4.6Theapparatusandproceduresmaybesuitableformeasuringcreeppropertiesofadhesivesonsubstratesotherthanwood,suchasmetal,plastic,andglass,butsuchconsid-erationsarenotwithinthescopeofthistestmethod.5.Apparatus
5.1TestingMachine—Atestingmachine,orothersuitableloadingmachine,capableofapplyingcompressionloadsfrom0to5000lbf(22kN)andcross-headspeedsfrom0.01to0.40in./min(0.3to10.2mm/min)issufficientforthistestmethod.Aminimumverticalspaceof20in.(508mm)isrequiredtocompresstheloadingspringinthecreep-testapparatus.
5.2CompressionShearingTool—Thetestingmachineisequippedwithashearingtoolcapableofapplyingauniformly
distributedcompressionloadtotheloadingledgesoftheblock-shearspecimen.Ashearingtoolequippedwithaself-aligningseatintheshearingbladeensuresuniformloading.5.3Creep-TestApparatus—Staticloadsshallbeappliedandmaintainedonblock-shearspecimensbymeansofthecompression-loadedcreep-testcylindershowninFig.1.5Theapparatusisspring-loadedandcansustainanyloaduptothedesigncapacityofthespring.Thisparticularspringhasadesignloadof2300lbf(10kN);however,othersofgreaterorlesscapacitymaybesubstituted.Varyingspringcapacities
Thiscreep-testapparatusmaybepurchasedfromHullMachineShop,P.O.373,Hull,GA30646,orothersuitablesuppliers.DetailedworkingdrawingsoftheapparatusareavailablefromASTM,100BarrHarborDrive,POBoxC700,WestConshohocken,PA19428–2959.OrderADJ4680.
5FIG.1CreepTestApparatus
withoutsidediametersnogreaterthanthecylinderinsidediameterareavailable.
5.3.1Forcreeptestsaboveroomtemperature,itisnotnecessarytoadjustthespringorloadtocompensatefortheeffectsofchangingtemperature.Itisonlynecessarythattheapparatus,withincludedspecimen,bepreconditionedtothetesttemperaturebeforethetestloadisappliedtothespring.Thepreheatedapparatusmustbewrappedwithapieceofflexiblethermalinsulationmaterialwhilethetestloadisappliedtothespecimen.Afterloadingandmeasurements,returntheloadedapparatustothetestenvironment.Sincetherearenosignificantchangesintemperaturebeforeorafterloading,noadjustmentsareneededinthespring.
5.3.2Thecreep-testapparatusismadeofcorrosion-resistantcomponentssothatitcanbeusedinhigh-temperatureandhumidenvironmentsforprolongedperiodswithoutconcernfordamagingtheapparatusorinterferingwiththeeffectivenessofthetest.
5.3.3Thecreep-testapparatushasbeencompactlydesignedwithitsload-applyingmechanismbuilt-in.Thus,severaloftheunitsmaybestackedonracksinasmallenvironmentalchambersuchasanoven,incubator,orhumiditycabinet.Theapparatusmaybetransferredfromoneexposurechambertoanother,ormayberemovedfromanexposurechamberformeasurementswithoutdisturbingthespecimenunderstaticload.
5.3.4Thecreep-testapparatusshowninFig.1hasamicroswitchmountedatitsbasewhichisactivatedwhenthecreepspecimenfails.Asmallpinislocatedinthelowerspecimenseatwhichisdrivenagainstthemicroswitchwhenthefailedspecimenstrikesit.Themicroswitchmustbeconnectedtoanautomatictimer-recorder.
5.4AutomaticTimer-Recorder—Ifcreeportime-to-failuremeasurementsaretobemade,anautomaticandmulti-channeltimer-recorderisconnectedtothemicroswitchoneachcreep-testapparatus.Thetimer-recorderiscapableofautomaticallyscanningtheseveralconnectedcircuitsatselectedintervalsoftime.Whenthecreepspecimenfailsandactivatesthemi-croswitch,thetimer-recorderautomaticallyrecordsthetimeatwhichthecircuitisbroken.
5.5Microscope—Amicroscopeisrequiredtomeasuredis-placementofscribemarksacrossthetwoadherendsofaspecimenascreepoccurs.Accuratemeasurementsarealsorequiredforbondlinethicknesses.Makemeasurementstothenearest0.001mm(0.0004in.).Alineartravelingbinocularmicroscopeisideallysuitedtocreepmeasurements;however,amicroscopefittedwithanappropriatelygraduatedscaleissatisfactory.Anobjectivelensofatleast73magnificationisrequired.
5.6EnvironmentalChambers—Controloftemperatureandrelativehumidityisrequiredincreeptestsofadhesivebondsonwoodsubstrates.Temperaturehasaprofoundeffectoncreeppropertiesofadhesives.Humidityalsoaffectscreepofcertainadhesives,butitcanalsoaffectdimensionalchangeinwoodadherends.Conditioningequipmentshouldbecapableofmaintainingaconstanttemperaturewithin63.0°F(61.7°C)oftheset-pointandconstantrelativehumiditywithin65%oftheset-pointatagiventemperature.
6.Materials
6.1Adherends—Selectsugarmaple(Acersaccharum)asthestandardadherendmaterialwiththegrainofthewoodstraightandparalleltothedirectionofshearandfreeofalldefectssuchasknots,splits,anddiscolorations.Sugarmapleisauniformlytexturedandhigh-densitywoodpreferredincreeptestsbecauseitislesslikelytodeformnearthebondlineorfailinthewoodbeforethetestiscompleted.
6.1.1Sugarmapleadherendmaterialhasaminimumspe-cificgravityof0.65(basedonoven-dryweightandvolume).Amethodforselectingmapleblocksatthisspecificgravity,includingappropriateadjustmentsinspecificgravityforvari-ousmoisturecontents,isdescribedintheAppendixofTestMethodD905.Ifmorecompleteproceduresarerequiredforrefereedtests,specificgravitymaybedeterminedinaccor-dancewithSection116ofMethodsD143.
6.1.2Anyotherspeciesofwoodmaybeusedasadherendmaterial,particularlyinthosecaseswhereitisnecessarytoknowthecreepbehaviorofanadhesiveincontactwithaspecificwoodspecies.However,itshouldberecognizedthatwoodsthatarelessuniformintextureandlowerindensitythansugarmaple,aremorelikelytodeformandfailprematurely.6.1.3Whenconductingcreepandtime-to-failuretests,itisimportantnottooverloadtheadherendandcausedeformationatthebondlineorfailureinthewoodbeforethetestiscompleted.Asaguidetoselectingmaximumlevelsofstress,itisrecommendedthattheloadnotexceedtheaverageshearstrengthparallel-to-grainforthespeciesofwoodwhenad-justedforanychangeinmoisturecontentfrom12%.Averageshearstrengthsat12%moisturecontentforsugarmapleandotherspeciesofwoodarepresentedinTable4-2oftheWoodHandbook.6Formoisturecontentsotherthan12%,itwillbenecessarytoadjustshearstrengthsbytheformuladescribedinpp.4-32and4-33oftheWoodHandbook.66.1.4Shouldprematurefailuresoccurinthewoodaftermeanstrengthvalueshavebeenadjustedformoisturecontent,thenthe5%exclusionlimitmaybeusedtoselectthemaximumlevelofstressforaparticularspeciesofwood.ThemaximumlevelofstressSequation:
max0.05isdeterminedbythefollowingSmax0.055Sm21.645~0.14Sm!
(1)
whereSmisthemeanshearstrengthparallel-to-grainaspresentedinTable4-2oftheWoodHandbook.66.2Adhesives—Creeppropertiesofanyadhesivecanbemeasuredonanyspeciesofwoodaslongasthelevelofshearstressdoesnotexceedtheshearstrengthofthewoodinthespecimen.
7.TestSpecimens
7.1Thestandardspecimenforbothcreepandtime-to-failuretestsshallbetheblock-shearspecimenillustratedinFig.2.Usethetestadhesivetobondthetwohardmapleadherendstogether.Theshearareais1.00by1.00in.(2.54by
6WoodHandbook:WoodAsAnEngineeringMaterial,USDAAgricultureHandbook72,rev.,U.S.ForestProductsLab.,1974,Table4-2andp.4-32and4-33.AvailablefromtheSuperintendentofDocuments,U.S.Govt.PrintingOffice,Washington,DC20402.
FIG.2Block-ShearSpecimenforCreepandTimeToFailureTests
2.54cm),equalling1.00in.2(6.45cm2).
7.2Thefinishedspecimenforcreeptestshaswell-definedscribemarksoneachsideofthespecimenformeasuringdisplacementofthetwoadherendsascreeptakesplace.
7.3Scribemarksarenotnecessaryforspecimensintendedforthetime-to-failuretests.7.4NumberofSpecimens:
7.4.1Testatleasttenspecimensateachsetoftestcondi-tions,thatis,eachcombinationofstresslevel,temperature,andrelativehumidity,foreithercreeptestortime-to-failuretest.Itissuggestedthatatleastonetestjointassembly(containstwelvespecimens)bepreparedforeachcombinationoftestconditions.
7.4.2Randomlyassignthetenspecimensforeachsetoftestconditionstothatsetfromtheentirelotofspecimenspreparedfortestingofagivenadhesive.
7.4.3Formanyadhesives,measuredcreeppropertieswillbehighlyvariable,andtenspecimensmaynotbealargeenoughsampletogiveahighdegreeofconfidencetothemeanofameasuredproperty.Creeptestsgenerallyareexpensiveandtime-consuming,andaspecimencountoftenrepresentsapracticalcompromise.
7.4.4Ifitisdeterminedfromexperimentalevidencethatareasonabledegreeofconfidencecanbegiventoameanbyusingalessernumberofspecimens,thenitispermissibletousethelessernumber.
8.ConditioningofMaterialsandSpecimens
8.1Conditionhardmapleboardsat7363°F(2361.7°C)
and6562%relativehumidityuntiltheyreachmoistureequilibrium(approximately12%moisturecontent),asindi-catedbynofurthersignificantchangesinweight.UseTestMethodsD2016forguidancetodeterminewhenequilibriumhasbeenreached.Otherrelativehumiditiesmaybesubstitutedaslongas(1)thewoodreachesequilibriumbetween10and12%moisturecontent,(2)thealternativeconditionsaresubstitutedconsistentlythroughoutthetestmethod,and(3)thesubstitutedstandardconditionsarereported.Ifthefacilitiesarenotavailableforconditioningwoodtothe10to12%moisturecontentrange,thenlowerrelativehumiditiesmaybesubsti-tutedtolowertheequilibriummoisturecontentofthewoodtothe7to9%range.Conditions(2)and(3)abovemustapplytothissubstitution.
8.2Itisimportantthatwoodmoisturecontentafterprecon-ditioningandduringcreeptestsremainessentiallythesame.Otherwiseadherendswillundergodimensionalchangeswithmoistureduringstaticloadingwiththeinevitableresultthatcreeppropertieswillbeaffected.Theexposureenvironmentforcreeptestsshallbethesameasindicatedin8.1,unlessotherwiseprescribed.
NOTE1—GuidelinesforselectingtestconditionsaregiveninAnnexA1.
9.SpecimenPreparation
9.1Planehardmapleboardsofspecifiedspecificgravityonbothsidestoathicknessof0.63in.(16mm).Ifaplanerisnotavailable,useahand-fedjointer.Planingshouldbecarriedout
nolongerthan24hbeforebonding.Cuttheboardsintoblocks12in.longby3in.wide(30.5by7.6cm).Thegraindirectionshallbeparalleltothelongdimensionsoftheblock.Keeptheplanedsurfacesfreeofdustanddirtuntiltheycanbebonded.9.2Preparethetestadhesiveandapplyittoadherendsinaccordancewithproceduresrecommendedbythemanufactureroftheadhesive.Sinceadhesivesofdifferenttypesmaybetestedbythismethod,nogivensetofspreadrates,assemblytimes,clamppressures,pressuretimes,andcuringtimes,canbespecifiedtocoveralladhesives.Normally,theadhesivemanufacturerdescribesproceduresformakingsoundbondsandtheseshouldbecarefullyfollowed.
9.3Afterpropercuring,cuteachtestjointassemblyintotwelvespecimenblanks.ThemethodofcuttingspecimensfromanassemblyisshowninFig.3ofTestMethodD905,althoughassemblyandspecimendimensionsaredifferent.Tomakecuttingofspecimenblankseasier,trimoneoutsideedgefromthejointassembly.Fromthisedge,makethesecondandthirdfull-lengthcutstheexactwidthofblock-shearspecimens.Afterwards,thesetwostripscanbecross-cuttotheexactlengthofspecimens.Withspecimenblankscuttoexactoveralldimensions,notchtheloadingledgestogiveasheararea1.00in.(2.54cm)longasillustratedinFig.2.Aftercutting,returnthefinishedblock-shearspecimenstothestandardatmospheregivenin8.1.Conditionthemtherefor7daystoensurethatthemoisturecontenthasstabilizedbeforecreeptestsbegin.Ifcreeptestsaretobeconductedatequilibriummoistureconditionsotherthanthosein8.1,thensubsequentcondition-ingwillberequiredasdescribedin10.1.2.
9.4Cutscribemarksalongthetwosidesofeachspecimenintendedforcreeptests.Asharp,single-edgedrazorblademakeswell-definedmarks.Themarksextendinastraightlineacross,andperpendiculartothebondlineandadherends.Inspectthecutswithanymicroscopefittedwithanobjectivelensof73magnification.Ifatleastoneedgeofthecutisnotclearlydefined,thenmakeanothercutnexttothefirst.Besuretousethesamecutedgeeverytimemeasurementsaremade.10.Procedure
10.1CreepTests:
10.1.1Randomlyselectandassignatleasttenspecimenstoeachsetoftestconditionsfromthetotalnumberofspecimensthataretobeusedforthecreeptests.Selectanadditionalsetofspecimenstoreplacedefectiveormis-testedspecimens.10.1.2Sincecreepspecimensofwoodundergodimensionalchangeswithchangesinenvironment,itisessentialthatspecimensbepreconditionedto,andinequilibriumwith,thosetestconditionsatthetimecreeptestsbegin.Ifcreepistobemeasuredatconditionsotherthanstandardconditionsin8.1,thenconditionspecimensatthosetestconditionsuntilspeci-mensreachequilibriummoisturecontent(indicatedbynochangeinweight)andtemperature.
10.1.3Afterspecimenshavebeenpreconditioned,measuretheshearareaofeachcreepspecimentothenearest0.01in.(0.25mm)usingadial-indicatingcaliperedmicrometer.Cal-culatetheshearareatothenearest0.01in.2(0.06cm2).
10.1.4Measurebondlinethicknessatthefourcornersoftheshearareaoneachspecimentothenearest0.0004in.(0.001mm)withalineartravelingmicroscope,orwithamicroscopehavingagraduatedscale.Thebondlinethicknessforeachspecimenshallbetheaverageofthefourmeasurements.10.1.5Ifcreepistobedeterminedforelevatedtempera-tures,butspecimenshavetoberemovedfromtestconditionsforperiodicmeasurementsofcreep,thecreeptestapparatusalongwithaproperlyseatedspecimenmustbeconditionedtoequilibriumtemperaturebeforeremovalfromtestconditions.Everyeffortshouldbemadetomaintaintesttemperaturewhenapparatusandspecimenareremovedformeasurements.Thiscanbeaccomplishedbywrappingtheapparatusinaflexiblethermalinsulationmaterialsuchaspolyurethanefoamuntilmeasurementscanbecompleted.Creepmeasurementsrequireonlyafewminutes,sodimensionalchangesfromlossorgainofmoistureshouldnotbeofanyconsequence.
10.1.6Mountaproperlyconditionedandmeasuredcreepspecimenontheloadingseatsofthecreep-testapparatus.Positiontheapparatusinthetestingmachinetoapplyacompressionloadtotheloadingshaftoftheapparatus.Ifamicroswitchisattachedtothebaseofthecreepapparatus,supporttheapparatusonthelowerbaseshowninFig.1whenapplyingtestloads.Applythepredeterminedtestloadtotheloadingshaft(whichcompressestheloadingspring)atauniformrateofspeedsuchthatthetestloadisreachedwithin1min.Whenthetestloadisreached,beginmeasurementsoftime.Turnthekeeper-nutontheloadingshaftuntilitfirmlyseatsagainsttheupperend-plug.Thisensuresthatthedeflec-tionoftheloadingspringwillremainconstantthroughoutthecreeptest.Removetheapparatusfromthetestingmachine.10.1.7Immediatelybeginmeasuringdisplacementofscribemarksoneachsideofthespecimenwiththetravelingmicroscope.Makemeasurementsatthefollowingtimeinter-vals:1,2,5,10,20,50,100,200,500,1000,2000–100,000min.Theseintervalsandtotalloadingtimesaresuggested,andmaybeadjustedasdeemedappropriate.Theintervalsprovideapproximatelyequallyspacedpointswhenplottedalongthelogarithmicbase−10scaleoftime.Ifdiscontinuitiesinthecreepstrain-versus-timecurvearesuspected,takereadingsmorefrequentlythanscheduledabove.
10.1.8Oncompletionofinitiallyplannedcreepmeasure-ments,andbeforefailureoccurs,itmaybedesirabletomeasurerecoveryatsomepointincreepdevelopment.Ifso,repositionthecreepapparatusinthetestingmachineandlowerthecross-headtojustcontacttheloadingshaft.Loosenthekeeper-nutenoughtoallowthespringtomakefullrecovery.Thenreversethecross-headdirectionandremovetheloadwithin1min.Begintimingimmediatelyandmakemeasure-mentsofdisplacementrecoveryaccordingtothesametimeintervalscheduleshownin10.1.7.Aftereachrecoverymea-surement,returntheapparatuswithspecimentotheappropri-atetestenvironmentuntilallmeasurementsarecompleted.Afterrecoverymeasurements,ifspecimensaretobere-loadedforadditionalcreepmeasurements,repeatproceduresin10.1.6and10.1.7.
10.1.9Continuethecreeptestuntilfailureifdesired.10.2Time-to-FailureTests:
10.2.1Randomlyselectandassigntenspecimenstoeachsetoftestconditionsfromthetotalnumberofspecimensthataretobeusedfortime-to-failuretests.Fromthesamelarger
group,randomlyselect20specimensfordeterminingaverageultimateshearstrength.Anadditionalsetoftenspecimensshouldbeselectedtoreplacedefectiveormis-testedspeci-mens.
10.2.2Measuretheshearareaandbondlinethicknessofeachspecimenasdescribedin10.1.3and10.1.4.
10.2.3Withproperlyconditioned,measured,andrandomlyselectedspecimens,determineaverageultimateshearstrengthfrom20specimensaccordingtoproceduresinSections2and7ofTestMethodD905.Forrigidadhesives,usethespecifiedcross-headspeedof0.015in.(0.00635mm/min)inTestMethodD905.Thisspeedshouldproducefailureinabout1min.Forlessrigidadhesiveswhereappropriatecross-headspeedsarenotknown,selectaspeedthatwillproducefailurewithin1min.Calculatetheshearstrengthatfailureinpounds-forcepersquareinch(kilopascals),basedonthebondlineareameasuredin10.1.3.
10.2.4Useatleastfourpoints,andpreferablymore,toestablishacurveoftimetofailureversuslevelofstress.Thesepointsshouldbeevenlyspacedovertherangeofstressestobetested.Itissuggestedthatspecimensbeloadedat90,80,70,and60%oftheultimateshearstrengthoftheadhesive,asdeterminedin10.2.4.Otherstresslevelsmaybeselected,butitisdesirablethattheloweststresslevelproduceaveragetimetofailurenear3000h.
10.2.5Applyappropriatestaticloadstoproperlycondi-tionedandmeasuredspecimenswiththecreep-testapparatus,asdescribedin10.1.6.Attachthemicroswitchtotheapparatusbaseandconnectittotheautomatictime-recordingdevicebeforeapplyingtestloads.Activatethetimerwhenthetestloadisreached.
10.2.6Afterremovingthecreep-testapparatusfromthetestingmachine,returntheloadspecimentotheappropriatetestingenvironment.Handletheapparatuscarefullytoavoidbumpsorvibrationsthatmayproduceprematurefailureinspecimensunderstaticloading.Continueloadinguntilfailureoccurs.Recordthetimeoffailure.
10.2.7Thefailedspecimenmustberemovedfromthecreep-testapparatususingthetestingmachine.Repositiontheapparatusinthetestingmachineandlowerthecross-headtojustcontacttheloadingshaft.Loosenthekeeper-nutsothespringcanmakeafullrecovery.Raisethecross-headandremovetheapparatus.
11.Calculation
11.1Calculatestressesforeachspecimeninpounds-forcepersquareinch(kilopascals)bydividingtheloadbytheinitialsheararea.Calculatepercentageofultimateshearstressbydividingtestshearstressbyultimateshearstressandmultiply-ingthequotientby100.
11.2Calculatecreepstrainininchesperinch(millimetrespermillimetre)bydividingtotaldisplacementofscribemarks(averageoftwosides)attimesspecifiedin10.1.7bybondlinethickness(averageoffourcorners).
11.3Calculatecreepmodulusinpounds-forcepersquareinch(kilopascals)bydividingtheinitialstressbycreepstrainatthetimesspecifiedin10.1.7.
11.4Calculatealeast-squareslinearregressionequationofstress(expressedaspercentofultimateshearstress)overlog
base−10timetofailure.Abetter-fittingcurvemaybeobtainedbycalculatingalinearregressionequationoflogbase−10ofstressleveloverlogbase−10oftimetofailure.Calculateanequationforeachtemperature(andrelativehumidity)atwhichtime-to-failuretestswereconducted.Fromtheseequations,calculatethetime-to-failureatprojectedlowerstresslevelsthanthosetested.Usuallyworkingstresslevelswillbeconsiderablylower,inwhichcaseextrapolationsmayextendbeyondactualdata.Normally,extrapolationshouldbelimitedtoonebase−10logcycle.
11.5Calculatetherateofcreepfromthecurveofcreepstrainversustime.Drawastraightlinetocoincidewiththeregionofsecondarycreepinthecreepcurve.Thisregionofsecondarycreepusuallyhastheleastslope,comparedtoslopesofthecurveinprimaryandtertiaryregions.Theminimumcreeprateisthequotientofcreepstrainovertimeatanypointalongthestraightlineinthesecondarycreepregion.12.Report
12.1Prepareacompletereportofalltests.Creepdatacannotbecollectedasquicklyasneeded,sotestsmayspanuptotwoormoreyears.Thesetestsarequiteexpensiveintermsofpersonnelandequipmentrequirements.Also,severaldiffer-entpeoplemayworkontheproject,andequipmentmaybechanged.Forthesereasons,aclearanddetailedreportcanprovidetheneededcontinuity,andperhapsavoidrepeatingtests.
12.2Suchvaluesascreeprateandtimetofailurecandifferbyasmuchasseveraltimesfromonespecimentoanother,evenwhenspecimensarepreparedatthesametimeandinthesameway.Therefore,descriptionsofmaterialsandequipmentshouldbeascompleteaspossible,andshouldincludethefollowing:
12.2.1Descriptionofadherends,includingwoodspecies,density,graindirection,orientationofgrainwithrespecttobondline,andequilibriummoisturecontentattimeofassem-bly.
12.2.2Descriptionofadhesives,includingtype,composi-tion,tradename,dateofmanufacture,andlotnumber.
12.2.3Completedetailsofadhesivemixing,application,andjointassembly,usedinpreparingthespecimens.Includedescriptionofmixingequipment,timeandtemperatureofmixing,pot-lifeofadhesive,andtimedelaybetweenapplyingadhesivetofirstandlastspecimens(numbersequentially).De-scribedmethodofspreadingadhesive,adhesivespreadrate,timeofopenorclosedassembly,temperatureandrelativehumidityattimeofassembly.Includedetailsonadhesivesetting,suchasmethodofapplyingpressuretojoints,pressure,timeofpressure,andtemperatureandrelativehumiditywhilesetting.Ingeneral,provideenoughinformationtoenableanotheroperatortorepeat,ortraceadiscrepancyin,specimenpreparationprocedures.
12.2.4Dimensionsofshearareaandbondlinethicknessinallspecimens.
12.2.5Preconditioningandtestconditions,includingtem-perature,relativehumidity,andtimeofexposuretotheseconditions.
12.2.6Descriptionoftestingequipment,includingmake,model,andcapacityofmanufacturedequipment,suchas
FIG.3LogarithmicCreepModulusVersusTimeCurvesatVariousStressLevels
testingmachine,shearingtool,timer-recorder,microscope,andenvironmentalchambers.Adrawingofthecreep-testapparatusshowninFig.1shallbeincludedinthereport.
12.2.7Numberofspecimenstestedateachtestcondition.12.2.8Ratesofloadingforcreepandtime-to-failuretests.Alsorateofloadingfordeterminingultimateshearstrengthofadhesives.
12.2.9Stresslevelsforalltestsofcreepandtimetofailureateachsetoftemperatureandrelativehumidityconditions.12.2.10Measurementsofcreepstrainandtime,andtimetofailure,foralllevelsofstress,temperatureandrelativehumidity,foreachspecimen.
12.2.11Foreachspecimen,aplotofsheardisplacementversustimewithminimumcreeprateindicatedbyastraightlinedrawntocoincidewiththeregionofsecondarycreep.12.2.12Foreachtesttemperature,afamilyofstress−straincurveswithtimeasaparameter,asillustratedinFig.4.
12.2.13Foreachtesttemperature,aplotoflogbase−10ofcreepstrainversuslogbase−10oftimewithstressasaparameter,asillustratedinFig.5.
12.2.14Foreachtesttemperature,aplotoflogbase−10ofcreepmodulusversuslogbase−10oftimewithstressasaparameter,asillustratedinFig.3.
12.2.15Aplotofcreepmodulusversustemperaturewithlevelofstressasaparameter.
12.2.16Aplotofcreepmodulusversusstresswithtempera-tureasaparameter.
12.2.17Foreachtemperature,aplotoftheregression
FIG.4Stress-StrainCurvesatVariousTimes
FIG.5LogarithmicCreepStrainVersusTimeCurvesatVariousStressLevels
equationforlogbase−10ofstresslevel(expressedaspercentageofultimateshearstrength)versuslogbase−10oftimetofailure,asillustratedinFig.6.Showtheextrapolationtoalowerstresslevelasadottedlineextendingfromthesolidlineofactualdata.
13.PrecisionandBias
13.1Atthepresenttime,thereisnobasisforstatementsconcerningprecisionandbiasoftestresultsobtainedfromeitherwithin-laboratoryorbetween-laboratorytesting.Asthistestmethodisusedbytheoriginatinglaboratory7andothers,limitedstatementsofvariabilityinmeasurementsandtestresultscanbemade.
13.2Conventionalmeansandequipmenthavebeenusedinthistestmethodtomeasurecreepdeformation,timetofailure,andtestconditions.SuchmeansandequipmentaresimilartothoseusedinexistingASTMtestmethodswherecreepismeasuredinmetals,plastics,andadhesivebondstometals.Precisioninmeasurementscanbeexpectedtobesimilar,sinceproceduresdescribedhereinaresimilar.
TheoriginatinglaboratorywasMaterialsScienceandEngineering,USDAForestService,SoutheasternForestExperimentStation,ForestrySciencesLabora-tory,CarltonStreet,Athens,GA30602.
7FIG.6StressVersusLogofTimetoFailure
13.3Variabilityintestresultsinthistestmethod,asincreeptestsonothertypesofmaterials,canbeexpectedtobemoreaconsequenceofvariabilityinthematerialitselfthanfromlackofprecisioninmeasurement.Deformationunderload,andparticularlytimetofailure,maybeexpectedtoshowextremevariability,sometimesseveraltimesgreaterfromonespecimentoanother.Thisvariabilityisstronglyinfluencedbylevelofstress,temperature,relativehumidity,durationofload,bond-
linethickness,andofcourse,theadhesivematerialitself.Thehigherthesevalues,thegreaterthevariabilitybecomes.How-ever,thisvariabilityiscommontocreepandtime-to-failuretestsofmoststructuralmaterials.14.Keywords
14.1adhesive;compression;creep;shear;strain;stress
ANNEX
(MandatoryInformation)
A1.GUIDELINESFORSELECTINGTESTCONDITIONS
A1.1Selectingconditionsofstress,durationofload,temperature,andrelativehumidityforcreepandtime-to-failuretestsdependsontheinformationneededforaspecificapplication.Quiteoftenspecificrequirementsareknownbe-foretesting,anditisonlynecessarytodetermineiftheadhesivemeetsthoserequirementsatspecificconditions.Manytimes,however,theobjectiveistocharacterizeadhe-sives,ortoobtaindesigninformationthatcanbeusedtouniformlycomparecreepandtime-to-failuredataoftwoormoreadhesives.Mostofthefollowingremarksareapplicabletothelattercase.
A1.2Temperature—Temperaturecanhavedramaticeffectsoncreeppropertiesofadhesives,particularlythosecontainingthermoplasticmaterials.Whenpropertiesaretobedefinedasfunctionsoftemperature,selectenoughtemperaturessothatcurvesofthepropertiesversustemperaturescovertheusefultemperaturerangefortheadhesive.Usuallythisrequiresaminimumofthreetemperatures,butpreferablyfour.Fordesigndata,testtemperaturesshouldbethesameasthoseintheintendedapplication.
A1.3RelativeHumidity—Highlevelsofatmosphericmois-turecanhavedramaticeffectsonperformanceofadhesivesunderlong-termstaticloading,particularlyadhesivescontain-ingthermoplasticmaterials.Moistureeffectsareevenmorepronouncedatelevatedtemperatures.Foradhesivesthatarenotmoisture-sensitive,itisnotnecessarytodefinecreeppropertiesoverarangeofrelativehumidityconditions.Iftheadhesiveismoisturesensitivehowever,itshouldbetestedattheupperlimitsofmoistureandtemperatureoftheintendedserviceenvironment.
A1.4StressLevelsforCreepTests—Selectionofstresslevelsformeasuringcreepdependonintendedusesforthedata.Whenadhesivesmustsustainspecificdesignloads,eithershort-termorindefinite,creeppropertiesshouldbemeasuredatthoseloadsandserviceconditionsofintendeduse.Toobtainmorecompleteinformation,creepmaybemeasuredatthreestresslevelsforeachtemperatureforadhesivesthatshowsmallstressdependency.Creepmeasurementsatfourorfivestresslevelswillbenecessaryforadhesivesthataresignificantlyaffectedbystress.A1.5StressLevelsforTime-to-FailureTests—Theobjec-tiveofthistestistoproduceacurveofstressversustimetofailure(ateachtesttemperature)fromrelativelyshort-termloadingdata,andtoextrapolatethesedatatoapproximatethelong-termload-carryingcapabilityofanadhesiveatnormaldesignloads.Thatthelogarithmoftimetofailureforamaterialunderconstantdeadloadstressvariesinverselyasthestresslevel,isawellverifiedrelationship.Ithasbeenusedwithreasonableaccuracytopredictthelifetimeofmaterialssuchaswood,paper,textilefibers,plastics,andsteelunderdeadloadstress.Verylittledatahasbeenpublishedtoshowaccuracyoftherelationshipinpredictingbehaviorofadhesive-bondedjoints.Thereliabilityoflong-rangeextrapolationswilldependupontheaccuracyofshort-termmeasurements.Errorscanbeintroducedfrominaccuratedeterminationsofstresslevels,fromvariabilityinmaterials,andfromunanticipatedchangesinthefailuremechanismfromoutsidethelimitsofobserva-tion.Therefore,extrapolationsmustbeapproachedthroughcarefulconsideration.Nevertheless,safedesignstressesbasedonsuchextrapolations(andappropriatefactorsofsafety)havebeenusedforyearsformanymaterials.
A1.5.1Whendataonstressversustimetofailureatagiventemperatureareplottedonlogarithmicscales,astraightlineofincreasingnegativeslopeistheresult.Sometimes,however,astraighterlineisproducedwhenstressisplottedonalinearscaleandtimetofailureisplottedonalogarithmicscale.A1.5.2Adhesivesofhighcreepmodulususuallydonotfailinlessthanayearatstresslevelslessthan50%ofultimateshearstrength,atroomtemperature.Therefore,time-to-failuremeasurementsareusuallymadebetween90and50%ofultimatestress.Toestablishacurveofstressversustimetofailure,acommonpracticeistoloadspecimensatfourormoreevenlyspacedintervalsofstressbeginningat90%.Stressisexpressedasapercentageoftheaverageshort-termultimateshearstrengthofadhesivebonds.Itisdesirablethatatleastonedatasetateachstresslevelfallwithineachbase−10logoftimecycle.Fig.6isanexampleofthetypeofcurveproducedwhenstressisplottedonalinearscaleandtimetofailureonalogarithmicscale.Thedottedlinerepresentstheextrapolationtopredictedservicelifeatnormalstresslevels.
A1.5.3Theservicelifeofanadhesiveunderstaticloadingisaffectedbytemperatureofservice,andthiseffectshouldbe
determinedintime-to-failuretests.Curvesofstressversustimetofailureshouldbedeterminedatseveraltemperaturesovertherangeofexpectedservicetemperatures.Iftemperaturehasaminimumeffect,thenthecurveswillbenearlyparallelandformaconsistentfamilyofcurves.Iftherearechangesintheslopeofcurvesathighertemperatures,thensimilarchangesin
slopemayoccuratlowertemperatureswheretime-to-failurecurveswillbeextrapolatedtolongertimeperiods.Ifnochangesoccurintheslopeofcurvesathighertemperatures,theextrapolationsofcurvesatlowtemperaturescanbemadewithgreaterconfidence.
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