機械設(shè)計制造及其自動化英文

1、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 prop

2、erties 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 Mecha

3、nical 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 betwee

4、n 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

5、 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

6、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 dimens

7、ions 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

8、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 dependi

9、ng 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 th

10、ey 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 t

11、he 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 spea

12、king, 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 mater

13、ial; 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 decr

14、eased. 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

15、 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

16、*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 hyd

17、raulic 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

18、 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)

19、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). T

20、his 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

21、 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 t

22、he 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 t

23、o 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 upp

24、er 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. th

25、e external work required for actuation of the piston (assuming isothermal change) is W1=-0x0(P-P0)Adx=-v1v0(P-P0)dv譯文: 材料的機械性能 材料的機械性能可以被分成三個方面：物理性能，化學(xué)性能，機械性能。 物理性能 密度或比重、溫度等可以歸為這一類。化學(xué)性能這一種類包括很多化學(xué)性能。其中包括酸堿性、化學(xué)反應(yīng)性、腐蝕性。其中最重要的是腐蝕性，在外行人看來，腐蝕性被解釋為在某處的零件抵抗腐蝕的能力。機械性能機械性能包括拉伸性能、壓縮性能、剪切性能、扭轉(zhuǎn)性能、沖擊性能、疲勞性能和蠕變。材

26、料的拉伸強度可以通過試件的橫截面積出試件承受的最大載荷得到，這是在拉伸試驗中，應(yīng)力沿Y軸，應(yīng)邊沿X軸變化的曲線。一種材料加載時開始發(fā)生變化的初值取決于負(fù)載的大小。當(dāng)負(fù)載去掉時可以看到變形消失。對于很多材料而言，在達(dá)到彈性極限的一定應(yīng)力值A(chǔ)之前，一直表現(xiàn)為這樣。在應(yīng)力-應(yīng)變圖中，這是可以用線性關(guān)系來描述的。這之后又一個小的偏移。在彈性范圍內(nèi)，達(dá)到應(yīng)力的極限之前，應(yīng)力和應(yīng)變是成比例的，這被稱為比例極限Ap。在這個區(qū)域，零件符合胡克定律，即應(yīng)力與應(yīng)變是成比例的，在彈性范圍內(nèi)（材料能完全恢復(fù)到最初的尺寸，當(dāng)負(fù)載去掉時）。曲線中的實際點，比例極限在彈性極限處。這可以認(rèn)為是材料恢復(fù)初值時落后于前者。這種影

27、響在不含鐵的材料中經(jīng)常提到。鐵和鎳有明顯的彈性范圍，而銅、鋅、錫等，即使在相對低的應(yīng)力下也表現(xiàn)為不完全彈性。實際上，能否清楚地分辯彈性極限和比例極限取決于測量設(shè)備的靈敏度。當(dāng)負(fù)載超過彈性極限時，塑性變形開始，逐漸的試件被硬化。變形比負(fù)載增加得更快時的點被稱成為屈服點Q。金屬開始抵抗負(fù)載轉(zhuǎn)變成快速變形，這時的屈服力成為屈服極限Ay。試件的延伸率 繼續(xù)由Q到T再到，在這種塑性流動時，應(yīng)力應(yīng)變關(guān)系在曲線上處于QRST區(qū)域。在點，試件破壞且這種負(fù)載稱為破壞負(fù)載。最大負(fù)載S除以試件初始的截面積，被定義為這種金屬的最終拉伸極限或試樣的拉伸強度Au。按邏輯說，在應(yīng)力不增加的情況下，一旦超出彈性極限，金屬開始屈服，并最終破壞。但是當(dāng)超出彈性極限后，在紀(jì)錄曲線上應(yīng)增大。這種變化主要有兩個原因：材料的應(yīng)力硬化由于塑性變形而引起的試件橫截面積的變小由于加工硬化，金屬塑性變化越大，硬化越嚴(yán)重。金屬拉伸越長，他的直徑（橫截面積）越小。直到到達(dá)點