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1、1,单词复习：concurrent engineering designdeemIntegralreciprocating motion/circular motion internal combustionengineAccreditation Board for Engineering and Technologymodern design theory and methodology,上一课回顾,2,Lesson 7 Engineering Design and safety factors 工程设计和安全系数,重点词汇：alludesaftey factoryield strength
2、/ultimate tensile strengthrequisite/requirement/requireevaluation/evaluatecrystalbizarre,3,Paragraph 1,Any consideration of engineering materials and engineering design of systems utilizing materials must initially deal with(涉及到)Murphys Law for Materials systems and the laws of materials application
3、s: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,Murphys Law:If any material can fail,it will.Laws of materials applications(1)All materials are unstable.(2)The materials system is only as strong or as stable as its weakest or most unstable component.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
4、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,Paragraph 2:,Although they are both obvious and incontrovertible(无可争辫的),we might elaborate upon these laws,especially the laws of materials applications.All materials are indeed unstable.Even such materials as platinum can be degraded in particular environments.虽然这些法则是显见
5、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,Under stress(应力，strain 应变),all materials respond to that stress.Creep(蠕变)is an example.Given enough time,failure(失效)can be calculated to occur in all materials under creep stress.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
6、有足够的时间，所有材料一定会产生屈服。,7,Creep deformation蠕变变形,Creep fracture蠕变断裂,8,Stress and environment can act in concert to cause failure,for example,stress corrosion cracking.Creep-temperature effects and stress-temperature effects can contribute to the degradation of properties and components and system failure
7、.压力和环境共同（in concert）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,The design processParagraph 3:,The design process usually begins with a specification of a solution.We sometimes allude to a design cycle,but the process may contain a design cycle plus design implementation,which involves actual
8、 production based upon the design.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,The design cycle can involve the original thoughts,sketches,and knowledge that in the specification stage produce engineering drawings.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-aided design(CAD,C
9、AM计算机辅助制造)is now employed to implement a cycle in which various designs or design ideas may be tested or simulated.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,Paragraph 4:,Even in the early design cycle,it is useful to have some concept of available materials.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。Designs that cannot
10、 be achieved because materials with requisite properties do not exist are only concepts that will never be a reality unless a material or materials are developed.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,Prior to a fabrication step,materials specifications must be made depending upon the
11、design requirements.在制造之前必须依据设计要求给出材料的技术要求（specification）。,13,These may have to be altered in the design simulation if the requisite properties cannot be identified in existing materials,or fabrication may have to delay until new materials can be designed and fabricated.Materials are the reality in
12、a design.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,Paragraph 5,Figure 1 illustrates a simple schematic representation of a design process that includes design implementation.图1所示为一个包括设计实施的设计过程简图（schematic representation）。The role of materials in the implementation is of co
13、urse crucial because they make the design a reality.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,The design or design implementation diagram shown in Figure 1 may not characterize the final product.图1示意图或许不能描述最终产品。The first process may produce a prototype(Rapid Prototyping)that will be tested and improved upon to
14、develop a final product.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,a final product works reliably and is economical(or profitable)and safe.一般来讲，最终产品要求工作可靠、经济（有利可图）和安全。,17,Safety factors in engineering designsParagraph 6,The concept of safety factor(安全系数)is normally applied to the evaluation and
15、consideration of strength of materials used in a design.安全系数常用于估算和考虑用于设计中的材料强度。Safety factors are normally applied to the yield strength of a material but can be applied to the ultimate tensile strength.安全系数一般用于材料的屈服强度（yield strength），也能用于极限抗拉强度。,18,The yield stress(屈服应力)is the stress,above which th
16、e material(particularly a metal or alloy)will deform permanently.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。Having exceeded the yield stress or yield strength(屈服强度),materials begin to creep.超过屈服应力或屈服强度，材料开始蠕变。,19,That is,dislocations(晶格错移)will be created,and application of a load to continue the stress above
17、 yield will allow the material to slip.Over time,it may slip(滑动)enough to simply come apart.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,So if a material is used in a service environment in which the yield stress is never exceed,it is unlikely it will fail by being over stress
18、ed.所以始终在不超过极限应力的状态下工作，材料不会屈服。We sometimes base a design on fractions of the yield stress.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,For example,if we assume a material will never exceed half the value of the yield stress and use this in the actual design,we can realize a safety factor of 2.例如，假设材料永远不会超过屈服应力值的一半
19、，并在实际设计中应用，这样安全系数为2。In an extreme case,we might allow the maximum stress in the system to be y/4,and the safety factor would be 4.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,Obviously a design that would put a material into a service environment in which the normal stresses would be near the ultimate tensile
20、 stress for sustained periods of time would be unsafe.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,On the other hand,large safety factors could render a design unworkable because no materials could be identified with the requisite strengths.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。In addition,even fulfilling the stre
21、ngth requirements does not necessarily justify a materials use.另外即使该强度可以实现，材料选用也是不合理的。,24,The material may be too expensive,or too corrosive,or unstable in other ways for the intended application.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。Or perhaps,in addition to high strength,high electrical conductivity(导电率
22、)would be required.或许除高强度外，需要高的导电率。,25,Finally,lets look at the concept of a materials system.A system in the most general sense involves an arrangement of and connection of parts or elements into a whole.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。,The materials system Paragraph 7,26,A machine is t
23、herefore a mechanical system having connected components.机器是相互联系的零件构成的机械系统。A materials system is therefore an arrangement of connected materials.材料也是材料之间相互联系构成的系统。,27,The system can be partitioned into different materials where they connect,and in the most ideal system materials are separated by an
24、interface(界面)or boundary that connects one component to another or provides a transition from one regime to another.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,Fracture can occur at crystal interfaces,and therefore even a polycrystalline(多晶的)metal or alloy might be thought of
25、as a system of connected crystals(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,Paragraph 8,In the context of failure as we have discussed,it is important to realize that failure can occur not only within a regime(a component in a connected arrangement),but at the connection itself.我们讨论屈服时，这种
26、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,So the interface is also a part of the system and must be considered in the context of the weakest link corollary and Murphys Law.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,Paragraph 9,Failure prevention is certainly not a hopeless issue even in very complex mater
27、ials system but does require a great deal of testing,evaluation,and fundamental understanding of materials behavior under a variety of conditions.防止失效不是没有希望的，即使在很复杂的材料系统中，它需要大量的实验、计算、弄清各种条件下材料的变化。,32,Selecting optimum conditions,underutilizing components in a system(designing for the weakest link an
28、d employing safety factors),introducing safety factors,consideration for environmental changes and even bizarre conditions of use,all contribute to successful applications of products.选择最优（optimum）条件，不使用极限状态（面向最薄弱环节设计，使用安全系数），引入安全系数，考虑环境变化，甚至特殊条件的应用，所有这些都有利于产品的成功使用。,33,One needs only to take a glimpse at the technology around us to appreciate how successful we have been in designing efficient,dependable,and useful products and fixtures.人们只要看一下我们身边的技术，就会欣赏到：我们设计的有效的、可靠的、用途广泛的产品和设备是多么的成功！,34,课后作业：,1、复习本课单词，预习下一课单词,
