外文翻译---机械零件的强度(编辑修改稿)

外文翻译---机械零件的强度(编辑修改稿)内容摘要：

f 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 quantities that testing is not justified at all。 or the design must be pleted so rapidly that these is not enough time for testing. (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. 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, and percentage elongation. With this meager information the engineer is expected to design against static and dynamic loads, biaxial stress states, high and low temperatures, and large and small parts! The data usually available for design have been obtained form the simple tension test, where the load was applied gradually and the strain given time to develop. Yet these same data must be used in designing parts with plicated dynamic loads applied thousands of times per minute. No wonder machine parts sometimes fail. To sum up, the fundamental problem of the designer is to use the simple tensiontest data and relate them to the strength of the part, regardless of the stress state or the loading situation. It is possible for 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 ductility. Ductility is measured by the percentage elongation which occurs in the material at fracture. The usual dividing line between ductility and brittleness is 5 percent elongation. A material having less than 5 percent elongation at fracture is said to be brittle, while one having more is said to be ductile. The elongation of a material is usually measured over 50mm gauge length. Since this is not a measure of the actual strain, 外文资料 7 another method of determining ductility is sometimes used. After the specimen has been fractured, measurements are made of the area of the cross section at the fracture. Ductility can then be expressed as the percentage reduction in crosssectional area. The characteristic of a ductile material which permits it to absorb large overloads is an additional safety factor in design. Ductility is also。