机械设计制造及其自动化专业英语

Unit1MetalsTheuseofmetalshasalwaysbeenakeyfactorinthedevelopmentofthesocialsystemsofman.Oftheroughly100basicelementsofwhichallmatteriscomposed,abouthalfareclassifiedasmetals.Thedistinctionbetweenametalandanonmetalisnotalwaysclear-cut.Themostbasicdefinitioncentersaround[以…为中心]thetypeofbondingexistingbetweentheatomsoftheelement,andaroundthecharacteristicsofcertainoftheelectrons[某种电子]associatedwiththeseatoms.Inamorepracticalway,however,ametalcanbedefinedasanelementwhichhasaparticularpackageofproperties.在人类社会的开展中，金属的应用起着关键性的作用。构成物质的大约100种根本元素中，大约有一半为金属。金属和非金属之间的区别不是特别明显。最根本的定义集中在元素原子间存在的连接形式和与这些原子相关联的电子的某些特性。然而，在实际应用中，可以将具有某些特性集合金属定义为某种元素。Metalsarecrystalline[adj.晶体的]wheninthesolidstateand,withfewexceptions[极少例外](e.g.mercury),aresolidatambient[周围环境的]temperatures.Theyaregoodconductorsofheatandelectricityandareopaque[不透明的]tolight.Theyusuallyhaveacomparativelyhighdensity.Manymetalsareductile[柔软的；易延展的]-thatis,theirshapecanbechangedpermanently[永久地，长期不变地]bytheapplicationofaforcewithoutbreaking.Theforcesrequiredtocausethisdeformationandthoserequiredtobreakorfractureametalarecomparativelyhigh,although,thefractureforcesisnotnearly[=farfrom,muchlessthan]ashighaswouldbeexpectedfromsimpleconsiderationoftheforcesrequiredtotearaparttheatomsofthemetal.除了少数例外金属在常温下是固态的。它们是热和电的良导体，不透光。它们往往具有较高的密度。许多金属具有延展性，也就是说，在不被破坏的情况下它们的形状在外力的作用下可以发生变化。引起永久变形所需的力和最终使金属断裂所需的力相当大，尽管发生断裂所需的力远没有像所预期的撕开金属原子所需的力那么大。Oneofthemoresignificantofthesecharacteristicsfromourpointofviewisthatofcrystallinity[crystallinity[,krist?'lin?ti]].Acrystallinesolidisoneinwhichtheconstituentatomsarelocatedinaregularthree-dimensionalarrayasiftheywerelocatedatthecornersofthesquaresofathree-dimensionalchessboard[n.棋盘].Thespacingoftheatomsinthearrayisofthesameorderasthesizeoftheatoms,theactualspacingbeingacharacteristicoftheparticularmetal.Thedirectionsoftheaxesofthearraydefinetheorientationofthecrystalinspace.Themetalscommonlyusedinengineeringpracticearecomposedofalargenumberofsuchcrystals,calledgrains[晶粒].Inthemostgeneralcase[一般情况下],thecrystalsofthevariousgrainsarerandomlyorientedinspace.Thegrainsareeverywhereinintimatecontactwithoneanotherandjoinedtogetheronanatomicscale.Theregionatwhichtheyjoinisknownasagrainboundary.从我们的观点来看，在所有的特性中结晶性是最重要的。结晶体是这样一种结构，组成它的原子定位在规那么的三维排列中，仿佛位于三维棋盘的方格的角上。原子间距随着原子大小呈规律性变化，原子间距是金属的一种特性。三维排列的轴线决定了晶体在空间中的方向。在工程实践中应用的金属由大量的晶体组成，这些晶体称之为晶粒。在大多数情况下，晶粒在空间中是自由排列的。在原子范围内，晶粒之间相互接触紧密结合。晶粒之间连接区域被称为晶界。Anabsolutelypuremetal(i.e.[也就是]onecomposedofonlyonetypeofatom)hasneverbeenproduced.Engineerswouldnotbeparticularlyinterestedinsuchametalevenifitweretobeproduced,becauseitwouldbesoftandweak.Themetalsusedcommerciallyinevitably[不可防止地]containsmallamountsofoneormoreforeignelements,eithermetallic[metallic[mi't?lik,me-]adj.金属的，含金属的]ornonmetallic.Theseforeignelementsmaybedetrimental[有害的],theymaybebeneficial,ortheymayhavenoinfluenceatallonaparticularproperty.Ifdisadvantageous,theforeignelementstendtobeknownasimpurities[杂质].Ifadvantageous,theytendtobeknownasalloyingelements[合金元素].Alloyingelementsarecommonlyaddeddeliberately[成心地，蓄意地]insubstantial[adj.大量的；实质的；内容充实的substantialevidence:实〔质〕体证据|实质性证据|实质证据substantialorder:大宗订单|大订单]amountsinengineeringmaterials.Theresultisknownasanalloy.绝对纯洁的金属从来也没有被生产出来过。即使绝对纯洁的金属可以生产出来，工程师们对它们也并不会特别感兴趣，因为它们很柔软、脆弱。实际应用中的金属往往都包含着一定数量的一种或多种外来金属或非金属元素，这些外来元素可能是有害的也可能是有益的或者它们对某种特定的属性没有影响。如果是有害的，这些外来元素被认为是杂质。如果是有益的，它们被认为是合金元素。在工程材料中往往被特意地参加一定数量的合金元素。得到的物质被叫做合金。Thedistinctionbetweenthedescriptors“metal”and“alloy”isnotclear-cut.Theterm“metal”maybeusedtoencompassbothacommerciallypuremetalanditsalloys.Perhapsitcanbesaidthatthemoredeliberatelyanalloyingadditionhasbeenmadeandthelargertheamountoftheaddition,themorelikelyitisthattheproductwillspecificallybecalledanalloy.Inanyevent[无论如何，不管怎样],thechemicalcompositionofametaloranalloymustbeknownandcontrolledwithincertainlimitsifconsistent[一致的]performanceistobeachievedinservice.Thuschemicalcompositionhastobetakenintoaccountwhendevelopinganunderstandingofthefactorswhichdeterminethepropertiesofmetalsandtheiralloys.金属和合金区别不大。金属这个词可以包括工业用纯金属和它的合金。也许可以这样说，合金元素越成心的被添加，被添加的合金元素的量越大，那么生产出来的产品越倾向于被称之为合金。不管怎样，如果想使一种金属或合金在使用中表现出稳定一致的特性，在其中添加何种化学成分，它的量多大都应该在控制范围之内。因此，当想了解决定金属和合金性质的因素时，应充分考虑它们的化学组成。Ofthe50orsometallicelements,onlyafewareproducedandusedinlargequantitiesinengineeringpractice.Themostimportantbyfarisiron[铁],onwhicharebasedtheubiquitous[普遍的，无处不在的]steelsandcastirons(basicallyalloysofironandcarbon).Theyaccountforabout98%byweightofallmetalsproduced.Nextinimportanceforstructuraluses(thatis,forstructuresthatareexpectedtocarryloads)arealuminum[铝],copper,nickel,andtitanium[[tai'teini?m,ti-]n.[化]钛〔金属元素〕].Aluminumaccountsforabout0.8%byweightofallmetalsproduced,andcopperabout0.7%,leavingonly0.5%forallothermetals.Asmightbeexpected,theremaindersareallusedinrather[相当]specialapplications.Forexample,nickelalloysareusedprincipally[主要地]incorrosion-andheat-resistantapplications,whiletitaniumisusedextensively[广泛地]intheaerospaceindustrybecauseitsalloyshavegoodcombinationsofhighstrengthandlowdensity.Bothnickelandtitaniumareusedinhigh-cost,high-qualityapplications,and,indeed,itistheirhighcostthattendstorestricttheirapplication.在50种左右的金属元素里，工程实践中只有少数金属被大量生产和使用。到目前为止最重要的是铁，以它为根底构成了处处可见的钢和铸铁。〔主要由铁和碳构成的合金〕它们的重量占所有生产出来的金属重量的98%。在结构应用〔也就是说，可以承受载荷的结构〕中居于其次位置的是铝、铜、镍和钛。在所有的金属产量中，铝占0.8％，铜占0.7％，剩下的占0.5％。剩下的金属用于相对特殊的用途。例如，镍合金主要用于抗磨损和耐高温的用途，由于钛合金具有高强度和低密度的综合特性，钛被广泛应用于航空工业中。镍合钛有高本钱和高质量的使用特性，事实上，它们高的本钱限制了它们的应用。Wecannotdiscussthesemoreesoteric[adj.秘传的；限于圈内人的；难懂的EsotericBuddhism:密宗|密宗，特别是指西藏的喇嘛教。]propertieshere.Sufficeittosay[Sufficeittosaythatyouloveme.只要说你爱我就够了。Sufficeittosaythatthegunwashis.只需说这枪是他的就够了。]thatawholecomplexofpropertiesinadditiontostructuralstrengthisrequiredofanalloybeforeitwillbeacceptedinto,andsurvivein,engineeringpractice.Itmay,forexample,havetobestrongandyethavereasonablecorrosionresistance;itmayhavetobeabletobefabricatedbyaparticularprocesssuchasdeepdrawing,machining,orwelding;itmayhavetobereadily[容易地]recyclable;anditscostandavailabilitymaybeofcriticalimportance.我们不能在这里讨论这些深奥的特性。在合金材料被采用和应用于工程实际之前，掌握其结构强度和它的综合性质就够了。举例来说，它可以强度很高，并且有好的耐磨性；它可以被例如拉伸加工，机械加工，或焊接等特殊工艺来加工出来；它可以被循环利用；它的本钱和实用性是首要的。Unit2SelectionofConstructionMaterialsThereisnotagreatdifferencebetween“this”steeland“that”steel;allareverysimilarinmechanicalproperties.Selectionmustbemadeonfactorssuchashardenability[[,hɑ:d?n?'bil?ti]n.可硬化性；淬硬性],price,andavailability[[?,veil?'bil?ti]是产品在被调用时能够运行〔即未处于失效或修复状态〕的概率。此量度考虑了产品的可靠性〔多久会失效〕和可用性〔多久能被修复〕。],andnotwiththeideathat“this”steelcandosomethingnoothercandobecauseitcontains2percentinsteadof1percentofacertainalloyingelement,orbecauseithasamysterious[[mi'sti?ri?s]adj.神秘的；不可思议的；难解的](神秘的，不可思议的)name.Atremendous[adj.极大的，巨大的；惊人的]rangeofpropertiesisavailableinanysteelafterheattreatment;thisisparticularlytrueofalloysteels. 在钢之间没有太大的区别；所有的钢在机械性能方面都是近似的。它们的选取标准是诸如脆硬性，价格，和可用性等。不仅仅是因为这种钢含有2％的合金元素另一种钢含有1％而使前者具有了后者没有的某些能力，或者是某种钢具有神奇的名字。经过热处理后，任何一种钢都具有大范围的特性；这种性质同样在合金钢中存在。 Considerationsinfabrication(制造)Thepropertiesofthefinalpart(hardness,strength,andmachinability[[m?,?i:n?'bil?ti]n.切削性；机械加工性]),ratherthanpropertiesrequiredbyforging,governtheselectionofmaterial.Thepropertiesrequiredforforginghaveverylittlerelationtothefinalpropertiesofthematerial;therefore,notmuchcanbedone[无论做什么都是白费力，都改变不了既定的现实]toimproveitsforgeability.Higher-carbonsteelisdifficulttoforge.Largegrainsizeisbestifsubsequentheattreatmentwillrefinethegrainsize.关于加工的考虑 最后零件的特性〔硬度、强度和可加工性〕而不是锻造特性决定了材料的选择。可锻性与材料的最后特性联系不大；因此，提高金属的可锻造性价值不大。高碳钢很难锻造。如果在随后的热处理过程进行细化，大尺寸晶粒是最好的。Low-carbon,nickel-chromium(铬)steelsarejustabout[几乎，差不多j]asplasticathightemperatureunderasingle520-ft·lb(1ft·lb=1.35582J)blow[atasingleblow=atoneblowatasingleblow:一下子|一举|一击]asplainsteelsofsimilarcarboncontent.Nickeldecreasesforgeabilityofmedium-carbonsteels,buthaslittleeffectonlow-carbonsteels.Chromiumseemstohardensteelatforgingtemperatures,butvanadium[](钒)hasnodiscernible(可区分的)effect;neitherhasthemethodofmanufactureanyeffectonhigh-carbonsteel. 在高温下低碳，镍铬合金钢在受到520-ft·lb的冲击下表现出与相同碳含量普通钢几乎同样的塑性。镍减少了中碳钢的可锻性，但对低碳钢影响不大。铬在锻造温度下时使钢硬化，但钒没有明显的效果；两种加工方法对高碳钢没有影响。FormabilityThecold-formabilityofsteelisafunction(功能)ofitstensilestrengthcombinedwithductility.Thetensilestrengthandyieldpointmustnotbehighortoomuchworkwillberequiredinbending〔弯曲〕;likewise〔同样地〕,thesteelmusthavesufficient〔充足的〕ductilitytoflowtotherequiredshapewithoutcracking.Theforcerequireddependsontheyieldpoint,becausedeformationstartsintheplasticrangeabovetheyieldpointofsteel.Work-hardeningalsooccurshere,progressively〔日益增多地〕stiffening〔使变硬〕themetalandcausingdifficulty,particularly〔独特的，显著的〕inthelow-carbonsteels. 成形 钢的冷成形是它的拉伸强度和延展性相结合的结果。拉伸强度和屈服点不能太高否那么在发生弯曲时需要做很多工作；与之相类似，钢应该有高延展性，使其在没有断裂的情况下成形。加工力的大小取决于屈服点，因为钢在屈服点之上才开始变形。与此同时，加工硬化也同时发生，金属变得越来越硬，增加加工难度，尤其在低碳钢中容易发生。Itisquiteinterestinginthisconnection〔关于这一点，就此而论〕todiscoverthatdeepdrawscansometimesbemadeinonerapidoperationthatcouldnotpossiblybedoneleisurely〔缓慢地，沉着不迫地〕intwoorthree.Ifadrawishalfmadeandthenstopped,itmaybenecessarytoanneal〔退火〕beforeproceeding,thatis〔换句话说〕,ifthepieceisgiventimetowork-harden.Thismaynotbeascientificstatement,butitisactuallywhatseemstohappen.在这方面，相当有趣的是你将发现有时可通过一次快速加载完成大拉伸，但以缓慢的方式两三次加载却不能实现。如果拉伸进行了一半就停止了，那么在再加工之前应先退火，也就是说工件是否有时间进行加工硬化。这不是一种科学的表达方法，但确实是发生了。 InternalstressesColdformingisdoneabovetheyieldpointinthework-hardeningrange,sointernalstressescanbebuiltupeasily.Evidenceofthisisthespringback〔回弹〕astheworkleavestheformingoperationandthewarpage〔翘曲，扭曲〕inany〔任何一种〕subsequentheattreatment.Evenasimplewashermight,byvirtueof[由于，凭借](依靠)theinternalstressesresultingfrompunching〔冲压〕andthenflattening〔整平〕,warp〔弯曲〕severely〔严格地，剧烈地〕duringheattreating.〔virtuen.德行,美德,贞操,优点,成效,效力,英勇believedinthevirtueofprayer.相信祈祷的力量 内应力在高于屈服点的加工硬化区进行冷加工很容易产生内应力。例如工件停止成型加工后会发生回弹，在随后的热处理后，工件会发生翘曲。即使是一个简单的垫圈，由于打孔和随后的平整加工中产生内应力，也会在热处理中呈现严重的翘曲。Whendoubtexistsasto〔关于〕whetherinternalstresseswillcausewarpage,apiececanbecheckedbyheatingittoabout1100andthenlettingitcool.Ifthereareinternalstresses,thepieceislikelyto〔可能〕deform.Piecesthatwillwarpseverelywhilebeingheatedhavebeenseen,yet〔然而〕theheat-treater[热处理炉]wasexpectedto[有望做某事；被期待做某事]putthemthrough[putthrough完成]andbringthemoutbetterthantheywereinthefirstplace.当是否内应力会引起翘曲的疑心存在时，可以通过将工件加工至1100然后进行冷却来验证。如果存在内应力，工件会发生变形。经过热处理的工件像我们看到的那样会发生严重的翘曲，但是我们仍然希望工件被扔到热处理炉中被处理，这样好过它存在内应力的状态。 WeldingThemaximumcarboncontentofplaincarbonsteelsafeforweldingwithoutpreheatingorsubsequentheattreatmentis0.3%.higher-carbonsteelisweldedeveryday,butonlywithproperpreheating.Therearetwoimportantfactors:theamountofheatsthatisputin;therateatwhichitisremoved. 焊接不需要预热或之后进行热处理就能平安焊接的最高碳含量为0.3％。高碳钢通过适宜的预热通常也可焊接。有两点值得注意：吸收热量的多少；移除速度。Weldingataslowerrateputsinmoreheatandheatsalargevolumeofmetal,sothecoolingrateduetolossofheattothebasemetalisdecreased(减少).Apreheatwilldothesamething.Forexample,sae4150steel,preheatedto600or800,canbeweldedreadily〔容易地〕.Whentheflameorarcistakenawayfromtheweld,thecoolingrateisnotsogreat,owingtothehighertemperatureofthesurroundingmetalandslowercoolingresults.Eventhemostrapidair-hardening〔风硬钢〕steelsareweldableifpreheatedandweldedataslowrate. 低速焊接带来了更多的热量，这对金属的大量体积进行了加热，所以冷却速度降低。预热可以取得与之相当的效果。例如当被预热至或时可以很好的焊接。由于周围金属的较高温度，当焊接弧移开焊接点后，冷却速度不会太快，产生了低速冷却的结果。即使是冷作硬化速度最快的金属也可以通过预热和慢速焊接到达良好的焊接效果。 MachinabilityMachinability〔机械加工性能〕meansseveralthings.Toproductionmenitgenerallymeansbeingabletoremovemetalatthefastestrate,leavethebestpossiblefinish,andobtainthelongestpossibletoollife.Machinabilityappliesto〔应用于〕thetool-work〔工具，零件〕combination.可加工性可加工性意味着几件事情。对于加工者来说，它意味着可以快速的移除金属，取得最好的加工效果，得到最长的刀具寿命。可加工性是刀具和零件的结合。Itisnotdeterminedbyhardness〔硬度〕alone,butbythetoughness〔韧性〕,microstructure,chemicalcomposition〔成分〕,andtendency〔倾向〕ofametaltohardenundercoldwork.Inthemisleadingexpression“toohardtomachine”,theword“hard”isusuallymeanttobesynonymous〔同义的〕with“difficult”.Manytimesamaterialisactuallytoosofttomachinereadily.Softnessandtoughnessmaycausethemetaltotear〔撕裂〕andflowaheadofthecuttingtoolratherthancutcleanly.Metalthatareinherently〔天性地，固有地〕softandtougharesometimesalloyedtoimprovetheirmachinabilityatsomesacrifice[['s?krifais]n.牺牲；祭品；供奉Allgainsaremadeatsomesacrifice.Mostpeoplemightchoosetodivertthecourseofthetrain,andsacrificeonlyonechild.]〔牺牲〕inductility.Examplesareuseoflead〔铅〕inbrass〔黄铜〕andofsulfur[['s?lf?]]〔硫磺〕insteel.加工性不仅仅只由硬度决定，它还由韧性，微观结构，化学成分和在冷加工下金属所呈现的硬化特性所决定。在容易混淆的表示“难加工”中，“hard”与“difficult”同义。许多时候，因为材料过软而难于稳定加工。材料柔软性和韧性能够产生金属撕裂，使金属在完成切削前流动至刀具前端。柔软的金属往往会被参加合金从而牺牲它的延展性来提高加工性能。如黄铜中参加铅钢中参加硫磺。Machinabilityisatermusedtoindicatetherelative〔比拟的〕ease〔不费力〕withwhichamaterialcanbemachinedbysharpcuttingtoolsinoperationssuchasturning〔车〕,drilling〔钻〕,milling〔铣〕,broaching[['b?ut?i?]n.拉削；拉孔；推削；扩孔v.钻孔；开口]〔拉削〕,andreaming〔铰〕.机械加工性能是在指对工件材料使用刀具进行诸如车、钻、铣、拉削、铰加工时的难易程度。Inthemachiningofmetal,themetalbeingcut,thecuttingtool,thecoolant,theprocessandtypeofmachinetool〔机床〕,andthecuttingconditionsallinfluencetheresults.Bychanginganyoneofthesefactors,differentresultswillbeobtained.Thecriterion[[krai'ti?ri?n]n.〔批评判断的〕标准；准那么；标准；准据]〔标准〕uponwhichtheratings〔等级〕listedarebased(等级评定的标准)istherelative[adj.相对的；有关系的；成比例的]volumeofvarious〔不同种〕materialsthatmayberemovedbyturningunderfixedconditionstoproduceanarbitrary〔任意的〕fixedamountoftoolwear.在对金属进行加工时，被切削的金属，切削刀具，冷却液，使用的机床的种类，切削条件均影响着切削效果。改变任何一种均会产生不同的切削效果。切削效果评定的准那么是：车削时在固定的切削条件下产生一定量的刀具磨损时，被加工试件相应的材料去除量。Unit3MechanicalPropertiesofMaterialsThematerialpropertiescanbeclassifiedintothreemajorheadings:(i)Physical,(ii)Chemical,(iii)Mechanical.Physicalproperties Densityorspecificgravity,moisture[['m?ist??]n.水分；湿度；潮湿；降雨量]content,etc.,canbeclassifiedunderthiscategory.材料特性主要分为三类：(i)物理特性，(ii)化学特性，(iii)力学性能。物理特性 密度或特定的重力，湿度等都属于此范畴。Chemicalproperties Manychemicalpropertiescomeunder[归入]thiscategory.Theseincludeacidity[[?'sid?ti]n.酸度；酸性；酸过多；胃酸过多]oralkalinity[[,?lk?'lin?ti]n.碱度；碱性],reactivity[化学反响的活性]andcorrosion.Themostimportantoftheseiscorrosionwhichcanbeexplainedinlayman’stermsastheresistanceofthematerialtodecay[腐朽，腐烂]whileincontinuoususeinaparticularatmosphere.化学特性 许多化学特性都归入到这个范畴。其中包括酸性或碱性，活性和耐腐蚀性。而在这其中最重要的是耐腐蚀性，通俗的解释是材料在特定大气中长期使用时，抵抗腐蚀的能力。Mechanicalproperties Mechanicalpropertiesincludethestrengthpropertiesliketensile,compression,shear,torsion,impact,fatigueandcreep[n.爬行；毛骨悚然的感觉；谄媚者vi.爬行；蔓延；慢慢地移动；起鸡皮疙瘩creep:蠕变|潜变|渐变creepstrength:蠕变强度|潜变强度|蠕变弱度creepstrain:蠕变变形|蠕动应变|蠕变应变].Thetensilestrengthofamaterialisobtainedbydividingthemaximumload,whichthespecimenbearsbytheareaofcross-sectionofthespecimen.力学特性 力学特性包括诸如拉伸，压缩，剪切，扭转，冲击，疲劳和蠕变等强度特性。一种材料的拉伸强度由试件承载的最大载荷除以试件的横截面积得到。 ThisisacurveplottedbetweenthestressalongtheY-axis(ordinate)andthestrainalongtheX-axis(abscissa[[?b'sis?]n.[数]横坐标；横线])inatensiletest.Amaterialtendstochangeorchangesitsdimensionswhenitisloaded,dependinguponthemagnitude[大小]oftheload.Whentheloadisremoveditcanbeseenthatthedeformationdisappears.Formanymaterialsthisoccursupto[一直到；相当于；忙于，在做；由决定的]acertainvalueofthestresscalledtheelasticlimit.Thisisdepictedbythestraightlinerelationshipandasmalldeviation[[,di:vi'ei??n]n.偏差；误差；背离averagedeviation:平均偏差|平均差|平均偏差，均差]thereafter,inthestress-straincurve.如下图为在拉伸试验中沿着X轴〔横轴〕的应变和沿着Y轴〔纵轴〕的应力之间的关系曲线。材料在加载时，随着载荷大小的变化，尺寸会发生改变。当卸载时，变形消失。对于许多材料来说，上述情况发生的应力极限值称为弹性极限。在应力-应变曲线中，直线关系和随后的小小的弯曲描述了上述的加载和卸载。Withintheelasticrange,thelimitingvalueofthestressuptowhichthestressandstrainareproportional,iscalledthelimitofproportionality.Inthisregion,themetalobeysHooke’slaw,whichstatesthatthestressisproportionaltostrainintheelasticrangeofloading(thematerialcompletelyregainsitsoriginaldimensionsaftertheloadisremoved). Intheactualplottingofthecurve,theproportionalitylimitisobtainedataslightlylowervalueoftheloadthantheelasticlimit.Thismaybeattributedtothetime-lagintheregainingoftheoriginaldimensionsofthematerial.Thiseffectisveryfrequentlynoticedinsomeno-ferrous[有色金属][铁的，含铁的]metals.在弹性范围内，应力应变成比例的应力极限值称为比例极限。在这个区域中，金属服从胡克定律—阐述了在加载的弹性范围内，应力和应变成比例关系〔材料在卸载后，能够完全回复它原来的尺寸〕。在曲线的实际绘制中，比例极限值要稍微比弹性极限值低。这可能是由于材料回复原尺寸需要的时间延迟。这种现象在一些有色金属中常常出现。 Whileironandnickelexhibit[[iɡ'zibit]n.展览品；证据；展示会vt.展览；显示；提出〔证据等〕vi.展出；开展览会]clearrangesofelasticity,copper,zinc〔锌〕,tin〔锡〕,etc,arefoundtobeimperfectly[[im'p?:fiktli]adv.有缺点地；不完美地；未完成地]elasticevenatrelatively〔相当地〕lowvaluesofstresses.Actuallytheelasticlimitisdistinguishable〔可区分的〕fromtheproportionality〔比例性〕limitmoreclearlydependinguponthesensitivity〔灵敏性〕ofthemeasuringinstrument.铁和镍存在明显的弹性范围，铜，锌，锡等即使在相当低的应力值下弹性也表现得不是很充分。实际上依靠测试仪器的精确性可以使比例极限和弹性极限区分得更清晰。 Whentheloadisincreasedbeyondtheelasticlimit,plasticdeformationstarts.Simultaneouslythespecimengetswork-hardened.Apointisreachedwhen〔这时〕thedeformationstartstooccurmorerapidlythantheincreasingload.ThispointiscalledtheyieldpointQ.themetalwhichwasresistingtheloadtillthen,startstodeformsomewhatrapidly,i.e.,yield.Theyieldstressiscalledyieldlimit.当在弹性极限之上增大载荷时会产生塑性变形。同时，试件发生加工硬化。到达某点后变形的速度快于载荷增加的速度。这一点叫做屈服极限点。一开始一直在抵抗载荷的金属在这一点后开始迅速地发生形变，也就是，屈服。屈服应力叫做屈服极限。 TheelongationofthespecimencontinuesformQtoSandthentoT.thestess-strainrelationinthisplasticflowperiodisindicatedbytheportionQRSTofthecurve.AtTthespecimenbreaks,andthisloadiscalledthebreakingload.ThevalueofthemaximumloadSdividedbytheoriginalcross-sectionalareaofthespecimenisreferredtoastheultimatetensilestrengthofthemetalorsimplythetensilestrength.试件从Q到S在到T不断地延长。在这个塑性流动期间的应力-应变关系表示为曲线上的QRST段。在T点试件断裂，此时的载荷称为断裂载荷。最大荷载值S除以试件的横截面积为金属的最大拉伸强度或简单地称为拉伸强度。Logicallyspeaking,oncetheelasticlimitisexceeded,themetalshouldstartstoyield,andfinallybreak,withoutanyincreaseinthevalueofstress.Butthecurverecordsanincreasedstressevenaftertheelasticlimitisexceeded.Tworeasonscanbegivenforthisbehavior:thestrainhardeningofthematerial;thediminishing[[di'mini?i?]adj.逐渐缩小的；衰减的v.减少；递减；衰减；削弱的权势]cross-sectionalareaofthespecimen,sufferedonaccountof[Onaccountoftheweather,wehadtocanceltheprojectedouting.]theplasticdeformation.逻辑上来说，一旦超过弹性极限，金属应该就会屈服直至最后断裂，在应力值上应该没有增加。但是实际的曲线却记录了在超过弹性极限后的增加了的应力。这种现象的发生可能有两种原因：1材料的应变强化；2由于塑性变形引起的试件横截面积的缩小。 Themoreplasticdeformationthemetalundergoes,theharderitbecomes,duetowork-hardening.Themorethemetalgetselongatedthemoreitsdiameter(andhence,cross-sectionalarea)isdecreased.ThiscontinuesuntilthepointSisreached.由于加工硬化，金属在发生塑性变形时会变得越来越硬。金属拉伸越长它的直径越小。这种现象一直持续到曲线上的S点。AfterS,therateatwhichtheredutioninareatakesplace,exceedstherateatwhichthestressincreases.Strainbecomessohighthattheredutioninareabeginstoproducealocalizedeffectatsomepoint.Thisiscalledneching.超过S点后，面积减少的速度超过了应力增加的速度。应变变得很大，面积的减少在某些点产生了局部效应。叫做颈缩。 Reductionincross-sectionalareatakesplaceveryrapidly;sorapidlythattheloadvalueactullydrops.ThisisindicatedbyST.FailureoccursatthispointT. Thenpercentageelongationandreductioninareaindicatetheductilityorplasticityofthematerial:Whereandaretheoriginalandthefinallengthofthespecimen;andaretheoriginalandthefinalcross-sectionalarea.横截面积减小的速度非常快；以至于实际上载荷降低。 伸长率和面积减少率表示了材料的延展性。Uint5DesignofmachineandmachineelementsMachinedesignMachinedesignistheartofplanningordevisingneworimprovedmachinestoaccomplishspecificpurposes.Ingeneral,amachinewillconsistofacombinationofseveraldifferentmechanicalelementsproperlydesignedandarrangedtoworktogether,asawhole.Duringtheinitialplanningofamachine,fundamentaldecisionsmustbemadeconcerningloading,typeofkinematicelementstobeused,andcorrectutilizationofthepropertiesofengineeringmaterials.Economicconsiderationsareusuallyofprimeimportancewhenthedesignofnewmachineryisundertaken.Ingeneral,thelowestover-allcostsaredesigned.Considerationshouldbegivennotonlytothecostofdesign,manufacturethenecessarysafetyfeaturesandbeofpleasingexternalappearance.Theobjectiveistoproduceamachinewhichisnotonlysufficientlyruggedtofunctionproperlyforareasonablelife,butisatthesametimecheapenoughtobeeconomicallyfeasible.Theengineerinchargeofthedesignofamachineshouldnotonlyhaveadequatetechnicaltraining,butmustbeamanofsoundjudgmentandwideexperience,qualitieswhichareusuallyacquiredonlyafterconsiderabletimehasbeenspentinactualprofessionalwork.DesignofmachineelementsTheprinciplesofdesignare,ofcourse,universal.Thesametheoryorequationsmaybeappliedtoaverysmallpart,asinaninstrument,or,toalargerbutsimilarpartusedinapieceofheavyequipment.Innoease,however,shouldmathematicalcalculationsbelookeduponasabsoluteandfinal.Theyareallsubjecttotheaccuracyofthevariousassumptions,whichmustnecessarilybemadeinengineeringwork.Sometimesonlyaportionofthetotalnumberofpartsinamachinearedesignedonthebasisofanalyticcalculations.Theformandsizeoftheremainingpartsaredesignedonthebasisofanalyticcalculations.Ontheotherhand,ifthemachineisveryexpensive,orifweightisafactor,asinairplanes,designcomputationsmaythenbemadeforalmostalltheparts.Thepurposeofthedesigncalculationsis,ofcourse,toattempttopredictthestressordeformationinthepartinorderthatitmaysagelycarrytheloads,whichwillbeimposedonit,andthatitmaylastfortheexpectedlifeofthemachine.Allcalculationsare,ofcourse,dependentonthephysicalpropertiesoftheconstructionmaterialsasdeterminedbylaboratorytests.Arationalmethodofdesignattemptstotaketheresultsofrelativelysimpleandfundamentaltestssuchastension,compression,torsion,andfatigueandapplythemtoallthecomplicatedandinvolvedsituationsencounteredinpresent-daymachinery.Inaddition,ithasbeenamplyprovedthatsuchdetailsassurfacecondition,fillets,notches,manufacturingtolerances,andheattreatmenthaveamarketeffectonthestrengthandusefullifeofamachinepart.Thedesignanddraftingdepartmentsmustspecifycompletelyallsuchparticulars,mustspecifycompletelyallsuchparticulars,andthusexercisethenecessaryclosecontroloverthefinishedproduct.Asmentionedabove,machinedesignisavastfieldofengineeringtechnology.Assuch,itbeginswiththeconceptionofanideaandfollowsthroughthevariousphasesofdesignanalysis,manufacturing,marketingandconsumerism.Thefollowingisalistofthemajorareasofconsiderationinthegeneralfieldofmachinedesign:①Initialdesignconception;②Strengthanalysis;③Materialsselection;④Appearance;⑤Manufacturing;⑥Safety;⑦Environmenteffects;⑨Reliabilityandlife;Strengthisameasureoftheabilitytoresist,withoutfails,forceswhichcausestressesandstrains.Theforcesmaybe;①Graduallyapplied;②Suddenlyapplied;③Appliedunderimpact;④Appliedwithcontinuousdirectionreversals;⑤Appliedatloworelevatedtemperatures.Ifacriticalpartofamachinefails,thewholemachinemustbeshutdownuntilarepairismade.Thus,whendesigninganewmachine,itisextremelyimportantthatcriticalpartsbemadestrongenoughtopreventfailure.Thedesignershoulddetermineaspreciselyaspossiblethenature,magnitude,directionandpointofapplicationofallforces.Machinedesignismot,however,anexactscienceanditis,therefore,rarelypossibletodetermineexactlyalltheappliedforces.Inaddition,differentsamplesofaspecifiedmaterialwillexhibitsomewhatdifferentabilitiestoresistloads,temperaturesandotherenvironmentconditions.Inspiteofthis,designcalculationsbasedonappropriateassumptionsareinvaluableintheproperdesignofmachine.Moreover,itisabsolutelyessentialthatadesignengineerknowshowandwhypartsfailsothatreliablemachineswhichrequireminimummaintenancecanbedesigned.Sometimes,afailurecanbeserious,suchaswhenatireblowsoutonanautomobiletravelingathighspeeds.Ontheotherhand,afailuremaybenomorethananuisance.Anexampleisthelooseningoftheradiatorhoseintheautomobilecoolingsystem.Theconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant,aconditionwhichisreadilydetectedandcorrected.Thetypeofloadapartabsorbsisjustassignificantasthemagnitude.Generallyspeaking,dynamicloadswithdirectionreversalscausegreaterdifficultiesthanstaticloadsand,therefore,fatiguestrengthmustbeconsidered.Anotherconcerniswhetherthematerialisductileorbrittle.Forexample,brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolved.Ingeneral,thedesignengineermustconsiderallpossiblemodesoffailure,whichincludethefollowing:①Stress;②Deformation;③Wear;④Corrosion;⑤Vibration;⑥Environmentaldamage;⑦Looseningoffasteningdevices.Thepartsizesandshapesselectedmustalsotakeinto