The Need for Surface Strain Measurement.docx
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The Need for Surface Strain Measurement.docx
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TheNeedforSurfaceStrainMeasurement
TheNeedforSurfaceStrainMeasurement
ByDavidW.Manthey,ProductDevelopment
RebeccaM.Pearce,BusinessDevelopment
CamSys,Inc.,Troy,NY,
Dr.DaeyongLee,ProfessorofMechanicalEngineering
RensselaerPolytechnicInstitute,Troy,NY
Oneofthemostcommonanalysisandevaluationmethodsusedforthesheetmetalstampingprocessismeasuringtheextentofdeformationincriticalareasonastampedpart.Theextentofdeformationissometimesrelatedtoformingseverity,wherethelevelofdeformationiscategorizedassafe,marginalorfailure.Inmanypressshops,strainmeasurementisusedasawaytoassesstheformabilityofastamping.
Fig.1--Mylartapeusedtomeasuremajorandminorstrainsfromdeformedcircles.
Surfacestraindatacanbeusedeffectivelytodiagnoseproductionproblems,andidentifypotentialfailuresitesandcontributingfactors.Thesedataalsoareusedtoverifypredictedresultsfromfiniteelementanalysisprograms(FEA).Manytechniqueshavebeendevelopedformeasuringstrainonsheetmetalparts.Tousethesetechniques,itisusuallynecessarytoapplyapatterntothesheetsurfacebeforeforming.Patternsofcirclesorsquaresarecommonlyused[1,2].Anexampleofacommonmanualstrainmeasurementtool,themylartape,isshowninFig.1,andadescriptionofstrainmeasurementusingcirclesisshowninFig.2[3].
Fig.2--Measurementofsurfacestrainsusingcircles.
Duringthestampingoperation,thegridpatterndeformswiththematerial,thenstraincanbemeasuredfromthedeformedpatternbycomparingittotheoriginalsizeofthepattern.Mostofthestrainmeasurementsdonetodayareperformedmanually,wherethemaximumandminimumlengthsacrossadeformedcirclearemeasured.Typesoftoolsusedinmanualmeasurementincludetravelingmicroscopes,mylartapewithgraduations,andruleranddividers.Thestraindatacanthenbeplottedonaforminglimitdiagram(FLD).Thisdiagramrelatesmeasuredstrainstothematerialformabilitylimits.AnalyzingthedataonanFLDisthemostcommonwaytoevaluatetheformabilityofastampedpart[4,5].
Sincemanualstrainmeasurementmethodscanbelaborintensiveandtheresultsaresignificantlyaffectedbyuserinterpretation,considerableefforthasbeendirectedtowarddevelopingautomatedmethodsforstrainmeasurement.OnemethodthatrepresentsthelatestdevelopmentsinthistechnologyistheAutomatedStrainAnalysisandMeasurementEnvironment(ASAME).Thissystemhasbeendescribedinpreviouspapers[6,7].
Currently,ASAMEisbeingusedineitheraproductionorresearchenvironmentatFord,GeneralMotors,MotorWheel,ALCOA,Fiat,Peugeot,Hyundai,Nissan,Toyota,NipponSteelandmanyotherleadingcompaniesthroughouttheworld.Customerapplicationsincludeproductiontroubleshooting,verificationofpredictedresults,dietryoutanddevelopmentofmaterialforminglimits.
GeometryandStrainMeasurementwithASAME
Tomeasurethesurfacegeometryandstrainonanareaofaformed(stamped)partwithASAME,firstasquareorcirclegridisappliedtothesurfaceofthesheetmaterialbeforeforming.Subsequently,ASAMEisusedtotakeseveralphotographsofanareaonthepart.Advancedimageprocessingsoftwareisusedtoidentifyandcomputethethree-dimensionalcoordinatesofintersectingpointsonthegridpattern[6,7].
SurfacegeometryandstraincanbemeasuredusingtwodifferentASAMEmethods.BothASAMEmethodsrequirethatagridpatternispresentonthesurfaceofapartbeforedeformation.
MethodA-Position-basedSystem
Fig.3--PositionbasedmeasurementsystemconsistsofaCCDcameramountedonpositioningequipment,andacomputercapableofacceptingvideoinputandpositioningencoderdata.
ThismeasurementsystemconsistsofaCCDcameramountedonpositioningequipment,andacomputercapableofacceptingvideoinputandpositioningencoderdata,Fig.3.Theparttobemeasuredisplacedonaturntable,andthentwoormorephotographsaretakenfromdifferentangles.Thephotographsthenareprocessedthroughtheuseofcomputersoftwaretolocatethe2-Dgridcoordinates.The3-Dcoordinatesaredeterminedusingthe2-Dcoordinates,camerapositionandinternalcameraparameters.Theinternalcameraparametersaredeterminedthoughacalibrationprocedurepriortomeasuringapart.
Ingeneral,theposition-basedsystemyieldsanaccuracyofabout±2percentstrainusingtwophotographsand±1.5percentstrainusingthreephotographspermeasurement.
MethodB-Target-basedSystemforMeasuringPrintedGrids
Fig.4--Recentlydevelopedtarget-basedsystemformeasuringprintedgridsdoesnotrequireknowledgeofcamerapositionorinternalcamerageometry.
Anewmeasurementmethod(MethodB)hasbeendevelopedrecentlythatdoesnotrequireknowledgeofthecamerapositionorinternalcamerageometrypriortomakingameasurement.ThismethodusesaKodakDCS420cdigitalcamera,acomputercapableofreadingPCMCIAharddrives,andaphotogrammetrytarget.Thephotogrammetrytargetisanobjectofknowndimensionswitheasilyidentifiablemarkings:
a25mmcubewitha5mmgridmachinedintoeachside,Fig.4.
Thetargetcubeisplacednexttotheparttobemeasured,andphotographsthenaretakenfromtwoormorelocationsusingthedigitalcamera.Eachphotographmustincludethetargetaswellastheareatobemeasured.TheKodakcamerastoresthephotographsonaPCMCIAharddrive,whichthenisremovedfromthecameraandinsertedintothecomputerforprocessingbytheuserwiththeASAMEsoftwaretogeneratethesurfacegeometryandstraindata.
Ingeneral,thetarget-basedsystemyieldsanaccuracyofabout±2.5percentstrainusingtwophotographsand±1.5percentstrainusingthreephotographs.
MethodC-Target-basedSystemforMeasuringProjectedGrids
Fig.5--Newtarget-basedASAMEcanbeusedformeasuringcoordinatedatabyprojectingagridontothesurfaceofthepart.Thus,griddoesnothavetobepermanentlyappliedtothesurface.
Thenewtarget-basedASAMEalsocanbeusedformeasuringcoordinatedatabyprojectingagridontothesurfaceofthepart,Fig5.Inthisway,agriddoesnothavetobepermanentlyappliedtothepartsurface.Asquaregridfroma35mmslideisprojectedontheobject,andtheKodakDCSisusedtotaketwoormorephoto-graphsofanobjectfromanyorientation.ThephotographsthencantransferredtothecomputerforprocessingasinMethodB.[7]
Fig.6--Asamplephotographfromprojectedgridmeasurementofanautomobiledoorpanel.
GeometryMeasurement
Tomeasuresurfacegeometry,theposition-basedmeasurementsystemrequiresthatagridbepermanentlyappliedontheoriginalsurfaceofapart.Inpractice,anon-contactmethodofmakingmeasurementsisdesirable,wherethematerialsurfaceisnotaffected.Thenewtarget-basedmeasurementsystemcanbeusedtomeasurethesurfacegeometryofanon-griddedpartbyprojectingagridoflightfromaslideprojectorontothepartareatobemeasured.
Anareaonanautomobilepanelwasmeasuredwiththetarget-basedsystemusingbothaprintedgridandaprojectedgrid.Thegridwasprojectedsuchthatthesquaresonthepartsurfacewereasimilarsizetothoseoftheprintedgrid.However,thetwogridpatternswerenotaligned,norweretheyidenticalinsize.Eachofthetwomeasurementswasmadewithfivephotographs.Thetargetwasnotinexactlythesamepositionforbothmeasurements.AsamplephotographfromtheprojectedgridmeasurementisshowninFig.6.
ComparisonwithCoordinateMeasurementMachines(CMM)
Fig.7--B-carMBRfrontsideINRforminglimitdiagram:
materialSPRC35-EisshownontheleftandmaterialSPRC35-Risshownontheright.
FormanyCMMs,thereareatleasttwomajorissuestobeexamined:
oneistheaccuracyandtheotherthetimerequiredtoproducesatisfactorydata.Asfarastheaccuracyisconcerned,mostofthecommercialmechanicalsystemsproducehighaccuracy.Manymanufacturersstatealinearaccuracyrangeof0.025mmto0.0125mm[8].Onemanufacturerclaimsanaccuracyof0.1micrometer[9].Alaser-basedsystemreportsanaccuracyof0.005mmto0.0125mm[9].
AsummaryofthecomparisonofthetwomethodsforcoordinatemeasurementisgiveninTableIforseveralmainissues.
TableI--ComparisonofDCSCameraSystemandConventionalCMMs
Issues
DCSCameraSystem
ConventionalCMMs
Probe-BasedandLaser-Based)
LinearAccuracy
±0.05mmto0.3mm1
±0.1umto0.0125mm
MeasurementArea
BasedonCamerafield-of-view2
Limitedbypositioninghardware
MeasurementTime
-20minutesperarea
Relativelylongtime
(dependentonnumberofprobecontacts)
Portability
Completelyportable
NotPortable
Cost
$50,000-$60,000
$100,000-$500,000
AdditionalFeatures
Surfacestrainscanbemeasuredifagridwasappliedtotheunderformedblank
Nostrainmeasurementcapability
1Dependsonthenumberofphotographsusedtomakemeasurementsandthesizeofthefieldofview.
2Areaslargerthanthefieldofviewcanbemeasuredbycombiningsmallermeasurements.Alargertargetcanbeusedtoallowalargerareatobemeasured.
ApplicationsEliminatingTearingforCarFrontSide
Fig.8a--CurrentmaterialstraindataoverlaidonnewmaterialFLD.Allpointsarewithinthesafetylimitforthenewmaterial.
In1994,HyundaiMotorCorporationpublishedapaperentitled"AProblemSolvingPlaninAutomotivePanelFormingUsingtheAutomatedStrainAnalysisandMeasurementEnvironment"[10].Inonecasestudyoutlinedinthispaper,ahighstrengthsteel(SPRC35-R)wasusedtoproducetheMBRfrontsideoftheB-car.Itwasobservedthattearingoccurredfrequent
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