【机械类毕业论文中英文对照文献翻译】残余应力
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机械类毕业论文中英文对照文献翻译
机械类
毕业论文
中英文
对照
文献
翻译
残余
应力
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【机械类毕业论文中英文对照文献翻译】残余应力,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,文献,翻译,残余,应力
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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 “last to cool is in tension,” although there is an exception if certain transformations of microstructure occur. Methods for minimizing or reversing these stresses include annealing for stress relief and hammer or shot peening of the weakened surface. Annealing requires heating mild steel to 11001200F, followed by slow cooling, Some preheating of the parts to be joined may minimize the tensile stresses in welds. A thin but highly effective surface layer of compressive stress may be induced by cold-rolling, coining, and peening processes. It is seen that these processes work-harden an outer layer, thus causing compressive stresses to remain, together with minor tensile stresses in adjacent interior layers. Since the compressive layer is readily obtained all around, these processes are suitable for reversing loads and rotating components where the stress varies between tension and compression. The processes must be carefully controlled in respect to roller pressures and feeds, shot size and speed, etc., for which extensive information is available in engineering books and periodicals. Cold-rolling is applied primarily to cylindrical and other shapes that can be rotated, such as threads and shaft fillets. The shape, size, and pressure of the roller and the yield strength of the shaft determine the depth of penetration, which can be calculated. A special fixture may be attached to the carriages of a lathe and made to slowly traverse the desired rolling of bolts and screws has long been part of a forming process that not only forms but strengthens the threads by deformation and grain flow around the roots and by inducing compressive residual stresses. Coining of holes, also called ball drifting, is a manufacturing process of forcing a hard, tungsten carbide or AIDI 52100 steel, slightly oversize ball through a hole in a plate, bushing, or tubing to give the holes final size and a fine finish. The length of the hole may be from 1/20 to 10 times its diameter. The machine is often set up for a high production of small parts with unskilled labor. An incidental result is that the process increases hardness, hence wear resistance, and induces around the hole a compressive residual stress that is usually advantageous, as in roller-chain links. The links ate highly stressed in pulsating tension with a concentration of the stress at and near the hole surfaces. With the compressive stress from ball drifting, the net tensile stress in service is decreased, and failure is minimized. Peening is the most widely used method for prestressing by mechanically induced yielding. By the impact of rounded striking objects, the surface is deformed in a multitude of shallow dimples, which in trying to expand put the surface under compression. Hammer peening, usually by air-driven tool with a rounded end, is useful on limited areas, such as a weld in shaft or on areas found weakened by corrosion, decarburization, or minor fatigue damage. With a hard spherical end to the tool, the depth of the compressed layer, which occurs below the surface, is about half the strain-hardened region. Shot peening is done on steels by the high-velocity impingement of small, round, steel or chilled cast-iron shot with diameters from 0.007to 0.175 in. The compressed layer has a depth from a few thousandths to a few hundredths of an inch, less than with hammer peening, but roughly proportioned to the shot size used and its velocity. Again the residual stress produced is about half of the strain-hardened yield strength. Shot peening is extensively used because it may be applied with minimum cost to most metals and shapes, except some interior ones. On soft metals, glass beads may be used. Helical springs are commonly shot peened, with up to a 60%increase in allowable stress under pulsating loads. Part of the improvement may be due to the removal of the weakening longitudinal scratches left from the wire-drawing operation.Similarly, coarse-machined and coarse-ground surfaces are smoothed and improved by shot peening, which may be a more economical method than producing a final finish by machining or grinding. Peening is not used on bearing and other closely fitting surfaces where high precision is required. A final grinding for accuracy after peening would remove part or all of the residual stress. Machines are available for the automatic and continuous peening of small-and medium-size parts moving on a conveyor or turntable through the blast. 残余应力 残余应力是结构或者机器中在没有外部载荷或者内部温差时存在的一种应力，它通常是在制造或者装配过程中所产生的。有时它被称为初始应力，而这个过程则被成为施加预应力。当这些结构或者机器投入使用时，工作载荷就会与残余应力相叠加。如果参与应力相加，则这种残余应力是有害的；如果残余应力与工作应力相减，则这种残余应力是有利的。 塑性变形中外力所作的功除大部分转化成热之外，还有一小部分以畸变能的形式储存在形变材料内部。这部分能量叫做储存能。储存能的具体表现方式为：宏观残余应力、微观残余应力及点阵畸变。按照残余应力平衡范围的不同，通常可将其分为三种： （1）第一类内应力，又称宏观残余应力，它是由工件不同部分的宏观变形不均匀性引起的，故其应力平衡范围包括整个工件。例如，将金属棒施以弯曲载荷，则上边受拉而伸长，下边受到压缩；变形超过弹性极限产生了塑性变形时，则外力去除后被伸长的一边就存在压应力，短边为张应力。这类残余应力所对应的畸变能不大，仅占总储存能的0.1左右。 （2）第二类内应力，又称微观残余应力，它是由晶粒或亚晶粒之间的变形不均匀性产生的。其作用范围与晶粒尺寸相当，即在晶粒或亚晶粒之间保持平衡。这种内应力有时可达到很大的数值，甚至可能造成显微裂纹并导致工件破坏。 （3）第三类内应力，又称点阵畸变。其作用范围是几十至几百纳米，它是由于工件在塑性变形中形成的大量点阵缺陷（如空位、间隙原子、位错等）引起的。变形金属中储存能的绝大部分（8090）用于形成点阵畸变。这部分能量提高了变形晶体的能量，使之处于热力学不稳定状态，故它有一种使变形金属重新恢复到自由焓最低的稳定结构状态的自发趋势，并导致塑性变形金属在加热时的回复及再结晶过程。这里仅列举几个有害的残余应力的例子，在机器的装配过程中，当两轴不在一条直线上或者在平行度上相差千分之几英寸时，采用刚性联轴器强使他们联接在一起。当轴旋转时，轴上所产生的应力就是一个方向不断改变的应力。在通常情况下无法经济地实现两轴线之间的精确找正对准时，补救的方法是采用能够补偿一定找正误差的弹性连轴器。出现上述情况时仅产生弹性应力，而残余应力是由于受到轴承约束而得以保存的。在机械加工造成塑性屈服的操作中，当约束去掉后，仍然留有应力。例如，在轴和曲轴的锻造及随后的冷却过程中可能会产生残余应力，切削加工会改变其平衡状态，使轴产生某种程度的翘曲。因此，通常的作法是在进行最终的切削加工之前，在压床上将轴矫直。矫直时需要采用足以使轴产生永久性变形或者屈服的弯矩。不均匀加热或者冷却通常会产生残余应力。焊缝是一个最常见的例子。在焊缝凝固后，焊缝金属和邻近区域的温度要比金属主体的温度高的多。金属沿着焊缝长度的自然收缩将被邻近的体积较大、温度较低的金属限制了一部分。因此沿着焊缝产生了残余拉应力。 通常，局部或浅表面加热会使受热部分和表面膨胀。如果膨胀部分能够自由移动，其移动距离会大大超过邻近的较大体积的材料的移动距离，造成了受热材料的屈服和变厚。由于在高温时屈服强度会降低，因此，这种现象很容易出现。在冷却过程中，温度降低的相同体积的材料能够阻止受热变厚区域完全收缩，结果产生了拉应力。因此，火焰切割后的表面处于受拉状态，其强度降低。一般规律是“最后冷却的部位处于受拉状态”，但是如果显微组织发生某种变化就会出现例外的情况。将这些残余拉应力减至最小或者使之反向的方法有：通过退火消除应力，对强度降低的表面进行垂击或喷丸处理。在退火过程中，要求把低碳钢加热到11001200F，某些合金钢要加热到1600F，然后对每英寸厚度保温一小时，随后进行缓慢冷却。对被联接零件进行某些预热可以将焊缝中的拉应力降至最低。冷压、挤压和喷丸强化都会产生一层薄的，但是十分有效的压应力表面层。可见看出，这些工艺方法使工件外层产生加工硬化
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