composites observed by computed tomography Fatigue and fatigue after impact behaviour of Thin- and Thick-Ply Composites Part C: Open Access

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Composites Part C: Open Access

journalhomepage:www.elsevier.com/locate/jcomc

Fatigue and fatigue after impact behaviour of Thin- and Thick-Ply composites observed by computed tomography

Benedikt Kötter

, Janina Endres

, Johann Körbelin

, Florian Bittner

, Hans-Josef Endres

, Bodo Fiedler

aHamburg University of Technology, Institute of Polymer and Composites, Denickestraße 15, Hamburg 21073, Germany

bLeibniz University Hannover, Institute of Plastics and Circular Economy (IKK), An der Universität 2, Garbsen 30823, Germany

a r t i c le i n f o

Keywords:

Low-velocity impact Delamination Damage progression Load ratio Constant-life diagram

a b s t r a ct

This study investigates theinfluence of loadratio andimpactdamage on the fatiguebehaviour of high- performancecarbonfibrereinforcedpolymers(CFRP)witharealfibreweightsbetween30gsmand360gsm.For undamagedsamples,theultimatetensileandcompressivestrength,aswellasthefatigueproperties,areevaluated withregardtotheirlayerthicknesses.Thefatiguetestswereperformedundertension-tension(R=0.1),tension- compression(R=-0.5)andcompression-compression(R=10)regime.Theresultsareillustratedasaconstant-life diagram,andapiecewiselinearinterpolationexaminesafirstprediction.Theresultsshowthatstaticandfa- tigueperformanceimproveswithdecreasinglayerthickness.Particularlyundertension-compressionloading, significantimprovementsareobserved,duetothesuppressionofmatrixcracksanddelaminationswiththinner layers.Inaddition,theeffectoflow-energyimpactonthefatiguebehaviourofThin-andThick-Plylaminatesis investigated.Thetestsdemonstratethatalthoughthedelaminationareaislarger,Thin-Plylaminatescansustain higherstressesandstillreachthesamenumberofloadcyclesincontrasttoThick-Plylaminates.Computedto- mographymeasurementsvisualize3-dimensionalthedamageprogressionaftervariouscyclesandprovethatthe Thin-Plycompositesshownoincreaseinthedamagedareaduringfatigue.Theinterlaminarstressatthedelami- nationisnotsufficientforexpansion.Incontrast,inthecaseofthickerlayers,thedamagegrowthsprogressively throughoutthewholesamplewithincreasingnumberofcycles.

1. Introduction

Fatigueandimpacttestsareofparticularinterestinthedesignpro- cessofstructuralcomponents.Fatiguetestsareutilisedtodeterminethe lifetimeofamaterialunderdifferentloadratios,andlow-velocityim- pacttestsrepresentcriticaldamagetothestructure[1].Defectsinlami- natedcompositestructureslikevoidsandimpactdamageshaveamajor influenceonthefatiguebehaviour[2,3].Duetotheirlightweightper- formance,carbonfibrereinforcedpolymersareincreasinglyused.How- ever,theirimpacttolerance,lifetimeandfailurebehaviourdifferssig- nificantlyfromconventionalconstructionmaterialssuchasmetals[4]. Thebehaviouroffibrereinforcedpolymers(FRP)iscomplex.Forexam- ple,theimpactcausesinter-fibrefractures,delaminationsand,depend- ingontheenergyapplied,fibrebreakages.Thedefectsaredistributed withinthematerial,wherebythedamagepattern’ssizeandshapede- pendonseveralfactors.Theseinclude,forexample,thepropertiesofthe samplestobetested,suchasstiffnessandlayerstructure,theclamping systemusedandtheappliedimpactenergy.Thedetectionofsuchdam- ageisdifficult.Here,methodssuchasultrasonicexamination,active

∗Correspondingauthor.

E-mailaddress:benedikt.koetter@tuhh.de(B.Kötter).

thermographyorcomputedtomographyareused[2,5–8].Studiesshow thatimpactdamagecanreducetheload-bearingcapabilityofCFRPby upto50%undertensileload[9].Fibrecompositesalsoexhibitcomplex failurebehaviourundercyclicload.Incontrast tometals,whereone criticalcrackforms,growsandleadstofinalfailure,FRPshaveacom- plexdelaminationdominantfailurebehaviour.Atalowstresslevelor numbersofloadcycles,therelativelylowstrengthofthematrixleads tointer-fibrefractures,whichforminthe90^◦layerperpendiculartothe loaddirection.Delaminationsareinitiatedwiththeincreasinglengthof thecracksandtheassociatedhighstressesatthecracktip.Earlierstud- iesshowedthatthedamageprogressionisassociatedwithadecrease inYoung’smodulus[10–13].Thematerialfailsprematurelybelowthe nominalelongationofthefibres.Thefullpotentialofthefibrescan- notbeexploited.Prematurefailureleadstoaconservativecomponent design[14].Defectsinlaminatedcompositestructureslikevoidsand impactdamageshaveamajorinfluenceonthefatiguebehaviour[15].

OneapproachtoincreasetheperformanceofFRPsistoreducethe layerthickness.StudiesbyKawabeetal.andSihnetal.[16,17]describe arovingspreadingprocess,allowingconventionalrovingstobeusedfor

https://doi.org/10.1016/j.jcomc.2021.100139

Received4February2021;Receivedinrevisedform22March2021;Accepted22March2021

2666-6820/© 2021TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/)

thinlayerthicknessesofupto20μm.Usually,layerthicknessesbelow 60μmaredescribedasThin-Plymaterials.Usingthinnerlayers,theman- ufacturingofthinnerlaminatesispossible,andthefreedomindesign ofthelay-up isincreased[18–21]. Regardingthemechanicalfailure behaviour,thefailuremechanismsofundamagedsampleschangefrom complexdelaminationdominatedtobrittle failureandpre-damageis reducedwithdecreasinglayerthickness.Duetothesuppressionofpre- damage,thestiffnessof thespecimensdoesnotdecrease underload [22].Yokozekietal.[23]showedbyacousticemissionteststhatthe initiationofdamageshiftstohigherstrainsandthus higherstresses.

Furthermore,thenumberofinter-fibrefracturesatlowerstressesde- creaseswithdecreasinglayerthicknessandcanbeexplainedbytheso- called”in-situstrength”,wherebythetransversestrengthofthematrix increaseswithdecreasinglayerthickness.Thein-situstrengthhasbeen investigatedindetailinmanystudies[24–26].Saitoetal.[27]showed thattheenergyatthecracktipishighestatalengthof75%ofthelayer thicknessbecausetheneighbouringlayerssuppressthecrack-opening.

Theenergyofshortercracksisnotsufficienttoinitiatedelaminations, andtherefore,theyaresuppressed.Evenundercompressiveload,the strengthis improved.Laminatequality, lay-upandfailurebehaviour haveapositiveinfluenceonthecompressivestrength.Resin-richregions andvoidsaresmaller,andthefibresaremorehomogeneousdistributed duetothespreadingprocess[18,20].Accordingtotheliterature,six parametersaremost importantforinvestigatingfatiguebehaviourof fibrereinforcedcomposites.Theseincludetheloadingpattern,thecon- trolmode,thestressratio,strainrate,waveformandthetemperature [28].InthecaseofThin-Plysamples,aseventhfactor,thelayerthick- ness, isacknowledged.Sihn etal. andYamaguchiet al.investigated Thin-Plylaminatesunderfatigueloading(tension-tension).Thesam- plesperformedasifunderstatictensileload.Theyshowedanincreased lifetime,asmallerdecreaseinstiffnessreductionandexhibitabrittle failure[17,29].

Toestimatetheinfluenceofdifferentloadratiosonthefatiguelife- timeofamaterialforwhichnoexperimentaldataisavailable,constant- lifediagrams(CLD) area fastandpowerful tool.Especiallyinfields wherefewfatigueinvestigationshavebeencarriedout,asisthecase withThin-PlyCFRPmaterialsandthesamplesexpensivetomanufac- ture,CLDsrepresentanexcellentfirstapproximation.Themainparam- etersdefiningaCLDarethemeancyclicstress,thecyclicstressampli- tudeandtheloadratio(R-ratio)asthequotientbetweentheminimum andmaximumcyclicstress.Dependingonthematerialbehaviour,dif- ferentinterpolationapproachescanbechosen.Thesimplesttypeofin- terpolationisthelinearCLDmodel,whichonlyrequiresoneS-Ncurve resp.Wöhlercurve,butassumesthatthematerialbehaves thesame undertensileandcompressiveloads[28].Differentapproacheswere proposed,whichcanbeuseddependingontheexaminedmaterialbe- haviour.Themostcommonlyusednon-linearinterpolationapproaches includeHarris’sCLD[30],Kawai’sCLD[31],Boerstra’sCLD[32],and Kassapoglou’sCLD[33].Withinthisstudy,thepiecewiselinearinterpo- lation[34]isused.Thepiecewiselinearinterpolationmodelrequiresa limitednumberofS-Ncurvesandtheultimatetensileandcompressive strength.Formostofthecompositestudies,thepredictionsbasedon piecewiselinearinterpolationarethemostaccurate[28].

Philippidisetal.proposedananalyticaldescriptionofthepiecewise linearinterpolationmethodfortheindividualregionsoftheCLD,en- ablingunknown loadratios(𝑅^′)tobecalculated[34].If𝑅^′isinthe tensile-tensileregionandbetween𝑅=1andthefirstknownmeasured value(𝑅1TT)counterclockwise,formula1applies.

𝜎a^′= 𝑈𝑇𝑆

𝑈𝑇 𝑆

𝜎a,1TT+𝑟^′−𝑟1TT

(1)

𝜎a^′displaystheinterpolatedand𝜎a,1TTtheexperimentaldetermined amplitudestressesand𝑈𝑇𝑆theultimatetensilestrength.Thefurther parametersarecalculatedaccordingby𝑟i=(1+𝑅i)∕(1−𝑅i)and𝑟^′= (1+𝑅^′)∕(1−𝑅^′).Inthefirstcase,𝑅i=𝑅1TTapplies.

If𝑅^′isbetweentwoknownR-ratios,𝑅iand𝑅i+1,formula2applies.

𝜎^′_a= 𝜎a,i⋅(𝑟i−𝑟i+1) (𝑟i−𝑟^′)⋅_𝜎^𝜎a,i

a,i+1+(𝑟^′−𝑟i+1) (2)

If𝑅^′isincompressive-compressiveregionandclockwisebetween 𝑅=1andthefirstmeasuredvalueincompressiveregion,𝑅1CC,formula 3applies.𝑈𝐶𝑆representtheultimatecompressivestrength.

𝜎^′_a= 𝑈𝐶𝑆

𝑈𝐶𝑆

𝜎a,1CC−𝑟^′+𝑟1CC

(3)

InthecaseofThick-PlyFRPs,severalstudieshaveinvestigatedthe fatigueafterimpact(FAI)behaviour.Delaminationssignificantlyreduce thelifetime[35–37]. Atthesametime,studiesonimpactdamageof Thin-Plyshowthatthedelaminationareasincreasewiththinnerlayers [18,38].Incombinationwiththelargerdelaminationarea,theimpact behaviour couldbe alimitingfactorusingThin-Plystructures. How- ever,previousstudiesshowthatundertensileandcompressiveloading ofundamagedsamples,lessdamageisinitiatedwithdecreasinglayer thicknessandexistingdamagegrowsmoreslowly[17,18,39].Thiscan havepositiveeffectsonthecyclicperformanceofstructuresdamaged byanimpact.Forthisreason,thecyclicbehaviourofimpactedsamples isinvestigatedasafunctionofthelayerthickness.

2. Materialsandandmethods 2.1. Materialsandspecimenpreparation

The materialused in thisstudyis a CFRPunidirectional prepreg systemproducedbyNTPT(NorthThinPlyTechnology,Switzerland), whichiscomposedofNTPT’sepoxyresinTP402andT700Scarbonfi- bresfromTorayCarbonFibersAmerica,Inc.Thetargetforthefibrevol- umecontentoftheprepregswas55%.Theproducedfibreareaweights are30gsm,60gsmand120gsm.Higherfibreareaweightssuchas240 gsmand360gsmwereproducedbyblock-scalinganaccordingtothe numberofsame-orientedlayers.TheprepregswerecutbyCNCcutter (AristomatTL1625fromARISTOGraphicSystemeGmbH&Co.KGi.I., Germany)andmanufacturedbyhandlay-up.Thelayersweredebulked byavacuumbageveryfourthlayer.Table1showstheusedlay-ups.

Thecuringofthelaminateswasaccordingtotheprocesssuggestedby themanufacturer NTPT.Themaximum temperaturewas160^◦C,and thedifferentialpressurewas7bar.Thecuredlaminatesweresawnus- ingATM’sBrilliant265precisionsawwithacorundumbladeatafeed rateof1.5mm/s.Dependingonthetestmethod,tabswerebondedon thesamplesbeforecutting,toreducestressconcentrationsintheload introductionareaandthuspreventprematurefailure.Inthecaseoften- silesamplesandthefatiguesamples,whichareloadedundertension- tension(R=0.1),acombinationof±45^◦lay-up,1mmthickGFRPand 1mmthickaluminiumtabswereused.Forthecompressiontestsandfa- tiguecompression-compression(R=10)tests,theusedtabmaterialwas a±45^◦lay-upGFRPlaminatewithathicknessof2mm.Inbothcases, thetabsarebondedat80^◦CwiththeepoxyresinadhesiveUHUEndfest plus300fromUHUGmbH&Co.KG,Germany.Thespecimendimen- sionsofthetensileandcompressiontestscorrespondtotheASTMstan- dardsused.Thefatiguetestssamplesundertensile-tensileload(R=0.1) havethesamegeometryasthesamplesforthestatictensiletests.An exceptionisthe30gsmsamples.Duetothestressconcentrationsinload introductionareainconnectionwiththedamagesuppressionbehaviour ofthethinlayers,non-standardfailuresoccurintheareaoftheload introduction.Toachieveastandard-conformingfailure,aCNCmilling machine(IselEuroModMP30fromiselGermanyAG,Germany)milled thesamplestotheshape ofadogbone.Thegauge lengthis 130mm andthegaugewidth21mm.Thespecimensforthecyclictestsunder tensile-compressiveloadalsocorrespondtothegeometryofthestatic tensiletests.Dependingonthebucklingsupportused,tabsareusedor not.Thebucklingsupportsareexplainedinmoredetailinthefollowing

Table1 Laminatelay-ups.

Fibre areal weight in gsm

Test Method 30 60 120 240 (2x120) 360 (3x120)

Tension [45∕90∕ − 45∕0] 12s [45∕90∕ − 45∕0] 6s [45∕90∕ − 45∕0] 3s [45 2∕ − 45 2∕90 2∕0 2] s [45 3∕90 3∕ − 45 3∕0 3] s

Compression [45∕90∕ − 45∕0] 22s [45∕90∕ − 45∕0] 11s [45∕90∕ − 45∕0] 6s [45 2∕90 2∕ − 45 2∕0 2] 3s [45 3∕90 3∕ − 45 3∕0 3] 2s

Fatigue [45∕90∕ − 45∕0] 12s [45∕90∕ − 45∕0] 6s [45∕90∕ − 45∕0] 3s – [45 3∕90 3∕ − 45 3∕0 3] s

FAI [45∕90∕ − 45∕0] 12s [45∕90∕ − 45∕0] 6s [45∕90∕ − 45∕0] 3s – [45 3∕90 3∕ − 45 3∕0 3] s

Table2

TestMethods andspecimen dimensions(^∗Tension-Compression30 gsm no tabs,

∗∗Tension-Tension30gsm).

Specimen Dimension Tension Compression Fatigue after Impact ASTM D3039-00 ASTM D3410-03 –

Overall length in mm 250 140 250

Width in mm 25 25 36

Thickness in mm 2.88 5.28 2.88

Tab length ^∗in mm 50 65 –

Free test length in mm 150 10 250

Gage length ^∗∗in mm 150 – –

Gage width ^∗∗in mm 21 – –

section.Inthecaseoftheimpactedsamples,thewidthwasincreased to36mmsothatthedamageoftheimpactdoesnotreachtheedges of thesamplesandgrowthof thedamagecan be determinedin the furtherinvestigations.Supposetheimpactalreadyreachestheedgeof thespecimen.Inthatcase,therewillbeinteractionsbetweentheedge ofthesampleandtheimpactdamage,whichwouldnotcorrespondto thecomponent’sstructuralbehaviour.Thefatiguesamples,whichare testedundercompression-compression,correspondtothegeometrical dimensionsofthesamplesofthestaticcompressivetests,exceptthat thethicknesshasbeenreducedto2.88mm(seeTable2).Aftersawing, theedgesofthesamplewerefinallygroundedwithabrasivepaper(grid size600and1200)toreduceedgeeffectscausedbypreparationsteps before.Theusedstandardsandsampledimensionsaresummarizedin Table2.

2.2. Experimentalmethods

Thestatictensiletests werecarriedoutinaccordancewithASTM DD3039-00[40].Theclampingpressureoftheservo-hydraulicclamp- ingjawsamounted140bar.ThetestswereperformedonaZwickRoell universaltestingmachineZ100andstrainswererecordedusingMulti- XtensExtensometerfromZwickRoell.Inthecaseofthethickestlayer thickness(360gsm),thecrossheaddisplacementhadtobeusedforthe strainmeasurement,duetoearlydelaminationoftheouterlayers.The compressiontestswerecarriedoutinaccordancewithASTMD6641- 16[41]performedusingaZwickRoellZ400universaltestingmachine.

IMADresdendevelopedtheusedclampingsystem(HCCF)forcompres- siontestsoffibrereinforcedcompositesathighcompressiveloads.The hydraulicpressurefortheclampingjawswas80bar.VishayPrecision Group(USA)straingaugeswitha resistanceof 350Ωmeasuredthe strains.Thefatiguetestswereperformedonservo-hydraulictestingma- chinesfromInstron.Threedifferentloadconditionsweretestedtoinves- tigatepossibleinfluencesofthelayerthicknessonthefailurebehaviour.

AloadratioofR=0.1waschosenfortensile-tensiletests.Thesam- plesweretestedonanInstron8800H2470understresscontrolmodeat 5Hzwithamaximumforceof100kNandservo-hydraulicclamps.The clampingpressurewas80bar.Thedisplacementofthecylinderrecorded thestrainsofthespecimens.Whenasampledidnotfailafterbeingsub- jectedto(10⁶cycles),thetestmachinestopped.

AloadratioofR=0.5waschosenforthetension-compressiontests.

Thetensileloadsaretwiceashighasthecompressiveloads.Assuming thatfibrecompositematerialsaregenerallydesignedfortensileloads andthereforeaccountforthemainportionofoperationalstresses,there-

Fig.1. Bucklingsupportsforfatiguetests.

forealoadratioof R=0.5isanapplication-basedapproach. Thehy- draulicuniversaltestingmachinefortension-compressiontestswasthe Instron8800H2470understresscontrolmodeat5Hzwithamaximum forceof100kNandtheInstron8802L2741understresscontrolmode at3Hzwithamaximumforceof250kN.Inbothcases,servo-hydraulic jawswithaclampingpressureof80barclampedthespecimens.Dueto theappliedcompressiveforcesandspecimengeometry,twodifferent bucklingsupportsareutilised(seeFig.1).Forthickerlayers,thebuck- lingdevicesupportsthetestlengthbetweenthetabs(Fig.1,left).How- ever,thiswasnotpossiblewiththethinnestlayerthickness(30gsm), becauseabucklingfailureoccurredinthesupport-freesectionbetween theclampingofthespecimenandthebucklingsupport.Thenewbuck- lingsupportwasdesignedaftertheOpenHoleCompression(ASTMD 6484-04[42])testjig,withthedifferencethattheloadisintroducedvia shearforces.Thebucklingsupporthastheadvantagethatthesampleis supportedalongitsentirelength,thuspreventinglocalbuckling,and transversecontractionisnothindered.Thesamesetupisusedforthe fatigueafterimpacttestswithaloadratioofR=0.5.

TheR-ratioofthecompression-compressiontestswaschosentobe R=10.Duetothegaugelengthof10mmandthethicknessof2.88mm, nobucklingsupportwasnecessary.Thespecimensweretestedonthe Instron8800H2470servo-hydraulictestingmachineunderstresscon- trolmodeat5Hzwithamaximumforceof100kNandservo-hydraulic jawsat120barclampingpressure.Thedisplacementofthecylinderde- terminedthestrain.

TheimpacttestingmachinewastheFWPrimus1700PlusfromCo- esfeldMaterialtest,Germany.Theimpacts wereapplied byusingan

Fig.2. Clampingdeviceforimpacttests.

Table3

Numberoftestedsamplesforeachplythickness andloadcase.

Fibre areal weight in gsm 30 60 120 240 360

Tension 7 6 6 6 5

Compression 6 6 6 6 5 Fatigue R = 0.1 20 10 10 – 9 Fatigue R = 0.5 19 7 7 – 14 Fatigue R = 10 10 13 10 – 7 FAI R = 0.5 8 6 8 – 5

impacterwithadiameterof16±0.1mm,ahardnessbetween60and 62HCRandweightof5.34kgandareboundapparatuspreventedmul- tipleimpacts.Themaximumimpactenergywas8J.Preliminarytests haveshownthatimpactenergyofover8Jleadstodelaminationupto theedgesofthesampleandduetotheimpactsystem,lowerenergies werenotpossible.Theclampingdesignofthesupportstructure was basedontheASTMD7136-05(Standardtestmethodformeasuringthe damageresistanceofafibre-reinforcedpolymermatrixcompositetoa drop-weightimpactevent[43])withcut-outdimensionsof27×60mm.

Theratiooflengthandwidthisidenticaltothestandard.Fourrubber tipsrestrainthespecimenduringimpactwithaminimumholdingca- pacityof1100N.Mechanicalendstopsalignedthesamples(seeFig.2).

Toanalysetheoccurringdamageandpossiblegrowth,sampleswere stoppedat0,5⋅10⁴,3⋅10⁵and8⋅10⁵cyclesandexaminedusingcom- putedtomography.ThesystemusedisaCT-AlphadevicefromProconX- Ray,Germany.Themeasurementsofthedamagedsampleregionswere takeninaxialandhelicalmodewithanx-raytubevoltageof65kVand avoxelsizeof27.3and24.3𝜇m,respectively.Thehelicalscanmode wasappliedtosamples(forexample360gsm)withalargeestimated damageareatocapturethecompletedamagedareawhilemaintaining asmallvoxelsize.Thevisualizationofthevolumedata,i.e.evaluation andgraphicalrepresentationofimpactdamageaswellasthedamage progressionaccordingtotherespectiveloadcycleswasdonewiththe VolumeGraphicsVGSTUDIOMAX3.3software.

3. Resultsanddiscussion 3.1. Tensiletests

TheresultsofthestatictensiletestsareshowninFig.3.Atleastfive samplesweretestedforeachconfiguration.Table3showsthenumberof testedsamplesforeachplythicknessandloadcase.Thetensilestrength increasessignificantlywithdecreasinglayerthicknessintherangebe- tween360gsmand60gsm.Evenlowerlayerthicknesses,30gsm,show

nofurtherimprovementcomparedtothe60gsmsamples.However,the strengthdoublesbetweenthe360gsmand30gsmor60gsmsamples.

Incase ofthe360gsmsamples, theouter±45^◦and90^◦ layersfully delaminateatlowstrains,andthereforetheloadismainlytransferred bythemiddle0^◦layers,whichsignificantlyreducethestrength.This behaviourisnotobservedatlowerlayerthicknesses.Fig.3showson theright-handsidethestress-straincurvesofthetensiletests.Itcanbe seen thatall configurationshavethesamestiffnessatlow strains,as thisismainlydependentonthefibresandtheirorientation.However, withincreasingload,initialdamageoccurs,andadecreaseinstiffness isrecordedforthickerlayers.Further,sampleswiththinnerlayers(30 and60gsm)donotshowanydamageuntilfinalfailure,andtherefore nodecreaseinstiffnesscanbedetermined.The30and60gsmspeci- mens’fracturepatternsshowabrittlefailurebehaviourandexhibitno oralmostnodelaminations.Inbothcases,theenergyatthecracktipas wellastheinterlaminarshearstressesaretoolowtoinitiatedelamina- tion.Asaresult,thefailurebehaviourisfibre-dominatedinbothcases, andthusnodifferencesbetweenthelayerthicknessescanbeobserved.

Althoughtheoreticallythestrengthofthe90^◦layersshouldincreaseex- ponentiallywithdecreasinglayerthicknessduetotheinsitueffect,the influenceistoosmallcomparedtotheinfluenceofthefibresin0^◦to measureasignificantchange.Thelayerthicknessatwhichdelamina- tioniscompletelysuppresseddependsonthematerialsusedandtheir properties,suchasthetoughnessofthematrix.Detailedinvestigations onthiscanbefoundinastudybyCugnonietal.[44].Withincreas- inglayerthickness,amoredelamination-dominatedfailurebehaviour occurs.Asaresult,itcanbeconcludedthatunderstatictensileload,a layerthicknessof60gsmincombinationwiththefibreandresinsystem usedinthisstudyalreadyshowsthemaximumachievablestrengthand areductionofthelayerthicknessdoesnotofferanyfurtheradvantage tothemechanicalpropertiesinvestigated.Thefreedomofdesigndueto thehighernumberoflayersisstillanadvantage.

3.2. Compressivetests

Theresultsofthecompressivetestsshowasignificantincreasein strengthwithdecreasinglayerthicknessfrom360gsmto30gsm.Each configurationwastestedwithaminimumoffivesamples(seeTable3).

Diagrams in Fig.4 show thecompressive strengthsand stress-strain curves.Thecurvecharacteristicsshowthatprogressivefailureoccurs withlowlayerthicknesses.Initialdamageorfirstlocalbucklingdoes notleadtofinalfailureasforthickerlayersundercompressiveload.

Due tothehighamount offibres andthedistributionofthe0^◦ lay- ers,thesampleshavearelativelyhighbendingstiffnessafterfirstdam- age,which leadstoadecrease instiffness butnottoafinal failure.

Afurtheradvantageregardingthebehaviourundercompressiveload is in the laminate quality. As already investigated in other studies, laminatequalityimproveswiththinnerlayerthicknesses[18,23].Fi- bre spreadingresults inamore homogeneousfibrearrangement and smallerresin-richregions.Inthecaseofthickerlayers,initialdamage alreadyleadstofinalfailure,asthebendingstiffnessissignificantlyre- duced.Defectsandresin-richregionsactasinitiatorsforpossibleinitial damage.

3.3. Fatiguetests

3.3.1. Tension– Tension

Theresultsofthetension-tensiontestsareshowninFig.5.Thedi- agramontheleft-handsiderepresentstheS-Ncurve(Wöhlercurve), heretheamplitudestressisplottedagainstthenumberofcyclestofail- ureonalogarithmicscale.Eachconfigurationwastestedwithatleast ninesamples.Duetothenon-standardfailureofthe30gsmsamplesat higherstresses,20samplesweretestedinthiscase.Thenon-filledpoints representsampleswhichhavereachedtherun-outcriterionof10⁶cy- cles.Solidlinesshowthecalculatedfailureprobabilitythat50%ofthe specimenswillfailatthisnumberofcyclesataspecificstressamplitude.

Fig.3. Tensilestrength(left)andtensilestress-straincurves(right)dependingonthelayerthickness.

Fig.4. Compressivestrength(left)andcompressivestress-straincurves(right)dependingonthelayerthickness.

The50%probabilityoffailureofthe360gsmsamplesissignificantly lowerthanthatoftheothersamples,althoughthecurveisrelatively flat.Duetothehighlayerthickness,all0^◦layersareinthemiddleof thelay-up.Theouterlayersfullydelaminateatlownumbersofcycles, resultinginatypeofunidirectionalsample.Still,theresultingunidi- rectionalload-bearinglayersresultsinaflatS-Ncurvesincethefatigue propertiesareessentiallydependentonthe0^◦carbonfibres.Thebe- haviourisalsoapparentinthestiffnessdegradationcurve.Ontheright- handsideistherelativestiffness,whichisthequotientofthestiffnessat cyclenandtheinitialstiffnessofthefirstcycle.Inthisstudy,stiffnessis definedastheslopeofthetangentofthestress-straincurve(hysteresis) at5%and50%ofthemaximumstressofeachhysteresisrecorded.The typicalshapeofaS-Ncurveconsistsofthreephases.Anearlydecrease instiffnesscombinedwithinitialdamagesuchasinter-fibrefractures, arelativelyconstantplateauonwhichthedamagegrows,andaslight butcontinuousdecreaseinstiffnessoccurs,andfinallyaconsiderable reductioninstiffnessandultimatefailure.Inthecaseofthe360gsm samples,afourthphasehasoccurred,aplateauwithconstantstiffness.

Inthisphase,allouterlayersarealreadydelaminated,andonlythe0^◦ layersinthemiddleofthespecimensareloaded.

Thereisasignificantimprovementinthefatiguestressofthe60gsm and120gsmcomparedtothe360gsmsamples.The50%probabilities offailureareshiftedtohigherstressamplitudes,andbothS-Ncurves aresteeperthanthecurveofthe360gsmsamples,wherethe60gsm specimenscanhandleevenhigherstresses.Bothlayerthicknesseshave

asimilarslopeoftheS-Ncurveandareequallysensitivetofailurebe- haviour.However,therelativestiffnessshowalsodifferencesbetween them. Thecurveofthe120gsm sampleshaveafourthphase,asdo the360gsmsamples,whichinturnisduetotherelativelythicksub- laminateinthemiddleofthelay-up.The60gsmsamplesdonotshow thisfourthphaseanymore.Theseconddecrease instiffnessleadsto finalfailure;itcanalsobeseeninthefracturepatterns.The120gsm samplesshowafailurebehaviourdominatedbydelaminations,whereas the60gsmsamplesfailrelativelybrittle,andonlyafewdelaminations arevisible.Asthestatictensiletestshavealreadyshown,theformation of delaminationsis suppressed,andbrittlefailureoccurs.Thefailure behaviourundertensile-tensileloadissimilartothefailurebehaviour understatictensileload.

Asdescribedinthesectionmaterialsandspecimenpreparation,the 30gsmsampleshadtobetestedwiththeshapeofadogbonesample.

Withoutthisgeometrychange,allspecimensfailintheloadintroduc- tionareaandthereforenotconformingtothestandard.Duetothebrit- tlepropertiesofthematerialandthesuppressionofdamage,highlocal stressconcentrationsoccurintheloadintroductionarea,whichleadsto prematurefailure.Amacheretal.investigatedfatigueopenholetensile (OHT)testsandobservedthatathigherstresses,thinnerlayerthick- nessesleadtoearlyfailurebecausenostressescanbedissipatedordi- vertedduetopre-damageintheareaofthestressconcentration.Below acertainstressamplitude,nodamageoccurs,andthefatigueproperties improvesignificantly[18].

Fig.5. S-NcurvesofthefatigueresultswithaloadratioofR=0.1(left)andrelativereductionofthespecimenstiffness(right).

Fig.6.S-NcurvesofthefatigueresultswithaloadratioofR=-0.5(left)andrelativereductionofthespecimenstiffness(right).

Despitethechangeinspecimengeometry,thespecimensfailednear theloadintroductionareaathighstresses(seeFig.5,open-symbols).

Thefracturepatternsofthetestedspecimensinthisstudyshowanin- creasedfailurein theareaof loadintroductionforthesampleswith highstresses(half-filledsquares)despitethesamplegeometry.Atlower stresses,thespecimensfailaccordingtothestandardwithinthesmaller cross-sectionarea. Thestressconcentrationsin theloadintroduction areaarebelowtheapparentlycriticalstress.Inthedevelopmentofthe stiffness decrease,brittle material behaviour can be recognized.The knowledgeofthesuppressionofpre-damageandtheresultsofAmacher etal.regardingstressconcentrations,suggeststhatthin-layerlaminates haveimprovedfatiguepropertiesatlowstressesandhighnumbersof cycles.Toinvestigatethefatiguebehaviourunderhigherstresses,the loadintroductionareaorthesamplegeometrymustbefurthermodified sothattheloadintroductionisnotthecriticalarea.

3.3.2. Tension-Compression

ThefatigueresultswithaloadratioofR=-0.5(Fig.6)showsimilar behaviourtothetension-tensiontests.Atleastsevenspecimenswere testedforeachconfiguration.However,asthe30gsmspecimensfailed inanon-standardwayathigherstresses,20specimensweretestedhere.

Ingeneral,thefatiguepropertiesimprovewithdecreasinglayerthick- ness.Inthecaseofthe60,120and360gsmsamples,nochangeinthe slopeoftheS-Ncurvesisapparent.Thecurvesareshiftedinheight.

OnlytheS-Ncurveofthe30gsmsamplesshowsalowergradientas undertension-tension.Heretheintersectionofthe30gsmandthe60 gsmS-Ncurveisatabout40,100cyclesandastresslevelof370MPa.

Also, inthis case,stressconcentrationsat thinlayerthicknesseswill lead topremature failureathigher stresses in theload introduction area.Inthetensile-tensiletests,specimensfailedatamplitudestressof about258MPanearthetabs,whichcorrespondstoamaximumstress of572MPa.Thetensile-compressivesamplesfailintheareaofthetabs atamplitudestressofabout381MPa,whichcorrespondstoamaximum stressof508MPa.However,thetwostressesarenotdirectlycompara- blesincethestressofthetensile-tensilespecimensreferstoasmaller cross-sectionareaduetothespecimendesignofadogbone.However, thetabsofthesampleshavethesamegeometricdimensionsand,when theforcepermmofspecimenthicknessiscalculated,thecriticalload is12.03kN/mmforthetensile-tensilespecimensand12.70kN/mmfor thetensile-compressivespecimens.Thusthesameproblemsariseun- dertensile-compressiveloadasundertensile-tensileload.Nevertheless, thinnerlayersdisplaysuperiorfatiguebehaviourathigherloadcycles, astheyoccurinindustrialapplications.

TherightdiagramofFig.6showsthatthestiffnessdecreasesoverthe numberofcycles.Thecurvesdonotshowatypicalhorizontalplateau asundertension-tension,butaregionwithaconstantstiffnessdecrease.

Thestiffnessdegradationofthe30,60and120gsmsampleslooksvery

Fig.7. Hysteresisofdifferentlayerthicknesses(upperleft:30gsm,upperright:60gsm,lowerleft:120gsmandlowerright:360gsm)at10¹,10²,10³,10⁴and 10⁵cycles.

similar.Noticeableisthestrongdecreaseofthe360gsmsampleinthe rangebetween1000and10,000cycles.Todescribethisreductionin stiffnessinmoredetail,Fig.7showsthehysteresisofthesamples.The diagramsshowthehysteresesof10¹,10²,10³,10⁴and10⁵cycles.The hysteresisofa30gsmsampleisintheupperleft,ofa60gsmsample intheupperright,ofa120gsmsampleinthelowerleftanda360gsm sampleinthelowerrightcorner.Withincreasinglayerthickness,the angularoffsetconcerningthefirstcyclesincreasesandtheareaofthe hysteresis(energypercycle)decreaseswithdecreasinglayerthickness.

Aflatterhysteresisexhibitsalowerstiffnessduetodamagelikeinter- fibrefracturesanddelaminations.Inthecaseofthe360gsmsamples (bottomright),thefirst1000hysteresisaresimilarlysuperimposed,but theangleofthehysteresisathighernumbersofcycleschangestrongly.

Thisisduetotheformationofdelaminationsbetweentheouterlayers andthe0^◦layersinthemiddleofthesamples.Thesmallopeningofthe hysteresisofthe360gsmsampleatahighnumberofcycles,indicates thatonlythe0^◦layersareloaded,andnoenergyisdissipateddueto openandcloseofinter-fibrefracturesordelaminations.Itcanbeseen thatthedeviationbetween thehysteresisissmallerwithlowerlayer thicknesses,whichindicatesaconstantmaterialbehaviourindependent ofthenumberofcycles.Asinthestatic,abrittlematerialbehaviourof thethinlayerthicknessescanbeseen.

3.3.3. Compression– Compression

Fig.8showstheS-Ncurvesofthefatiguetestsundercompressive- compressiveload(R=10).Foreachconfiguration,atleastsevensam- plesweretested.Theresultsdemonstrateanimprovementofthefatigue propertieswithdecreasinglayerthickness.Duetothetestsetupandthe

Fig.8. S-NcurvesofthefatigueresultswithaloadratioofR=10.

failurebehaviouroffibrecompositesundercompression,thereisasig- nificantscatterwithintheresults.Still,thegradientoftheS-Ncurves showsdifferences.Theslopesofthe30and60gsmsamplesaremuch steeper.FracturepatternsoftestedspecimensinFig.9showachang- ingfailuremechanism,ascanalsobefoundunderstaticcompressive loadingofquasi-isotropicsamples.Themicrographsrepresentfracture

Fig.9.FracturepatternofthefatiguetestedsampleswithaloadRatioofR=10 (compression-compression).

patternsoffourspecimensthathavebeenloadedaboutahalfamil- lioncyclestofailure.Inthecaseofthe360gsmspecimens,theouter layersdelaminatedatalow numberofcycles.Thefailurebehaviour isacombinationofthrough-thickness(0^◦ layer),andbrooming[45]. Duetotheabsenceofsupportinglayers,themiddle0^◦layersstarted tobend,andthefinalfailureoccurred.The60gsmand120gsmsam- plesshowasimilarfailurepattern.Inbothcases,broomingoccurs,and delaminationsandsub-laminateshaveformed.Inthecaseofthe120 gsmsamples,thesub-laminatesusuallyconsistoffourlayerswithone 0^◦layerontheoutside,sothatthesub-laminatesarefragileagainst buckling.Incontrast,thesub-laminatesofthe60gsmsamplesconsist ofmorethanfourlayersand0^◦ layersarenotonlyattheoutsideof thesub-laminates.Theselayerssignificantlyincreasethebendingstiff- nessofthesub-laminatesandimprovethemechanicalpropertiesunder compressiveload.Furthermore,thematerialsinthisstudy,aswellas previousstudiesbyotherlaboratories([16–18]),showthatthequality ofthelaminatesorprepregsimproveswithdecreasinglayerthickness.

Thehighermaterialqualitieshaveapositiveeffectonthebehaviourun- dercompression.Thefracturepatternofthe30gsmsamplediffersfrom theothersamples.Delaminationoccursinasmallarea,andthefracture patternsshownobucklingoftheouterlayersorsub-laminates.Thefrac- turepatternshowsalongitudinalsplitting,andthelayersslidetogether morelikeacomb.Thestopcriterionofthefatiguestress-controlledtests wassetwithamaximumdisplacement.The30 gsmsamplesshow a residualcompressivestrengthevenafterstoppingthetests.Bypushing thelayersorsub-laminatestogether,relativelyhighbendingstiffnessis stillpresent.However,duetothedisplacementoftheupperandlower partofthespecimen,thetesthadtobestopped.Forindustrialapplica- tions,thefailurebehaviourimpliesahighsafetyfactorconcerningthe useofThin-Plyundercycliccompressiveload.Despitedamage,thereis arelativelyhighresidualstiffnessandstrength.

3.3.4. Constant-lifediagram

Theconstant-lifediagraminFig.10summarizedthefatigueresults obtained.Themeanstressisplottedontheabscissaandtheamplitude stressontheordinate.Eachpointrepresentstheratiobetweenmean stressandamplitudestressasafunctionoftheloadratioat5⋅10⁵cycles.

Thetheoreticalinterpolationsbetweenthemeasuredvaluesarecalcu- latedaccordingtothepiecewiselinearinterpolationofPhilippidisetal.

[34].Areductionofthelayerthicknessresultsinanimprovementofthe fatiguepropertiesandhighernumberofloadcyclescanbetoleratedat thesamestresslevel.Onlytensile-tensileloading(R=0.1),the30gsm achievednosignificantimprovementcomparedtothe60gsmsamples.

However,aspreviouslypresented,thereducedlayerthicknessesunder highloadsinthetensileareaofthehysteresisresultedindamage in theareaofloadintroduction.Athighernumbersofloadcycles,thethin layerthicknesseswithinthisstudyshowadvantages.Itisalsonotice- ablethatespeciallyundertensile-compressiveloading(R=0.5)thethin layerthicknesseshaveasignificantimprovement.Duetothedamage suppression,nopre-damageinitiatesundertensionload,leadingtoa significantstiffnessreductionundercompressiveload.Especiallyinthe alternatingloadrange,theadvantagesofthinlayerthicknessesbecome apparent.

Fig.10. Constant-lifediagramforn=5⋅10⁵cycles,piecewiselinearinterpola- tion.

Furthermore,evenifthestatictensileandcompressivetestsshow onlyminorimprovementsofthe30gsmcomparedtothe60gsmspec- imen,theimprovementishigherunderfatigueloading(R=0.5).The resultsshowthattheloadcaseshouldbetakenintoaccountiftheper- formanceoffibrecompositesistobeimprovedbyreducingthelayer thickness.Inareaswithalternatingloads,itisreasonabletousethinner layers.Nevertheless,thefailurebehaviourshouldbetakenintoaccount.

Inthecaseoftensileloads,brittlefailureoccurs,especiallywiththin layers,whichisadisadvantageinpracticalapplications.Inthecaseof compressiveloads,ontheotherhand,aprogressivefailureoccurswith thethinnerlayers,whichistobeevaluatedpositivelyundercertaincon- ditions.Insummary,theresultsshowthattheuseofthinlayerscanbe useful.Still,ithastobeconsideredcarefullydependingontheapplied stressesandfailurebehaviour.

3.4. Fatigueafterimpact

3.4.1. Impact

Fig.11showsrepresentativeultrasonicimagesandmicrographsof theoccurringimpactdamage.Theultrasonicimagesillustratethede- fectdepthofthedamage.Theultrasoundimagesillustratethedefect depthofthedamage,andanapproximationofthedepthisvisualized bydifferentcolours.Ifthereareseveraldamagesontopofeachother, onlytheuppermostdamagecouldbedetected.Theultrasonicimages demonstratethattheextentofthedamagedareaandthedamagepattern dependsonthelayerthickness.The360gsmsamplesshowatypicalfail- urebehaviourforfibrereinforcedcomposites.Inter-fibrefracturesand delaminationsarevisible,whichincreasewiththedepthofthesample.

Dependingonthefibreorientation,thedamagespreadoutinadifferent direction.Numerousstudieshavedescribedthefailurebehaviouroffi- brereinforcedcompositestructuresunderimpactloadsothatreference willbemadetothemhere[4,46].Withdecreasinglayerthickness,the failurepatternofthesampleschanges.Thenumberandlengthofinter- fibrefracturesdecreaseuntiltheyareentirelysuppressedinthecaseof the30gsmsample,ashasalsobeenshownbyArteiroetal.andSaito etal.[20,27].The60gsmsamplesshowfewerinter-fibrefracturesbut manydelaminations.Sub-laminatesareformed,whichhaveathickness offourlayers,240𝜇m.Thehighnumberofdelaminationsisuntypical forCAIsamples(ASTMD7137-05[47])withthinlayers.However,the untypicalfracturepatternresultsfromthesmallerclampingdeviceused inthisstudy.

IncontrasttothelargerCAIspecimens,lessmaterialcanabsorben- ergyandlocallythedamageisincreased.Duetothesmalldeformations

Fig.11. UltrasonicC-Scanimages(left)andmicrographs (right)oftheimpactforapotentialimpactenergyof8J, fromtoptobottom:360gsm,120gsm,60gsmand30gsm.

andthesuppressionofinter-fibrefractures,lessenergycanbedissipated elastically,andhighshearstressesareapplied,whichinitiatedelamina- tions.Theshapeofthedelaminationshaschangedfromapeanuttoa circularshape.Inthecaseofthe30gsmsample,threelargecircular delaminationsoccurred.Thefirstdelaminationdevelops betweenthe middlelayers,thehighestshearstressarea,andtwosub-laminatesare formed.Afurtherloadincreasestheshearstresseswithinthetwosub- laminates.Iftheshearstressesalsoexceedthecriticalstressinthesub- laminatesbetweenthenewmiddlelayers,furtherdelaminationsoccur.

Theshearstressesbetweentheotherlayersarenotsufficienttoinitiate furtherdelaminationsaswiththe60gsmsamples.About50%ofthe30 gsmsamplesshowthisfailurepattern.Theother50%showasimilar failurepatterntothe60gsmsamples.Itappearsthatthetestsetupis atatippingpointbetweenthetwofailuremodes.Itwasnotpossibleto increasetheenergybecausehigherenergiescausedfibrebreaksonthe backsideofthespecimen.Lowerenergieswerenotpossiblebecauseof thetestsetupandmachineused.

3.4.2. Fatigueafterimpact

Fig.12showstheS-Ncurvesofthepre-damagedsamples.Atotal of27samplesweretested.8sampleseachwithafibrearealweightof 30and120gsm,6with60gsmand5with360gsm(seeTable3).The solidlinescorrespondtotheS-Ncurvesofthepre-damagedsamples andthedashedlinestotheS-Ncurvesofthesampleswithoutimpact damage.Asexpected,thelifetimeofthepre-damagedsamplesisshorter thanthatoftheundamagedsamples.However,thedifferencebetween thepre-damagedandnon-damagedspecimensvariesbetweenthelayer thicknesses.Consideredat10⁵cycles,the60gsmand360gsmsam- plesshowthehighestdecreaseoffatiguestresswith31.9%and28.1%

respectively.

Asseenin themicrographs,thesampleswithalayerthicknessof 360gsmshowlargedelaminationsandmatrixcracks(seeFig.11).The growthofdamagewithanincreasingnumberof loadcyclesreduces thefatiguestrength.Acomputedtomographysystemexaminedthepre- damagedsamplesafteracertainnumberofcyclesandcomparedthe

Fig.12. S-NcurvesofthefatigueafterimpactresultswithaloadratioofR=-0.5 andanpotentialimpactenergyof8J.

damagepatterns.Thesampleswerestoppedandanalysedafter0,5⋅10⁴, 3⋅10⁵and8⋅10⁵cycles.Fig.13presentsrepresentativetomographyim- agesofthe30gsmand360gsmpre-damagedsamplesafterloading.

Thedamagepatternsofthe360gsmspecimenshowlarge,orientation- drivendelaminations.Thedamagedareaincreasessignificantlywiththe increasingnumberofcycles.Thedelaminationsreachtheedgeofthe sampleafteronly5⋅10⁴cycles,andcracksarevisibleonthetopsur- faceafter3⋅10⁵cycles.Fromtherecordeddata,sectionalimagesofthe respectivesamplesweremade,whichshowthecurrentstateof dam- ageperpendiculartotheload,seeFig.14.Thedamageofthe360gsm

Fig.13. Computedtomographyimagesoftheimpactdamageofthe30gsm(top)andthe360gsmsample(bottom).Thelefttwoimagesshowtheinitialstateafter theimpact,thethreerightimagesshowthedamagepatternaftercyclicloadingof5⋅10⁴,3⋅10⁵and8⋅10⁵cycles.Coloursarechosenarbitraryandindicatelayers ofdamageatdifferentdepths.

Fig.14. Crosssectionsoftheimpactedspecimens(left30gsmandright360gsm)after0,5⋅10⁴and8⋅10⁵cycles.Coloursarechosenarbitraryandindicatelayers ofdamageatdifferentdepths.

samplegrowthperpendiculartothetensiledirection.Duetothefatigue loading,furtherinter-fibrefractureshaveformed,whichleadtodelami- nationsandexistingdelaminationsgrowduetohighinterlaminarshear stresses.Slightbucklingofthespecimen,ascausedbytheimpact(asym- metricdamage),increasesthestresseswithinthecracktipofdelamina- tion,which supportsdamage growth.Themicrographofthe60 gsm samplealsoshowsahighnumberofdelaminations.Eachsub-laminate consistsoffourlayersandthushasathicknessofabout240𝜇m.The individualdelaminationsarearrangedcircularlyaroundtheimpact(see Fig.11).Thisgeometricarrangementweakensthematerialinallspatial directionsandfavoursabucklingofthesampleorthelayers,resulting inprematurefailure.Additionally,theindividualsub-laminateshavea lowbendingstiffness;therespective0^◦layerislocatedontheoutside atthesub-laminatesandisnotsupportedagainstbuckling.

Asalreadydescribedinthepreviouspart,the30gsmsamplesshow twodifferentfailurepatterns.Somesamplesshowfewbutverylarge delaminations (Fig. 11), and other samples show more andsmaller delaminations like the 60 gsm samples. The computed tomography measurementsallowanon-destructively3-dimensionalvisualisationof thedamagepattern.Fig.14showscross-sectionsof theimpactsfrom

Fig.13perpendiculartotheloadingdirection.Thedelaminationsare highlighted,andthecoloursarechosenarbitraryandindicatedamageat differentdepths.Inadditiontothecross-sections,detailedimagesfrom therespectiveimpactareaareshown,andthethicknessesofthesub- laminatesaremeasured.TheCT-cross-sectionsrevealthatthethinnest sub-laminatesareapprox.220𝜇mthickandhaveatleasteightCFRP layers.However,thickersub-laminatesarealsovisibleupto720𝜇m.

DuetothehighernumberofCFRPlayersandthesupportofthein- ner0^◦layersagainstbuckling,thesub-laminateshaveahigherbending stiffnessthanthesub-laminatesofthe60gsmsamples.Aswiththe60 gsmsamples,acirculararrangementofthedelaminationscanbefound, whichwouldindicatealargereductioninfatigueproperties.However, thespecimenslifetimedecreasedbyonly20.3%.Thetomographycross- sectionsdemonstratethatwithanincreasingnumberofcycles,thedam- agedoesnotgrowanyfurtherandastaticstateofdamageisachieved.

Thelaminateexhibitnomatrixcracks,andtheformationofdelamina- tionsissuppressed.Theinterlaminarshearstressesarenotsufficientfor thegrowthofdelaminations.Duetothelargenumberoflayersandthe associatedhighnumberofinterfaces,theinterlaminarshearstressesare lowerbetweenthelayers.Thecross-sectionsofthetomographyscansof

the360gsmsamplesshowadamageprogression(greenandreddelam- ination)andalsonewdamagessuchasinter-fibrefracturesanddelami- nations(yellowdelaminations)withanincreasingnumberofcycles.As aresult,thelifetimeisreduced.

The120gsmsamplesexhibitthesmallestreductioninfatigueper- formance.Contrarytothistrend,themicrographsinFig.11showinter- fibrefracturesanddelaminations.Concerningtheresidualstiffnessof thesub-laminates,thedelaminationsarebetweenthe0^◦and45^◦layers.

Thedifferencesbetweenthe120gsmandsampleswiththinnerlayers (30and60gsm)canbeseenintheultrasoundimagesinFig.11.The delaminationsarenotcircularbuthavetheshapeof apeanut.Thus, althoughlargerdelaminationsarepresent,theyspreadpreferentiallyin thedirectionofthefibreorientationinthelayer.Thedelaminationareas aresmallintransversedirectiontothepreferreddelaminationorienta- tion,leadingtoahighlocalresidualstiffness.

4. Conclusion

Thefatiguebehaviourofundamagedandimpactedsamplesofquasi- isotropichigh-performancecompositeswasinvestigatedinthisstudy.

Statictensileandcompressivetests,aswellasfatiguetestsunderdif- ferentloadratiosandlayerthicknesseswerecarried out,andapre- dictionoffatiguebehaviourisgivenbyalinearinterpolationmethod.

Theundamagedtestresultsshow thatthestaticandfatiguestrength increasewithdecreasinglayerthicknessanddependsontheloadra- tio.Inthetensile-compressive loadrange(R=-0.5) inparticular,sig- nificantimprovements canbeachievedusingthinlayers.Theforma- tionofinter-fibrefracturesanddelaminationsaresuppressedunderten- sileload,leadingtolowerdamagegrowthundercompressiveloadand higherlifetimes.The30gsmspecimensexhibita58.94%improvement inlifetimeatamaximumstressof50%ofthetensilestrengthcompared tothe60gsmsamples.Thereductionoflayerthicknessisleadingtoa largerloadingspace.

Althoughtheimpactdamagesofthe30 gsmsampleshavesignifi- cantlylargerprojecteddelaminationareas,theyhavelessinfluenceon thefatiguebehaviour ofthesamples. Computed tomographyimages showthatthedamagedoesnot spreadwithintheThin-Plylayers.In thecaseofthe360gsmsamples,thedelaminationsalreadygrowatlow numbersofcyclesandreachtheedgeofthesampleafter5⋅10⁴cycles andleadtofinalfailure.Inadditiontodelaminationgrowth,theshape ofdelaminationsandlayerstructureofthesub-laminatesalsohaveasig- nificantinfluenceonthelifetime.Localhigherbendingstiffness,asin thecaseofpeanut-shapeddelaminationsorsub-laminateswithinternal 0^◦layers,leadtoanimprovementinlifetime.

Fromtheresultsobtainedfromthisstudy,itcanbeconcludedthat fatiguebehaviourofundamagedandimpactedsamplesissuperiorwith thinnerlayerthicknesses.Still,forindustrialapplications,thetrade-off betweenperformanceandcostshastobedone,i.e.thesuitablelayer thicknessdependsonloadratioandeffortofproduction.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingfinancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinfluence theworkreportedinthispaper.

Supplementarymaterial

Supplementarymaterialassociatedwiththisarticlecanbefound,in theonlineversion,at10.1016/j.jcomc.2021.100139

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