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翻译实践第一课----机械工程英语(Lesson 2)
作者:zxlzhou 提交日期:2006-9-16 22:10:00 | 分类: | 访问量:2618

LESSON 2 Properties of Materials
The properties of a material are those characteristics that help modify and distinguish one material from another. All properties are observable and most can be measured quantitatively.Properties are classified into two main groups,physical and chemical properties.Physical properties involve no change in the composition of the material.Chemical properties are associated with the transformation of one material into another.Physical properties are, in turn, arbitrarily subdivided into many categories. These subdivisions bear names such as mechanical,metallurgical,fabrication,general,magnetic,electrical,thermal,optical,thermonuclear,and electro-optical. Regardless of the name of subdivision,physical properties result from the response of the materials to some environmental variable,such as a mechanical force, a temperature change, or an electro-magnetic field. In the following, the mechanical property of materials will be discussed.
材料的特性是变改和区分一种材料和另一种材料的特征。所有的特性都是可见的,而且大多数是可以量化的。材料特性可分为物理特性和化学特性两大类。物理特性指的是材料成分不发生改变的特性。化学特性指的是能够结合一种材料变成另外一种的特性。物理特性,可以被任意地分为许多种类。这些细类名称包括机械特性,冶金特性,装配特性,通用特性,磁性特性,电子特性。这些特性是由于材料在环境变化下形成的反应而产生,比如机械力,温度的改变,电磁场。接下来,我们将要讨论材料的机械特性。
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Mechanical Property of Materials
In selecting a material for a product such as a piston in an internal combustion engine, a designer is interested in properties such as strength, ductility, hardness, or fatigue strength. Mechanical properties are defined as a measure of a material’s ability to carry or resist mechanical forces or stresses. When any matter is at rest, the atomic or molecular structure is in equilibrium. The bonding forces in this structure resist any attempt to disrupt this equilibrium. One such attempt may be an external force or load. Stress results from forces such as tension, compression, or shear that pull, push, twist, cut, or in some way deform or change the shape of a piece of material.
在为一个产品选择材料上,譬如内燃机上的一个活塞,一个设计者感兴趣的特性是材料的长度,延展性,硬度或者疲劳强度等。机械特性是用来衡量材料抵抗机械载荷或者压力性能的指标。当任何材料在静止状态时,原子或分子结构保持着平衡,它们之间的结合力阻止任何破坏这种平衡的意图。这个意图也许是一个外力或外载荷。压力产生于拉伸,压缩,或者拖,推,扭,削之类的剪切,或者某种变形以及材料的部分形状改变。
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Stress and strain Stress is defined as the resistance offered by a material to external forces or loads. It is measured in terms of the force exerted per area. Normal stress is that applied perpendicular to the surface to which it is applied, i.e., tension or compression. Another way of defining it is to say that stress is the amount of force divided by the area over which it acts. An assumption is made that the stress is the same on each particle of area making up the total area. If this is so, the stress in uniformly distributed. The force and the area over which the force acts can be used to calculate the stress produced in the material. With the use of polarized light and models made of photoelastic plastic, it is possible to detect the concentration of stress.
压力是用来定义抵抗材料面对外力和外载荷时的性能指标。它是根据外在受力的每一份面积来衡量的。正应力是垂直作用在材料表面上的,也就是指的拉伸和压缩。另外一种定义是说压力是根据力的作用面积来计算它的数值的。有一种猜想声称,压力是作用在每个粒子大小的面积上的力综合起来构成的整个作用面上的综合力。如果确实如此的话,压力就是均衡分布的。力和力作用的面积可以用来计算出材料内部产生的压力。随着分极光和光弹性塑料模型的应用,很有可能测算出压力的密度。
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Strain, or unit deformation, is defined as the unit change in the size or shape of material as a result of force on the material. Many times, we assume that a solid body is rigid; that is, when the body is loaded with some force, the body keeps its same size and shape. This is far from correct. Regardless of how small the force ,a body will alter its shape when subjected to a force. The change in a physical dimension is called deformation.
应变或者单位变形是材料上的压力造成材料大小和形状的单位改变量。很多时候,我们猜想固体是硬的。也就是说,当受到外在力时,材料机体保持它的大小和形状。其实事实并非如此。无论力有多小,一旦受到压力,机体都会改变它的形状。物理尺寸上的改变就叫做变形。
When a piece of material is subjected to a load, it will not only deform in the direction of the load(axial deformation),but it will also deform in a lateral direction. The ratio of the lateral unit deformation or strain to the unit longitudinal deformation or strain is known as Poisson’s ratio.
当材料的一部分受到载荷时,它不只是在轴向有变形,而且还存在横向变形。横向变形或应变与纵向变形或应变的比值称为泊松比。
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Stress-strain Diagrams The stress-strain diagram is used to determine how a certain material will react under load. Figure 2.1 is a stress-strain diagram for low-carbon steel. Strain values(mm/mm) and stress values(MPa) are plotted respectively along the horizontal axis and the vertical axis.
应力-应变图 应力-应变图是用来表示某一种材料在受力下的反应。图2.1是低碳钢的应力-应变图。应力值和应变值分别分布在水平轴和垂直轴上。
The straight-line portion of the diagram up to almost the yield point is known as the elastic region of the diagram. Within this range of stresses, the material will return to its original dimensions once the load, hence, the nominal stress, has been removed.
从图表中的直线开始上升一直到屈服极限点是弹性变形区域。在这个应力范围内,当载荷,公称应力消除之后,材料会恢复原始尺寸.
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Beyond the yield point, the material will continue to deform, but with less stress than before, because the material has begun to yield. In this region, known as the plastic region, plastic deformation takes place and when the load is removed, the material will not return to its original dimensions.
超过屈服点之后,材料会继续变形,但是因为材料开始屈服,应力会减小。在这个区域,称为塑性变形区域,当载荷消除之后会产生塑性变形,材料无法恢复原来尺寸。
The yield point represents the dividing line or transition from the elastic to the plastic region of the curve. When the stress reaches the yield point, a large increase in strain occurs with no increase in stress. The modulus, modulus of elasticity in tension, or coefficient of elasticity is the ratio of the stress to the strain in the elastic region of the stress-strain diagram. The tensile modulus approximately equal to the compressive modulus of elasticity within the proportional limit. This modulus is an indication of the stiffness of the material when subjected to a tensile load. The stiffness of a material is defined as the ratio of the load to the deformation produced.
屈服点是弹性变形区域到塑性变形区域之间曲线的过渡点。当应力达到屈服点,应变会急剧增大而应力保持不变。模量,拉伸载荷下的弹性模量,也就是应力-应变图中弹性变形区域的应力与应变之比,也称为弹性系数。拉伸载荷模量约等于比例极限内的弹性模量。它反映的是材料在受到拉伸载荷时的硬度。材料的硬度是其所受载荷和产生的相应变形之间的比值。
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Not all materials produce stress-strain diagrams on which there is a clear indication of the start of yielding as the load is increased. Cast iron is an example. This situation should not be interpreted to mean that cast iron does not exhibit elastic properties under moderate loads. The modulus of elasticity for these materials is sometimes taken as the slope of a tangent to the stress-strain curve at the origin.
不是所有的材料都会产生应力-应变图,即当载荷增加时显现出开始屈服的迹象。铸铁就是个例子。这种情况并不代表铸铁在适度的载荷下不会出现弹性性征。有时这些材料的弹性模量表现为应力-应变曲线的原点处的斜切线。
Ultimate Strength of Tensile Strength Ultimate strength or tensile strength is the maximum stress developed in a material during a tensile test. It is a good indicator of the presence of defects in the crystal structure of a metal material, but it is not used too much in design because considerable plastic deformation has occurred in reaching this stress. In many applications the amount of plastic deformation must be limited too much smaller values than that accompanying the maximum stress.
抗拉强度的极限强度
抗拉强度的极限强度是指材料拉伸测试中达到的最大应力 。它揭示了金属的晶体结构中存在的缺陷,但因其在达到最大应力的过程中会产生极大的塑性变形,在设计构造中并不常用。在大多数使用情况中,塑性变形量的数值必须降到远小于伴随其产生的最大应力。
Yield Strength Many materials do not have a yield point. This poses a problem in deciding when plastic deformation begins for such materials. By agreement, a practical approximation of the elastic limit, called the offset yield strength, is used. It is the stress at which a material exhibits a specified plastic strain. For most applications, a plastic strain of 0.002 in./in. can be tolerated, and the stress that produces this strain is the yield strength. This is sometimes expressed as 0.2% strain. The yield strength is determined by drawing a straight line, called the offset line, from the 0.2% strain value on the horizontal axis parallel to the straight-line portion of the stress-strain curve. The stress at which this offset line intersects the stress-strain curve is designated as the yield strength of the material at 0.2% offset.
屈服强度
许多材料没有屈服点。这引出一个如何决定这种材料产生塑性变形时间的问题。因此我们定义了一个实际存在的,弹性极限的近似值,称为残余变形屈服强度。它是一种材料表现其特定塑性应变的塑性应力。大多数应用中,塑性应力在0.002in./in以内的是可以承受的,这个应力产生的应变就是屈服强度。这就是有时被称为0.2%的应变。屈服强度是一条直线,称为偏移线,它是应力-应变曲线中的水平轴上与垂直线部分平行的0.2%应变值。这条偏移线与应力-应变曲线交叉处的应力被定义为材料在0.2%偏移处的屈服强度。

Shear Stress
A second family of stresses is known as shear stress or shearing stress. A shearing force produces a shear stress in a material, which, in turn, results in a shearing deformation. A stress-strain diagram can be plotted using shear stress and shear strain. Such a diagram will show a definite straight-line portion(elastic region)in which the shearing stress is directly proportional to the shearing strain, Like the normal stress-strain ratio, the ratio of the two shear quantities, is known as the modulus rigidity or shear modulus of elasticity.
第二种应力称为剪切应力。剪切力由于在材料中形成剪切变形而产生了一个剪切应力。应力-应变图可以用来表现剪切应力与剪切变形。这个图表中的直线部分(弹性区域)明确表示出剪切应力与其产生的相应的剪切应变。象常规的应力应变比值一样,此两者剪切量之间的比值,被称为弹性剪切模量或者硬度系数。
Ductility
A material that can undergo large plastic deformation without fracture is called a ductile material. A brittle material, on the other hand, shows an absence of ductility. Consequently, a brittle material shows little evidence of forthcoming fracture by yielding, as a ductile material would do. A ductile material, by yielding slightly, can relieve excess stress that would ultimately cause fracture. This yielding can be accomplished without any degradation of other strength properties.
一种可以经受较大的塑性变形而不发生破裂的材料叫做塑性材料。另一方面,脆性材料是没有延展性的。因此,当接近屈服时脆性材料并不会出现即将破裂的迹象,而塑性材料却是这样的。脆性材料通过轻微地屈服可以减少多余的压力,以免形成最终的破裂。在其他强度特性不减退的情况下,这个屈服过程是可以完成的。

Toughness.
The ability or capacity of a material to absorb energy during plastic deformation is known as toughness. The modulus of toughness, equal to the total area under the stress-strain curve up to the point of rupture, represents the amount of work per unit volume of a material required to produce fracture under static conditions. Toughness can also be expressed in terms of the ease or difficulty in propagating a crack. It can be measured by the amount of energy absorbed by a material in creating a unit area of crack. A tough material would have no defects in its microstructure. Impact is defined as a sudden application of a load confined to a localized area of a material. Exemplified by the striking of a material with a hammer, this relatively quick application of force, as opposed to a slow or static loading of a material can cause considerable damage to a material that cannot adequately redistribute the stresses caused by the impact. Ductile materials usually survive impact due to their microstructure, which allows slip to take place. Most metals have good toughness and thus have good impact resistance. Due to their inherent nature as compounds of metals and nonmetals, ceramics do not possess the ability to redistribute stresses and plastically deform. Consequently, they have poor toughness, poor impact resistance, and poor fracture toughness.
韧性/刚性
一种材料在塑性变形中吸取能量的能力或容量称为韧性。韧性模量,等于从
应力-应变曲线图下方上升到断裂点之间的总面积,表现出了静态条件下单位体积的
材料能够产生破裂所需的做功。韧性还表现出破裂减弱和难于扩展的特性。它可以通过材料在其单位面积破裂时能量的吸收量。韧性材料在其微观结构上是没有任何缺陷的。冲击是指施加在材料限定区域上的突然载荷。比如突然在材料上用铁锤施加的击打,这种相关的突然作用的外力,与慢态或者静态施加在材料上的载荷相反,因为不能充分地重新分配冲击所带来的应力而能够引起对材料的巨大破坏。塑性材料是能够承受冲击的,这是由于其微观结构中允许分子滑移。大多数金属具有良好的韧性,因此对冲击具有良好的抵抗能力。由于陶瓷具有金属和非金属混合而成的固有特性,它缺乏对应力和塑性变形重新分配的能力,因此,它们的很差的韧性,冲击抵抗力差和断裂韧度。
Malleability.
Malleability, workability, and formability are some terms related to ductility that describe the ability of materials to withstand plastic deformation without the occurrence of negative consequences as a result of undergoing various mechanical processing techniques. Terms such as weldability, brazability , and machinability, although more properly classified under processing properties, are mentioned here as additional examples of terms used to generally describe the reaction of materials to various manufacturing and/or fabricating processes in industry.
延展性
延展性,可使用性,可成型性是一些与塑性有关的概念,它们用来描述材料在多样的机械加工工艺条件下未出现消极结果的情况下承受塑性变形的能力。尽管存在更加合适的工艺性能分类方式,可焊性,可钎焊性,可加工性作为另外几种概念,在这里可以用来泛指材料在各种不同的工业制造工艺条件下的性能反应。

Flexural or Bending Strength
Figure 2.2 is a sketch of a simple supported beam. The transverse load or force P bends the beam, thus resulting in normal stresses in compression near the top surface and normal stresses(tensile)at the bottom of the beam. Assuming that the beam material is homogeneous and isotropic, the normal stresses will be at a maximum near the top and bottom surfaces of the beam. These normal stresses are known as flexural or bending stresses. The maximum bending stress developed at failure is known as the flexural strength. For those materials that do not crack, the maximum bending or flexural stress is called the flexural yield strength.
弯曲强度
图2.2是一个简单的支撑梁的大致结构图。横向载荷(力)P使得梁产生弯曲,然后产生在靠近顶面部分压力作用的正应力,而在梁的底部产生拉力作用的正应力。假定梁的材料内部分布均匀和等方性的,正应力会在靠近顶部和底部表面达到一个最大值。这些正应力就是弯曲应力。材料弯曲失效时的最大弯曲应力叫做挠曲强度。由于这些材料并没有断裂,这个最大弯曲应力或者挠曲应力被称为弯曲屈服强度。

Fatigue Strength
The fatigue or endurance limit is the maximum stress below which a material can presumably endure an infinite number of stress cycles. Fatigue strength is the maximum stresses that can be sustained for a specified number of cycles without fracture. In other words, fatigue strength can be any value on the ordinate of the stress-cycles diagram. The fatigue limit, as determined empirically, is generally below the yield strength. Most design stresses are lower than the fatigue or yield strength of a material primarily because of the adverse effects of surface conditions on the strength of materials. Another term used in describing failures is endurance ratio, which is the quotient of endurance limit to tensile strength. For many ferrous alloys, the endurance limit is about one-half the tensile strength of the metal.
疲劳强度
疲劳忍耐极限是指材料在趋向无穷多的应力循环下能够承受的最大应力。疲劳强度是指材料特定的应力循环下未断裂可以持续的最大应力。也就是说,疲劳强度可以是应力循环图中纵坐标上的任意值。疲劳极限,经验决定了它大致在屈服强度以下。大多数的材料设计应力都是略小于其疲劳或者屈服强度,主要是由于考虑到材料表面条件的反作用。另外一种表示失效的条件是耐久比,它是耐久极限与抗拉强度的比值。对于许多铁合金来说,耐久极限大致等于金属弹性强度的一半。

Creep(Creep Strength)
Creep is a slow process of plastic deformation that takes place when a material is subjected to a constant condition of loading(stress) below its normal yield strength. After a certain amount of time has elapsed under constant load, the creep strain(plastic deformation) will increase and some materials will rupture, or fracture, is known as creep rupture. Creep occurs at any temperatures. However, at low temperatures, slip is impeded by impurity atoms and grain boundaries. At high temperatures, the diffusion of atoms and vacancies permits the dislocations to move around impurity atoms and beyond grain boundaries, which results in much higher creep rates. Different types of materials have different creep characteristics, dependent on the structure of materials.
抗蠕变强度
蠕变是一种缓慢的塑性变形过程。当材料受到常规屈服强度以下的恒定载荷或应力时,蠕变就会产生。当经受一段时间的恒定载荷之后,蠕变应变(塑性变形)将会加剧,一些材料将会出现破裂或断裂,这就是蠕变破裂。蠕变在任何温度条件下都可以发生。然而,低温下,滑移会被杂质原子和晶体界限所阻碍。高温时,原子的扩散导致空间产生混乱而在杂质原子四处移动以及游离于晶体界限之外,这就大大提高了蠕变产生的几率。不同种类的材料具有不同的蠕变特性,取决于它们的结构。

Torsional Strength
Torsion describes the process of twisting. The torsional stress is the shear stress produced in the material by the applied torque and is calculated using the torsion formula. Torsional yield strength roughly corresponds to the yield strength in shear. The ultimate torsional strength or modulus of rupture expresses a measure of the ability of material to withstand a twisting load. It is roughly equivalent to the ultimate shear strength. The torsional modulus of elasticity is approximately equal to the shear modulus or the modulus of elasticity in shear.
抗扭强度
扭转是扭曲的过程。扭曲应力是在材料受到扭矩时产生的剪切应力,可以通过转矩公式来计算。扭转屈服强度大致相当于剪切屈服应力。断裂时的扭转极限强度或模量描述的是材料受到扭曲载荷时的抗扭能力。它大致等于剪切极限强度。弹性扭转模量接近于剪切模量或者弹性剪切模量。

Hardness
Hardness is measure of a material’s resistance to penetration(local plastic deformation) or scratching. One of the oldest and most common hardness tests, based on measuring the degree of penetration of a material as an indication of hardness, is the Brinell. Brinell hardness numbers(HB) are a measure of the size of penetration made by a 10mm steel or tungsten carbide sphere with different loads, depending on the material under test. The indentation size is measured using a microscope containing an ocular scale. Vickers hardness numbers(HV) employ a diamond pyramid indenter. Rockwell hardness testers, using a variety of indenters and loads with corresponding scales, are direct reading instruments(i.e., the hardness is read directly from a dial).
硬度
硬度是材料抵抗渗透(局部塑性变形)或刮伤的能力。最古老和最普遍的硬度测试,是基于测试材料的渗透度用来表达硬度值,这就是布氏硬度。布氏硬度值(HB)是在一个10mm的钢铁或者硬质合金碳化物球体上施加不同的载荷时测试出来的渗透值。渗透产生的缺口尺寸是由肉眼通过显微镜观察测量出来的。维氏硬度值(HV)使用的是一个角锥状金刚石压痕计测量出来的。洛氏硬度试验机,使用一系列的压头施加相应比例的载荷,然后直接在量具上读取数值(比如,直接从刻度盘上读取硬度值)。



#日志日期:2006-9-23 星期六(Saturday) 晴 送小红花 推荐指数:复制链接 举报



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