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1、Lesson 13 The Strength of Mechanical Elements,TerminologyText tour,机械工程专业英语教程,1,2018/10/19,Terminology Justify To demonstrate or prove to be just, right, or validBiaxial Having two axestriaxial 三轴的，三维的，空间的,机械工程专业英语教程,2,2018/10/19,TerminologyDuctile Easily drawn into wire or hammered thin 有延伸性的：易被拉成线
2、或锤薄的，可锻造的，可塑的，易变性的 Ductility 延展性，可锻性，韧性brittle: 脆性的，易碎的，易损坏的,机械工程专业英语教程,3,2018/10/19,Terminologyspecimen 样品，样件，试样，试件elongation: 拉伸，伸长，延伸率,机械工程专业英语教程,4,2018/10/19,TerminologyReliability: 可靠性，安全性Cross-sectional: 横截面的，剖面的Cold-work: 冷变形加工,机械工程专业英语教程,5,2018/10/19,Terminologyerosion: 腐蚀，侵蚀，磨损，磨蚀indentatio
3、n: 压痕，凹痕nondestructive: 非破坏的，无损的,机械工程专业英语教程,6,2018/10/19,TerminologyYield strength, ultimate strength, and percentage elongation: 屈服强度，极限强度，和延伸百分率Gauge length: 标距长度，计量长度,机械工程专业英语教程,7,2018/10/19,Lesson 9 The strength of mechanical elements 机械零件的强度,One of the primary considerations in designing any ma
4、chine or structure is that the strength must be sufficiently greater than the stress（承受应力）to assure both safety and reliability.,Lesson 9 The strength of mechanical elements 机械零件的强度,To assure that mechanical parts do not fail in service, it is necessary to learn why they sometimes do fail. Then we s
5、hall be able to relate the stresses with the strengths to achieve safety.,Lesson 9 The strength of mechanical elements 机械零件的强度,Ideally, in designing any machine element, the engineer should have at his disposal（在手头） the results of a great many strength tests of the particular material chosen. These
6、tests should have been made under specimens having the same heat treatment, surface finish, 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 service. ,Lesson 9 The strength of mechanical elements 机械零件
7、的强度,This means that, if the part is to experience a bending load, it should be tested with a bending load. If it is to be subjected to combined bending and torsion, it should be tested under combined bending and torsion. Such tests will provide very useful and precise information.,Lesson 9 The stren
8、gth of mechanical elements 机械零件的强度,They tell the engineer what factor of safety to use and what the reliability is for a given service life. Whenever such data are available for design purposes, the engineer can be assured that he is doing the best possible job of engineering.,Lesson 9 The strength
9、of mechanical elements 机械零件的强度,The cost of gathering such extensive data prior to design is justified（值得）if failure of the part may endanger human life, or if the part is manufactured in sufficiently large quantities.,Lesson 9 The strength of mechanical elements 机械零件的强度,Automobiles and refrigerators
10、, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. The cost of making these tests is very low when it is divided by the total number of parts manufactured.,Lesson 9 The strength of mechanical e
11、lements 机械零件的强度,You can now appreciate（鉴别评价）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（完善的）testing program is justified during design.,Lesson 9 The strength of mechanical elemen
12、ts 机械零件的强度,(2) The part is made in large enough quantities so that a moderate series of tests is feasible.,Lesson 9 The strength of mechanical elements 机械零件的强度,(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
13、not enough time for testing.,Lesson 9 The strength of mechanical elements 机械零件的强度,（4）the part has already been designed, manufactured, and tested and found to be unsatisfactory. Analysis is required to understand why the part is unsatisfactory and what to do to improve it.,Lesson 9 The strength of m
14、echanical elements 机械零件的强度,It is with the last three categories that we shall be mostly concerned. This means that the designer will usually have only published values of yield strength, ultimate strength（极限强度）, and percentage elongation（延伸率）. With this meager（贫乏的）information the engineer is expecte
15、d to design against static and dynamic loads, biaxial and triaxial stress states, high and low temperatures, and large and small parts!,Lesson 9 The strength of mechanical elements 机械零件的强度,The data usually available for design have been obtained from the simple tension test, where the load was appli
16、ed gradually and the strain given time to develop有充分的时间产生应变. Yet these same data must be used in designing parts with complicated dynamic loads applied thousands of times per minute. No wonder machine parts sometimes fail.,Lesson 9 The strength of mechanical elements 机械零件的强度,To 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, regardless of the stress state or the loading situation.,
