机械工程英语翻译.docx
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机械工程英语翻译
第一单元:
TypesofMaterials材料的类型
Materialsmaybegroupedinseveralways.Scientistsoftenclassifymaterialsbytheirstate:
solid,liquid,orgas.Theyalsoseparatethemintoorganic(onceliving)andinorganic(neverliving)materials.
Forindustrialpurposes,materialsaredividedintoengineeringmaterialsornonengineeringmaterials.Engineeringmaterialsarethoseusedinmanufactureandbecomepartsofproducts.
Nonengineeringmaterialsarethechemicals,fuels,lubricants,andothermaterialsusedinthemanufacturingprocess,whichdonotbecomepartoftheproduct.
Engineeringmaterialsmaybefurthersubdividedinto:
①Metal②Ceramics③Composite④Polymers,etc.
•MetalsandMetalAlloys金属和金属合金
Metalsareelementsthatgenerallyhavegoodelectricalandthermalconductivity.Manymetalshavehighstrength,highstiffness,andhavegoodductility.
Somemetals,suchasiron,cobaltandnickel,aremagnetic.Atlowtemperatures,somemetalsandintermetalliccompoundsbecomesuperconductors.
Whatisthedifferencebetweenanalloyandapuremetal?
Puremetalsareelementswhichcomefromaparticularareaoftheperiodictable.Examplesofpuremetalsincludecopperinelectricalwiresandaluminumincookingfoilandbeveragecans.
Alloyscontainmorethanonemetallicelement.Theirpropertiescanbechangedbychangingtheelementspresentinthealloy.Examplesofmetalalloysincludestainlesssteelwhichisanalloyofiron,nickel,andchromium;andgoldjewelrywhichusuallycontainsanalloyofgoldandnickel.
Whyaremetalsandalloysused?
Manymetalsandalloyshavehighdensitiesandareusedinapplicationswhichrequireahighmass-to-volumeratio.
Somemetalalloys,suchasthosebasedonaluminum,havelowdensitiesandareusedinaerospaceapplicationsforfueleconomy.Manyalloysalsohavehighfracturetoughness,whichmeanstheycanwithstandimpactandaredurable.
Whataresomeimportantpropertiesofmetals?
金属有哪些重要特性?
Densityisdefinedasamaterial’smassdividedbyitsvolume.Mostmetalshaverelativelyhighdensities,especiallycomparedtopolymers.
Materialswithhighdensitiesoftencontainatomswithhighatomicnumbers,suchasgoldorlead.However,somemetalssuchasaluminumormagnesiumhavelowdensities,andareusedinapplicationsthatrequireothermetallicpropertiesbutalsorequirelowweight.
Fracturetoughnesscanbedescribedasamaterial’sabilitytoavoidfracture,especiallywhenaflawisintroduced.Metalscangenerallycontainnicksanddentswithoutweakeningverymuch,andareimpactresistant.Afootballplayercountsonthiswhenhetruststhathisfacemaskwon’tshatter.
Plasticdeformationistheabilityofbendordeformbeforebreaking.Asengineers,weusuallydesignmaterialssothattheydon’tdeformundernormalconditions.Youdon’twantyourcartoleantotheeastafterastrongwestwind.
However,sometimeswecantakeadvantageofplasticdeformation.Thecrumplezonesinacarabsorbenergybyundergoingplasticdeformationbeforetheybreak.
Theatomicbondingofmetalsalsoaffectstheirproperties.Inmetals,theoutervalenceelectronsaresharedamongallatoms,andarefreetotraveleverywhere.Sinceelectronsconductheatandelectricity,metalsmakegoodcookingpansandelectricalwires.
Itisimpossibletoseethroughmetals,sincethesevalenceelectronsabsorbanyphotonsoflightwhichreachthemetal.Nophotonspassthrough.
Alloysarecompoundsconsistingofmorethanonemetal.Addingothermetalscanaffectthedensity,strength,fracturetoughness,plasticdeformation,electricalconductivityandenvironmentaldegradation.
Forexample,addingasmallamountofirontoaluminumwillmakeitstronger.Also,addingsomechromiumtosteelwillslowtherustingprocess,butwillmakeitmorebrittle.
•CeramicsandGlasses陶瓷和玻璃
Aceramicisoftenbroadlydefinedasanyinorganicnonmetallicmaterial.Bythisdefinition,ceramicmaterialswouldalsoincludeglasses;however,manymaterialsscientistsaddthestipulationthat“ceramic”mustalsobecrystalline.
Aglassisaninorganicnonmetallicmaterialthatdoesnothaveacrystallinestructure.Suchmaterialsaresaidtobeamorphous.
PropertiesofCeramicsandGlasses
Someoftheusefulpropertiesofceramicsandglassesincludehighmeltingtemperature,lowdensity,highstrength,stiffness,hardness,wearresistance,andcorrosionresistance.
Manyceramicsaregoodelectricalandthermalinsulators.Someceramicshavespecialproperties:
someceramicsaremagneticmaterials;somearepiezoelectricmaterials;andafewspecialceramicsaresuperconductorsatverylowtemperatures.Ceramicsandglasseshaveonemajordrawback:
theyarebrittle.
Ceramicsarenottypicallyformedfromthemelt.Thisisbecausemostceramicswillcrackextensively(i.e.formapowder)uponcoolingfromtheliquidstate.
Hence,allthesimpleandefficientmanufacturingtechniquesusedforglassproductionsuchascastingandblowing,whichinvolvethemoltenstate,cannotbeusedfortheproductionofcrystallineceramics.Instead,“sintering”or“firing”istheprocesstypicallyused.
Insintering,ceramicpowdersareprocessedintocompactedshapesandthenheatedtotemperaturesjustbelowthemeltingpoint.Atsuchtemperatures,thepowdersreactinternallytoremoveporosityandfullydensearticlescanbeobtained.
Anopticalfibercontainsthreelayers:
acoremadeofhighlypureglasswithahighrefractiveindexforthelighttotravel,amiddlelayerofglasswithalowerrefractiveindexknownasthecladdingwhichprotectsthecoreglassfromscratchesandothersurfaceimperfections,andanoutpolymerjackettoprotectthefiberfromdamage.
Inorderforthecoreglasstohaveahigherrefractiveindexthanthecladding,thecoreglassisdopedwithasmall,controlledamountofanimpurity,ordopant,whichcauseslighttotravelslower,butdoesnotabsorbthelight.
Becausetherefractiveindexofthecoreglassisgreaterthanthatofthecladding,lighttravelinginthecoreglasswillremaininthecoreglassduetototalinternalreflectionaslongasthelightstrikesthecore/claddinginterfaceatananglegreaterthanthecriticalangle.
Thetotalinternalreflectionphenomenon,aswellasthehighpurityofthecoreglass,enableslighttotravellongdistanceswithlittlelossofintensity.
•Composites复合材料
Compositesareformedfromtwoormoretypesofmaterials.Examplesincludepolymer/ceramicandmetal/ceramiccomposites.Compositesareusedbecauseoverallpropertiesofthecompositesaresuperiortothoseoftheindividualcomponents.
Forexample:
polymer/ceramiccompositeshaveagreatermodulusthanthepolymercomponent,butaren’tasbrittleasceramics.
Twotypesofcompositesare:
fiber-reinforcedcompositesandparticle-reinforcedcomposites.
Fiber-reinforcedComposites纤维加强型复合材料
Reinforcingfiberscanbemadeofmetals,ceramics,glasses,orpolymersthathavebeenturnedintographiteandknownascarbonfibers.Fibersincreasethemodulusofthematrixmaterial.
Thestrongcovalentbondsalongthefiber’slengthgivethemaveryhighmodulusinthisdirectionbecausetobreakorextendthefiberthebondsmustalsobebrokenormoved.
Fibersaredifficulttoprocessintocomposites,makingfiber-reinforcedcompositesrelativelyexpensive.
Fiber-reinforcedcompositesareusedinsomeofthemostadvanced,andthereforemostexpensivesportsequipment,suchasatime-trialracingbicycleframewhichconsistsofcarbonfibersinathermosetpolymermatrix.
Bodypartsofracecarsandsomeautomobilesarecompositesmadeofglassfibers(orfiberglass)inathermosetmatrix.
Fibershaveaveryhighmodulusalongtheiraxis,buthavealowmodulusperpendiculartotheiraxis.Fibercompositemanufacturersoftenrotatelayersoffiberstoavoiddirectionalvariationsinthemodulus.
Particle-reinforcedcomposites微粒加强型复合材料
Particlesusedforreinforcingincludeceramicsandglassessuchassmallmineralparticles,metalparticlessuchasaluminum,andamorphousmaterials,includingpolymersandcarbonblack.
Particlesareusedtoincreasethemodulusofthematrix,todecreasethepermeabilityofthematrix,todecreasetheductilityofthematrix.Anexampleofparticle-reinforcedcompositesisanautomobiletirewhichhascarbonblackparticlesinamatrixofpolyisobutyleneelastomericpolymer.
•Polymers聚合材料
Apolymerhasarepeatingstructure,usuallybasedonacarbonbackbone.Therepeatingstructureresultsinlargechainlikemolecules.Polymersareusefulbecausetheyarelightweight,corrosionresistant,easytoprocessatlowtemperaturesandgenerallyinexpensive.
Someimportantcharacteristicsofpolymersincludetheirsize(ormolecularweight),softeningandmeltingpoints,crystallinity,andstructure.Themechanicalpropertiesofpolymersgenerallyincludelowstrengthandhightoughness.Theirstrengthisoftenimprovedusingreinforcedcompositestructures.
ImportantCharacteristicsofPolymers
Size.Singlepolymermoleculestypicallyhavemolecularweightsbetween10,000and1,000,000g/mol—thatcanbemorethan2,000repeatingunitsdependingonthepolymerstructure!
Themechanicalpropertiesofapolymeraresignificantlyaffectedbythemolecularweight,withbetterengineeringpropertiesathighermolecularweights.
Thermaltransitions.Thesofteningpoint(glasstransitiontemperature)andthemeltingpointofapolymerwilldeterminewhichitwillbesuitableforapplications.Thesetemperaturesusuallydeterminetheupperlimitforwhichapolymercanbeused.
Forexample,manyindustriallyimportantpolymershaveglasstransitiontemperaturesneartheboilingpointofwater(100℃,212℉),andtheya