外文翻譯---機械零件的強度(留存版)

　　

【正文】 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, 外文資料 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 important because it is a measure of that property of a material which permits it to be coldworked. Such operation as bending and drawing are metalprocessing operations which require ductile materials. When a material is to be selected to resist wear, erosion, or plastic deformation, hardness is generally the most important property. Several methods of hardness testing are available, depending upon which particular property is most desired. The four hardness numbers in greatest use are the Brinell , Rockwell, Vickers, and Knoop. Most hardnesstesting systems employ a standard load which is applied to a ball or pyramid in contact with the material to be tested. The hardness is then expressed as a function of the size of the resulting indentation. This means that hardness is an easy property to measure, because the test is nondestructive and test specimens are not required. Usually the test can be conducted directly on an actual machine element. Shafts and Couplings Virtually all machines shafts. The most mon shape for shafts is circular and the cross section can be either solid or hollow(hollow shafts can result in weight savings). Rectangular shafts are sometimes used, as in screwdriver blades, socket wrenches and control knob stems. A shaft must have adequate torsional strength to transmit torque and not be overstressed. It must also be torsionally stiff enough so that one mounted ponent does not deviate e