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1、4 Stress Limits in Design,第4课 设计中的应力极限,How large can we permit the stresses to be? Or conversely: How large must a part be to withstand a given set of loads? What are the overall conditions or limits that will determine the size and material for a part? 我们可以允许多大应力？或者反过来说，构件尺寸该多大才能抵抗给定的一组荷载？决定构件尺寸和材料
2、的总体条件和限值是什么？,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
3、不同构件必须满足不同的功能要求，不同构件界定承载能力的条件可能差别很大。,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。,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 with
4、 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。,Obviously, we should be concerned with the maximum “deflection” that we, as individuals, find acceptable. This level wil
5、l 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。,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。,An airplane wing structure is
6、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. 一个机翼结构显然不同，你如从机窗向外观看恶劣天气中的机翼，你
7、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。,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作状态，而不管飞行员做什么或者天气怎么样。,In fact, we
8、 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？,Engineers who are responsible for the design of airplane wing structures must know, wi
9、th 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。,It takes great patience and dedication for many years to assemble enough test data and failure analyses to be able to predict the “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
10、够预测最坏的情况。,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小时一次，还是10000小时一次，等等。,As we said earlier, our object is to avoid fail
11、ure. 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：,Error in design: We the designer
12、s 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 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
13、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。,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: 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：,Maybe the wrong material was used; mayb
14、e 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 threads 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
15、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。,Error in use: As we all know, we can damage almost anything if we try hard enough, and sometimes we do so accidentally: 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：,We went too fast; we lost control; we fell asleep; we were not watc
16、hing 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，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。,Any of the above can happen：Nothing is designed perfectly; nothing is made p
17、erfectly; 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。,When failure does occur, and we try to determine the cause, we can usually examine the 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。,
