汽車專業(yè)英語(yǔ)課件：New Materials of Automobile -

汽車專業(yè)英語(yǔ)

NewMaterialsofAutomobileTheaimofthisunitisto:Introducethebroadrangeofmaterialsthatdesignerscandrawupon;Introducethepropertiesofmaterialsthatarerequiredforvehicledesign;Demonstrateparticularusesofmaterialpropertiesbycasestudies;Demonstratethematerialselectionprocessanditsinteractivitywithdesign.7.1IntroductionThemainthemeofthispartwillbethestudyofthevariousinterrelationshipsbetweenthestructureofengineeringmaterials,themethodsofcomponentmanufactureandtheirultimatedesignedbehaviorinservice.Thefourmajorgroupsofengineeringmaterialsaremetalsandalloys;ceramicsandglasses;plasticsandpolymersandmoderncomposites,suchassiliconcarbidereinforcedaluminumalloys.Illustrativecasestudieswillmakeupasignificantsectionofthispart.Thefullrangeoftheseengineeringmaterialsisusedintheconstructionofmotorvehicles.Itisacommonmyththattheaerospace,defenseandnuclearindustriesleadthewayintheuseofmaterialsforaggressiveenvironmentsandloadingregimes.Theautomotiveindustryhasitsownagendawiththeaddedcriteriaofconsumerdemandsofacceptablecostsaswellascriticalenvironmentalissues.Engineers,ingeneral,arefamiliarwithmetalssincetheyhavetheallroundproperties,whicharerequiredforloadbearingandotherapplications.Thissituationishelpedeconomicallybythefactthatofthehundredorsoelementswithintheearth’scrust,themajorityaremetals.Thismeansthatwhilstsomearemoredifficulttoextractthanothers,awiderangeofmetalsisavailabletosupplementiron,aluminium,copperandtheirwiderangingalloys.Metalshaveadequatestrength,stiffnessandductilityunderbothstaticanddynamicconditions.Otherphysicalpropertiesarealsoacceptablesuchasfracturetoughness,density,expansioncoefficient,electricalconductivityandcorrosionandenvironmentalstability.Awiderangeofformingandmanufacturingprocesseshavebeendevelopedaswellasanextensivedatabaseofdesignproperties.Thereisalsoawell-establishedscrapandrecyclingbusiness.金屬材料在靜態(tài)和動(dòng)態(tài)加載條件下都具有充足的強(qiáng)度、剛度和韌性。其他的力學(xué)性能，如斷裂韌性、密度、膨脹系數(shù)、電導(dǎo)率、腐蝕性和環(huán)境穩(wěn)定性也都較好。大范圍的成型和制備工藝作為設(shè)計(jì)性能的全面數(shù)據(jù)庫(kù)已經(jīng)發(fā)展了起來(lái)。同時(shí)存在對(duì)于報(bào)廢和再循環(huán)產(chǎn)業(yè)的固定模式。Onlywhenextremepropertiessuchaslowdensity,lowthermalandelectricalconductivity,hightransparencyorhightemperatureandchemicalresistancearerequired,andwhereeaseofmanufactureandperhapslowcostareimportant,doengineersconsiderfundamentallydifferentmaterials,suchaspolymersandceramics.只有當(dāng)特殊的性能，如低密度、低熱導(dǎo)和電導(dǎo)、高的透明度或高溫和耐化學(xué)是必須的，并且易于制造，可能低成本最重要，工程師們將考慮完全不同的材料，比如高聚物和陶瓷。Thesetwogroupsofmaterialshavealternativeengineeringlimitationssuchaslowstrengthorbrittleness.Consequently,combinationsofthesethreematerialsgroupshavebeenusedtoformthefourthgroupofengineeringmaterialsknownascomposites,ofwhichthemajortonnagegroupisglassreinforcedpolymers.Ceramicreinforcedmetalsalsoformasignificanttechnicalgroupofcompositematerials.Allfourgroupsofthesematerialshaveanessentialparttoplayinthedesign,constructionandserviceuseinvehicleengineering.這兩組材料具有替代工程局限性，如低強(qiáng)度或脆性。因此，這三種材料的組合體已經(jīng)被用于形成第四組工程材料稱為復(fù)合材料，其中占主要地位的是玻璃纖維聚合物。陶瓷增強(qiáng)金屬也形成了一個(gè)重要復(fù)合材料的技術(shù)組。所有四組材料在車輛工程的設(shè)計(jì)、施工和服務(wù)使用中起了主要的作用。essentialpart主要部分Inadditiontothedirectengineeringissues,thevehicledesignerneedstoconsiderthepoliticalissuessuchaspollutionandrecyclingduetothevastquantitiesofmaterialsusedinautomotivemanufacture.InWesternEurope,theECpoliticiansnowexpectvehiclestobeclean,safe,energyefficient,affordableandalso‘intelligent’,whichmeansthattheyshouldbeabletoanticipatetheactionsofthedriverandotherroadusers.Thishasleadtosignificantresearchfundingwhich,inthematerialsarea,hasinvolvedworkintheareasofcombustionenginematerials,batteriesandfuelcells,wearresistantmaterialsandlightweightvehiclebodymaterials.Suchworkisexpectedtocontinue.However,whilstengineering,environmentalandsafetyissueswillbeofgeneralconcern,themanufacturerwillcontinuetobemotivatedbyprofitwhilstthedriverwillstillexpectpersonalfreedom.Onthisissuegovernmentiscaughtbetweentheenvironmentallobbiesandthecarindustry,whichmakesaconsiderablecontributiontogrossdomesticproduct.Thus,roadusageislikelytocontinuetoincrease,sothatsomeformofoveralltrafficmanagementmaywellbecomeessentialasroadbuildingprogrammersarescaleddownduetoeconomicandenvironmentalpressures.7.2StructureandManufacturingTechnologyofAutomotiveMaterialsEngineeringmaterialsareevolvingrapidly,enablingnewvehiclecomponentdesigns,forloadbearingstructuresandbodywork,engines,fuelsupply,exhaustsystems,electricalandelectronicdevicesandmanufacturingsystems.Modernmaterialsincludefibercomposites,technicalceramics,engineeringpolymersandhightemperaturemetalalloys.Thevehicledesignermustbeawareofthesedevelopmentsandbeabletoselectthecorrectmaterialforagivenapplication,balancingpropertieswithprocessing,usingabasicunderstandingofthestructuralinterrelationships.工程材料迅速發(fā)展，使新車輛組件的設(shè)計(jì)，針對(duì)于負(fù)載軸承結(jié)構(gòu)和車體、發(fā)動(dòng)機(jī)、燃料供應(yīng)系統(tǒng)，排氣系統(tǒng)、電氣和電子設(shè)備和生產(chǎn)系統(tǒng)而展開。現(xiàn)代材料包括纖維復(fù)合材料、技術(shù)陶瓷、工程聚合物和高溫金屬合金。汽車設(shè)計(jì)師必須了解這些發(fā)展并能夠?qū)τ诮o定用途選擇正確的材料、調(diào)解加工時(shí)的性能，對(duì)于結(jié)構(gòu)的相互關(guān)系有基本的理解。7.2.1MetalsandAlloysManymetalsarenotabundantandsocanonlybeusedforspecialistapplicationssuchasincatalyticconvertersandpowerfulpermanentmagnets.Incontrast,iron,copperandaluminumareveryabundantandmoreeasilyobtainedandsoarewidelyusedinbothpureandalloyforms.Iron-basedorferrousmetalsarethecheapestandthemostwidelyusedatpresent.Forlowloadapplications,suchasbodyworkandwheels,mildorlowcarbonsteelissufficientlystrongwithyieldstrengthsvaryingbetween220and300MPa.Itisalsoeasytocut,bend,machineandweld.Fordriveshaftsandgearwheels,thehigherloadsrequiremediumcarbon,highcarbonoralloysteels,whichhaveyieldstrengthsofabout400MPa.Higherstrengthandwearresistanceareneededforbearingsurfaces.Mediumandhighcarbonsteelscanbehardenedbyheattreatmentandquenchingtoincreasetheyieldstrengthstoabout1000MPa.Unfortunately,thesehardenedsteelsbecomebrittlefollowingthisheattreatment,sothatafurthermildre-heating,calledtempering,isrequired.Thisreducesthebrittlenesswhilstmaintainingmostofthestrengthandhardness.Stainlesssteelsarealloyswithavarietyofforms,Austenitic,Ferritic,MartensiticandthenewerDuplexsteels.Acommoncompositioncontains18%chromiumand8%nickel,asshowninBS970,1991.Theircorrosionresistanceandcreepresistancearesuperiortoplaincarbonsteels,particularlyathightemperatures.However,highermaterialandmanufacturingcostslimittheiruseinvehicleengineeringtospecialistapplicationssuchaslongerlifeexhaustsystems.Castironshave2to4%carbon,incontrasttothe1%orlessforotherferrousmetalsmentionedabove.Thismakesthembrittle,withpoorimpactproperties,unlessheat-treatedtoproduceductileiron.Itismorereadilycastthansteel,sincethehighercarboncontentreducesthemeltingpoint,makingpouringintocomplexshapedmouldsmucheasier.Inaddition,thecarbonintheformofgraphitemakesanidealboundarylubricant,sothatcylindersandpistonshavegoodwearcharacteristics,foruseindieselengines.However,itisnowlargelyreplacedbythemuchlighteraluminiumalloysfortheseapplicationsinpetrolengines.Copperanditsalloysformasecondgroupofvehicleengineeringmetals,includingcopperitself,brass,bronzeandthecupro-nickels.Copperismoreexpensivethansteel,butisductileandeasilyshaped.Italsohashighthermalconductivity,givinggoodheat-transferforradiators,althoughmorerecentlyreplacedbythelighteraluminuminthisapplication.Itshighelectricalconductivityismadeuseofinwiringandcablingsystems.Brassisacopperalloy,commonlywith35%zinc,whichmakesiteasiertomachineyetstrongerthanpurecopper.Thus,complexshapescanbeproducedforelectricalfittings.However,suchalloyssufferfromalongtermproblem,knownas‘dezincification’,inwater.Corrosioncanbeminimizedbyusingthemoreexpensivecopperalloy,bronze,wheretinisthealloyingelement,althoughthismaterialmaybehardertomachine.Copper-nickelalloyshavegoodcreepresistanceathightemperatureswheretheyarealsocorrosionresistant.Thelatterpropertyismadeuseofinbrakefluidpipework.Aluminiumanditsalloyshaveamajoradvantageoversteelsandcopperalloys,asvehicleengineeringmaterials.Theirmuchlowerdensitiesleadtolowerweightcomponentsandconsequentfuelenergysavings.Whilstaluminiumoresareabundant,theextractionofpurealuminiumisveryenergyintensive,beingelectro-chemicalinnatureratherthanthepurelychemicalprocessusedforsteels.Copperoccupiesanintermediatepositiononthispoint.Thus,purealuminiumismoreexpensivethanironandcopperandhaslowerinherentstrengthandstiffness.However,itdoeshavecorrosionresistancewithgoodthermalandelectricalconductivity.Awiderangeofalloysisnowavailablewithvariousheattreatmentsandmanufacturingopportunities.Thesematerialshavenowreplacedsteelsandcopperalloysinmanyvehiclecomponentapplications,wheretheirhighermaterialscostscanbedesignedout,seeFig.7-1.Nevertheless,materialsdevelopmentsaresuchthataluminumalloysarethemselvesincompetitionwithpolymersandcompositematerialsforsuchapplicationsasvehiclebodywork.Considerablepricefluctuationsinmaterialsoccurfromtimetotimeduetofuelpricevariationssothatthecostvaluesshouldbeconsideredinrelativeterms.Theselectionofametalforadesignapplicationrequiresexperimentaldata.Thefirststagewilldeterminewhichgroupofmetalsshouldbeused,steels,copperoraluminium.Thenaspecificselectionwillrequiremoredetailedinformation.Testingofmaterialsandcomponentswillthereforeberequired.Somepropertiesarelargelyindependentofcomposition,microstructureandprocessing.Theseincludedensity,modulus,thermalexpansionandspecificheat.However,manypropertiesareverydependentonalloycomposition,microstructure,heat-treatmentandmechanicalhistory.Thesepropertiesincludeyieldandtensilestrength,ductility,fracturetoughness,creepandfatiguestrength,sothatspecificinformationisrequired.由于燃料價(jià)格的變化，材料的相當(dāng)大的價(jià)格波動(dòng)時(shí)常發(fā)生，所以成本值應(yīng)該按相對(duì)價(jià)值計(jì)算。對(duì)于設(shè)計(jì)應(yīng)用的金屬材料的選擇需要實(shí)驗(yàn)數(shù)據(jù)。第一階段將確定哪些金屬應(yīng)該被使用、鋼、銅或鋁。然后一個(gè)特定的選擇將需要更詳細(xì)的信息。材料試驗(yàn)和組件將因此被要求。一些屬性是很大程度上獨(dú)立的成分、顯微組織和處理。這些包括密度、彈性模量、熱膨脹和比熱。然而,許多屬性非常依賴合金成分、組織、熱處理及機(jī)械的歷史。這些屬性包括屈服和抗拉強(qiáng)度、韌性、斷裂韌性、蠕變、疲勞強(qiáng)度，因此特定的信息是必要的。inrelativeterms按相對(duì)價(jià)值計(jì)算7.2.2PlasticsandPolymersAnimalandvegetablematerialsarecomposedofawiderangeofnaturalpolymermolecules,suchasproteins,fatsandcarbohydrates.Theseoccurinthestructuresoftimber,leather,rubber,cotton,woolandsilk,whichareallloadbearing,inservice.Thesenaturalpolymersarewidelyusedinengineeringandtechnicallydemandingapplications,suchasbuildingproducts,sportsequipment,vehicletyresandinternalcartrims.About100yearsago,thefirstman-madeorsyntheticpolymerswereproduced,suchascelluloseproductsandthephenolics.Thesearestillusedinfabricsandelectricalproductsrespectively.Therearenowabout30differentgroupsofpolymericmaterialsincommonusage,manyofwhichfindapplicationinvehicleengineering.Thesematerialsarelessstrongthanmetalsandalloysbyafactorof10,althoughtheycanbereinforcedbyfibrousandparticulatematerials,suchasglass,carbonandaramid(Kevlar)fibres.Theseagainreplicatenaturalmaterials,suchaswood,whichisatwo-phasecomposite.Compositesarestiff,strong,ductileandlight-weight,andalthoughexpensiveinsomecases,areusedextensivelyinvehicleengineeringforsuchapplicationsasbodywork,bumpers,prop-shaftsandfuelinletmanifolds.Thewiderangeofcommercialpolymershasresultedfromagreaterunderstandingofpolymerstructures,fromatomicthroughmoleculartosolidstatelevels,suchthat‘tailor-making’ofpolymersisnowpossible,ataprice.Thus,polymersarecurrentlyavailablewhichwillprocessreadilyandhavetherequiredpropertiesandbehaviorinservice.Polymersarealsolessstiffthanmetals,byafactorof100,sothattheiruserequiresnewdesignprocedures.Polymersandparticularlytheircompositescanalsobeveryanisotropicinbehavior,leadingtodirectionalproperties.Theyarealsomuchmoretemperature,timeandfrequencysensitivethanmetalsandceramics.Againpricevaluesaresubjecttoconsiderablefluctuationsandthepropertyvaluesshouldbeconsideredasarankingduetothewiderangeofavailablegradeswithineachpolymergroup.Polymericmaterialsaremadeupofverylongchainmoleculeswithabackboneofprincipallycarbonatoms,whichareheldtogetherbyprimaryforcesorbonds,comparableinstrengthtothoseinmetals.Siliconepolymershaveasiliconoxidebackbone.However,theselongchainmoleculesareheldtogether,inthermoplastics,bymuchweaker,secondaryforcesproducingamoreopenstructure,whichleadstotheinherentlylowerdensity,strengthandstiffnessvalues,asshowninTables7-1and7-2.Inthermosettingpolymers,suchasphenolics,theseweakersecondaryforcesarechemicallysupplementedwithstrongerprimaryforces,duringcuring,formingathree-dimensionalmolecularnetwork,therebyincreasingtheirstrengthandstiffness.Rubberymaterialsfortyres,hoses,beltingandenginemountingsaresimilarlycross-linkedbyavulcanizationprocesswithsulphur,butsincetheyarealreadyabovetheirsofteningormeltingpointsatroomtemperature,theyremaintypicallyflexible.Thus,manyofthegeneralpropertiesofpolymersarethoseofmaterialsneartheirsofteningpoints.Theycreepunderloadinservice,aproblemwhichrequiresamorecomplex,pseudo-elasticdesignapproachcomparedtothatusedformetalsandceramics.Mostpolymersarenowmadefromoilandnaturalgasandformthebasisofseveralmajorindustries,namelyplastics,rubbers,fibers,coatingsandadhesives,allofwhichsupplythemotorvehicleindustrywitharangeofproducts.Theplasticsgroupiscommonlydividedintothermoplastics,whichsoftenonheatingandre-hardenagainoncooling,andthermosettingpolymersorresins,whicharenotsoftenedonre-heatingaftertheoriginalformingprocess.Thermoplasticsmayhaveacrystallinephasemeltingpoint,Tm,aboveroomtemperature,aswellasanamorphousphasesofteningpoint,Tg.Thelatterisbelowroomtemperatureinductilethermoplastics,suchaspolyethylene,butwellaboveroomtemperatureinbrittlethermoplastics,suchaspolystyrene.Naturalrubberandsyntheticelastomershavemeltingandsofteningpointswellbelowroomtemperatureandsoarenormallyflexible,thepolymerchainsbeingheldtogetherbyaloosearrangementofstrongcross-links.Textilefibresincludingmanynaturalpolymerssuchascotton,woolandsilkarethermoplasticinnature.Syntheticfibressuchasnylon,polyestersandpolypropyleneareextrudedtoformveryfinefilaments,asrequired.Theyareadditionally,mechanicallydrawn,duringcoldorhotprocessing,toorientthepolymermoleculesalongtheaxisofthefibretogiveadditionalstrengthandstiffness,whichresultsinanisotropicproperties.Coatingsandadhesivescanbeconsideredasthinfilmsofeitherthermoplasticorthermosettingpolymers.Withthecorrectformulationtheycanformcorrosionresistant,decorativebarriersaswellasstructuraljoiningmaterials.Commercially,thermoplasticscanbedividedintotwogroups.Thereisthetonnageorcommoditygroup,involvingthepolyolefins,suchaslowandhighdensitypolyethylenes,PE,polypropylene,PP,polyvinylchlorides,PVC,bothplasticizedandunplasticizedandthepolystyrenes,PS,includingthegeneralpurposeandhighimpactgrades.Engineeringthermoplasticsareusedinsmallerquantitiesformoredemandingapplications.Suchmaterialsincludenylons,PES,PTFE,PEEKandpolyacetals,suchasDelrin.Becauseofthelongnamesofpolymericmaterials,aninternationallyrecognizedsystemoflettersymbolsisused,asindicatedaboveincludingPESforpolyethersulphone.Thermoplasticsmoleculesarelinearorbranched.Chemicalengineersproducepolymermoleculesbythechemicalprocessofpolymerization.Polymermoleculesareeasilymeltedtoviscousfluidsandcanbeprocessedbyarangeoftechniquesintocomplexshapes.Processesincludeinjectionmoulding,extrusionandthermoformingandtherearewellestablishedweldingandjoiningtechniques.Themoleculeshavearangeoflengthswithinabroadbandandtheymaysolidifytoamorphoussolids(PVCandPS)orpartiallycrystallinesolids(PE,PPandnylons).Thisrangeofmolecularandsolidstatestructuresmeansthatmeltingpoints,Tm,andothertemperaturetransitions,Tg,arenotsharp,incontrasttothoseformetals.Commonthermosetsarephenolics(Bakelite),epoxyresins(Araldite)andunsaturatedpolyestersusedinGRPcomposites.Thesematerialsalsofinduseincoatingsandadhesives.Thermosetsarenormallymadebymixingtwocomponents,aresinandahardener,whichreactandhardenatroomtemperatureoronheating.Thehardenedorcuredresinmaterialconsistsofpolymermolecules,heavilycross-linkedtoformathree-dimensionalmolecularnetwork.Thiscomplexpolymerizationandcross-linkingprocesspreventscrystallization,leavingthesolidmaterialamorphous,likeinorganicglassesandsoareinherentlybrittle,requiringreinforcementwithwood-flour,paper,glassormica,dependingontheenduse.Thus,incontrasttothermoplastics,re-heatingcausesminimalsofteningandtheextensivecross-linkednetworkstructurepreventsmeltingorviscousflow.Consequently,thesematerialscannotbehot-workedorrecycled.Excessiveheatingwill,ofcourse,leadtodecompositionaswiththermoplastics.Theterm‘rubber’normallyreferstonaturalrubber,whereas‘elastomer’isatermusuallyreservedforsyntheticrubbers,suchaschloroprene(Neoprene),nitrileandbutadienerubbers,widelyusedintyres,hoses,sealsandbelting,aswellasgeneralmechanicalssuchasenginemountings.Thesematerialsconsistofveryhighmolecularlengthpolymermoleculeswithoccasional,chemicalandphysicalcross-links,givingaverylooseandopennetwork.Atroomtemperature,thematerialsarewellabovetheirsofteningpoints(Tg)andmeltingpoints(Tm).Thus,theywouldbeviscousliquidsbutforthecross-links.Thelatter,however,leadtoflexiblesolidswithanactive‘memory’,whichreturnsthemtotheiroriginalshape,rapidlyandcompletelyonunloading.Textilefibresaremadefrombothnaturalandsyntheticpolymericrawmaterials.Syntheticfibresconsistofsimple,thermoplasticpolymermolecules,suchasnylons,polyestersandacrylics.Theyarecharacterizedbybeingveryanisotropic,wherethephysicalandmechanicalpropertiesareverydirectional.Thestrengthandstiffnessvaluesalongthefibreareverymuchgreaterthanacrossthefibre.Thisisapotentialproblemsinceaxialshrinkageathightemperaturesisconsiderable,againduetothe‘memory’effect.Thishassignificanceinfabricandclotheswashing.Textileandpackagingfilmsarecommonlybiaxiallyorientedduringmanufacture,againforimprovedpropertiesandforgreatereconomies,sincethinnerfilmscanbeused.Significantshrinkageisagainapotentialproblem.Incontrast,thermoplasticbulkmouldingsmustbemanufacturedtogiveisotropiccomponentstoavoiddimensionaldistortionandpoorimpactproperties.織物纖維是由天然和合成聚合物原料制得的。合成纖維由簡(jiǎn)單的、熱塑性聚合物分子,如尼龍、聚酯和丙烯酸樹脂組成。他們以明顯的各向異性為特征，物理和機(jī)械性上具有定向性。沿纖維防線的強(qiáng)度和剛度值大于垂直纖維方向的。這是因?yàn)樵诟邷叵螺S向收縮相當(dāng)大，又可歸因于“記憶”效應(yīng)，而產(chǎn)生的潛在問(wèn)題。這對(duì)于織物和布的洗滌織物十分有意義。紡織和包裝薄膜通常采用雙軸取向的制造過(guò)程，為改善性能和獲得更高的經(jīng)濟(jì)性，可以使用更薄的薄膜。顯著收縮再次成為一個(gè)潛在的問(wèn)題。相比之下，熱塑性體模具必須制造給各向同性組件來(lái)避免尺寸畸變和不良的沖擊性能。Thepropertiesandbehaviourinserviceofpolymericmaterialsaremoredependentontheirmolecularstructuresandmethodsofmanufacture,whichmayintroducesignificantanisotropy,incomparisontometals.Thepropertyvaluesarealsoaffectedbythemethodsoftesting,particularlythetesttemperatureandtherateorfrequencyofmechanicalloading.Thus,whilstitispossibletomakegeneralcomparisonsbetweenonegroupofpolymersandanother,saybetweenpolypropylene,polyvinylchlorideandnylons,informationonaspecificpolymergrademustbeobtainedfordesignpurposes.Temperatureandtime(rateorfrequency)effectscanbeexplainedstructurallyasfollows.Crystallinematerials,commonlymetals,haveacharacteristicmeltingpoint,Tm.Theymayalsohavephasechangesinthesolidstate,suchasthebodycentredcubictofacecentredcubiccrystalstructurechange,at910℃,inpureiron.Amorphousmaterialssuchasglassesandmanypolymers,suchasacrylics,alsohavesolidstatetransitions.Inpolymers,themainamorphoustransitionisknownastheglass/rubberorbrittle/ductiletransitionattheTgtemperature.Thus,semi-crystallinematerialssuchasceramicsandsomepolymers,typicallypolypropylene,nylonsandtoaminorextentPVC,willhavebothcrystallineandamorphousphasetemperaturetransitions.Withmetals,glassesandceramics,theTmandTgvaluesarewellabovetheroomorservicetemperatures,althoughitshouldbenotedthatthemeltingpointoftinislowat232℃,leadat327℃andsoldersataround180℃.Thus,thepropertiesofmostloadbearingmetalsandceramics,inservice,arelargelyunaffectedbytemperatureandtemperaturechange.Incontrast,themeltingpointsofsemi-crystallinepolymersarelower,withPTFEat327℃,nylon66at260℃,PPandPVCat175℃,PEat143℃andnaturalrubberat–39℃.TheirTgvaluesareevenlower,alittleaboveroomtemperatureforthermoplasticsandwellbelowroomtemperatureforrubberymaterials.Hence,thermoplasticpolymersaregenerallyductileandtheirpropertiesareverymuchaffectedbysmalltemperaturechanges.Consequently,the‘elastic’modulusvaluesforthermoplasticpolymersatroomtemperaturesareconsiderablylowerthanformetalsandceramics,atbetween1and3GPa.Additionally,underconstantloadatroomtemperature,significantcreepoccursduetomolecularuncoilingslipbetweenpolymermolecules.Asaresult,themodulusvaluefallswithtime,makingittimedependent.Literaturevaluesofmodulusarenormallyquotedatshortloadingtimesof100seconds.Forlongerloadingtimes,say1000hours,themodulusvaluecoulddropto1/3oftheshort-termvalue.AttemperaturesincreasingfromtheTgtotheTm,intherubberystate,themodulusdropsdramatically,fromabout1to3GPadownto1to3MPa.Thesetemperatureandtimeeffectshaveconsiderableinfluenceonthedesignproceduresusedforpolymericmaterials.Thermoplasticsmaterialsaresaidtobe‘visco-elastic’anda‘pseudo-elastic’approachisusedtodesignsignificantlyloadedcomponentssuchaspressurepipe-workforgasandwaterdistribution.Thedesignprinciplesandequationsarethesameasformetals,butthetemperatureandtimeeffectsmustbepartofthepropertygatheringprocedures,sothattheappropriatematerialsdataareusedinthedesigncalculations.Otherphysicalpropertiesofpolymersarealsoinfluencedbytheirstructure,aswellastemperatureandtime.Incontrasttometals,mechanicalstrainsofpolymersarehigherbutrecoverable,specificheatsarelarger,whichinfluencesprocessingandtheircoefficientsofthermalexpansionarehigher,whichmayinfluencetheinteractionwithothermaterials.Theirthermalandelectricalconductivity,however,aremuchsmallerthanformetalssotheymaybeusedasinsulatingmaterials.Thestructure/processing/propertyrelationshipsaremorecomplexthanthoseformetalsandceramics.However,whentheserelationshipsareunderstood,thepotentialfornewdesigns,newprocessesandnewproductsisconsiderable,wellbeyondbeingdirectsubstitutematerialsforexistingmetallicorceramiccomponents.7.2.3CeramicsandGlassesThesematerialsarebrittle,sodotheyhaveaplaceinengineeringletaloneautomotiveengineering?ThefactsarethatthePyramidsandtheGreatWallofChinastillstand,asdotheNormancathedrals,makinguseofgranite,sandstoneandthelessdurablelimestone.Someearlyclaypotterystillsurvivesasdoearlyweaponsandcuttingtoolsmadeofflint.Glassandceramicsarenottoughandductilelikemetalsandsomepolymers.Theirinter-atomicbondstructuresdonotnormallypermittheoperationofenoughslipsystemstogiveageneralchangeofshapeorplasticdeformation.Thisleavesonlybonddeformationunderexternalloadingandinter-atomicbondsbreakatverylowstrains.Eventakingthisbondingfactorintoaccount,thestrengthsandstiffnessofceramicsarestillmuchlowerthanmightbetheoreticallyexpected,althoughthisisnowfullyunderstood.Incontrasttometalsandpolymers,however,ceramicsdohaveintheirfavorgoodwearandchemicalresistanceincorrosiveenvironments.Theyhavehightemperatureresistanceandaregoodelectricalinsulators.Vehiclecomponentdesignhasneedofsuchmaterialproperties,sothatthebrittlebehaviorisbeingresolved,firstlybyunderstandingthenatureofthisengineeringproblem.Inadditiontothefundamentalbondingproblem,brittlefailureisexacerbatedbythepresenceofdefects,atallstructurallevels,includingthosecausedbymanufacture.Controloftheatomiccomposition,thethermalhistoryandthemanufacturingmethodscanminimizethesize,thesizedistribution,thenumberandtheshapeofthedefects,soreducingthepotentialforbrittlefailureatlowstresslevels.Suchcontrolisnowimplementedintheproductionoftechnicalceramics.However,thisstillleavesthefundamentalcauseofbrittleness,limitedbonddeformation,tobeovercomeoravoided.Thelattertacticisusedatpresentbyusingceramicsinbothmetalandpolymercomposites.Thus,someoftheadvantageousphysicalandmechanicalpropertiesofceramicsarenowbeingutilized.Manufacturingprocessesaremakinguseofceramiccuttingtoolsmadefromsiliconcarbide,siliconnitride,sialon(Si/Al/O/N),zirconiaanddensealumina(aluminiumoxidewithminimalporosity),