单轴拉伸试验下砖的实验研究英文中文翻译.docx

单轴拉伸试验下砖的实验研究英文中文翻译.docx

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单轴拉伸试验下砖的实验研究英文中文翻译

ExperimentalInvestigationofBricksUnder

UniaxialTensileTesting

BSTRACT

Softeningisagradualdecreaseofmechanicalresistanceresultingfromacontinuousincreaseofdeformationimposedonamaterialspecimenorstructure.Itisasalientfeatureofquasi-brittlematerialslikeclaybrick,mortar,ceramics,stoneorconcretewhichfailduetoaprocessofprogressiveInternalcrackgrowth.Suchmechanicalbehaviouriscommonlyattributedtotheheterogeneityofthematerial,duetothepresenceofdifferentphasesandmaterialdefects,suchasflawsandvoids.Fortensilefailurethisphenomenonhasbeenwellidentifiedforconcretebutveryfewresultsexistsforclaybrick..Inthepresentpaper,theresultsofanextensivesetoftestscarriedoutatUniversityofMinhoandincludingthreedifferenttypesofbackunderniaxialtensionwillbepresented.Bothtensilestrengthandfractureenergyarequantified,withrecommendationsfortheadoptionofpracticalvalues.

INTRODUCTION

Thetensilebehaviourofconcreteandotherquasi-brittlematerialsthathaveadisorderedInternalstructure,suchasbrick.canbewelldescribedbythecohesivecrackmodelproposedinitiallybyHILLERBORG[1].Thismodelhasbeenwidelyusedasthefundamentalmodelthatdescribesthenon-linearfracturemechanicsofquasi-brittlematerials,e.g.[2,3].Accordingtothismodelandduetocrackinglocalization,whichisacharacteristicoffractureprocessInquasi-brittlematerials,thetensilebehaviourIscharacterizedbytwoconstitutivelawsassociatedwithdifferentzonesofthematerialduringtheloadingprocess.seeFigure1.Theelastic-plasticstress-strainrelationshipofFigurelaisvaliduntilthepeakloadisreached.ItisnotedthatbeforethepeakInelasticbehaviouroccursduetomicro-crackingandtheenergydissipatedinthisprocessisusuallyneglectedforthecalculationofthefractureenergy.Thestress-crackopeningdisplacementrelationshipofFigurelbdescribesthestrainsofteningbehaviourinthefractureprocesszoneafterthepeak.Thecohesivestress-openingdisplacementdiagramIscharacterizedbythegradualdecreaseofstressfromftmaximumvalue,tozero,correspondingtotheIncreaseofthedistancebetweenthetwoedgesofthecrackfromzerotothecriticalopening,u,ThesofteningdiagramassumesafundamentalroleInthedescriptionofthefractureprocessandIscharacterizedbythetensilestrength,fr,andthefractureenergy,Gr,whichIsgivenbytheareaunderthesofteningdiagram,seeFigure1b.Thecriticalcrackopening,ue,canbereplacedbytheductilityindexd,[4]givenastheratioGrlfr,whichrepresentsthefractureenergynormalizedbythetensilestrength.Thisparameterallowsthecharacterizationofthebrittlenessofthematerialandisdirectlyrelatedtotheshapeofthedescendingportionofthestress-deformationdiagram.

Thereareseveralexperimentalmethodsthathavebeenusedtomeasurethefractureproperties（tensilestrength,fractureenergyandductilityIndex）thatallowthedefinitionoftheconstitutivelawsofthematerial,namelydirecttensiletests,indirecttensiletestssuchasthethree-pointloadtest,andtheBraziliansplittingtest.Althoughtensilefailureresultsfromaloadcombinationandamultiplicity,offactors.meaningthatdirecttensionisnottheonlycauseoftensilecracking,adirecttensiletestseemstobethemoslappropriatetesttocharacterizethebasicfailuremechanism（modeI）ofquasi-brittlematerials.ThistestIsdefinedasthereferencemethodtofollow（5jbeingadoptedinthisworkfarthecharacterizationofthetensilebehaviourofbricks.

Differentissuesrelatedtothespecimensandthetestprocedureshavebeendiscussedinthepast,namelythetestingequipment,thecontrolmethod,thelocationoftheLinearVariableDisplacementTransducers（LVDTs）,thealignmentofthespecimenand,especially,theattachmentofthespecimenstothesteelplatens.TherelevanceofthelatterIsaddressedInFigure2[6].ThebehaviourinFigure2a（rotatingplatensorhinges）Isjustifiedbytherotationofthespecimenduringtheloadingoperation,wherethecrackproceedsfromonesideofthespecimentotheotherside.InthecaseofFigure2busingfixed（non-rotating）platens,abendingmomentisintroducedandmultiplecrackswillappear.Thisresultsinaslightlylargertensilestrengthandahighervalueofenergydissipated（fractureenergy）.Finally,ItisnotedthatalthoughthetensilestrengthandfractureenergyareconsideredIntrinsicpropertiesofthematerial,itIswellknownthatfracturepropertiesaresizeandscaledependent[6,7].

Tensilefractureparametersofmasonryconstituents,namelyunitsandthemortar-unitinterface,arekeyparametersforadvancednumericalmodellingofmasonryandforadeeperunderstandingofthebehaviourofmasonrystructures.inmepresentpaper,anexperimentalprogrammeusingthreetypesofclaybrickIsdiscussedwiththeobjectiveofincreasingthedataavailableintheliterature.

TESTSET-UPANDSPECIMENS

TensiletestswereperformedwithsolidbricksproducedbyValedaGandara,Portugal（S）,hollowbricksproducedbyJ.MonteiroeFilhos,Portugal（HP）,andhollowbricksproducedbySuceram,Spain（HS）.Allbricksareextrudedandtheyweretestedinverticalorthickness（V）andinhorizontalorlength（H）directionresultinginsixserieswiththefollowingnotation:

SV,SH;HPV,HPH;HSV,HSH.Table1givesthedimensionsofthebricksandthefreewaterabsorption.Thenetcompressivestrengthofthebricks,alongtheextrusiondirectionwas78N/mm282N/mm2and58N/mm2,respectivelyforS.HPandHS.Hereitisnotedthatthesevaluesaremerelyindicative,asthefirsttwovalueswerefromindependenttestsbydifferentresearchersandinsufficientInformationaboutthetestingproceduresisavailable,see（8,9].Thethirdvalueofcompressivestrengthwasprovidedbythemanufacturer.

Itisnotedthat:

（a）bricksHPareextrudedwiththeholesparalleltothelargerdimensionandbricksHSareextrudedwiththeholesparalleltothesmallerdimension;（b）bricksHPandHShavesmallgroovesintheuppersurface（sideoppositetothefacingside）inordertoincreaseadhesionbetweentheunitandthebackingmortar,seeFlgure3.

Testingequipmentandappliedmeasuringdevices

ThetestswereperformedinthelaboratoryoftheCivilEngineeringDepartmentofUniversityofMinho,usingaCS7400-Sshearingtestingmachine.Thismachinehastwoindependenthydraulicactuators,positionedinverticalandhorizontaldirections.Ithasaloadcellconnectedtotheverticalactuatorwithamaximumcapacityof25kN,beingparticularlysuitedtosmallspecimens（maximumsizeof90x150x150mm）.Theadoptionofaconstantcrosssectionforthespecimensleadstouncertaintyaboutthelocationofthemicro-cracks.Thisrepresentstheusualsupplementarydifficultyforthecontrolmethodofthistypeoftest.Sincethe

controlsystemallowsonlyoneLinearVariableDisplacementTransducer（LVDT）asdisplacementcontrol,itwasdecidedtointroduce,bymeansadiamondsawingmachine,two

lateralnotcheswithadepthof8mmandathicknessof3mmatmidheightofthespecimeninordertolocalizethefracturesurface.Withthenotches,thestressanddeformationdistributionisnolongeruniform,withstressandstraingradientsoccurringverylocalizednearthenotchtips.Sincethree-dimensionalnpn-uniformcrackopeningcanoccurontensiletests[10],thetensiletestcontrolusingtheaverageofthedeformationsregisteredonthefourcornersofthespecimenisthemostappropriateprocedure,seeFigure4.However,theavailableequipmentcanonlycontrolonedisplacementtransducer（LVDT）,locatedatanotchedside.Thetransducershaveameasurebaseof1mmwithalinearityof0.17%ofthefullstroke.Adeformationrateof0.5um/swasusedinthetests.Theforceappliedwasmeasuredonaloadcellof25kNmaximumloadbearingcapacity,withanaccuracyof0.03%.

Afterpreparationofthespecimens'ends,glueadhesionconditionswereenhancedbymakingaseriesofsuperficialslotswithasaw.Then,thespecimenswerecarefullyfixedtothesteelplatensusinganepoxyresin（DEVCOM）insuchawaythattheplatenswerekeptperfectlyparallel.Here,ItIsnotedthatthesteelplatensarefixed（non-rotating）,meaningthatloadeccentricityIsnotspecimens.Theonlysourceofanissueforpnsmadceccentricityisparallelismbetweenthesteelplatenswhichwethelackof,uldinduceabendingmomentInthespecimenintheclampingoperation.

Specimendimensions

Takingintoconsiderationthebrickdimensionsandthetestset-up,40x40x70mmSbrickspecimenswereextractedasshownInFigure5.HPandHSbricksarehollowand,therefore,thespecimensextractedfromthebricksmustberepresentativeofthebrickshell,achannelorUspecimens,andthebrickweb1specimens,seeFigure6.Here,itisnotedthattheusageofchannelspecimensinquestionablebecausealoadeccentricityisintroducedbythefactthetopandbottomflangesarefullygluedtothesteelspecimens.Nevertheless,becausetheendplatensarefullyfixed,theeccentricityisverylow.alinearelasticFEMcalculationIndicatesthatthenormalizedloadeccentricity（measuredbyeccentricity/webwidth）isonly0.03.

RESULTS

Fromtheforce-elongationrelationshipobtainedinthetensiletests,thefollowingparameterswereevaluated:

tensilestrength,fractureenergy,andresidualstressatultimatescanreading.ThenotchesreducetheYoung'smodulusofthebrick（Eb）byabout20%-40%[11].Asthemeasureof

Ebisquestionable,itisnotshownhere.

Figure7illustratestheprocedureadoptedforevaluatingthefractureenergy,G,.Inthecohesivecr