机械零件的强度机械英汉翻译.doc
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1、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 fail
2、Then 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
3、 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 b
4、ending 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 enginee
5、r 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 v
6、ery 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
7、)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 quanti
8、ties 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
9、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
10、 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 p
11、arts 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
12、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 ductili
13、ty 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
14、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 la
15、rge 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
16、 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 ha
17、rdnesstesting 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 a
18、nd 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 forgingF
19、ew 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
20、 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 t
21、he 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 previou
22、s 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 rat
23、e 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 alt
24、ernately 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 ran
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- 机械零件 强度 机械 英汉翻译
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