Instruction Bulletin B-129-8 Surface Preparation for Strain Gage

SurfacePreparationforStrainGageBonding

Document No.:11129 1 of 7 Revision 19-Dec-11

Micro-Measurements [email protected] www.micro-measurements.com

Instruction Bulletin B-129-8

Micro-Measurements

SurfacePreparationforStrainGageBonding

1.0 Introduction

Straingagescanbesatisfactorilybondedtoalmostany

solidmaterialifthematerialsurfaceisproperlyprepared.

Whileaproperlypreparedsurfacecanbeachievedinmore

thanoneway,thespecificproceduresandtechniques

describedhereofferanumberofadvantages.Tobeginwith,

theyconstituteacarefullydeveloped

andthoroughlyproven

system;and,whentheinstructionsarefollowedprecisely

(alongwiththoseforgageandadhesivehandling),the

consistentresultwillbestrongstablebonds.Theprocedures

aresimpletolearn,easytoperform,andreadilyreproducible.

Furthermore,thesurfacepreparationmaterialsusedinthese

proceduresare,unlessotherwisenoted,generallylowin

toxicity,anddonotrequirespecialventilationsystemsor

otherstringentsafetymeasures.Ofcourse,aswithany

materialscontainingsolventsorproducingvapors,adequate

ventilationisnecessary.

Theimportanceofattentiontodetail,andpreciseadherence

toinstructions,cannotbeoverstressedinsurfacepreparation

for

straingagebonding.Lessthorough,orevencasual,

approachestosurfacepreparationmaysometimesyield

satisfactorygageinstallations;butforconsistentsuccessin

achievinghigh‐qualitybonds,themethodsgivenherecan

berecommendedwithoutqualification.Fundamentaltothe

Micro‐Measurementssystemofsurfacepreparation

isanunderstandingofcleanliness

andcontamination.All

opensurfacesnotthoroughlyandfreshlycleanedmustbe

consideredcontaminated,andrequirecleaningimmediately

priortogagebonding.Similarly,itisimperativethatthe

materialsusedinthesurfacepreparationbefresh,clean,

anduncontaminated.Itisworthnotingthatstraingagesas

receivedfromMicro‐Measurementsarechemically

clean,andspeciallytreatedontheundersidetopromote

adhesion.Simplytouchingthegageswiththefingers

(whicharealwayscontaminated)canbedetrimentalto

bondquality.

TheMicro‐Measurementssystemofsurfacepreparation

includesfivebasicoperations.Theseare,intheusual

orderofexecution:

1.Solvent

degreasing

2.Abrading(dryandwet)

3.Applicationofgagelayoutlines

4.Conditioning

5.Neutralizing

Thesefiveoperationsarevariedandmodifiedforcompatibility

withdifferenttestmaterialproperties,andexceptions

areintroducedasappropriateforcertainspecialmaterials

andsituations.

Thesurfacepreparationoperationsaredescribedindividually

in

Section2.0,followingasummaryofthegeneral

principlesapplicabletotheentireprocess.Section3.0

discussesspecialprecautionsandconsiderationswhichshould

beborneinmindwhenworkingwithunusualmaterials

and/orsurfaceconditions.

Thevarioussurfacepreparationandinstallationaccessories

referredtothroughoutthisApplicationNoteare

Micro‐

MeasurementsAccessories,listedinStrainGage

AccessoriesDataBookandavailabledirectlyfromMicro‐

Measurements.

Asaconveniencetothegageinstallerinquicklydetermining

thespecificsurfacepreparationstepsapplicabletoany

particulartestmaterial,Section4.0includesachartlisting

approximately75common(anduncommon)materialsand

thecorresponding

surfacepreparationtreatments.



2.0BasicSurfacePreparation

OperationsandTechniques

2.1GeneralPrinciplesofSurface

PreparationforStrainGageBonding



Thepurposeofsurfacepreparationistodevelopachemically

cleansurfacehavingaroughnessappropriatetothe

gageinstallationrequirements,asurfacealkalinity

correspondingtoapHof

7orso,andvisiblegagelayoutlines

forlocatingandorientingthestraingage.Itistowardthis

purposethattheoperationsdescribedherearedirected.

Asnotedearlier,cleanlinessisvitalthroughoutthesurface

preparationprocess.Itisalsoimportanttoguardagainst

recontaminationofaonce‐cleanedsurface.

Followingare

severalexamplesofsurfacerecontaminationtobeavoided:

a.Touchingthecleanedsurfacewiththefingers.

b.Wipingbackandforthwithagauzesponge,orreusinga

once‐usedsurfaceofthesponge(orofacottonswab).

c.Draggingcontaminantsintothecleanedareafromthe



uncleanedboundaryofthatarea.

d.Allowingacleaningsolutiontoevaporateonthesurface.

e.Allowingacleanedsurfacetositformorethanafew

minutesbeforegageinstallation,orallowingapartially

preparedsurfacetositbetweenstepsinthecleaning

procedure.

Beyondtheabove,itis

goodpracticetoapproachthesurface

preparationtaskwithfreshlywashedhands,andtowash

handsasneededduringtheprocedure.



2.2SolventDegreasing

Degreasingisperformedtoremoveoils,greases,organic

contaminants,andsoluablechemicalresidues.Degreasing

shouldalwaysbethefirstoperation.Thisistoavoidhaving

subsequent

abradingoperationsdrivesurfacecontaminants

intothesurfacematerial.Porousmaterialssuchastitanium,

SurfacePreparationforStrainGageBonding

Document No.:11129 2 of 7 Revision 19-Dec-11

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Instruction Bulletin B-129-8

Micro-Measurements

castiron,andcastaluminummayrequireheatingtodrive off

absorbedhydrocarbonsorotherliquids.

Degreasingcanbeaccomplishedusingahotvapordegreaser,

anultrasonicallyagitatedliquidbath,aerosoltypespraycans

ofCSM‐2Degreaser,orwipingwithGC‐6IsopropylAlcohol.

One‐wayapplicators,suchasthe

aerosoltype,ofcleaning

solventsarealwayspreferablebecausedissolvedcontaminants

cannotbecarriedbackintotheparentsolvent.Whenever

possible,theentiretestpieceshouldbedegreased.Inthecase

oflargebulkyobjectswhichcannotbecompletelydegreased,

anareacovering4to6in[100to150mm]onallsidesofthe

gageareashouldbecleaned.Thiswillminimizethechanceof

contaminationinsubsequentoperations,andwillprovidean

areaadequatelylargeforapplyingprotectivecoatingsinthe

finalstageofgageinstallation.



2.3SurfaceAbrading

General—Inpreparationforgageinstallation,thesurface

is

abradedtoremoveanylooselybondedadherents(scale,rust,

paint,galvanizedcoatings,oxides,etc.),andtodevelopa

surfacetexturesuitableforbonding.Theabradingoperation

canbeperformedinavarietyofways,dependinguponthe

initialconditionofthesurfaceandthedesiredfinishforgage

installation.For

roughorcoarsesurfaces,itmaybenecessary

tostartwithagrinder,discsander,orfile.(Note:Before

performinganyabradingoperations,seeSection3.1forsafety

precautions.)Finishabradingisdonewithsilicon‐carbidepaper

oftheappropriategrit,andrecommendedgritsizesforspecific

materialsaregiveninSection4.0.

Ifgritblastingisusedinsteadofabrading,eithercleanalumina

orsilica(100to400grit)issatisf actory.Inanycase,theair

supplyshouldbewellfilteredtoremoveoilandother

contaminantvaporscomingfromtheaircompressor.

Thegritusedinblasting

shouldnotberecycledorused

againinsurfacepreparationforbondingstraingages.

Theoptimumsurfacefinishforgagebondingdepends

somewhatuponthenatureandpurposeoftheinstallation.

Forgeneralstressanalysisapplications,arelativelysmooth

surface(intheorderof100μin[2.5μm]rms)issuitable,



andhastheadvantageoverroughersurfacesthatitcanbe

cleanedmoreeasilyandthoroughly.Smoothersurfaces,

compatiblewiththethin"gluelines"requiredforminimum

creep,areusedfortransducerinstallations.Incontrast,

whenveryhighelongationsmustbemeasured,arougher

(andpreferablycross‐hatched)surfaceshouldbe

prepared.

Therecommendedsurfacefinishesforseveralclassesof

gageinstallationsaresummarizedinTableI,below.



TABLEI

CLASSOF

INSTALLATION

SURFACEFINISH,rms

µin µm

Generalstressanalysis 63‐125 1.6‐3.2

Highelongation

>250 >6.4

cross‐hatched

Transducers 16‐63 0.4‐1.6



WetAbrading—WheneverM‐PrepConditionerAis

compatiblewiththetestmaterial(seeSection4.0),the

abradingshouldbedonewhilekeepingthesurfacewetwith

thissolution,ifphysicallypracticable.ConditionerAisa

mildlyacidicsolutionwhichgenerallyacceleratesthecleaning

processand,onsomematerials,

actsasagentleetchant.Itis

notrecommendedforuseonmagnesium,syntheticrubber,or

wood.

2.4Gage‐LocationLayoutLines

Thenormalmethodofaccuratelylocatingandorientinga

straingageonthetestsurfaceistofirstmarkthesurface

withapairofcrossedreferencelines

atthepointwherethe

strainmeasurementistobemade.Thelinesaremade

perpendiculartooneanother,withonelineorientedinthe

directionofstrainmeasurement.Thegageistheninstalled

sothatthetriangularindexmarksdefiningthelongitudinal

andtransverseaxesofthegridarealigned

withthereference

linesonthetestsurface.

Thereference,orlayout,linesshouldbemadewithatoolthat

burnishes,ratherthanscoresorscribes,thesurface.Ascribed

linemayraiseaburrorcreateastressconcentration.Ineither

case,suchalinecanbedetrimentaltostraingage

performanceandtothefatiguelifeofthetestpart.On

aluminumandmostothernonferrousalloys,a4Hdrafting

pencilisasatisfactoryandconvenientburnishingtool.

However,graphitepencilsshouldneverbeusedonhigh

temperaturealloys,wheretheoperatingtemperaturemight

causeacarbonembrittlement

problem.Ontheseandother

hardalloys,burnishedalignmentmarkscanbemadewitha

ballpointpenoraround‐pointedbrassrod.Layoutlinesare

ordinarilyappliedfollowingtheabradingoperationandbefore

finalcleaning.Allresiduefromtheburnishingoperationshould

beremovedbyscrubbingwithConditionerAasdescribedin

thefollowingsection.



2.5SurfaceConditioning

Afterthelayoutlinesaremarked,ConditionerAshouldbe

appliedrepeatedly,andthesurfacescrubbedwithcotton

tippedapplicatorsuntilacleantipisnolongerdiscolored

bythescrubbing.Duringthisprocessthesurfaceshouldbe

keptconstantlywetwith

ConditionerAuntilthecleaningis

completed.Cleaningsolutionsshouldneverbeallowedto

dryonthesurface.Whenclean,thesurfaceshouldbedried

bywipingthroughthecleanedareawithasingleslow

strokeofagauzesponge.Thestrokeshouldbegininside

SurfacePreparationforStrainGageBonding

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thecleanedareatoavoiddraggingcontaminantsinfromthe

boundaryofthearea.Then,withafreshsponge,asingle

slowstrokeismadeintheoppositedirection.Thesponge

shouldneverbewipedbackandforth,sincethismayredeposit

thecontaminantsonthecleanedsurface.



2.6Neutralizing

Thefinalstepinsurfacepreparationistobringthesurface

conditionbacktoanoptimumalkalinityof7.0to7.5pH,

whichissuitableforallMicro‐Measurementsstrain

gageadhesivesystems.Thisshouldbedonebyliberally

applyingM‐PrepNeutralizer5Atothecleanedsurface,and

scrubbingthesurface

withacleancotton‐tippedapplicator.

Thecleanedsurfaceshouldbekeptcompletelywetwith

Neutralizer5Athroughoutthisoperation.Whenneutralized,

thesurfaceshouldbedriedbywipingthroughthecleaned

areawithasingleslowstrokeofacleangauzesponge.

Withafreshsponge,asinglestrokeshouldthenbemadein

theoppositedirection,beginningwiththecleanedareato

avoidrecontaminationfromtheuncleanedboundary.

Iftheforegoinginstructionsarefollowedprecisely,thesurface

isnowproperlypreparedforgagebonding,andthe

gageorgagesshouldbeinstalledassoonaspossible.



3.0Special

PrecautionsandConsiderations



3.1SafetyPrecautions

Asinanytechnicalactivity,safetyshouldalwaysbea

primeconsiderationinsurfacepreparationforstraingage

bonding.Forexample,whengrinding,discsanding,or

filing,theoperatorshouldwearsafetyglassesandtakesuch

othersafetyprecautionsasspecifiedbyhisorganization

or

bytheOccupationalSafetyandHealthAdministration

(OSHA).

Whendealingwithtoxicmaterialssuchasberyllium,lead,

uranium,plutonium,etc.,allproceduresandsafetymeasures

shouldbeapprovedbythesafetyofficeroftheestablishment

beforecommencingsurfacepreparation.



3.2SurfacesRequiringSpecialTreatment

Concrete—Concretesurfacesare

usuallyuneven,rough,

andporous.Inordertodevelopapropersubstrateforgage

bonding,itisnecessarytoapplyalevelingandsealingprecoat

ofepoxyadhesivetotheconcrete.Beforeapplyingthe

precoat,theconcretesurfacemustbepreparedbya

procedurewhichaccountsfortheporosityofthis

material.

Contaminationfromoils,greases,plantgrowth,andother

soilsshouldberemovedbyvigorousscrubbingwitha

stiff‐bristledbrushandamilddetergentsolution.Thesurface

isthenrinsedwithcleanwater.Surfaceirregularities

canberemovedbywirebrushing,discsanding,orgritblasting,

afterwhichallloosedustshouldbeblownorbrushed

fromthesurface.



ThenextstepistoapplyConditionerAgenerouslytothe

surfaceinandaroundthegagingarea,andscrubthearea

withastiff‐bristledbrush.ContaminatedConditionerA

shouldbeblottedwithgauzesponges,andthenthesurface

shouldberinsedthoroughlywithcleanwater.Following

thewaterrinse,thesurfaceaciditymustbereducedby

scrubbingwithNeutralizer5A,blottingwithga uzesponges,

andrinsingwithwater.Afinalthoroughrinsewith

distilledwaterisusefultoremovetheresidualtracesof

water‐solublecleaningsolutions.Beforeprecoating,the

cleanedsurfacemustbethoroughlydried.Warmingthe

surfacegentlywithapropanetorchorelectricheatgunwill

hastenevaporation.

Micro‐MeasurementsM‐BondAE‐10room‐temperature‐curing

epoxyadhesiveisanidealmaterialforprecoatingthe

concrete.Forthosecasesinwhichthetesttemperaturemay

exceedthespecifiedmaximumoperatingtemperatureofAE‐

10(+200°F[+95°C]),itwillbenecessarytofillthesurfacewith

ahighertemperatureresinsystemsuchasM‐BondGA‐61.

Inapplyingthecoatingtotheporousmaterial,theadhesive

shouldbeworkedintoanyvoids,andleveledtoformasmooth

surface.Whentheadhesiveiscompletelycured,itshouldbe

abradeduntilthebasematerialbeginstobeexposedagain.

Followingthis,theepoxysurfaceiscleanedandprepared

conventionally,accordingtothe procedurespecifiedinSection

4.0forbondinggagestoepoxies.

PlatedSurfaces—Ingeneral,

platedsurfacesaredetrimental

tostraingagestability,anditispreferabletoremovethe

platingatthegagelocation,ifthisispermissible.Cadmium

andnickelplatingareparticularlysubjecttocreep,andeven

harderplatingsmaycreepbecauseoftheimperfectbond

betweentheplatingandthebasemetal.

Whenitisnot

permissibletoremovetheplating,thesurfaceshouldbe

preparedaccordingtotheproceduregiveninSection4.0

forthespecificplatinginvolved.Notethatitmaybenecessary

toadjusttestingprocedurestominimizetheeffectsof

creep.

UseofSolventsonPlastics—Plastics

varywidelyintheir

reactivitytosolventssuchasthoseemployedinthesurface

preparationproceduresdescribedhere.Beforeapplyinga

solventtoanyplastic,Section4.0,whichincludesmost

commonplastics,shouldbereferredtofortherecommended

compatiblesolvent.ForplasticsnotlistedinSection

4.0,themanufacturer

ofthematerialshouldbeconsulted,

ortestsshouldbeperformedtoverifynonreactivitybetween

thesolventandtheplastic.

DimensionalorMechanicalChangesDuetoSurface

Preparation—Formostmaterials,strainmeasurementresults

areusuallynotsignificantlychan gedbythesurface

preparationproceduresdescribedinthisApplicationNote.

Even

withappreciablematerialremoval,effectsonthestatic

mechanicalpropertiesofthetestpartaregenerallynegligi‐

blecomparedtoothererrorsourcesintheexperiment.It

SurfacePreparationforStrainGageBonding

Document No.:11129 4 of 7 Revision 19-Dec-11

Micro-Measurements [email protected] www.micro-measurements.com

Instruction Bulletin B-129-8

Micro-Measurements

shouldbeunderstood,however,thatremovalofaplatedor

hardenedsurfacelayer,orofasurfacelayerwithsignificant 

residualstresses,maynoticeablyaffectthefatiguelifeor

thewearcharacteristicsofthepartwhenoperatedunder

dynamicserviceconditions.

SiliconeContamination—Thepropertiesofsilicones

whichmakethemexcellent

lubricantsandmold‐release

agentsalsomakethemtheenemiesofadhesion,and

thereforepotentiallythemostseriousofcontaminantstobe

encounteredinthepracticeofbondingstraingages.The

problemiscompoundedbythehighnaturalaffinityof the

siliconesformostmaterials,andbytheirtendencyto

migrate.Furthermore,sincesiliconesarerelativelyinert

chemically,andunaffectedbymostsolvents,theyare

amongthemostdifficultsurfacecontaminantstoremove.

Thebestpracticeistokeepthegage‐bondingareafreeof

silicones.Thismaynotbeaseasyasitsounds,sincethe

widelyusedsiliconescanbeintroducedfromavarietyof

sources.Forinstance,manyhandcreamsandcosmetics

containsilicones,andtheseshouldnotbeusedbypersons

involvedingageinstallation.Someofthemachininglubricants

alsocontainsilicones,andsuchlubricantsshouldbe

avoidedwhenmachiningpartsthataretohavestrain

gages

installed.Similarly,silicone‐saturatedcleaningtissuesfor

eyeglassesshouldnotbeusedinthegage‐bondingareaor

bygage‐installationpersonnel.

Regardlessofeffortstoavoidsilicones,contaminationmay

stilloccur.Lightcontaminationcansometimesberemoved

bycleaningwithConditionerA,preferablyheatedto

+200°F[+95°C].Moreseverecasesmayrequirespecial

cleaningsolutionsandprocedures,recommendationsfor

whichshouldbeobtainedfromthemanufacturerofthe

siliconecompoundinvolvedinthecontamination.



4.0IndexofTestMaterialsand

SurfacePreparationProcedures

Inthissection,thespecificstep‐by‐stepsurfacepreparation

proceduresaregivenforapproximately75

differentmaterials.

Forcompactness,andconvenient,quickaccesstothe

procedureforanyparticularmaterial,theinformationis

presentedinchartforminTableII.Thetestmaterialsare

listedalphabetically,fromABSPlasticstoZirconium;and

thecompleteprocedureforeachmaterialisdefinedbyone

ormoredigits

ineachoftheapplicableoperations columns

ofthetable.Eachdigitidentifiestherequiredoperationand

specifiesthestepnumberforthatoperationinthecomplete

procedure.

Forexample,assumethatthenecessityarisesforbonding

oneormorestraingagestoabrasstestspecimen.Reading

downtheSpecimen

MaterialcolumnofTableIItoBrass,

andfollowingthatrowacrossthetabletotheright,thefirst

stepinsurfacepreparationconsistsofdegreasingthe

specimenwithCSM‐2Degreaser.Thesymbol(1)inthe

IsopropylAlcoholcolumnindicatesthatthisisasuitable

substitutedegreasingoperation.Continuing

acrosstherow,

thesecondoperationcallsforabradingthespecimensurface

with320‐gritsilicon‐carbidepaper.Inthethirdoperation

thespecimenisreabradedwith400‐gritsilicon‐carbide

paper,wetlappingwithConditionerAiffeasible.The

fourthandfifthoperationsconsistofapplyinglayoutlines

forlocating

thegages,andscrubbingthesurfacecleanwith

ConditionerA.Cleaningwithisopropylalcoholisthefinal

operationintheprocedure.

IntheRemarkscolumn,itisrecommendedthatthegages

beinstalledwithin20minutesaftercompletingthesurface

preparation,becausethefreshlybaredbrasssurfacetendsto

oxidize

rapidly.Inaddition,intheGritBlastcolumn,the

gageinstallerisspecificallyadvisednottosubstitutegrit

blastingforothersurfaceabradingmethods,inorderto

avoidsignificantlyalteringthesurfaceconditionofthis

relativelysoftmaterial.Surfacepreparationproceduresfor

othermaterialsaredefinedsimilarlyinthetable,and,in

manycases,accompaniedbyspecialwarningsor

recommendationsintheRemarkscolumn.Whenanoperation

notincludedinthefirsttencolumnheadingsisrequired,itis

indexedintheOthercolumn,withanarrowpointingtothe

Remarkscolumnwheretheoperationisspecified.



AdditionalReferences

Foradditional

information,refertoInstructionBulletins

listedbelow:

B‐127,"StrainGageInstallationswithM‐Bond200Adhesive".

B‐130,"StrainGageInstallationswithM‐Bond43‐B,600,

and610Adhesives".

B‐137,"StrainGageApplicationswithM‐BondAE‐10,

AE‐15,andGA‐2AdhesiveSystems".



ImportantNotice



Theprocedures,operations,andchemicalagents

recommendedinthisApplicationNoteare,tothe

bestknowledgeofMicro‐Measurements,

reliableandfitforthepurposesforwhichrecommended.

Thisinformationonsurfacepreparationforstraingage

bondingispresentedingoodfaithasanaidtothestrain

gageinstaller;butnowarranty,expressedorimplied,is

given,norshallMicro‐Measurementsbeliableforany

injury,loss,ordamage,directorconsequential,connected

withtheuseoftheinformation.Beforeapplyingthe

procedurestoanymaterial,theuserisurgedtocarefully

reviewtheapplicationwithrespecttohuman

healthandsafety,andtoenvironmentalquality



SurfacePreparationforStrainGageBonding

Document No.:11129 5 of 7 Revision 19-Dec-11

Micro-Measurements [email protected] www.micro-measurements.com

Instruction Bulletin B-129-8

Micro-Measurements



TABLEII

IndexofTestMaterialsandSurfacePreparationProcedures(Sheet1of3)



SurfacePreparationforStrainGageBonding

Document No.:11129 6 of 7 Revision 19-Dec-11

Micro-Measurements [email protected] www.micro-measurements.com

Instruction Bulletin B-129-8

Micro-Measurements

IndexofTestMaterialsandSurfacePreparationProcedures(Sheet2of3)

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SurfacePreparationforStrainGageBonding

Document No.:11129 7 of 7 Revision 19-Dec-11

Micro-Measurements [email protected] www.micro-measurements.com

Instruction Bulletin B-129-8

Micro-Measurements

IndexofTestMaterialsandSurfacePreparationProcedures(Sheet3of3)

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