Extreme shearography

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Delft University of Technology

Extreme shearography

Development of a high-speed shearography instrument for quantitative surface strain

measurements during an impact event

Anisimov, Andrei G.; Groves, Roger M.

DOI

10.1016/j.optlaseng.2020.106502

Publication date

2021

Document Version

Final published version

Published in

Optics and Lasers in Engineering

Citation (APA)

Anisimov, A. G., & Groves, R. M. (2021). Extreme shearography: Development of a high-speed

shearography instrument for quantitative surface strain measurements during an impact event. Optics and

Lasers in Engineering, 140, [106502]. https://doi.org/10.1016/j.optlaseng.2020.106502

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OpticsandLasersinEngineering140(2021)106502

ContentslistsavailableatScienceDirect

Optics

and

Lasers

in

Engineering

journalhomepage:www.elsevier.com/locate/optlaseng

Extreme

shearography:

Development

of

a

high-speed

shearography

instrument

for

qua

ntitative

surface

strain

measurements

during

an

impact

event

Andrei

G. Anisimov

∗

_,

_Roger

_M.

_Groves

Aerospace Non-Destructive Testing Laboratory, Faculty of Aerospace Engineering, Delft University of Technology, the Netherlands

a

r

t

i

c

l

e

i

n

f

o

Keywords: High-speed shearography Double-pulse shearography Flexural waves Impact Composite materials Surface strain

a

b

s

t

r

a

c

t

Monitoringofextremedynamicloadingsoncompositematerialswithhightemporalandspatialresolution pro-videsanimportantinsightintotheunderstandingofthematerialbehaviour.Quantitativemeasurementofthe surfacestrainatthefirstmomentsoftheimpacteventmayrevealtheinitiationofthefailuremechanismsleading todamage.Forthispurpose,wedevelopedashearographyinstrumentforstrainmeasurementsduringasevere impacteventatμstemporalresolution.Thispaperpresentsthedesign,developmentandexperimental mea-surementofthesurfacestrainduringanimpactonaluminiumandcompositesamples.Thefinaldesignrealises measurementsofthein-andout-of-planesurfacestraincomponentstoimprovecouplingofexperimentaldata withthenumericalmodels.Theexperimentsonaluminiumandcompositespecimensrevealedthemainelastic materialresponsetobeinthefirst1-2μsaftertheimpactfollowedbytheinitiationandpropagationofflexural wavescausingin-andout-of-planedeformation.Furtheranalysisofthewavefrontswillbeusedasinputand validationdatafornewnumericalandanalyticalmodelsoftheimpactresponseofcompositesandforvalidation ofotherexperimentaltechniquesasacousticemissionandembeddedpiezosensors.

Thesetoftechnicalparametersofthedevelopedshearographyinstrumentmakesitoneofthemostextreme ap-plicationsofshearographyformaterialcharacterisation.Theframeworkforthisworkisthe“EXTREMEDynamic Loading– PushingtheBoundariesofAerospaceCompositeMaterialStructures” Horizon2020project.

1. Introduction

Improvingtheimpactresponseofcompositematerialsisan impor-tantdirectionofthematerialsdevelopmenttowardssaferandlighter aircraft.Compositematerialsarevulnerabletoextremedynamic load-ingssuchasbladeoﬀ eventsorforeignobjectdamage(hail,runway debris,birdstrike)[1].Thedevelopmentofnewinstrumentsto recon-structextremedynamiceventsandtomeasurefailureparameterswill provideanimportantinsightintotheunderstandingofthebehaviourof composites.Thestraindevelopmentduringtheimpactisofparticular interestforbothmaterialsmodellingandforexperimentalresearchers [2,3].Theshearography(specklepatternshearinginterferometry) tech-nique[4–6]isusedinthisprojecttoprovideaquantitative measure-mentofthesurfacestraindevelopmentattheﬁrstmomentsofthe im-pactevent(𝜇stimescale)whichmayrevealtheinitiationofthefailure mechanismsincompositematerials.

Inpractice,shearographyhasactivelybeenusedfornon-destructive inspectionortesting(NDI,NDT)ofcompositematerialsovermorethan 20years[5,7,8],mostlyfordefectdetection,localisationand

charac-∗_{Corresponding}_author_at:_Kluyverweg_1,₂₆₂₉_HS,_Delft,_the_Netherlands

E-mailaddress:a.g.anisimov@tudelft.nl(A.G.Anisimov).

terisation.Aposteriorinspectionofimpactdamagehasbeenreported in theliteratureforbothlow [9–11]andhigh-velocity impacts[12]. Shearographyhasalsoalreadybeenusedfortheexperimental mechan-icsofcompositesatslowstrainrates[5,13–15],butnotduringan im-pacteventathighimpactspeedsandenergies.Previouslyholographic techniqueswereusedtocapturethepropagationofflexuralwaves dur-ingtheimpactforcewithclassical(film)pulsedholography[16,17]and laterwiththeelectronicspecklepatterninterferometry(ESPI)by intro-ducingacarrierfrequency[18].DoublepulseESPIhasbeenactively usedforimpactmonitoring[19,20]andnumericalcomparisonwiththe availablefiniteelementmodels[21,22].

Whenshearographyisusedforthemonitoringofdynamicevents,the phasecorrespondingtothesurfacedisplacementgradientoftheobject canbeextractedfromcapturedinterferogramswithouttime-averaging byusingaspatialphase-shiftandFourier-basedprocessing [23].The ﬁrstspatialphase-shiftforshearographyusedacarrierspatialfrequency generated bya Mach-Zehnder interferometer[24]. Recently, various conﬁgurationsofspatialphase-shiftwithaMach-Zehnder interferome-terweredeveloped[25,26]andlatertheMichelsoninterferometerwas

https://doi.org/10.1016/j.optlaseng.2020.106502

Received4July2020;Receivedinrevisedform11October2020;Accepted23November2020

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alsoadaptedforthispurpose[27]andimprovedbyusingaslitinstead ofacircularaperturetoimprovetheenergyeﬃciencyandqualityof therevealedphasemaps[28].Alternativemethodswerealsoreported withacameracoupledwithanarrayofmicro-polarisers[29],a dual-wavelengthmethodwithasyntheticwavelength[30]andwith diﬀrac-tiveopticalelements[31].

Dynamicloadingwithahighspeedofdeformationrequiresthe high-estpossibleacquisitionrate,reaching𝜇sandsub-𝜇stemporal resolu-tion.Varioussolutionsforfastshearographyhavebeenreportedwhich employedhigh-speed[32]ormoreoftendoubleframecameraswitha higherspatialresolution[33].Thedoubleframeapproachwasalsoused forvibrationanalysisin afrequencyrangeuptoafewkHz[34–36], includingfortransientvibrations[37,38]andimpact-inducedﬂexural wavesatlowimpactenergies[39,40]. Mostofthereported shearog-raphyinstruments with spatialphase-shift implemented out-of-plane deformationanalysiswiththeshear appliedin onedirection per ex-perimentduetothecomplexityoftheopticalpartsandphase extrac-tion.However,therewererecentresultswiththeshearsimultaneously appliedinboth𝑥-and𝑦-directionsbymultiplexingusingpolarisation [41,42],diﬀerentspectralranges[43]andspatialphase-shifting[44].

This paper presents thefinal results of the development and ex-perimentaltestingofthenewhigh-speed EXTREMEshearography in-strument,whichwasperformedintwoconfigurationswithamodified MichelsonanddoubleimagingMach-Zehnderinterferometers, respec-tively[45,46].Bothconfigurationsarepresentedandcomparedwhen usedforhigh-speedshearography.Thelatteronewasusedforthe fi-naldesignasitallowsanindependentadjustmentoftheshearamount. Thedevelopmentwasdrivenbytheneedtomaximisethevalueofthe expectedresults forthevalidation ofnumericalmodels– to measure bothin-andout-of-planesurfacestraincomponentsatimpactspeeds upto200m/swith𝜇stemporalresolutionoverthefieldofviewaround 100×100mm.Thereforetwoviewingdirections(shearing interferome-ters)withadouble-frameapproachwereusedtocapturethe interfero-gramsduringtheimpact.Themainresultsincludeexperimentally mea-suredsurfacestrainmapsoverthefieldofviewduringimpacteventson aluminiumandcompositesamples.

Theframeworkforthisworkisthe“EXTREMEDynamicLoading– PushingtheBoundariesofAerospace CompositeMaterial Structures” Horizon2020project[47].Withintheproject,theshearographydatais fusedwithhigh-speed3Ddigitalimagecorrelation(DIC)[48]and in-situimpactdatafromﬁbreopticalsensorsbasedonFibreBraggGratings (FBG),embeddedandsurfacemountedpiezo-electricsensors[49].

2. High-speedshearographytheory

Shearographyisacoherent-opticaltechniquethatrealisesfull-ﬁeld directmeasurementofthesurfacedisplacementgradientwhenthe ob-jectisdeformed.Inthisproject,theobjectisdeformedbyimpact load-ingwithagas-gunusinganimpactor.Asintheliterature[33–36],a pulsedlaserisusedheretoproduceaspecklepatternbyilluminatingthe objectwithanexpandedlaserbeam.Interferogramsarerecordedwith camerasthroughshearingdevicesattheinitialmomentoftheimpact (from0to10μsaftertheimpact).Duringtheimpactevent,eachcamera recordsinterferogramssynchronouslywiththelaserpulsesandrealises spatialphase-shiftingtogivetheshearographyphasecorrespondingto thesurfacedisplacementgradientwhichwasbuiltupinbetweenlaser pulses.Thisphasemapisfurtherprocessedundercertaindesign consid-erations,e.g.camerasandlasersorientation,togivethesurfacestrain components.

Ingeneral,tomeasurethein-andout-of-planesurfacestrain com-ponents,amulticomponent3Dshearographyconﬁgurationisneeded, e.g.withthreeshearingcameras[5].Duetotheexpectedcomplexityof thehigh-speedshearographyinstrument[45,46],thenumberof shear-ingcameraswasreducedtotwowiththemainsensitivitytothein-and out-of-planecomponents.AschematicoftheEXTREMEshearography instrumentispresentedinFig.1.

Whentwoshearingcamerasareused,eachofthemwillprovidea phasechangeΔ𝜙_𝑥whichcanbeextractedfromtheFourierside-spectra of therecordedinterferograms[25].Iftheillumination andviewing directionsareinthe𝑥𝑧-planeandarelativelysmallsheardistance𝑑𝑥

isappliedinthe𝑥-direction(theshearissigniﬁcantlysmallerthanthe distance totheobject),thephase changeΔ𝜙_𝑥 becomesafunctionof thein-andout-of-planesurfacestraincomponents(𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥, respectively): Δ𝜙_𝑥=𝜙’_𝑥−𝜙ref_𝑥 = 2𝜋 𝜆 ( 𝑘𝑥𝜕𝑢∕𝜕𝑥+𝑘𝑦𝜕𝑣∕𝜕𝑥+𝑘𝑧𝜕𝑤∕𝜕𝑥)𝑑𝑥 ifinxzplane ≈ 2𝜋 𝜆 ( 𝑘𝑥𝜕𝑢∕𝜕𝑥+𝑘𝑧𝜕𝑤∕𝜕𝑥)𝑑𝑥, (1) where𝜙′

𝑥 and𝜙𝑟𝑒𝑓𝑥 arethesignalandreferencephasediﬀerences

ob-tainedbeforeandafterdeformation,subscript𝑥referstotheshear di-rection,𝜆 isthelaserwavelength,and(𝑘_𝑥𝑘_𝑦𝑘_𝑧)arecomponentsofthe sensitivityvector,thatisthebisectoroftheviewingangleΘbetweenthe illuminationandviewingdirections[5].Thesurfacestraincomponents 𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥fortheshear𝑑𝑥𝑗inthe𝑥-directioncanbecalculated

byprocessingthephasechangesΔ𝜙_𝑥𝑗obtainedateachcamera𝑗=1,2: [ 𝜕𝑢∕_𝜕𝑥 𝜕𝑤∕𝜕𝑥 ] = 𝜆 2𝜋 [ 𝑘𝑥1 𝑘𝑧1 𝑘𝑥2 𝑘𝑧2 ]−1[ Δ_𝜙_𝑥₁∕_𝑑_𝑥₁ Δ𝜙_𝑥2∕𝑑𝑥2 ] ifsymmetric setupin𝑥𝑧−plane ≈ 𝜆 4_𝜋 [ 1∕sin(Θ) 1∕sin(−Θ) 1∕(cos(Θ)+1) 1∕(cos(Θ)+1) ][ Δ𝜙_𝑥₁∕𝑑_𝑥₁ Δ𝜙_𝑥2∕𝑑𝑥2 ] . (2)

Eqs(1)and(2)arevalidaslongastheilluminationandthe symmet-ricviewingdirectionslayin𝑥𝑧-plane.Incaseofangularmisalignment betweentheviewingdirectionsandtheilluminationoffewerthan5 de-grees(rotationoverthe𝑥axis),theshearstrain𝜕𝑣∕𝜕𝑥ismixedupwith theout-of-planecomponent𝜕𝑤∕𝜕𝑥upto5%(assumingthemagnitude ofthesecomponentsiscomparable).

2.1. High-speedshearography

Giventhefactthatthepulsedilluminationat𝜇stimescaleispossible withmodernlasersthatcandelivertwoorseveralsub-pulses[50],the mainchoiceisofthecameraarchitecture.Acomparisonofthediﬀerent approachesforhigh-speedshearographyisdoneinTable1.

Inthisproject,thethirdarchitecturewiththedoubleframecameras waschosenduetotheabilitytoachievehighnumberofpixelsandsmall pixelsizeastheydeﬁnethehighestspatialfrequencyandthefrequency range,respectively. These arethemainlimitationsofthehigh-speed camerasinoptions1and2.Thelimitationoftwoframespercameracan beovercomebyincreasingthenumberofcameras[56]orbyrepeating theexperimentswithcontrolleddelays.

3. Instrumentdesignanddevelopment 3.1. Spatialphase-shiftshearography

Fig.2showstheshearographyconfigurationforoneofthecameras withspatialphase-shiftingusingtheMichelson[27]orthedouble imag-ingMach-Zehnder[25]interferometers.Bothconfigurationsimplement theshearingprinciplebyoneofthemirrorswhichistiltedwithrespect tothereferenceone.Thespatialphase-shiftisrealisedbyslitsinboth interferometers(Fig.2(b,c))whichlimitthefrequencybandwidthof therecordedinterferogramwithacut-off frequency𝑓𝑐𝑢𝑡=𝐷∕2𝜆𝑓[24],

where𝐷 istheslitwidthprojectedtothefocalplaneand𝑓 isthe fo-callengthoftheimaginglens.InthecaseofMichelsoninterferometer, thefilteringisdoneinthefrontfocalplaneofthefirstlensofthe4f system,intheMach-Zehnderscheme– thisisdonedirectlyinthefront focalplaneofeachimaginglens.Theoffset(carrier)frequency𝑓_𝑜isset bytheangularoffset𝛽 as𝑓𝑜=sin𝛽∕𝜆.FortheMichelsoninterferometer

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A.G. Anisimov and R.M. Groves Optics and Lasers in Engineering 140 (2021) 106502

Fig.1.TheEXTREMEshearographyinstrumentforin-andout-of-planemeasurements.

Table1

Comparisonoftheshearingcamerasarchitectures.Parametersmaydependonspeciﬁccameramodels.

Camera architecture Main advantages Main limitations

1. One or more high-speed cameras with a trade-off between the spatial resolution and the frame rate [51–53]

•Flexibility in various frame rates with a trade-off for the resolution

•Ability to capture a sequence of interferograms to record the whole impact event

•Frame rates around 10 6 fps result in low spatial

resolution (order of 128 ×48 pixels), which is not enough for digital speckle interferometry 2. One or more high-speed cameras with a

ﬁxed resolution at extremely high frame rates (buffer on the sensor) [55]

•Moderate spatial resolution (close to 1 Mpixel) at extremely high frame rates (higher than 5 ∙10 6 fps)

•Ability to capture a sequence of interferograms (up to 100-200 frames)

•[Valid for architectures 1-2] Large pixel size (order of 20 ×20 𝜇m) and low ﬁll factor limit the maximum spatial

•frequencies for the spatial phase-shift

•High price, especially if 2 or 3 cameras are needed for the 3D shearography [ 5 , 6 , 54 ] 3. One or several double frame CCD cameras

with an interline frame transfer [ 18–22 , 34–40 ]

•High spatial resolution (several Mpixel)

•Small pixel size (up to 3 ×3 𝜇m)

•Affordable price

•The number of frames is limited to 2 per camera

•Long exposure time for the second frame if no extra shutter is used (exposure equal to a readout time of the ﬁrst frame, reaches 100-200 ms)

Fig.2. Shearographyconﬁguration(a)foroneofthecameraswithspatialphase-shiftingwithmodiﬁed

(b)Michelsonor(c)doubleimagingMach-Zehnderinterferometers:schematicrepresentationoftheopticalpathsinbothconfigurations,(d)relativeoff-axisoffsetof theslitsinMach-Zehnderschemeand(e)dataprocessingflowintheFourierdomain(FOV– fieldofview,FT– Fouriertransform,IFT– inverseFouriertransform).

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Fig. 3.Comparison of the phase maps cap-turedwith(a)the Michelsonand(b) Mach-Zehnder interferometers (Fig. 2(b,c), respec-tively)correspondingtotheout-of-plane de-formationduringimpactsonaluminiumplates withtheshearinthe𝑥-direction.

(Fig.2(b)),𝑓𝑜dependsontheactualshearamount𝛽 expressedinan

angularform.FortheMach-Zehnderinterferometer,𝑓𝑜isindependently

setbytheangularoffsetoftheslits𝛽 (Fig.2(c,d))[25].Both interfer-ometersweremodifiedbyusingaslitinsteadofcircularaperturesto improvetheenergyefficiencyandqualityoftherevealedphasemaps byincreasingtheareaandfrequencyrangeofthesidemaximumsinthe Fourierspectrum(redrectanglesinFig.2(b,e))[57].

AftertheinverseFouriertransformation,thesidemaximumsyield acomplex functionthatcontains thephaseinformation𝜙𝑟𝑒𝑓𝑥 and𝜙′𝑥

fortheinterferogramscaptured beforeandafterthedeformation, re-spectively. Toisolate these maximums with a windowed transform, theoﬀset frequency 𝑓𝑜 has tobe at least twice higher than𝑓𝑐, i.e.

2𝑓_𝑐𝑢𝑡≤𝑓_𝑜⇒ 𝐷≤𝑓sin𝛽[24].

Thedynamicrangeofthemeasuredstrainduringanimpactevent hastobemaximized.Forthat,thesheardistancetypicallyhastobe de-creased.FortheMichelsoninterferometerthisisuptoareliablespectra separation(asinFig.2(b)),fortheMach-Zehnderinterferometerthis isuptotheexperimentallyfoundlimitofnotlessthan2specklesizes. Whentheslitisused,thespecklesizeisdeﬁnedasthelongestsizeof thespeckle(herehorizontal).Tominimizethesubjectivespecklesize Δ𝑠=𝜆𝑓∕𝐷,theslitwidth𝐷 hastobemaximized.However,the mini-mumspecklesizeΔ𝑠forshearographyhastobemorethan6pixels[24]. Thismakesthetrade-oﬀ fortheslitwidth𝐷:

𝜆𝑓∕6𝑝_𝑥≤𝐷≤𝑓sin𝛽, (3)

where𝑝𝑥isthepixelsizeinthe𝑥-direction.Therefore,forreliable

spec-tralseparation,itispreferabletousecameraswithaminimalpixelsize andahighnumberofpixels(seeoption3inTable1).

3.2. Experimentalcomparisonoftheinterferometers

OneMichelsonandoneMach-Zehnder interferometerswere simi-larlyassembledtoexperimentallycomparetheirperformance.To sim-plifythepreliminarytests,theinterferometerswereoriented perpendic-ularlytotheobject,sothephasemaps(Fig.3)recordedduringimpact eventscorrespondtotheout-of-planedeformation.Eachinterferometer wastestedindividuallyinthesametestconditionsandimpactenergy withnewaluminiumspecimens,whichisasimplematerialcase.The meansheardistanceovertheﬁeldofviewfortheMichelson interfer-ometerwasdecreasedto2.7mmfromthepreviouslyreportedresults [45]up tothelimit ofthespectraseparation(Fig.2(b)).The Mach-Zehnderinterferometerhadashearwiththemeanvalueof1.4mm.

The phase maps (Fig. 3) reveal the propagation of a symmetric circularﬂexuralwaveduringthetime intervalaround3to4μs

af-terthemomentoftheimpact.Fig.3(b)wascapturedwiththe Mach-Zehnder interferometer andreveals the increased dynamic range of the resolved phase fringes in comparison with theMichelson inter-ferometer. According to the comparison of the shear distances, the Michelson interferometeris expected to have a higher phase value (morefringes,Eq.(1)),however,thesefringesarenotresolvedinthe phasemap.

Foraccuratestrainestimationoverthefieldofview(Eq.(2)) two correctionshavetobeimplemented.First,thesensitivityvectorhasto becorrectedduetothevaryingviewingangleΘfordifferentpointsof thespecimen[5,6].Second,thenon-uniformityofthesheardistancehas tobecalibrated,e.g.bycapturinganobjectwithaknown2Dpattern viareferenceandshearedfieldofviews.Fig.4showsthesheardistance variationforbothinterferometerswhencheckerboardimageswere cap-turedindependentlythroughthereferenceandtheshearedfieldofview andtherelativeshiftsofallcornerswereidentifiedandinterpolated [54].

Fig.4showsthatbothinterferometershavenon-uniformshear dis-tanceswhichisacumulativeresultofthemisalignmentsand imperfec-tionsofthespecificopticalparts(e.g.mirrorsnon-flatness).According toFig.4,thedistributedopticalschemeinaMach-Zehnder interferom-etermayresultina15%differenceoverthefieldofviewincomparison with1%withtheMichelsonone.Incaseoftheimpact-monitoringwhen theopticalsetupislocatedclosetotheimpactevent,afinalalignment ofthesetupaftereachimpactischallenging,thereforeaccordingtoour practice,thepost-calibrationof theactualshearamountispreferable andeasierimplementedthantheopticsrealignment.

3.3. EXTREMEshearographyinstrumentdesign

Based on the performance comparison of the two interferome-ters(Figures3and4),thearchitecturewiththeMach-Zehnder inter-ferometer was used furtherfor the instrumentdevelopment. This is mainlydue totheabilitytoadjusttheshear distance independently from the oﬀset frequency which is critical for the high strain sce-nario as an impact event. Tomaximise the couplingof the experi-mentaldatawiththenumericalmodels,thedevelopedconﬁguration of theshearographyinstrument(Fig.5)realisesmeasurementsof the in- andout-of-planesurfacestraincomponents(𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥or 𝜕𝑣∕𝜕𝑦and𝜕𝑤∕𝜕𝑦dependingonthesheardirectionduringtheimpact, Eq.(2)).Thereforetwoshearinginterferometerswereplaced symmet-ricallyinahorizontalplane.Theinstrumentwasbuilttogetherwitha gas-gunasanimpactchamberwiththreezones drivenbythesafety measures:

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Fig.4.Variationofthesheardistanceinthe𝑥-directionfor(a)theMichelsonand(b)Mach-Zehnderinterferometers(Fig.2(b,c)).Theshearvalueswerenormalized foradirectcomparison.

Fig.5. EXTREMEshearographyinstrumentforin-andout-of-planesurfacestrainmeasurements: (a)overviewoftheinstrument,thegas-gunandtheimpactchamber,

(b)theimpactzonewiththespecimenclampingandtheopticalsynchronisationunit, (c)themonitoringzoneasseenfromoneoftheshearinginterferometers,

(d)twosymmetricshearinginterferometersimplementingthedouble-imagingscheme.

a) an impact zone with the optical synchronisation unit pro-viding real-time information about the approaching impactor (Fig.5(b));

b) amonitoringzonewherethebacksurfaceofthespecimenis ob-servedduringtheimpact(Fig.5(c));

c) ashearographyzonewheretheshearingcamerasandthelaser beampathareplaced(Fig.5(d)).

Thegas-gunisadaptedforimpactspeedsupto200m/sandoperates underpressurisedair or nitrogen,impactreleaseis controlledbyan electromagneticvalve.Theimpactordesignwithahemispheretipwith aradiusofcurvatureof5mmwasusedfollowingstandardpracticein theaerospaceindustry.

Eachdouble-imagingMach-Zehnderinterferometer[25]consistsofa double-framecameraBOBCATB3420byImperx.Thiscamerahasan

in-terlineframetransferwitharesolutionof3388×2712pixels,apixelsize of3.69×3.69𝜇mandaninterframingtimeof200ns.Thespeckle pat-ternisproducedbyilluminatingthespecimenwithanexpandedbeam fromapulsedlaserinadoublepulseregime(customisedSpitLight600 Nd:YAG-LasersystembyInnoLasLaserGmbH,wavelength532nm).An additionalgroundglassdiﬀuser(Fig.1(a))isusedtonormalisethe en-ergydistributionovertheilluminatedareatoovercometheinitial circu-larfringesinthebeamduetocomplexmodescombination.Twoportions ofthesinglepulsewiththeinitialenergyfrom100upto500mJ sequen-tiallyilluminatethespecimenunderthecontrolofaPockelscellwith theminimumseparationtimeof1𝜇s.Ineachinterferometer,twoslits wereplacedatthefrontfocalplanesoftheimaginglensestoachievea betterqualityofthephasemapsincomparisonwithacircularaperture [57].Thetrade-oﬀ betweentheinterferometerparameters(Eq.(3))was madeasfollows:

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Fig.6. Timingoftheshootingroutine:(a)diagramwiththemaintriggersandsignals(timescaleisnotlinear),

(b)electricalsignalscapturedduringanimpact:cameraexposure(blueline),laserpulses(red)andAEsignals(brownandgreen;timeaxisistoscale).

a) theminimumdistancebetweenthecameraandtheobjectof 500-600mmwasbasedonsafetymeasures;

b) thefocallengthwasmaximisedupto50mm(Xenoplan2.8/50 bySchneider)anddeﬁnedbythevisibleareaofØ150mm; c) eachslitwasshiftedfromthejointaxisby1.9mm(𝛽=2.3°)

re-sultingin theoﬀsetfrequencyclosetoahalfofthemaximum one,seeFig.2(c-e);

d) the1.8mmslitwaschosenasarationalcompromiseresultingin theabsenceoftheoverlapofthecentralandsidemaximumsof theFourierspectraandtheaveragesubjectivespecklesizeof7 pixels(correspondingto0.35mm;alongwiththe𝑥-axis). e) thesheardistance𝑑𝑥wasdecreasedincomparisonwiththe

pre-liminarytests(section3.2)to17to21pixels(0.9..1.1mm)and calibratedovertheﬁeldofviewforbothcamerasasinFig.4(b).

During the tests, square specimens of 200×200 mm are tightly clampedbetweentwo squareclampswithcircularwindowson both impactandobservingsides(Fig.5(b,c)).Theclampingitselfisﬁxedto thewallwhichdividestheimpactandmonitoringzones.Toimprove thelaserlightscattering,thespecimensarepaintedwithmattewhite paint.

3.4. “On-the-ﬂy” synchronisationbasedontheapproachingimpactor Missingtheimpactmeanslossofthespecimen,thereforeallparts oftheinstrumentrequirereliablereal-timesynchronisationwithan ac-curacyupto1𝜇stocapturetheinitialmomentofimpact.Theoverall shootingproceduretakesmorethan10seconds(Fig.6(a))andincludes: phase1.Warming:10sofpre-heatofthelaserforthermalstabilisation bytriggeringthelaser(greensignalinFig.6(a))resultingin100 dou-blepulses(redsignal)at10Hz;phase2.Shot:openingofthegas-gun valve,theimpactorstartstravelling;phase3.Travelling:theimpactor travelstowardsthespecimenbeingcontinuouslymonitoredbythe op-ticalinterrupterstocalculatetheexpectedimpacttimeandtheneeded “dynamicdelay” beforetheﬁnal“laser-camera” trigger;phase4. Im-pact:capturingoftwoframessynchronouslywithtwolasersub-pulses andacousticemission(AE)signalsfromtwosensors(Fig.5(c)).

“On-the-ﬂy” synchronisationisdonebyprocessingsignalsfrom20 opticalinterrupters(pairsofLEDandfastphototransistors)whichcross thepathoftheimpactor(Fig.5(b))usingareal-timeprocessing unit withanFPGAarchitecture[45,46].Theactualspeedoftheimpactoris calculatedbasedontherelativedelaysbetweenthese20signals,then theappropriatedelaystotriggerthelaserandthecameraarecalculated

inareal-timerightbeforetheimpact.Aftertheimpact,theactual mo-mentofimpactisestimatedbyprocessingsignals(brownandgreenin Fig.6(b))fromtwominiatureacousticemissionsensorsPICOHF-1.2by PhysicalAcousticsoperatingintherangeof500-1850kHzwhichare tapeddirectlytothespecimensymmetricallyfromthespecimencenter (Fig.5(c)).

4. Experimentalresults

Theexperimentalresultspresentedherearetoinvestigatethe instru-mentperformance.Forthattwoaerospacegradematerialswereselected withisotropicandanisotropicproperties.Aseparatestudywillcoverthe modellingaspectofthematerialbehaviour.

ThedevelopedinstrumentwiththeMach-Zehnderarchitecturewas used forthemaintests.Thein-andout-of-plane surfacestrain com-ponents weremeasuredduringimpact tests on4mm thick 6082-T6 aluminiumspecimens(Fig.7)and3mmthickcarbonﬁbrereinforced composite(CFRP)specimenswith0/90/0/90/0layupofunidirectional plies(Fig.8).Thestrainmapsrevealtheevolutionof𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥 in betweenthelaserpulses (1𝜇s, Pockelscellseparationtime)with varyingdelaystartingfromthemomentoftheimpactandnotthetotal strainwhichwasbuiltupduringtheimpact.Eachimpactexperiment withanewspecimenresultedinapairofmaps(𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥) pre-sentedinsub-columnsinFigures7(e.g.aandeintheredrectangle,b andf,etc.)and8.Straincomponents𝜕𝑣∕𝜕𝑦and𝜕𝑤∕𝜕𝑦inFig.8were measuredbyrotatingthespecimensby90degreesandmaintainingthe samecalibratedsheardistance𝑑𝑥.Notethephaseunwrappingalgorithm

hasnotbeenabletoresolveallfringesandfullyidentifyallthephase steps,whichresultedinthepartiallylostdata.

TheimpactorandthedamageintheCFRPspecimenareshownin Fig.9.NopenetrationoftheimpactorwasobservedintheCFRPand aluminiumspecimens.Duringthetests,theimpactorspeedwasinthe rangeof58.5±0.5m/scorrespondingtoa53.8..55.7Jimpactenergy rangewithanimpactorof32g.

ThestrainmapsinFigures7and8reflectbothspatialand tempo-ralinformationontheflexuralwaves.Toassessthetemporalaspect, thewavespeedsforthealuminiumspecimens(Fig.7)wereestimated, first, based on theAE signals recordedbythe piezo-electricsensors andthencomparedwiththeonederivedfromthegraphicallycaptured strainmaps(Fig.7).Forbothcases,thecross-sectionsofthestrainmaps wereplottedtogetherinFig.10andmatchedwiththeAEsignalsfrom thesametests (shownontheleftandrightsides).First,thespeedof 5490m/swascalculatedbyaveragingthetimeofarrivalofthewaveto

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Fig.7. In-andout-of-planesurfacestrainmaps(𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥)of4mmaluminiumplates(6082-T6) atthetimefrom0to2.9..3.9μsaftertheimpact.Axesunitsmm.

Fig.8. In-andout-of-planesurfacestrainmaps(𝜕𝑢∕𝜕𝑥and𝜕𝑤∕𝜕𝑥,𝜕𝑣∕𝜕𝑦and𝜕𝑤∕𝜕𝑦)of3mmCFRPspecimensatthetimefrom0to6..7μsaftertheimpactwith anadditionalzoom(m)intotheunwrappedphasemap.Axesunitsmm.

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Fig.9. Theimpactor(a),(b)theimpactedand(c)themonitoredsideofthe3mmCFRPspecimenrevealingthedamageaftertheimpact.

Fig.10. Matchingthegeometricalandtimedata:cross-sectionsofthestrainmapsperimpactfromFigure7,thesignalsfromaphotodiodeandtheacousticemission (AE)sensorscapturedduringtheimpactevents.Theunresolvedstrainvaluesareplottedinlightgreycolour.

theAEsensorsplacedsymmetricallybetween65and70mm(greyarea) fromthespecimencenterduringthefirstimpacttest(toplineinFig.10). Thisvaluewasusedtorecalculatethespatial𝑥-axisinmm(bottom)to thetimeaxis(top).Thesecondcalculationwasdonebasedonthe lo-cationofthewavefrontinthecapturedstrainmaps(linestwotofive) withanassumption,thatthespeedofthegraphicallycapturedwave maydifferfromthemeasuredwithAEsignals.Fourcross-sectionsfrom 1.1to3.9𝜇swereaveragedtogivethespeedestimationof5380m/s, whichdiffersby2%fromthevaluemeasuredwiththeAEsensors.

Twophotodiode signals(redline inFig. 10; shown onlyfor one experiment)with1μs separationtimerepresentthelasersub-pulses recordedduringtheimpact(alsoinFig.6(b))togetherwiththeAE sig-nals(plottedatthesametimescale).Oncethecross-sectionsofstrain mapswerematchedwiththecorrespondingtime(topandbottomaxes), therisingedgeofthesecondphotodiodesignal(secondsub-pulse) cor-respondstothelocationofthewavefrontinthespatialdomain.

5. Discussion

ThefirststrainmapsinFigures7and8(a,e;b,f)and5(a,e)reveal theelasticresponseofthematerialduringtheinitiationoftheimpact eventduringthefirst0..2𝜇saftertheimpact.Furtherintime,the prop-agationoftheflexuralwaveisobservedatvaryingdelaysafterthe im-pact.Forthealuminiumspecimens(Fig.7),theexpectedaxial symmet-ricwaveswereobserved.Thewavefrontforthecompositespecimens (Fig.8)reveals theanisotropyof thematerialpropertiesresulting in differentwavevelocitiesindifferentdirections.Thewavestravelfaster alongthereinforcementfibredirections(verticalandhorizontal)and

slowerinthediagonaldirections[58,59].Themeasuredstrainmaps willbeusedasoneofseveralvalidationtechniquesforthenew numeri-calandanalyticalmodelsbeingdevelopedwithintheEXTREMEproject [2,3,47].Forcomposites,thisinformationabouttheearlyresponsemay supplementthefailuremodesanalysis(ﬁbreormatrixfracture, delam-ination,plasticity).

The estimated speed of the captured waves of 5380-5490 m/s (Fig.10)issmallerthantheexpected6240m/s(speedofthe longitu-dinalultrasonicwavefor6082-T6aluminiumcalculatedwithYoung’s modulus 71GPa,Poisson’sratio0.33,density2700kg/m3_)._The dif-ferencecanbeexplainedbytheuncertaintiesintheestimationofthe timeofarrival(about±0.5𝜇s),themomentoftheactualimpact, sen-sorpositioningerror(±1mm),repeatabilityoftheimpactlocation(± 2-3mm)andtherepeatabilityofthetestsingeneral.Furtherinvestigation isneededtoidentifythemodalcompositionoftheemergingacoustic waves.

Theaforementionedrepeatabilityofthetestsisacrucialissue.The comparisonofthestrainmapsinFigures7,8,and10isbasedonthe con-ditionthatwasmade,thatthetestsarerepeatableandreproducible.In ageneralcase,theyarenotduetovaryingimpactorspeedof58.5±0.5 m/sandcorrespondingimpactenergyof53.8..55.7J(duetovarying pressure in thegasgun vessel),variationof thematerial properties, specimengeometriesandotherexperimentalconditions.However,the instrumentandtheexperimentalprocedureweredesignedand devel-opedina waytominimizethese deviations.Inourexperiments,the desired delaybetween theimpactandthelasersub-pulsescould dif-fer fromtheactualonebyuptoseveralmicroseconds.However,the recordedtimedataofthetravellingimpactor,triggers,lasersub-pulses

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A.G. Anisimov and R.M. Groves Optics and Lasers in Engineering 140 (2021) 106502 andAEsignalsallowedustopost-calculateandreconstructtheactual

timingwiththeuncertaintyofaround±0.5𝜇s.

Themainproblemofthecapturedphase mapsandfurther calcu-latedstrainmapsisthelossofthemeasurementresultsinthecenter after2-3𝜇s(dependingonthematerialproperties).Thishappens be-causeofahighnumberoffringesattheimpactlocationinabutterfly pattern,whichischaracteristicforshearography.Fig.8(m)showsa fil-teredwrappedphasemapfromoneofthecameraswherethesymmetric butterflypatterncanbeidentified.Latertheunresolvedfringesatthe centerareclusteredduringtheunwrappingprocedurewhichresultsin thelostdata.Basedonourexperience[45,46]theoveralldynamicrange ofthepresentedMach-Zehnderschemeisimprovedincomparisonwith theMichelsonconfiguration.Furtherimprovementscouldbe:

a) increasingthelaserwavelength from532nmto1064to dou-blethedynamicrangeoftherecordingsurfacestrain,however, attentionhastobepaidtothedecreasedbyahalfspecklesize; b) increasingthefocallengthoftheimaginglensestoincreasethe

spatialresolution(Eq.(3)),butlimitingtheﬁeldofview. AnalternativeapproachistoinvestigatethecapabilitiesofESPIto monitorthepresentedimpactscenario.Accordingtotheexperimental results[18–22],thephasemapsobtainedwithESPIoftenhavemore resolvedfringesthanwithshearography.However,accordingtoour ex-perience,therequiredalignmentoftheopticalsetupaftereachimpact testmaybechallenging.

Anadditionaltechnicalcomment hastobemade aboutthe dou-bleimagingMach-Zehnderinterferometer[25]incomparisonwiththe Michelsonscheme.Thebeamsplitterthathastobeplacedbehindthe imaginglenses(Fig.2(c))causespracticaldiﬃcultiesintheuseof stan-dardC/CS mount objectivelensesandsinglelenseswithshortfocal lengths(lessthan30mm)incombinationwithstandardC/CSmount cameras.Thisisbecauseofthegapbetweenthelastmechanicalpartof theobjectivelensandtheimageplaneofthecamerasensorhastobeat least16.7mmwhichcorrespondstothereducedthicknessofa1-inch prisminair.Whenadditionalmarginsforthemechanicalassembly,in practice10-20mmaretakenintoaccount,therequiredgapismorethan 30mm.ThismakesmostofthestandardC-mountobjectivelensesnot applicableforthistask(asthebackfocaldistanceis16mm).Asa solu-tion,singlelenseswiththefocallengthofmorethan25-30mmcanbe usedwiththeexpectedpoorimagequality.Theobjectivelensusedin thisproject(Xenoplan2.8/50bySchneider)togetherwiththecamera (BOBCATB3420byImperx)havehighintegrationcapabilitybypartial disassemblyofthepartswhichretainstheappropriateimagequality.

6. Conclusions

Inthispaper,thedesign,developmentandexperimentalresultsof anewshearographyinstrumentforhigh-speedimpactmonitoringare presented.Thenewinstrumentiscapableofcapturingtheimpact re-sponseofmaterialsusingtworecentlyreportedconfigurationsof shear-inginterferometerswhichwereadaptedandexperimentallycompared. ThedoubleimagingMach-Zehnderconfigurationwasusedforthefinal designduetotheindependentadjustmentoftheshearamount,which isthekeyparametertobeminimizedwhenhighstrainvaluesare ex-pected.

Themainvalueofthepresentedexperimentalresults isinthe𝜇s mappingof theﬂexuralwavesduringtheﬁrstmomentafterthe im-pact.Asexpected, waveswiththeaxialsymmetry werecaptured for aluminiumspecimens,andaxiallyasymmetricforthecomposites. Fur-theranalysisofthewavefrontswillbeusedasinputandvalidationdata fornewnumericalandanalyticalmodelsoftheimpactresponseof com-positesandforvalidationofotherexperimentaltechniquesasacoustic emissionandembeddedpiezosensors[49].Currently,theEXTREME shearographyinstrumentrealisesmeasurementsofthein-and out-of-planesurfacestraincomponentsduring theimpactusing thedouble frameapproach.Theoverallsetoftechnicalparametersofthedeveloped

shearographyinstrumentmakesitoneofthemostextremeapplications ofshearographyformaterialcharacterisation.

Futurestepsoftheinstrumentdevelopmentincludeoptimisationof theinterferometertoincreaseitsdynamicrange,exploringsimultaneous measurementsin multiplesharingdirections[60]andrecordingofa sequenceofinterferograms(morethan2)ina“pulsetrain” regime.The materialbehaviourduringtheimpactwillbenumericallymodelledand comparedwiththeexperimentalresults.

Fundingstatement

Theproject“EXTEME” leadingtothispaperhasreceivedfunding fromtheEuropeanUnion’sHorizon2020researchandinnovation pro-gramunderagreementNo.636549.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingﬁnancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinﬂuence theworkreportedinthispaper.

Supplementarymaterials

Supplementarymaterialassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.optlaseng.2020.106502.

CRediTauthorshipcontributionstatement

AndreiG.Anisimov:Methodology,Software,Investigation, Valida-tion,Writing-originaldraft.RogerM.Groves:Conceptualization, Val-idation,Supervision,Fundingacquisition,Writing-review&editing.

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Dr Andrei G. Anisimov is a Senior Researcher in Optical Metrology at Delft University of Technology, The Netherlands. He obtained MS and PhD degrees in optical engineering from University ITMO, Russia. In 2014 he joined the Aerospace NDT Laboratory at TU Delft. His research interests include machine vision and laser interferometry techniques for high precision measurement and non-destructive testing of aerospace and civil engineering materials and structures. Current projects include characterization of composite materials, evaluation of damage and structural behaviour.

Dr Roger M. Groves is Associate Professor in Aerospace NDT/SHM and Heritage Diagnostics at Delft University of Technology, The Netherlands. His PhD is in Optical Instrumen- tation from Cranﬁeld University (2002) and he was a Senior Scientist at Institute for Applied Optics, University of Stuttgart, before joining TU Delft in 2008 as an Assistant Professor. Dr Groves heads a team of approximately 20 researchers in the Aerospace NDT Laboratory at TU Delft. His research interests are Optical Metrology, Fibre Optic Sensing and Ultrasonic Wave Propagation in Composite Materials. He has approximately 200 journal and conference publications in these top- ics. In 2020 he was awarded Fellow of SPIE.