机械专业英语完整7PPT课件

.,1,单词复习：concurrentengineeringdesigndeemIntegralreciprocatingmotion/circularmotioninternalcombustionengineAccreditationBoardforEngineeringandTechnologymoderndesigntheoryandmethodology,上一课回顾,.,2,Lesson7EngineeringDesignandsafetyfactors工程设计和安全系数,重点词汇：alludesafteyfactoryieldstrength/ultimatetensilestrengthrequisite/requirement/requireevaluation/evaluatecrystalbizarre,.,3,Paragraph1,Anyconsiderationofengineeringmaterialsandengineeringdesignofsystemsutilizingmaterialsmustinitiallydealwith(涉及到)MurphysLawforMaterialssystemsandthelawsofmaterialsapplications:工程材料和应用材料的系统工程设计最初都会涉及材料的墨菲法则和材料应用法则。,.,4,MurphysLaw:Ifanymaterialcanfail,itwill.Lawsofmaterialsapplications(1)Allmaterialsareunstable.(2)Thematerialssystemisonlyasstrongorasstableasitsweakestormostunstablecomponent.墨菲法则：如果任何材料有可能出问题，它（系统）终将出问题。材料应用法则：（1）任何材料都是不牢固的；（2）材料系统的强度和牢固程度仅仅像它最薄弱的、最不牢固的部分。,.,5,Paragraph2:,Althoughtheyarebothobviousandincontrovertible(无可争辫的),wemightelaborateupontheselaws,especiallythelawsofmaterialsapplications.Allmaterialsareindeedunstable.Evensuchmaterialsasplatinumcanbedegradedinparticularenvironments.虽然这些法则是显见的、不容置疑的，我们还要详细描述这些法则，特别是材料应用法则。所有材料确实是不牢固的。即使像白金（platinum）这样的材料在特殊环境中也会性能减弱。,.,6,Understress(应力，strain应变),allmaterialsrespondtothatstress.Creep(蠕变)isanexample.Givenenoughtime,failure(失效)canbecalculatedtooccurinallmaterialsundercreepstress.在压力作用下，所有材料都会有变化。蠕变(creep)是一个例子。在蠕变压力作用下，若有足够的时间，所有材料一定会产生屈服。,.,7,Creepdeformation蠕变变形,Creepfracture蠕变断裂,.,8,Stressandenvironmentcanactinconcerttocausefailure,forexample,stresscorrosioncracking.Creep-temperatureeffectsandstress-temperatureeffectscancontributetothedegradationofpropertiesandcomponentsandsystemfailure.压力和环境共同（inconcert）作用会造成屈服，例如，应力腐蚀裂纹。蠕变温度影响和应力温度影响都会导致性能降低、零件和系统失败。,.,9,ThedesignprocessParagraph3:,Thedesignprocessusuallybeginswithaspecificationofasolution.Wesometimesalludetoadesigncycle,buttheprocessmaycontainadesigncycleplusdesignimplementation,whichinvolvesactualproductionbaseduponthedesign.设计过程通常从解决方案的详细描述开始。有时我们把设计过程说成是设计循环，但它或许是一个设计循环外加依赖于设计而进行实际生产的设计实施过程。,.,10,Thedesigncyclecaninvolvetheoriginalthoughts,sketches,andknowledgethatinthespecificationstageproduceengineeringdrawings.设计循环涉及最初的构想、草图、在详细描述阶段绘制工作图的知识。Computer-aideddesign(CAD,CAM计算机辅助制造)isnowemployedtoimplementacycleinwhichvariousdesignsordesignideasmaybetestedorsimulated.计算机辅助设计现在已用于设计过程，各种设计和设计思想能被检验和仿真。,.,11,Paragraph4:,Evenintheearlydesigncycle,itisusefultohavesomeconceptofavailablematerials.即使在早期的设计循环中，具有一些材料的概念也是有用的。Designsthatcannotbeachievedbecausematerialswithrequisitepropertiesdonotexistareonlyconceptsthatwillneverbearealityunlessamaterialormaterialsaredeveloped.除非开发出新的材料，否则由于材料不具有必要的性能而不能实现的设计只能是一个永远不能成为现实的概念。,.,12,Priortoafabricationstep,materialsspecificationsmustbemadedependinguponthedesignrequirements.在制造之前必须依据设计要求给出材料的技术要求（specification）。,.,13,Thesemayhavetobealteredinthedesignsimulationiftherequisitepropertiescannotbeidentifiedinexistingmaterials,orfabricationmayhavetodelayuntilnewmaterialscanbedesignedandfabricated.Materialsaretherealityinadesign.如果现存的材料不具备必要的性能，或者只有新材料开发出来才能制造，在设计仿真阶段就必须改动这些材料的技术要求。在设计中材料是现实的。,.,14,Paragraph5,Figure1illustratesasimpleschematicrepresentationofadesignprocessthatincludesdesignimplementation.图1所示为一个包括设计实施的设计过程简图（schematicrepresentation）。Theroleofmaterialsintheimplementationisofcoursecrucialbecausetheymakethedesignareality.由于材料使设计成为现实，因此其地位当然是至关重要的。,.,15,ThedesignordesignimplementationdiagramshowninFigure1maynotcharacterizethefinalproduct.图1示意图或许不能描述最终产品。Thefirstprocessmayproduceaprototype(RapidPrototyping)thatwillbetestedandimprovedupontodevelopafinalproduct.设计之初或许要构想一个用于检验和进一步完善的原型，以开发最终产品。,.,16,Generally,afinalproductworksreliablyandiseconomical(orprofitable)andsafe.一般来讲，最终产品要求工作可靠、经济（有利可图）和安全。,.,17,SafetyfactorsinengineeringdesignsParagraph6,Theconceptofsafetyfactor(安全系数)isnormallyappliedtotheevaluationandconsiderationofstrengthofmaterialsusedinadesign.安全系数常用于估算和考虑用于设计中的材料强度。Safetyfactorsarenormallyappliedtotheyieldstrengthofamaterialbutcanbeappliedtotheultimatetensilestrength.安全系数一般用于材料的屈服强度（yieldstrength），也能用于极限抗拉强度。,.,18,Theyieldstress(屈服应力)isthestress,abovewhichthematerial(particularlyametaloralloy)willdeformpermanently.屈服应力是超过它以后材料（尤其是金属、合金）会产生永久性变形的应力。Havingexceededtheyieldstressoryieldstrength(屈服强度),materialsbegintocreep.超过屈服应力或屈服强度，材料开始蠕变。,.,19,Thatis,dislocations(晶格错移)willbecreated,andapplicationofaloadtocontinuethestressaboveyieldwillallowthematerialtoslip.Overtime,itmayslip(滑动)enoughtosimplycomeapart.也就是产生位移(dislocation)，即连续施加一超过屈服强度的载荷材料将产生滑移。再过一段时间或许会产生裂纹（slip）。,.,20,Soifamaterialisusedinaserviceenvironmentinwhichtheyieldstressisneverexceed,itisunlikelyitwillfailbybeingoverstressed.所以始终在不超过极限应力的状态下工作，材料不会屈服。Wesometimesbaseadesignonfractionsoftheyieldstress.人们有时以屈服强度的一部分（几分之一）为基础进行设计。,.,21,Forexample,ifweassumeamaterialwillneverexceedhalfthevalueoftheyieldstressandusethisintheactualdesign,wecanrealizeasafetyfactorof2.例如，假设材料永远不会超过屈服应力值的一半，并在实际设计中应用，这样安全系数为2。Inanextremecase,wemightallowthemaximumstressinthesystemtobey/4,andthesafetyfactorwouldbe4.在极限状态下，人们或许允许最大应力为y/4，这时安全系数为4。,.,22,Obviouslyadesignthatwouldputamaterialintoaserviceenvironmentinwhichthenormalstresseswouldbeneartheultimatetensilestressforsustainedperiodsoftimewouldbeunsafe.显然，一个设计让材料连续工作在极限抗拉应力附近是不安全的。,.,23,Ontheotherhand,largesafetyfactorscouldrenderadesignunworkablebecausenomaterialscouldbeidentifiedwiththerequisitestrengths.另一方面，由于没有材料具有规定的强度，很大的安全系数将使设计不能实施。Inaddition,evenfulfillingthestrengthrequirementsdoesnotnecessarilyjustifyamaterialsuse.另外即使该强度可以实现，材料选用也是不合理的。,.,24,Thematerialmaybetooexpensive,ortoocorrosive,orunstableinotherwaysfortheintendedapplication.为了这种想要的应用，从其它方面讲，材料或许太贵、易腐蚀、不稳定。Orperhaps,inadditiontohighstrength,highelectricalconductivity(导电率)wouldberequired.或许除高强度外，需要高的导电率。,.,25,Finally,letslookattheconceptofamaterialssystem.Asysteminthemostgeneralsenseinvolvesanarrangementofandconnectionofpartsorelementsintoawhole.最后，让我们看一下材料系统的概念。一个系统大多数情况下包括构成整体的要素和要素间的联系。,ThematerialssystemParagraph7,.,26,Amachineisthereforeamechanicalsystemhavingconnectedcomponents.机器是相互联系的零件构成的机械系统。Amaterialssystemisthereforeanarrangementofconnectedmaterials.材料也是材料之间相互联系构成的系统。,.,27,Thesystemcanbepartitionedintodifferentmaterialswheretheyconnect,andinthemostidealsystemmaterialsareseparatedbyaninterface(界面)orboundarythatconnectsonecomponenttoanotherorprovidesatransitionfromoneregimetoanother.系统可以分成相互联系的不同材料部分，在大多数理想系统中，材料由连接不同部分或在不同区域（regim）过渡的接触面或边界面分开。,.,28,Fracturecanoccuratcrystalinterfaces,andthereforeevenapolycrystalline(多晶的)metaloralloymightbethoughtofasasystemofconnectedcrystals(晶体).断裂会在晶粒接触面间产生，所以即使一个多晶的金属或合金材料也认为是一个相互联系的晶粒系统。,.,29,Paragraph8,Inthecontextoffailureaswehavediscussed,itisimportanttorealizethatfailurecanoccurnotonlywithinaregime(acomponentinaconnectedarrangement),butattheconnectionitself.我们讨论屈服时，这种认识是非常重要的：屈服不但产生在相互联接的区域，而且发生在联系本身。,.,30,SotheinterfaceisalsoapartofthesystemandmustbeconsideredinthecontextoftheweakestlinkcorollaryandMurphysLaw.接触面也是系统的一部分，所以在最薄弱环节界面分离是必然结果，符合墨菲法则。,.,31,Paragraph9,Failurepreventioniscertainlynotahopeless