机械专业外文文献翻译@外文翻译--残余应力

附录Residual Stresses A residual stress is one that exists without external loading or internal temperature differences on a structure or machine. It is usually a result of manufacturing or assembling operations. Sometimes it is called initial stress, and the operations, prestressing. When the structure or machine is put into service, the service loads superimpose stresses. If the residual stresses add to the service-load stresses, they are detrimental; if they subtract from the service-load stresses they are beneficial. In the plastic deformation the external force does the merit turns into outside the heat except the majority of extensions, but also some small part by the distortion can the form stores up in the deformation material. This part of energy named storage energy. The storage can the concrete manifestation way is: Macroscopic residual stress, microscopic residual stress and lattice distortion. According to the residual stress balance scope difference, usually may divide into it three kinds: (1) First kind of internal stress, also called the macroscopic residual stress, it is causes by the work piece different part macroscopic distortion nonuniformity, therefore its stress balance scope including entire work piece. For example, serves with Jin Shubang the curving load, then above is pulled elongates, under receives the compression; The distortion surpasses when the limit of elasticity has had the plastic deformation, after then the external force elimination by elongated one side on the existence compressed stress, the leg of right triangle is the tensile stress. This kind of residual stress corresponds the distortion can not be big, only accounts for always stores up can about 0.1%. (2) Second kind of internal stress, also called the microscopic residual stress, it is produces by between the crystal grain or the subgrain distortion nonuniformity. Its sphere of action and the crystal grain size quite, namely maintain the balance between the crystal grain or the subgrain. Sometimes this kind of internal stress may achieve the very great value, even possibly creates the micro crack and causes the work piece destruction. (3) Third kind of internal stress, also calls the lattice distortion. Its sphere of action is several dozens to several hundred nanometers, it is because the work piece forms in the plastic deformation the massive lattice flaw (for example vacancy, interstitial atom, dislocation and so on) cause. In the distortion metal the storage can the major part (80%90%) uses in forming the lattice distortion. This part of energy enhanced the distortion crystal energy, causes it to be at the thermodynamics non-steady state, therefore it has one kind to make the distortion metal to restore to the free enthalpy lowest stable structure condition spontaneous tendency, and causes the plastic deformation metal in heating time reply and the recrystallization process. Only a few examples of detrimental residual stresses will be given here .One, in the assembly of machinery, occurs when two shafts are not in line or are a few thousandths of an inch out of parallel, and they are forced into connection by rigid couplings. The resulting stresses in the shafts become reversing stresses when the shafts are rotated. The correction, when perfect alignment cannot be economically attained, as is frequently the case, is to use flexible couplings of a type necessary for the degree of misalignment. The preceding case occurs with elastic stresses only, and the residual stresses are maintained by bearing constraints. In applications where mechanical work causes plastic yielding .stresses remain when the constraints are removed. For example, the forging of shafts and crankshafts and the cooling after forging may induce residual stresses, the equilibrium of which id changed in machining, causing some warping of the shafts. It is then common practice to straighten the shafts in a press before the final machining operation. Straightening requires a bending moment large enough to cause permanent set or yielding. Detrimental residual stresses commonly result from differential heating or cooling. A weld is a common example, The weld metal and the areas immediately adjacent are, after solidification, at a much higher temperature than the main body of metal. The natural contraction of the metal along the length of the weld is partially prevented by the large adjacent body of cold metal. Hence residual tensile stresses are set up along the weld. In general, local or shallow heating which would expand the region or surface, if it were free, a distance well beyond that which the adjacent larger volume will allow causes yielding and upsetting of the heated material, This readily occurs because of the reduced yield strength at elevated temperatures. The same cooler volume prevents the upset, heated region from fully contracting during its cooling, and tensile general rule is that the “la