Large deformationmeasurementschemefor3Ddigitalimage correlation method correlation method

a b s t r a c t   Difficultiesoftenarisefordigitalimagecorrelation(DIC)techniquewhenseriousde-correlationoccurs between the referenceimageandthedeformedimageduetolargedeformation.Anupdatingreference
image schemecouldbeemployedtodealwithlargedeformationsituation,howeverthatwillintroduce accumulatederrors.

Alargedeformationmeasurementscheme,combiningimprovedcoarsesearch method andupdatingreferenceimagescheme,isproposedinthispaper.Foraseriesofdeformation images, thecorrelationcalculationbeginswithaseedpointandspreadsout.Animprovedcoarsesearch method isdevelopedtocalculatetheinitialcorrelationparametersfortheseedpoint,whichguarantees that thecorrelationcalculationcanbecarriedoutsuccessfullyeveninlargedeformationsituation.

Only for extremelylargedeformation,thereferenceimageisupdated.Usingthismethod,notonlyextremely large deformationcanbemeasuredsuccessfullybutalsotheaccumulatederrorcouldbecontrolled.Apolymer materialtensiletestandafoamcompressiontestareusedtoverifytheproposedscheme.

Experimentalresultsshowthatupto450%tensiledeformationand83%compressiondeformationcan be measuredsuccessfully.

1. Introduction

Digital imagecorrelation(DIC)method [1] can providefull-field displacementandsurfacestrainfieldofmeasurementobject,by comparingtwoobjectdigitalimagesbeforeandafterdeforma-tion. 2DDIC [2], whichisusedwithasinglecamera,canmeasure only inplanedisplacement/strainfieldsonplaneobjects,while
3D DIC [3] technique, whichcombinesDICwithstereovision,can measure the3Ddisplacementfieldandsurfacestrainfieldof3D object. Duetoitsadvantagesofsimpleequipment,highprecision and non-contactmeasurement,theDICmethodhasbeenused widely fordeformationmeasurementtheseyears.Sztefeketal.

[4] used theDICmethodtodeterminethesurfacestrainofbone during loading.Stanfordetal. [5] made useoftheDICmethodto
measure thewingdeformationofmicroairvehiclesandthe measurement resultiscomparedwiththenumericalresult.

Miehe etal. [6] applied theDICmethodtothesurfacestrain measurement ofamorphousglassypolymers.TheDICmethodis employed inBambach’sresearch [7] to capturethefull-surface transverse bucklingdeformationoftheflangesandwebsand the results areusedtodescribeandexplainthefundamentalbehavior of flangesandwebs.ExperimentsbasedonDICmethodwere performed byHargatherandSettles [8] to measurethedeforma- tion ofthinplatesinresponsetoexplosiveblast.

In recentyears,muchstudyhasbeencarriedoutonDIC method forimprovingitsaccuracy,robustnessandcomputational efficiency. Varioussub-pixelregistrationalgorithms [9] are devel-oped, andtheperformanceofdifferentmethodsisdiscussed.The
effect ofsubsetsizeusedinDIChasbeeninvestigated,andatheoretical modelhasbeenderivedforselectingpropersubset size toimprovemeasurementaccuracy [10]. Theimpactofinterpolation phaseerrorisstudiedbySchreieretal.

[11] and ahigh-order interpolationschemeishighlyrecommendbythem.Lu andCary [12] proposed asecond-ordermappingfunction, which makesthemeasurementmoreaccuratewhenhigherorder deformation occurs.Pan [13] developed areliability-guidedDIC (RG-DIC) thatmakesthecorrelationcalculationmorerobust.A two-step extendedDIC(X-DIC)wasdevelopedbyChenetal.

[14]to measurethefull-fielddisplacementsacrossthecrackdomain.A combinedDICapproachandanadaptiveDICapproachsuitable for sampleswithdiscontinuities/holeswereproposedbyZhang et al.

[15]. Morerecently,PanandLi [16] proposed afast DIC method,whosespeedisabout120–200timesfasterthan
that ofthetraditionalmethod,withoutanylossofmeasurement accuracy.

The DICmethodusestherandomspecklepatterntomatchthe corresponding pointsinthereferenceimageandthedeformed
image. Inordertotrackthelocationofapointinthedeformed Fig. 13 demonstrates thefoamspecimenandthecompression

device beforeandafterdeformation.Anaverage60%compression is imposedonthespecimen,thusthespecimenheightdecreases from 30mmto12mm.Totally105stagesarerecordedduringthe test.

Thede-correlationeffectisfoundtobemoreseriousforthe foam materialbecauseofitsporousstructure.Afterspecifying the calculationareaandtheseedpoint(see Fig. 13a), 1860points with subsetof2525 pixelandsubsetstepof13pixelsare calculated.

Thesameprocessandanalysismethodemployedin the tensiletestisperformedonthecompressiontest,andsimilar conclusion isobtained.Theimprovedcoarsesearchmethod performs muchbetterthanthetraditionalcoarsesearchmethod, yet combiningtheimprovedcoarsesearchmethodwiththe updating referenceimageschemegivesthebestperformance.

According tomethoddescribedin Section 3.2, thereferenceimage only needstobeupdatedtwice.Themeasurementresults including displacementfields(in x, y, z directions) andstrain fields (ex, ey, exy strain) ofthe60thand105thstageareillustrated in Fig.

14. Forthe105thstage,themaximumdisplacementin y direction (compressiondirection)isabout18.6mm,themax-imum displacementin z direction (out-of-planedisplacement)is about 0.9mm,andthemaximum ex strain (compressiondirec-
tion) isupto83%.

5. Conclusion

To solvetheproblemoflargedeformationmeasurementusing 3D DICmethod,alargedeformationmeasurementscheme,combining improvedcoarsesearchmethodwithupdatingrefer-ence imageschemeispresented.Toimprovethecalculation speed, aseedpointbasedmethodisadopted.Accordingtothe deformation continuityofadjacentstage,animprovedcoarse search methodisproposed,whichguaranteesthatthecorrelation calculation canbecarriedoutsuccessfullyinlargedeformation situation (upto300%tensiledeformation).Onlyforextremely large deformation(e.g. 4300% tensiledeformation),theupdating reference imageschemeisemployed.Bycombiningimproved coarse searchmethodwithupdatingreferenceimagescheme,not only largedeformationcanbemeasuredautomatically,butalso
the accumulatederrorintroducedbyupdatingreferenceimage can becontrolled.

Apolymermaterialtensiletestandafoam compression testarecarriedouttoverifytheproposedmethod. Measurement resultsofupto450%tensiledeformationand83% compression deformationconfirmthevalidityofthemethod.The proposed schemeinthisworkisveryusefulforthematerial properties analysisofpolymerorfoammaterialthatmayexperi-ence extremelylargedeformation.