机械零件的强度--机械英汉翻译.doc

原文The Strength of Mechanical ElementsOne of the primary considerations in designing any machine or structure is must be sufficiently greater than the stress to assure both safety and reliabilityTo assure that mechanical parts do not fail in service，it is necessary to learn why they sometimes do failThen we shallbe able to relate the stresses with the strengths to achieve safetyIdeally，in designing any machine element，the engineer should have at his disposal the results of a great many strength tests of the particular material chosenThese tests should have been made on specimens having the same heat and size as the element he proposes to design; and the tests should be made under exactly the same loading conditions as the part will experience in serviceThis means that，if the part is to experience a bending load, it should be tested with a bending loadIf it is to be subjected to combined bending and torsion，it should be tested under combined bending and torsionSuch tests will provide very useful and precise informationThey tell the engineer what factor of safety to use and what the reliability is for a given service lifeWhenever such data ale available for design purposes，the engineer Can be assured that he is doing the best possible job of engineering The cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life，or if the part is manufactured in sufficiently large quantitiesAutomobiles and refrigerators，for example，have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufactureThe cost of making these tests is very low when it is divided by the total number of parts manufacturedYou Can now appreciate the following four design categories：(1)Failure of the part would endanger human life, or the part is made in extremely large quantities；consequently，an elaborate testing program is justified during design(2)The part is made in large enough quantities so that a moderate series of tests is feasible (3)The part is made in such small quantities that testing is not justified at all；or the design must be completed so rapidly that there is not enough time for testing (4)The part has already been designed，manufactured，and tested and found to be unsatisfactoryAnalysis is required to understand why the part is unsatisfactory and what to do improve itIt is with the last three categories that we shall be mostly concernedThis means that the designer will usually have only published values of yield strength，ultimate strength，and percentage elongationWith this meager information the engineer is expected to design against static and dynamic loads，biaxial and triaxial stress states，high and low temperatures，and large and small parts! The data usually available for design have been obtained from the simple tension testwhere the load was applied gradually and the strain given time to developYet these same data must be used in designing parts with complicated dynamic loads applied thousands of times per minuteNo wonder machine parts sometimes failTo sum up，the fundamental problem of the designer is to use the simple tension-test data and relate them to the strength of the part，stress state or the loading situationIt is possible for two metals to have exactly the same strength and hardness，yet one of these metals may have a superior ability to absorb overloads，because of the property called ductilityDuctility is measured by the percentage elongation which occurs in the material at fractureThe usual dividing line between ductility and brittleness is 5 percent elongationA material having less than 5 percent elongation at fracture is said to be brittlewhile one having more is said to be ductileThe elongation of a material is usually measured over gauge lengthSince this is not a measure of the actual strain，another method of determining ductility is sometimes usedAfter the specimen has been fractured，measurements are made of the area of the cross section at the fractureDuctility can then be expressed as the percentage reduction in cross-sectional areaThe characteristic of a ductile material which permits it to absorb large overloads is all additional safety factor in designDuctility is also important because it is a measure of that property of a material which permits it to be cold-workedSuch operations as bending and drawing are metal-processing operations which require ductile materials Where a material is to be selected to resist wear，erosion，or plastic deformation，hardness is generally the most important propertySeveral methods of hardness testing are available，depending upon which particular property is most desiredThe four hardness numbers in greatest use are the B血ell，Rockwell，Vickers，and knoopMost hardnesstesting systems employ a standard load which is applied to a ball or pyramid in contact with the material to be testedThe hardness is then expressed as a function of the size of the resulting indentationThis means that hardness is an easy property to measurebecause the test is nondestructive and test specimens are not requiredUsually the test can be conducted directly on an actual machine elementHeat treatment is thermal cycling involving one or more reheating and cooling operations after forging for the purpose of obtaining desired microstructures and mechanical properties in a forgingFew forgings of the types are produced without some form of heat treatmentUntreated forgings are usually relatively low-carbon steel parts for noncritical applications or are parts intended for further hot mechanical work and subsequent heat treatmentThe chemical composition of the steel，the size and shape of the product，and the properties desired are important factors in determining which of the following production cycles to useThe object of heat treating metals is to impart certain desired physical properties to the metal or to eliminate undesirable structural conditions which may occur in the processing or fabrication of the materialIn the application of any heat treatment it is desirable that the“previous history，or structural condition of the material be known so that a method of treatment can be prescribed to produce the desired resultIn the absence of information as to the previous processing，a microscopic study of the structure is desirable to determine the correct procedure to be followedThe commercial heat treatments in common use involve the heating of the material to certain predetermined temperatures，“soaking”or holding at the temperatureand cooling at a prescribed rate in air，liquids，or retarding media Spheroidizing is heating of iron-based alloys at a temperature slightly below the critical temperature range followed by relatively slow cooling，usually in airSmall objects of high carbon steel are more rapidly spheroidized by prolonged heating to temperatures alternately within and slightly below the critical temperature range The purpose of this heat treatment is to produce a globular condition of the carbideNormalizing is heating iron-base alloys to temperatures approximately above the critical temperature range followed by cooling in air to below the rangeThe purpose is to put the metal structure in a normal condition by removing all internal strains and stresses given to the metal during some processing operationHardening is a process to increase its hardness and tensile strength，to reduce its ductility, and to obtain a fine grain structureThe procedure includes heating the metal above its critical point of temperature，followed by rapid coolingAs steel is heated，a physical and chemical change takes place between the iron and carbonThe critical point，or critical temperature，is the point at which the steel has the most desirable characteristicsWhen steel reaches this temperature，somewhere between 1400 and 1600the change is ideal to make for a hard，strong material if it is cooled quicklyIf the metal cools slowly，it changes back to its original stateBy plunging the hot metal into water，oil，or brine (quenching)，the desirable characteristics are retainedThe metal is very hard and strong and less ductile than beforeSteel that has been hardened by rapid quenching is brittle and not suitable for most usesBy tempering or“drawing”the hardness and brittleness may be reduced to the desired point for service conditionsAs these properties are reduced，there is also a decrease in tensile strength and an increase in the ductility and toughness of the steelThe operation consists of the reheating of quench-hardened steel to some temperature below the critical range，followed by any rate of coolingAlthough this process softens steel，it differs considerably from annealing in that the process lends itself to close control of the physical properties and in most cases does not soften the steel to the extent that annealing wouldThe final structure obtained from tempering fully hardened steel is called tempered martensiteTempering is possible because of the instability of the martensite，the principal constituent of hardened steelLow temperature draws，from 300 to 400 (150 to 205)，do not caused much decrease in hardness and are used principally to relieve internal strainsAs the tempering temperatures are increasedthe breakdown of the martensite takes place at a faster rate，and at about 600 (315)the change to a structure called tempered martensite is very rapidThe tempering operation may be described as one of precipitation and agglomeration，or coalescence of cementiteA substantial precipitation of cementite is at 600 (315)，which produces a decrease in hardnessIncreasing the temperature causes coalescence of the carbides，with continued decrease in hardnessThe primary purpose of annealing is to soften hard steel so that it may be machined or cold-workedThis is usually accomplished by heating the steel to slightly above the critical temperature to form austenite，holding it there until the temperature of the piece is uniform throughout，and then cooling at a slowly controlled rate so that the temperature of the surface and that of the center of the piece are approximately the sameThis process is known as full annealing because it wipes out all trace of previous structure，refines the crystalline structure，and softens the metalAnnealing also relieves internal stresses previously set up in the metalWhen hardened steel is reheated to above the critical range，the constituents are changed back into austenite，and slow cooling then provides ample time for complete transformation of the austenite into the softer constituentsFor the hypoeutectoid steels，these constituents are pearlite and ferriteThe temperature to which given steel should be heated in annealing depends on its composition，and for carbon steels it can be obtained readily from the iron-carbide equilibrium diagramThe heating rate should be consistent with the size of sections so that the center part is brought up to temperature as uniform as possibleWhen the annealing temperature has been reached，the steel should be held there until it is uniform throughoutFor maximum softness and ductility, the cooling rate should be very slow，such as allowing the parts to cool down with the furnaceThe higher the carbon content，the slower this rate must bea译文机械零件的强度 在设计任何机器或者结构时，所考虑的主要事项之一是其强度应该比它所承受的应力要大得多，以确保安全与可靠性。要保证机械零件在使用过程中不失效，就必须知道它们在某些时候为什么会失效的原因，然后，我们才能将应力与强度联系起来，以确保其安全。 设计任何机械零件的理想情况为，工程师可以利用大量的他所选用的这种材料的强度试验数据。这些试验应该采用与所设计的零件有着相同的热处理，表面形貌度和尺寸大小的试件进行，而且试验应该在与零件使用过程中承受的载荷完全相同的情况下进行。这表明，如果零件将要承受弯曲载荷，那么就应该进行弯曲载荷的试验。如果零件将要承受弯曲和扭转的复合载荷，那么就应该进行弯曲和扭转复合载荷的试验。这些种类的试验可以提供非常有用和精确的数据。它们可以告诉工程师应该使用的安全系数和对于给定使用寿命时的可靠性。在设计过程中，只要能够获得这种数据，工程师就可以尽可能好地进行工程设计工作。 如果零件的失效可能会危害人的生命安全，或者零件有足够大的产量，则在设计前收集这样广泛的数据所花费的费用是值得的。例如，汽车和冰箱的零件的产量非常大，可以在生产之前对它们进行大量的试验，使其具有较高的可靠性。如果把进行这些试验的费用分摊到所生产的零件上的话，则每个零件摊到的费用是非常低的。 你可以对下列四种类型的设计作出评价： (1)零件的失效可能会危害人的生命安全，或者零件的产量非常大，因此在设计时安排一个完善的试验程序会被认为是合理的。 (2)零件的产量足够大，可以进行适当的系列试验。 (3)零件的产量非常小，以至于进行试验根本不合算；或者要求很快地完成设计，以至于没有足够的时间进行试验。(4)零件已经完成了设计、制造和试验，但其结果不能令人满意。这时候需要采用分析的方法来弄清楚不能令人满意的原因和应该如何进行改进。 我们将主要对后三种类型进行讨论。这就是说，设计人员通常只能利用那些公开发表的屈服强度、极限强度和延伸率等数据资料。人们期望着工程师在利用这些不是很多的数据资料的基础上，对静载荷与动载荷、两维应力状态与三维应力状态、高温与低温以及大零件与小零件进行设计!而设计中所能利用的数据通常是从简单的拉伸试验中得到的，其载荷是逐渐加上去的，有充分的时间产生应变。到目前为止，还必须利用这些数据来设计每分钟承受几千次复杂的动载荷的作用的零件，因此机械零件有时会失效是不足为奇的。 概括地说，设计人员所遇到的基本问题是，不论对于哪一种应力状态或者载荷情况，都能利用通过简单拉伸试验所获得的数据并将其与零件的强度联系起来。 可能会有两种具有完全相同的强度和硬度值的金属，其中的一种由于其本身的延展性而具有很好的承受超载荷的能力。延展性是用材料断裂时的延伸率来量度的。通常将5的延伸率定义为延展性与脆性的分界线。断裂时延伸率小于5的材料称为脆性材料，大于5的称为延性材料。 材料的伸长量通常是在的计量长度上测量的。因为这并不是对实际应变量的测量，所以有时也采用另一种测量延展性的方法。这个方法是在试件断裂后，测量其断裂处的横截面的面积。因此，延展性可以表示为横截面的收缩率。 延性材料能够承受较大的超载荷这个特性是设计中的一个附加的安全因素。延性材料的重要性在于它是材料冷变形性能的衡量尺度。诸如弯曲和拉延这类金属加工都需要采用延性材料。 在选用抗磨损、抗侵蚀或者抗塑性变形的材料时，硬度通常是最主要的性能。有几种可供选用的硬度试验方法，采用哪一种方法取决于最希望测量的材料特性。最常用的四种硬度数值是布氏硬度、洛氏硬度、维氏硬度和努氏硬度。大多数硬度试验系统是将一个标准的载荷加在与被试验材料相接触的小球或者棱锥上。因此，硬度可以表示为所产生的压痕尺寸的函数。这表明由于硬度是非破坏性试验，而且不需要专门的试件，因而硬度是一个容易测量的性能。通常可以直接在实际的机械零件上进行硬度试验。 热处理是锻后一次或多次重新加热和泠却操作的热循环过程，使锻件获得所需的显微组织和机械性能。 几乎所有的锻件都需要进行某种形式的热处理。没有经过热处理的锻件，或者是应用场合不太重要