土木工程专业英语课PPT课件

1、 Design limits are based on avoiding failure of the part to perform its desired function.设计限值是以构件不发生破坏而执行其要求的功能为基础的。 Because different parts must satisfy different functional requirements, the conditions which limit load-carrying ability may be quite different for different elements. 因为不同构件必须满足不同的功能

2、要求，不同构件界定承载能力的条件可能差别很大。第1页/共33页 As an example, compare the design limits for the floor of a house with those for the wing of an airplane.举例来说，可以将住宅楼板和机翼的设计限值比较一下。 第2页/共33页 If we were to determine the size of the wooden beams in a home such that they simply did not break, we would not be very happy w

3、ith them; they would be too “springy”. Walking across the room would be like walking out on a diving board.如果我们要这样来确定木梁的尺寸，只要它不断裂就行，那我们可能就不会很喜欢这些木梁了，这些木梁可能弹性变形太大了。人们穿过房间就会像从跳水板下来一样。第3页/共33页 Obviously, we should be concerned with the maximum “deflection” that we, as individuals, find acceptable. This

4、 level will be rather subjective, and different people will give different answers. 显然，我们应该关心我们作为个体能够接受的最大挠度，这个水平的主观性相当大，不同人将给出不同的答案。第4页/共33页 In fact, the same people may give different answers depending on whether they are paying for the floor or not!事实上，同一群人也会给出不同的回答，这取决于他们是否支付楼板的费用。 第5页/共33页 An a

5、irplane wing structure is clearly different. If you look out an airplane window and watch the wing during turbulent weather, you will see large deflections; in fact you may wish that they were smaller. However, you know that the important issue is that of “structural integrity”, not deflection.一个机翼结

6、构显然不同，你如从机窗向外观看恶劣天气中的机翼，你就会看到很大的变形，而事实上你会希望这种变形更小些才好。然而，你知道重要的是结构的完整性，而不是变形。第6页/共33页 We want to be assured that the wing will remain intact. We want to be assured that no matter what the pilot and the weather do, that wing will continue to act like a good and proper wing. 我们要确保机翼完好无损，我们要确保机翼一直处于良好的工

7、作状态，而不管飞行员做什么或者天气怎么样。第7页/共33页 In fact, we really want to be assured that the wing will never fail under any conditions. Now that is a pretty tall order; who knows what the “worst” conditions might be?事实上，我们确实想要确保机翼在任何条件下都不会失效。现在这是不可能实现的事，谁又知道最恶劣的条件是什么呢？ 第8页/共33页 Engineers who are responsible for the

8、 design of airplane wing structures must know, with some degree of certainty, what the “worst” conditions are likely to be. 负责机翼结构设计的工程师们必须在某种确定的程度上了解最恶劣的条件该是什么样的。第9页/共33页 It takes great patience and dedication for many years to assemble enough test data and failure analyses to be able to predict th

9、e “worst” case. 多年来，人们花费很多耐心和辛勤工作，收集了充足的试验数据，进行了大量的失败分析，能够预测最坏的情况。第10页/共33页 The general procedure is to develop statistical data which allow us to say how frequently a given condition is likely to be encounteredonce every 1 000 hours, or once every 10 000 hours, etc.通常的程序是统计数据，以此分析特定条件发生的频率，是1000小时一

10、次，还是10000小时一次，等等。 第11页/共33页 As we said earlier, our object is to avoid failure. Suppose, however, that a part has failed in service, and we are asked: Why? “Error” as such can come from three distinctly different sources, any or all of which can cause failure:如前所述，我们的目的是不让失效发生。但是，如果假设一个部件在使用中失效了，我们会

11、被问道：为什么？这种“失误”可能来源于下列三种情况，其中一种或全部都可能引起失效：第12页/共33页1.Error in design: We the designers or the design analysts may have been a bit too optimistic: Maybe we ignored some loads; maybe our equations did not apply or were not properly applied; maybe we overestimated the intelligence of the user; maybe we

12、slipped a decimal point.设计失误：我们这些设计者或设计分析者可能都有一点过于乐观了。也许我们忽略某些荷载，也许我们的公式不合适或者被不恰当地使用了。也许我们过高估计了使用者的智力；也许我们点错了一个小数点。第13页/共33页2.Error in manufacture: When a device involves heavily stressed members, the effective strength of the members can be greatly reduced through improper manufacture and assembly:

13、 制造失误：当一个设备的构件需要承受非常大的应力时，可能通过不恰当的制造和装配而使构件的有效强度大大降低：第14页/共33页Maybe the wrong material was used; maybe the heat treatment was not as specified; maybe the surface finish was not as good as called for; maybe a part was “out of tolerance”; maybe the surface was damaged during machining; maybe the threa

14、ds were not lubricated at assembly; or perhaps the bolts were not properly tightened.也许采用了错误的材料，不是按要求进行的热处理，表面喷漆也不如所要求的那样好，某个构件的偏差超出了限值，加工时表面被损坏，安装时没有给螺纹润滑，或者螺栓没有恰当地旋紧。 第15页/共33页3.Error in use: As we all know, we can damage almost anything if we try hard enough, and sometimes we do so accidentally:

15、使用失误：正如我们所知，如果我们尽力的话，可以将任何东西损坏，有时这样的事意外发生了：第16页/共33页We went too fast; we lost control; we fell asleep; we were not watching the gages; the power went off; the computer crashed; he was taking a coffee break; she forgot to turn the machine off; you failed to lubricate it, etc. 我们飞得过快，失去了控制，睡着了，没有注意仪表，

16、断电了，计算机坏了，他正去喝咖啡了，她忘了关掉机器了，你没有好好做润滑，等等，等等。 第17页/共33页 Any of the above can happen：Nothing is designed perfectly; nothing is made perfectly; and nothing is used perfectly. 上面提到的任何失误都可能发生，任何产品都不能设计、制造、使用得完美无缺。第18页/共33页 When failure does occur, and we try to determine the cause, we can usually examine t

17、he design; we can usually examine the failed parts for manufacturing deficiencies; but we cannot usually determine how the device was used (or misused). 失效发生时，我们力图确定失败的原因。我们通常可以考察设计，检验失效的部件，找出制造缺陷；但我们往往不能确定设备是如何使用（或者错误使用）的。第19页/共33页 In serious cases, this can give rise to considerable differences of

18、 opinion, differences which frequently end in court.在很严重的情况下，这会造成很大的意见分歧，这些分歧往往只能在法庭上解决。 第20页/共33页 In an effort to account for all the above possibilities, we design every part with a safety factor. Simply put, the safety factor (SF) is the ratio of the load that we think the part can withstand to t

19、he load we expect it to experience. 在我们尽力考虑上面提及的所有可能之后，我们用一个安全系数设计每个部件。简单地说，安全系数SF是我们认为部件能承受的荷载与其可能经历的荷载的比值。第21页/共33页 The safety factor can be applied by increasing the design loads beyond those actually expected, or by designing to stress levels below those that the material actually can withstand

20、(frequently called “design stresses”). Safety factor = SF = failure load / design load = failure stress / design stress 当增大设计荷载使其超过实际荷载时，或者当设计比材料实际可承受的应力低的应力（通常称为设计应力）时，就使用安全系数。 安全系数SF= 设计应力设计应力失效应力失效应力设计荷载设计荷载失效荷载失效荷载/第22页/共33页It is difficult to determine an appropriate value for the safety factor.

21、 In general, we should use larger values when:要确定适当的安全系数值是困难的。一般地，在下列情况下我们应该采用较大的安全系数值：第23页/共33页1.The possible consequences of failure are high in terms of life or cost. 失效可能造成严重的人员伤亡和财产损失时；2.There are large uncertainties in the design analyses. 设计分析中不确定因素多时。 第24页/共33页 Values of SF generally range f

22、rom a low of about 1.5 to 5 or more. When the incentives to reduce structural weight are great (as in aircraft and spacecraft), there is an obvious conflict. 安全系数值通常为1.5到5或者更大。当减小结构重量意义重大时，比如航空、航天设备，就存在一个突出的矛盾。第25页/共33页 Safety dictates a large SF, while performance requires a small value. The only r

23、esolution involves reduction of uncertainty. 安全要求大安全系数，而机器工作性能又要求较小的安全系数值。只有降低不确定性才是唯一的解决办法。第26页/共33页 Because of extreme care and diligence in design, test, manufacture, and use, the aircraft industry is able to maintain very enviable safety records while using safety factors as low as 1.5.由于人们在设计、试

24、验、制造和使用过程中具有最大程度的细心和勤勉，才使得航空工业保持着非常骄人的安全记录，而这里使用的安全系数却仅仅是1.5。 第27页/共33页 We might note that the safety factor is frequently called the “ignorance factor”. This is not to imply that engineers are ignorant, but to help instill in them humility, caution, and care. 我们可能注意到，安全系数还常被称为“无知系数”。这并不是说工程师无知，而是要求他们谦卑、谨慎和细心。第28页/共33页 An engineer is responsible for his or her design decisions, both ethically and legally. Try to learn from the mistakes of others rather than making your