机械设计制造及其自动化英文.doc

.英文原文:Mechanical properties of materials The material properties can be classified into three major headings: (1) physical, (2) chemical, (3) mechanicalPhysical properties Density or specific gravity, moisture content, etc., can be classified under this category. Chemical propertiesMany chemical properties come under this category. These include acidity or alkalinity, react6ivity and corrosion. The most important of these is corrosion which can be explained in laymans terms as the resistance of the material to decay while in continuous use in a particular atmosphere. Mechanical properties Mechanical properties include in the strength properties like tensile, compression, shear, torsion, impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of cross-section of the specimen. This is a curve plotted between the stress along the This is a curve plotted between the stress along the Y-axis(ordinate) and the strain along the X-axis (abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappears. For many materials this occurs op to a certain value of the stress called the elastic limit Ap. This is depicted by the straight line relationship and a small deviation thereafter, in the stress-strain curve (fig.3.1). Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality Ap. In this region, the metal obeys hookess law, which states that the stress is proportional to strain in the elastic range of loading, (the material completely regains its original dimensions after the load is removed). In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the elastic limit. This may be attributed to the time-lagin the regaining of the original dimensions of the material. This effect is very frequently noticed in some non-ferrous metals. Which iron and nickel exhibit clear ranges of elasticity, copper, zinc, tin, are found to be imperfectly elastic even at relatively low values low values of stresses. Actually the elastic limit is distinguishable from the proportionality limit more clearly depending upon the sensitivity of the measuring instrument. When the load is increased beyond the elastic limit, plastic deformation starts. Simultaneously the specimen gets work-hardened. A point is reached when the deformation starts to occur more rapidly than the increasing load. This point is called they yield point Q. the metal which was resisting the load till then, starts to deform somewhat rapidly, i. e., yield. The yield stress is called yield limit Ay. The elongation of the specimen continues from Q to S and then to T. The stress-strain relation in this plastic flow period is indicated by the portion QRST of the curve. At the specimen breaks, and this load is called the breaking load. The value of the maximum load S divided by the original cross-sectional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength Au. Logically speaking, once the elastic limit is exceeded, the metal should start to yield, and finally break, without any increase in the value of stress. But the curve records an increased stress even after the elastic limit is exceeded. Two reasons can be given for this behavior: The strain hardening of the material; The diminishing cross-sectional area of the specimen, suffered on account of the plastic deformation. The more plastic deformation the metal undergoes, the harder it becomes, due to work-hardening. The more the metal gets elongated the more its diameter (and hence, cross-sectional area) is decreased. This continues until the point S is reached. After S, the rate at which the reduction in area takes place, exceeds the rate at which the stress increases. Strain becomes so high that the reduction in area begins to produce a localized effect at some point. This is called necking. Reduction in cross-sectional area takes place very rapidly; so rapidly that the load value actually drops. This is indicated by ST. failure occurs at this point T. Then percentage elongation A and reduction in reduction in area W indicate the ductility or plasticity of the material: A=(L-L0)/L0*100% W=(A0-A)/A0*100% Where L0 and L are the original and the final length of the specimen; A0 and A are the original and the final cross-section area. The Two Types Of Power Transmission In hydraulic power transmission the apparatus (pump) used for conversion of the mechanical (or electrical,thermal) energy to hydraulic energy is arranged on the input of the kinematic chain ,and the apparatus (motor) used for conversion of the hydraulic energy to mechanical energy is arranged on the output (fig.2-1) The theoretical design of the energy converters depends on the component of the bernouilli equation to be used for hydraulic power transmission. In systerms where, mainly, hydrostatic pressure is utilized, displacement (hydrostatic) pumps and motors are used, while in those where the hydrodynamic pressure is utilized is utilized gor power transmission hydrodynamic energy converters (e.g. centrifugal pumps) are used. The specific characteristic of the energy converters is the weight required for transmission of unit power. It can be demonstrated that the use of hydrostatic energy converters for the low and medium powers, and of hydrodynamic energy converters of high power are more favorite (fig.2-2). This is the main reason why hydrostatic energy converters are used in industrial apparatus. transformation of the energy in hydraulic transmission. 1. driving motor (electric, diesel engine);2. mechanical energy;3. pump; 4. hydraulic energy; 5. hydraulic motor; 6. mechanical energy; 7. load variation of the mass per unit power in hydrostatic and hydrodynamic energy converters 1、hydrostatic; 2.hydrodynamicOnly displacement energy converters are dealt with in the following. The elements performing converters provide one or several size. Expansion of the working chambers in a pump is produced by the external energy admitted, and in the motor by the hydraulic energy. Inflow of the fluid occurs during expansion of the working chamber, while the outflow (displacement) is realized during contraction. Such devices are usually called displacement energy converters. The Hydrostatic Power In order to have a fluid of volume V1 flowing in a vessel at pressure work spent on compression W1 and transfer of the process, let us imagine a piston mechanism (fig.2-3(a) which may be connected with the aid of valves Z0 and Z1 to the external medium under pressure P0 and reservoir of pressure p1.in the upper position of the piston (x=x0) with Z0 open the cylinder chamber is filled with fluid of volume V0 and pressure P0. now shut the value Z0 and start the piston moving downwards. If Z1 is shut the fluid volume in position X=X1 of the piston decreases from V0 to V1, while the pressure rises to P1. the external work required for actuation of the piston (assuming isothermal change) is W1=-0x0(P-P0)Adx=-v1v0(P-P0)dv译文: 材料的机械性能 材料的机械性能可以被分成三个方面：物理性能，化学性能，机械性能。 物理性能 密度或比重、温度等可以归为这一类。化学性能这一种类包括很多化学性能。其中包括酸碱性、化学反应性、腐蚀性。其中最重要的是腐蚀性，在外行人看来，腐蚀性被解释为在某处的零件抵抗腐蚀的能力。机械性能机械性能包括拉伸性能、压缩性能、剪切性能、扭转性能、冲击性能、疲劳性能和蠕变。材料的拉伸强度可以通过试件的横截面积出试件承受的最大载荷得到，这是在拉伸试验中，应力沿Y轴，应边沿X轴变化的曲线。一种材料加载时开始发生变化的初值取决于负载的大小。当负载去掉时可以看到变形消失。对于很多材料而言，在达到弹性极限的一定应力值A之前，一直表现为这样。在应力-应变图中，这是可以用线性关系来描述的。这之后又一个小的偏移。在弹性范围内，达到应力的极限之前，应力和应变是成比例的，这被称为比例极限Ap。在这个区域，零件符合胡克定律，即应力与应变是成比例的，在弹性范围内（材料能完全恢复到最初的尺寸，当负载去掉时）。曲线中的实际点，比例极限在弹性极限处。这可以认为是材料恢复初值时落后于前者。这种影响在不含铁的材料中经常提到。铁和镍有明显的弹性范围，而铜、锌、锡等，即使在相对低的应力下也表现为不完全弹性。实际上，能否清楚地分辩弹性极限和比例极限取决于测量设备的灵敏度。当负载超过弹性极限时，塑性变形开始，逐渐的试件被硬化。变形比负载增加得更快时的点被称成为屈服点Q。金属开始抵抗负载转变成快速变形，这时的屈服力成为屈服极限Ay。试件的延伸率 继续由Q到T再到，在这种塑性流动时，应力应变关系在曲线上处于QRST区域。在点，试件破坏且这种负载称为破坏负载。最大负载S除以试件初始的截面积，被定义为这种金属的最终拉伸极限或试样的拉伸强度Au。按逻辑说，在应力不增加的情况下，一旦超出弹性极限，金属开始屈服，并最终破坏。但是当超出弹性极限后，在纪录曲线上应增大。这种变化主要有两个原因：材料的应力硬化由于塑性变形而引起的试件横截面积的变小由于加工硬化，金属塑性变化越大，硬化越严重。金属拉伸越长，他的直径（横截面积）越小。直到到达点为