveispreparedbymixingequalvol-umesoftheresinandhardenerparts.Themixedadhesivecuresfullyin24–48hatroomtemperaturewithhandlingstrengthinabout8h.

Thealuminumpowderusedfor?llingtheepoxyadhesivewasobtainedfromAlliedBritanniaLimited.Thealuminumparticles(?ller)werespherical/roundishwithsizesmallerthan50?mindiameter.

Thealuminumsheetsusedasadherentsinmakingalu-minumjointswerecutfromlocallyobtainedaluminumplates.

2.2.Adhesivejointpreparation

Thefollowingprocedureswereusedforcleaningthealu-minumsheets(80mm×25.4mm)beforeadhesivelyjoiningthem(KozmaandOlefjord,1987a;Prakashetal.,1987;Semerdjiev,1970).

1.Degreasebydippingshortlyintrichloroethyleneandiso-propylalcohol,separately.2.Washwithwater.

3.Roughensurfacesbyabradercleaning(mechanicalclean-ing)by400gritsiliconcarbidegrindingpaper.

4.Degreasebydippingintrichloroethyleneandisopropylalcohol(30mineach).

journalofmaterialsprocessingtechnology205(2008)183–189

185

Fig.2–Finiteelementmodelofthesystem.

5.Immersefor2–4hinasolutionofH2SO4,sodiumdichro-mateanddistilledwaterinproportion:22.5,7.5and70byweight,respectively(chromic–sulphuricetchingprocess).6.Washwithdistilledwater.

7.Drywithcleanpaperortissueandkeepinadesiccatoruntiluse.Thesingle-lapshearadhesivejointswerethenpreparedbybondingsurfacecleaned/treatedaluminumsheetstogetherwithneatepoxyadhesiveandthenafteradditionofaluminumpowderatseveralfractions(10,25and50wt%).Equalvolumesoftheepoxyresinandthehardenerweremixedandthenalu-minumpowderwasaddedintotheadhesivemixingagain.Adesigned?xturewasusedtoassembletheadhesivejoints.Ithadtwo?xedendplatesandamovableonebetweenthem.Themetalsheetswerebondedtogetherbetweenthemovableplateandthefarendplate.The?xtureincludedamicrometerusedforcontrollingtheadhesivethickness.Theactualadhe-sivethicknessofthecuredjointwasmeasuredbyaDigimaticCaliper.

2.3.Mechanicalcharacterizationoftheadhesivejoints

Theadhesivejointswereassembledandtestedasspeci?edinASTMD1002(AnnualBookofASTMStandards,1992).Thejointcon?gurationisshowninFig.1whiletheappliedloadsareshowninFig.2.Endtabsofthesamethicknessandmate-rialoftheadherendswereusedtoplacetheloadaxisinthesameplaneoftheoverlaparea.TheequipmentusedformechanicalcharacterizationwasanInstron5567mechani-caltestingsystem.Jointstrengths,reportedinunitsofshearstress,werecalculatedasfollows:jointstrength=

failureloadadhesivelaparea

(1)

3.Stressanalysisby?niteelementmethod

Thein?uenceofmetal?llercontentandtheadhesivethick-nessonstressdistributionwithintheadhesivejointswasanalyzedby?niteelementmethod(FEM).Dueitsversatilityandcomputationalpower,theFEMiscommonlyusedintheanalysisofmodernengineeringandscienti?csystems.Inthisstudy,theFEMisusedtomodelandanalyzetheadhesivejointthroughthewell-known?niteelementpackageANSYS.Thesystemconsistingofthejointandtwo-bondedaluminumspecimensissubjectedtotension.TheresultingshearandvonMisesstressesarecomputedforvariousadhesivecompo-sitionsandthicknessestoseetheireffectsonthemechanicalstrengthofthejoint.Thesestressesarechosen,becauseit

is

Fig.3–Theelasticbeamelement.

expectedthattheshearstressisthemodeoffailurefortheadhesivejointandthevonMisesstressisameasureonthestateofequivalentstress.

The?niteelementmodelofthesystemisshowninFig.2.Thetwoaluminumspecimensseeninthe?gurearemodeledusingtheBEAM3elementinANSYS(ANSYS,2005).DepictedinFig.3,thiselasticbeamelementallowsin-planehorizontalandverticaldisplacements(uandv)androtationabouttheout-of-planeaxis(?)atthetwoendnodesof1and2.AsshowninFig.3,thegrippedportionoftheleftspecimenis?xedandtherightspecimenispulledwithaforce(F)of100Nresemblingtheactualtestingconditions.Intheactualtest,thisforceisalignedwiththecentralaxisofthejointandhenceabendingmomentofM=Ft/2isappliedtoaccountfortheeccentricityoftheforceatthe?niteelementmodel(wheretistheadhesivethickness).

Thetwodimensional(2D)quadrilateralelementwithmid-sidenodesandthicknessoption,PLANE82,wasusedformodelingtheadhesivebondgivingin-planedisplacementsattheeightnodes.Thiselementisahigherversionofthefour-nodedelementinANSYSandisbettersuitedfortheapplicationswherethebendingmomentexists(ANSYS,2005).Theadhesivebondismadeofepoxyandaluminum?llerwhoseweightpercentagesarechangedtogetherwiththebondthickness(t)toobservetheireffectsonthemechanicalstrengthofthejoint.ThematerialpropertiesforthealuminumandepoxyarelistedinTable1.Themodulusofelasticity(E)andPoisson’sratio(??)fortheadhesiveareassumedasE=caEa+ceEe(2)??=ca??a+ce??e

(3)

whereEaandEearethemoduliofelasticity,and??aand??earethePoisson’sratiosforthealuminumandepoxy,respectively.Thecoef?cientscaandceintheaboveequationsdenotethefractionscorrespondingtothealuminum?llerandepoxy.

Table1–Materialproperties

Aluminum

Epoxy

Modulusofelasticity(E,GPa)68.951.0858Poisson’sratio(?)

0.333

0.38

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Fig.4–Aplotofadhesivejointstrengthvs.adhesivethicknessforaluminumjointsbondedwithneatepoxywithno?ller.

4.

Resultsanddiscussion

4.1.

Mechanicalcharacterization

AdhesivejointstrengthversusadhesivethicknessisplottedinFig.4foraluminumjointsbondedwithneatepoxywithno?llercontent.Adhesivethicknessesinvestigatedrangedfrom0.03mmto1.3mm.Asseenintheplotthereisageneraltrendofdecreaseinsingle-lapadhesivejointshearstrengthwithadhesivethickness.Increaseofadhesivethicknessfrom0.03mmto1.3mmresultedadecreaseofabout35–40%inadhesivejointshearstrength.Howevertheeffectwasnotsig-ni?cantforadhesivethicknessesuptoabout0.7mm.

Inlightoftheaboveresult,thesmallestadhesivethickness(0.03–0.05mm)wasusedfortheexperimentsoftherestoftheinvestigation.

Aplotofadhesivejointstrengthversusamountofalu-minumpowderusedintheadhesiveispresentedinFig.5.Additionofaluminumparticlesasmuchas50wt%didnotcauseasigni?cantdecreaseinadhesivejointstrength.

It

Fig.5–Aplotofadhesivejointstrengthvs.adhesive?ller

content.

Fig.6–Fracturesurfacesofanaluminumjointbondedbyepoxywithnometal

?ller.

Fig.7–Fracturesurfacesofanaluminumjointbondedbyepoxywith25wt%aluminum?ller.

shouldbenotedthat50wt%Al?llercontentintheadhesivecorrespondstoabout44.45%involumebecauseofdensitydifferencebetweenaluminumparticlesandepoxyadhesive.

Fracturedspecimenswerevisuallyexaminedtodeterminethefailuremode.Thejointsfailedalmostcompletelyincohe-sivefailuremode.Cohesivefailureoccurswhentheadhesiveseparatesfromitself(failurewithinadhesive).AdhesivelayersleftonbothjointsurfacesareclearlyshowninFigs.6and7forneatepoxyandepoxywith25wt%aluminum?llercontent,asexamples.

4.2.FEManalysis

TheshearandvonMisesstressesforvariousbondthick-nessesandvariousadhesivecompositionswereanalyzed.Accordingtothestresscontourswhicharenotshownhere,themaximumshearandvonMisesstressesoccurattheadhesive–metalsubstrateinterface.Thisisespeciallytruefortheshearstresses,whichalmostvanishtowardthemiddleoftheadhesive.ThesametrendalsoexistsforthevonMises

journalofmaterialsprocessingtechnology205(2008)183–189

187

Fig.8–Maximumshearstressvs.adhesivethicknessforneatepoxy.

stresses,whichattainmaximumattheedgesanddecreaseawayfromtheedges.

Inordertoinvestigatetheeffectofbondthicknessonthebondstrength,themaximumshearandvonMisesstressesareplottedagainstthebondthicknessforneatepoxyadhesiveinFigs.8and9.Asseeninthese?guresshearstressesintheadhesiveincrease(decreasingthebondstrength)asthebondthicknessincreases.AsforthemaximumvonMisesstresses,theyshowslightlydifferentormixedbehavior(Fig.9),butthegeneraltrendissimilartothetrendfortheshearstresses.Inotherwords,thevonMisesstressesarealsoincreasedasthethicknessofthebondisincreased.ItshouldbenotedthattheshearstresscontributiontothevonMisesstressissigni?cantinthebondregionclosetothealuminumplate.This,inturn,resultsinpossiblefailureofbondinginthisregion.

Fig.10showstheplotsof?niteelementpredictionsandexperimentalresultsfortheadhesivejointstrengthofneatepoxyatdifferentadhesivethicknesses.Itcan

be

Fig.9–MaximumvonMisesstressvs.adhesivethicknessforneat

epoxy.

Fig.10–Adhesivejointstrengthvs.adhesivethicknessforneat

epoxy.

Fig.11–Variationofmaximumshearstresswith

aluminumcontentforadhesivethicknessof0.05mm.

observedthatthe?niteelementpredictionsagreewellwiththeexperimentalresults.Somediscrepanciesbetweenbothresultsarebecauseof(i)theexperimentalerror,whichisestimatedas7%(basedontheexperimentalrepeats)and(ii)theassumptionofthehomogeneousadhesiveprop-ertiesacrossthebondingsectioninthe?niteelementsimulations.

Stressanalysiswasalsodoneforvariousadhesivecompo-sitions.Themassaverageofmechanicalpropertieswasusedinsimulationstoaccountforthealuminumpowderconcen-trationintheadhesivecomposition.Figs.11and12showtheplotsofthemaximumshearandvonMisesstressesversusadhesivecompositionsforthebondthicknessof0.05mm.Itisevidentthatthesestressesgethigherastheweightpercent-ageofaluminumcontentintheadhesivebondincreases.Inotherwords,thealuminumcontentintheadhesiveadverselyaffectstheadhesionstrengthofthejoint.However,asthebondthicknessincreasesthiseffectdiminishesandtherateofincreaseinstressestendstodecreaseathighbondthick-

188

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183–189

Fig.12–MaximumvonMisesstressvs.aluminumcontentforadhesivethicknessof0.05mm.

nesses.Resultsfortheadhesivethicknessof1.0mmarepresentedinFigs.13and14.

Furthermore,asstatedinthemanuscriptinSection3,theshearstressisthepossiblecauseoffailurefortheadhesivejoint.Hence,asseeninFigs.11and13,themaximumshearstressis,ingeneral,higherfortheadhesivethicknessof1mmthanthatcorrespondingtothethicknessof0.05mm.Thisresultindicatestheadverseeffectoftheadhesivethicknessincreaseinbondstrength.

Althoughinthepresentstudy,aluminumpowderisusedas?llerintheadhesive,investigationintomechanicalresponseofthejointwhenotherpowdersuchascopperwillbefruitfulforfutureinvestigation.Inaddition,examinationofthemechanicalresponseofthejointsduetomixtureofaluminumandcopperpowdersusedas?llerintheadhe-sivewillbeinterestingforimprovedthermalandelectrical

conductivities.

Fig.13–Variationofmaximumshearstresswithaluminumcontentforadhesivethicknessof1

mm.

Fig.14–MaximumvonMisesstressvs.aluminumcontentforadhesivethicknessof1mm.

5.Conclusions

Thefollowingstatementscanbemadefromthe?ndingsofthisstudy:

1.AccordingtotheresultsofbothFEManalysisandtheexper-imentalinvestigation,adhesivethicknesshasanegativeeffectonsingle-lapadhesivejointshearstrength.

2.The?niteelementresultsonstressesshowthatthealu-minumcontentintheadhesiveadverselyaffectsthemechanicalstrengthofthebond.

3.However,apromisingresultwasobtainedthroughexper-imentalinvestigationthatepoxyadhesiveretainsitsadhesionstrengthevenwithasmuchas50wt%additionofaluminum?ller.

4.Eventhoughthe?niteelementanalysisshowshigherstressesattheadhesive–metalsubstrateinterface,actualfailureoccurswithintheadhesiveindicatingthatthestrengthofadhesiontothemetalsubstratesurfaceisstrongerthanthestrengthoftheadhesiveitself.

Acknowledgment

ThisstudyhasbeenfundedbyKingFahdUniversityofPetroleum&MineralsundertheProject#SAB-2003/12.

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