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1、Chapter7MechanicsPropertiesofWood(木材的力学性能)Structuralapplicationsofwoodproductsareomnipresentintoday’ssociety，andFigure10.1showstwosuchapplications.SomeofthemostimportantmechanicalproperiesofwoodproductsrealistedinTable10.1Force,expressedonthebasisofunitareaorvolume,isknowna
2、sastress(应力).Themeasureofdistortionreslutingfromappliedstressisknownasstrain.(应变)Figure10.2illustratesstressandstraininawoodtestspecimenundercompressionparalleltothegrain.Theelementcatalogiesofstressare:tensilestress(拉应力),compressivestress(压应力),bendstress(弯曲应力),torsinalstre
3、ss(扭转应力),shearstress(剪切应力)andsoon.1Stress(应力)andStrain(应变)1.1DefinitionofStressandStrain(应力和应变的定义)ＰＰＰＰＰＰshearstresstensilestresscompressivestressL⊿LP1.2Therelationshipbetweenstrainandstress(应力与应变的关系)2Elastictity,Plasticity,andCreep(弹性，塑性，蠕变)2.1Hooke’sLaw(虎克定理),ModulusofElas
4、ticity(弹性模量) Hooke’slawstatesthatthestrainεisproportionaltothestressσ:ε=α.σwhereα=ε/σisacompliance,i.e.,thestrainperunitstress.Inthetechnicalliteraturenormallythereciprocalvalue1/α=Eisused.2.2RhombieSymmetryofwood(木材的正交对称性),AnisotropicNatureofWood(各向异性),SystemsofElasticCons
5、tants(弹性体)Ashasbeenshownabove,woodisananisotropicmaterialbutthetrunkofatreeconsistsmoreorlessofconcentricEiscalledthemodulusofelasticity(弹性模量)orYoung’smodulus.(杨氏模量)cylindricalshellsthusimpartingacylindricalsymmetrytothewood.Thesymmetryisreflectedinmostphysicalpropertiesoft
6、hewoodasinoftheelasticproperties,thestrengthvalues,thethermalandelectricalconductivityvalues.Theanisotropyodconiferouswoodsismuchmoreprouncedthanthatofbroad-leavedwoods:thisissepeciallyvalidforthequadrantz-x,asFig.7.32.3InfluencesAffectingtheElasticPropertiesofWood(影响木材弹性的因
7、素)(1)GrainAngle2.4PlasticityandCreep(塑性和蠕变)(1)Stress-strainBehaviourHooke’slawcannotbeexpectedtobevalidinawiderangeforsuchcomparativelycomplicatedmaterialsaswoodandothernaturalhighpolymers.Thestress-straindiagramisthereforenotthesameasforanidealelasticbody.Ifidealelasticbeh
8、aviourisattributedtorubber,thenaloading–underloading.cyclecanbecarriedoutwithouten
