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1、WOOD375PROPERTIESOFWOOD,CERAMICS,PLASTICS,METALS,WATER,ANDAIRPropertiesofWoodMechanicalPropertiesofWood.—Woodiscomposedofcellulose,lignin,ash-formingminerals,andextractivesformedintoacellularstructure.(Extractivesaresubstancesthatcanberemovedfromwoodbyextractionwithsuchsolventsaswater,alcohol,aceton
2、e,benzene,andether.)Variationsinthecharacteristicsandvolumesofthefourcomponentsanddifferencesinthecellularstructureresultinsomewoodsbeingheavyandsomelight,somestiffandsomeflexible,andsomehardandsomesoft.Forasinglespecies,theprop-ertiesarerelativelyconstantwithinlimits;therefore,selectionofwoodbyspec
3、iesalonemaysometimesbeadequate.However,tousewoodmosteffectivelyinengineeringappli-cations,theeffectsofphysicalpropertiesorspecificcharacteristicsmustbeconsidered.Themechanicalpropertieslistedintheaccompanyingtablewereobtainedfromtestsonsmallpiecesofwoodtermed“clear”and“straightgrained”becausetheydid
4、notcontainsuchcharacteristicsasknots,crossgrain,checks,andsplits.However,thesetestpiecesdidcontainsuchcharacteristicsasgrowthringsthatoccurinconsistentpatternswithinthepiece.Sincewoodproductsmaycontainknots,crossgrain,etc.,thesecharacteristicsmustbetakenintoaccountwhenassessingactualpropertiesorwhen
5、estimatingactualperfor-mance.Inaddition,themethodsofdatacollectionandanalysishavechangedovertheyearsduringwhichthedatainthetablehavebeencollected;therefore,theappropriatenessofthedatashouldbereviewedwhenusedforcriticalapplicationssuchasstressgradesoflumber.Woodisanorthotropicmaterial;thatis,itsmecha
6、nicalpropertiesareuniqueandinde-pendentinthreemutuallyperpendiculardirections—longitudinal,radial,andtangential.Thesedirectionsareillustratedinthefollowingfigure.ModulusofRupture:Themodulusofruptureinbendingreflectsthemaximumload-car-ryingcapacityofamemberandisproportionaltothemaximummomentbornebyth
7、emember.Themodulusisanacceptedcriterionofstrength,althoughitisnotatruestressbecausetheformulausedtocalculateitisvalidonlytotheproportionallimit.WorktoMaximumLoadinBending:Theworktomaximumloadinbending
