专业外语课后阅读第二课page-18--20-Physical-Properties-of-Wood

2PhysicalPropertiesofWoodpage18--20-1-2PhysicalPropertiesofWoodpage18--20Physicalpropertiesarethequatitativecharacteristicsofwoodanditsbehaviortoexternalinfluencesotherthanappliedforces.Includedherearedirectionalproperties,moisturecontent,dimentionalstability,thermalandpyrolytic(fire)properties,desity,andelectrical,chemical,anddecayresistance.Familiaritywithphysicalpropertiesisimportantbecausetheycansignificantlyinfluencetheperformanceandstrengthofwoodusedinstructuralapplications.Thephysicalpropertiesofwoodmostrelevanttostructuraldesignandperformancearediscussedinthissection.A.DirectionalPropertiesWoodisanorthotropicandanisotropicmaterial.orthotropic[,ɔ:θə'trɔpik]adj.直生的，正交的；支架桥面合一的（指一种桥设计，支架结构同时也是桥面或路面）网络释义orthotropic:直生的|正交各向异性的|正交orthotropicplate:正交蛤异性板|正交各向异性板orthotropicslab:正交异性板anisotropic[æn,aisəu'trɔpik]adj.[植]各向异性的；非均质的网络释义anisotropic:非单折射性|各向异性|蛤异性的AnisotropicFiltering:各向异性过滤|非等向性过滤AnisotropicSteel:各向异性钢片基本翻译各向异性材料；蛤异性物质网络释义anisotropicmaterial:蛤异性材料|各向异性材料|蛤异性物质anisotropicsheetmaterial:各向异性薄板材料Becauseoftheotientationofthewoodfibersandthemanerinwhichatreeincreasesindiameterasitgrows,propertiesvaryalongthreemutuallyperpendicularaxes:longitudinal,radial,andtangential(Fig.2-2).Thelongitudinalaxisisparalleltothefiber(grain)direction,theradialaxisisperpendiculartothegraindirectionandnormaltothegrowthrings,andthetanentialaxisisperpendiculartothegraindirectionandtangenttothegrowthrings.Althoughmostwoodpropertiesdifferineachofthesethreeaxisdirections,differencesbetweentheradialandtangentialaxisandthelongitudinalaxisarerelativelyminorwhencomparedtodifferencesbetweentheradialortangentialaxisandthelongitudinalaxis.Propertiyvaluestabutatedforstructuralapplicationsareoftengivenonlyforaxisdirectionsparalleltograin(longitudinal)andperpendiculartograin(radialortangential).B.MoisturecontentThemoistrecontentofthewoodisdefinedastheweightofwateringwoodgivenasa2PhysicalPropertiesofWoodpage18--20-2-percentageofovendryweight.Inequationform,moistrecontent(MC)isexpressedasfollows:Fig.2-2Threeprincipalaxesofwoodwithrespecttograindirectionandgrowthrings100%moistweightdryweightMCdryweightWaterisrequiredforthegrowthanddevelopmentoflivingtreesandconstitutesamajorportionofgreenwoodanatomy.Inlivingtrees,moistrecontentdependsonthespeciesandthetypeof-wood,andmayrangefromapproximately25%tomorethan250%(twoandhalftimestheweightofthedrywoodmaterial).Inmostspecies,themoisturecontentofsapwoodishigherthanthatofheartwood.Waterexistsinwoodeitherasboundwater(inthecellwall)orfreewater(inthecellcavity).Asboundwater,itisbounded(viasecondaryorhydrogenbonds)withinthewoodcellwall.Asfreewater,itissimplypresentinthecellcavities.Whenwooddries,mostfreewaterseperatesatafasterratethanboundwaterbecauseofaccessibilityandtheabsenceofsecondarybonding.Themoisturecontentatwhichthecellwallsarestillsaturatedbutvirtuallynowaterexistsinthecellcavitiesiscalledthefibersaturationpoint.Thefibersaturationpointusuallyvariesbetween21%and28%.Woodisahydroscopicmaterialthatabsorbsmoistureinahumidenvironmentandlosesmoistureinadryenvironment.Asaresult,themoisturecontentofwoodisafunctionofatmosphericconditionsanddependsontherelativehumidityandtemperatureofthesurroundingair.Underconstantconditionsoftemperatureandhumidity,woodreachesanequilibiummoisturecontent(EMC)atwhichitisneithergainingnorlosingmoisture.TheEMCrepresentsabalancepointwherethewoodisinequilibiumwithitsenvironment.Instructuralapplications,themoisturecontentofwoodisalmostalwaysundergoingsomechangesastemperatureandhumidityconditionvary.Thesechangesareusuallygradualandshort-termfluctuationsthatinfluenceonlythesurfaceofthewood.ThetimerequiredforwoodtoreachtheEMCdependsonthesizeandpermeabilityofthemember,thetemperature,andthedifferencebetweenthemoisturecontentofthememberandEMCpotentialofthatenvironment.Changesinmoisturecontentcannotbeentirelystoppedbutcanberetarcedbycoatingsortreatmentsappliedtothewoodsurface.C.DimensionalStabilityAbovethefibersaturationpiont,woodwillnotshrinkorswellfromchangesinmoisturecontentbecausefreewaterisfoundonlyinthecellcavityandisnotassociatedwithinthecellwalls.However,woodchangesindimensionasmoisturecontentvariousbelowthefibersaturationpiont.Woodshrinksasitlosesmoisturebelowthefibersaturationpiontandswellsasitgainsmoistureuptothefibersaturationpiont.Thesedimensionalchangesmayresultinsplitting,checking,andwarping.ThephenomenaofdimensionalstabilityandEMCmustbeunderstood.Dimensionalstabilityofwoodisoneofthefewpropertiesthatsignificantlydiffersineachofthethreeaxisdirections.Dimensionalchangesinthelongitudinaldirectionbetweenthefibersaturationpiontandovendryarebetween0.1%and0.2%amdareofnopracticalsignificance;however,injuvenilewood,thesepercentagesmaybesignificantlyhigher.Thecombinedeffects2PhysicalPropertiesofWoodpage18--20-3-ofshrinkageintangentialandradialaxescandistorttheshapeofwoodpiecesbecauseofthedifferenceinshrinkageandthecurvatureoftheannualrings(Fig.2-3).Generally,tangentialshrinkage(varyingfrom4.4%to7.8%dependingonspecies)istwicethatofradialshrinkage(from2.2%to5.6%).