(木材的力學(xué)性能)

Chapter7

MechanicsPropertiesofWood(木材的力學(xué)性能)

Structuralapplicationsofwoodproductsareomnipresentintoday’ssociety，andFigure10.1showstwo

suchapplications.

SomeofthemostimportantmechanicalproperiesofwoodproductsrealistedinTable10.1

Force,expressedonthebasisofunitareaorvolume,isknownasastress(應(yīng)力).

Themeasureofdistortionreslutingfromappliedstressisknownasstrain.(應(yīng)變)

Figure10.2illustratesstressandstraininawoodtestspecimenundercompressionparalleltothegrain.

Theelementcatalogiesofstressare:tensilestress(拉應(yīng)力),compressivestress(壓應(yīng)力),bendstress(彎曲應(yīng)力),torsinalstress(扭轉(zhuǎn)應(yīng)力),shearstress(剪切應(yīng)力)andsoon.

1Stress(應(yīng)力)andStrain(應(yīng)變)1.1DefinitionofStressandStrain(應(yīng)力和應(yīng)變的定義)

ＰＰＰＰＰＰshearstresstensilestresscompressivestressL⊿LP1.2Therelationshipbetweenstrainandstress(應(yīng)力與應(yīng)變的關(guān)系)

2Elastictity,Plasticity,andCreep(彈性，塑性，蠕變)2.1Hooke’sLaw(虎克定理),ModulusofElasticity(彈性模量)

Hooke’slawstatesthatthestrainεisproportionaltothestressσ:ε=α.σ

whereα=ε/σisacompliance,i.e.,thestrainperunitstress.Inthetechnicalliteraturenormallythereciprocalvalue1/α=Eisused.

2.2RhombieSymmetryofwood(木材的正交對(duì)稱性),AnisotropicNatureofWood(各向異性),SystemsofElasticConstants(彈性體)

Ashasbeenshownabove,woodisananisotropicmaterialbutthetrunkofatreeconsistsmoreorlessofconcentric

Eiscalledthemodulusofelasticity(彈性模量)orYoung’smodulus.(楊氏模量)

cylindricalshellsthusimpartingacylindricalsymmetrytothewood.Thesymmetryisreflectedinmostphysicalpropertiesofthewoodasinoftheelasticproperties,thestrengthvalues,thethermalandelectricalconductivityvalues.Theanisotropyodconiferouswoodsismuchmoreprouncedthanthatofbroad-leavedwoods:thisissepeciallyvalidforthequadrantz-x,asFig.7.32.3InfluencesAffectingtheElasticPropertiesofWood(影響木材彈性的因素)

(1)GrainAngle2.4PlasticityandCreep(塑性和蠕變)

(1)Stress-strain

Behaviour

Hooke’slawcannotbeexpectedtobevalidinawiderangeforsuchcomparativelycomplicatedmaterialsaswoodandothernaturalhighpolymers.Thestress-straindiagramisthereforenotthesameasforanidealelasticbody.

Ifidealelasticbehaviourisattributedtorubber,thenaloading–underloading.cyclecanbecarriedoutwithoutenergyloss,andtimeisnotafactor.Forwoodandotherelastic-plasticmaterialsastress-straincycleasinFig7.27takespalce.ThermodynamicallytheareaofloopC’D’F’A’repreasentstheenergylossduringtheentirecycle.Ifthestressalwaysactsonlyinonedirectionandifthestress-straincyclesarerepeated,thenthepermanentsetmaybeincreasedascanbeseenfromFig.7.28

(2)CreepandCreepRecovery

Wood,accordingtothepresentpresentexperimentalresultsandresults,possessesbothelasticaswellasplasticproperties.

AscanbeseenfromFig7.30,ifweapplyastressatzero-timethereisaninstaneaouselasticdeformationOA.ThisisfollowedbyaretartedcreepABenderconstantstress.Onremovalofthestressattimeaninstaneaouselasticrecoverytakesplace,followedbyaretardedpartialcreeprecoveryfromtoDattime.Afterthat,furtherrecoveryissoinsignificantthatitcanbeneglected.ThusDErepresentsthepermanentdeformationleftattheendoftheloading-unloadingcycle.3TheStrengthofWood(木材強(qiáng)度)

3.1DefinitionofStrengh(木材強(qiáng)度的定義)

Theresistanceofthebodytotheappliedatressisknownasthestrengthofthematerial.

Sincethereareanumberofdefferentkindsofstressws,thestrengthofthematerialmustbestatedintermsofitscompressive,tensile,shear,orbendingstregth.

3.2EffectofSpecificGravityonStrengthofWood(比重對(duì)強(qiáng)度的影響)

Thespecificgravityofwood,becauseitisameasureoftherelativeamountofsolidcellwallmaterial,isthebestindexthatexistsforpredictingthestrengthpropertiesofwood.

Ingeneralterms,withoutregardtothekindofwood,therelationshipbetweenspecificgravityandstrengthcanbeexpressedbytheequation:S=KwhereSisanyoneofthestrengthpropertise,Kisaproportionalityconstantdifferingforeachstength

property,Gisthespecificgravity,andnisanexponentthatdefinestheshapeofthecurverepresentingtherelationship.Fig6-6presentscurvesforthreeimportantstrengthpropertiesattwolevelsofmoisturecontent.

TherelationshipofstrengthtospecificGravityseenTab10.2.

Theeffectiveofwoodinresistinganyparticularformofappliedforceisafunctionnotonlyofthetotalamountofthewallmaterial,butoftheproportionsofthecellwallcomponentsfoundinagivenpiece,andalsooftheamountofextractivesinthecelllumen..Ameasureoftheefficiencyofthewoodtoresiststressisgivenbyanindexcalledthespecificstrengthwhichistheratioofstrengthtospecficgravity.Thisindexisgeneraltermsastheweight-strengthratio(比強(qiáng)度)

InComparisonwithotherstructhralmaterials,theweight-strengthratioforwoodisveryfavorableforsomeapplications.Woodhasahighindexofrigidityincomparisonwithsolidstructuralmaterialsandiswellsuitedforuesinsituationsthatrequireelasticstability.Besides,woodsuffersincompasionwithmetalsforusesthatrequirehighshearandcompressionresistance,becausetheverydistributionofwoodsubstancethatincreasestherigidityinbendingreducestheshearandcompressionefficiency.3.3EffectofMoistureContentonStrengthofWood(含水率對(duì)強(qiáng)度的影響)Mostofthestrengthpropertiesandelasticcharacteristicsofwoodvaryinverselywiththemoisturecontentofthewoodbelowthefibersaturationpoint.Ineffect,thechangesinmoistureaffectspecificgravityvalues,ashasbeenpointedoutearlier,andresultinchangesinstrength.TheserelationshipsforthreedifferentstrengthpropertiesareplottedascurvesinFig6-7.3.4EffectofTimeonStrengthofWood(時(shí)間對(duì)強(qiáng)度的影響)，DeformationinWood（木材的形變）,Relaxation(松弛)

Deformationinwoodunderstressistheresultoftwoindependentcomponentsoperatingsimultaneously.

Theelasticbehaviour（彈性變形）ofwoodresultsfromthepresenceandbehaviourofthecellulosemicrofibrilswhichexhibitelasticresponsetoloadapplicationandremoval;i.e.,the

deformationnisfullyrecoverablewhentheloadisremoved.Thesecondcomponentofdeformatioonistheplasticdeformation（塑性變形）beginswiththefirstapplicationofloadandincreaseswithtime.Recoveryofplasticdeformationinwoodisslowandeventuallyamountstoonlyabouothalfofthetotaldeformation.

Apieceofwoodmaintainedataconstantdeformationwillshowadecreasingmagnitudeofstressresistancetothedeformationwithincreasingtime.Thisphenomenonofdecreasingstressresistanceofthewoodastheresultofplasticflow(塑性流動(dòng))isknownasrelaxation(松弛).3.5TensileStrength(拉伸強(qiáng)度)

Neverthelessthetensilestrengthofwoodparalleltothegrainisextremelyhighandmayreach,forsomespeciesintheair-drycondition,amaximumof3,000kP/.Thetensilestrengthofseparatedwoodfibersisevenhigherandmayvarybetweenabouot2,000and13,000.kP/．

DeterminationofTensileStrengthalongtheGrain(順紋方向的拉伸強(qiáng)度測(cè)量).AstheTensileStrengthalongtheGrainismuchgreaterthanthatthecompressivestrengthperpendiculartothegrainandtheshearingstrength,itisdifficulttocarryoutsatisfactorytestsintensionparalleltothegrain.Themeasurementareasfollows:3.6Compressivestrength(壓縮強(qiáng)度)

ThemaximumCompressivestrengthplaysanimportantroleintheutilizationofwoo