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附译文一Classification of Engineering MaterialsMaterials are the matter of the universe. These substances have properties that make them useful in structure, devices, products, and systems. The term properties describe behavior of materials when subjected to some external force or condition. Engineering materials is a term often used loosely to define most materials that go into products and systems. Engineering materials can also have a more specific meaning that refers to materials whose structure has been designed to develop specific properties for a given application.There are nearly a limitless variety of materials. The method to understand them is to consider all materials as members of a big family. Materials that possess common characteristics are placed into their own group within the family. Although overlaps exist in the grouping system, it is easier to understand materials when relationships are identified.Four groups, metallics, polymers, ceramics, and composites comprise the main group; a fifth group of other materials is used for materials that do not fit well into the four groups. Each group then divides into subgroups. Within any system such as transportation, communications, or construction, numerous materials systems exist. These systems must pull materials from the groups with the materials family to meet their special needs. The wise materials user recognizes the many options available because of the compatibility between groups and then proceeds to develop a materials system that meets a specific need.The grouping system used here follows closely grouping systems common to engineering. Slight differences exist in other grouping systems. For example, one system might divide materials into metals, polymers, ceramics, glass, wood, and concrete. Some systems divide all materials into three group: organics, metals, and ceramics. The they distinguish between natural and synthetic materials. Other group materials are as crystalline or amorphous. Regardless of the system, some inconsistencies develop because of the nearly limitless variety of materials. Still, the advantages of grouping outweigh the limitations.MetalsMetallics, or metallic materials, include metal alloys. In a strict definition, metal refers only to an element such as iron, gold, aluminum, and lead. The definition used for a metal will differ depending on the field of study. Chemists might use a different definition for metals than that used by physicists.Metals are elements that can be defined by their properties, such as ductility, toughness, malleability, electrical and heat conductivity, and thermal expansion.Metals are also large aggregations. Metals usually have fewer than four valence electrons, as opposed to nonmetals, which generally have four to seven. The metal atom is generally much larger than the atom of the nonmetal.Alloys consist of metal elements combined with other elements. Steel is an iron alloy made by combining iron, carbon, and some other elements. Aluminumlithium alloys provide a 10% saving in weight over conventional aluminum alloy.While metals comprise about three-fourths of the elements that we use, few find service in their pure form. There are several reasons for not using pure metals. Pure metals may be too hard or too soft, or they may be too costly because of their scarcity, but the key factor normally is that the desired property sought in engineering requires a blending of metals and elements. Thus, the combination forms find greatest use. Therefore, metals and metallics become interchangeable terms. Metals are broken into subgroups of ferrous and nonferrous metals.Ferrous Ferrous metals include iron and alloys of at least 50% iron, such as cast iron, wrought iron, steel, and stainless steel. Each of these alloys is highly dependent on possessing the element carbon. Steel is our most widely used alloy. Sheet steel forms car bodies, desk bodies, cabinets for refrigerators, stoves, and washing machines; it is used in doors, “tin” cans, shelving, and thousands of other products. Heavier steel products, such as plate, I beams, angle iron, pipe, and bar, form the structural frames of buildings, bridges, ships, automobiles, roadways, and many other structures.Non-ferrous Metal elements other that iron are called non-ferrous metals. The nonferrous subgroup includes common lightweight metals such as titanium and beryllium and common heavier metals such as copper, lead, tin, and zinc.Among the heavier metals is group of white metals, including tin, lead, and cadmium; they have lower melting points, about 230 to 330C. Among the high-temperature nonferrous metals are molybdenum, niobium, and tungsten. Tungsten has the highest melting point of all metals: 3400C. Metal alloys other than iron are called nonferrous alloys. The possible combinations of nonferrous alloys are practically endless.Powdered Metals Alloying of metals involves melting the main ingredients together so that on cooling, the metal alloy is generally, a nonporous solid. Powder metal is often sometimes called sintered metal. This process consists of producing small particles, compacting, and sintering. The squeezing pressure with added heat bonds the metal powder into a strong solid. Powdered metals can be ferrous, nonferrous or a combination of ferrous and nonferrous elements with nonmetallic elements.PolymericsPolymeric materials basically are materials that contain many parts. A polymer is a chainlike molecule made up of smaller molecular units. The monomers, made up of atoms, bond together covalently to form a polymer that usually has a carbon backbone. Thousands of polymers join together to form a plastic.Plastics The term plastic is used to define human-made polymer resins containing carbon atoms covalently bonded with other elements, along with organic and inorganic substances. The word plastic also means moldable or workable, such as with dough or wet clay. Plastic materials are either liquid or moldable during the processing state, state, after which they turn to a solid. After processing during the processing state, after which they turn to a solid. After processing, some plastics cannot be returned to the plastic or moldable state; they are thermosetting plastics or thermo-sets. Common thermosetting plastics include epoxy, phenollic, and polyurethane. Other plastics can be repeatedly reheated to return to the plastic state; they are thermoplastics. Examples of thermoplastic plastics are acrylics, nylon, and polyethylene.Wood Of all the materials used in industry, wood is the most familiar and most used. Wood is a natural polymer in the same manner that polymers of ethylene are joined to form polyethylene, glucose monomers polymerize in wood to form cellulose polymers. Glucose is a sugar made up of carbon, hydrogen, and oxygen. Cellulose polymers join in layers with the glue like substance lignin, with is another polymer.Elastomers An is defined as any polymeric material that can be stretched at room temperature to at least twice its original length and return to its original length after the stretching force has been removed. Elastomers are able to store energy, so they can return to their original length and/or shape repeatedly. Elastomers have a molecular, amorphous structure consists of long coiled-up chains of giant molecules that are entangled with each other. Adjacent polymers are not strongly bonded together. When a tensile force is applied, these coils straighten out and snap back like springs to their original coiled condition on removal of the force.Other Natural Polymers A most amazing natural polymer is human skin, which has no equal substitute. Animal skin or hide in the form of fur and leather has limited industrial use because synthetic materials have been developed that offer greater advantages to the designer than those of the natural polymers have. Medical science continues to study such natural polymers as bones, nails, and tissues of human beings and animals to synthesize these materials for replacement when they are damaged due to injury or illness. Bioengineering and biomechanics are newer fields that integrate engineering and medicine to solve material problems in the treatment of humans.CeramicsCeramics are crystalline compounds comining metallic and nonmetallic elements. Glass is grouped with ceramics because it has similar properties, but most glass is amorphous. Included in ceramics are porcelain such as pottery; abrasives such as emery used on sand-paper, refractories such as tantalum carbide, with a melting temperature of about 3870C, and structural clay such as brick. Ceramics, including glass, are hard, brittle, stiff, and have high melting points. Ceramics primarily have ionic bonds, but covalent bonding is also present. Silica is a basic unit in many ceramics. The internal structure of silica has a pyramid unit. These silicate tetrahedrons join chains. The silicon atom occupies the space opening between the oxygen atoms and shares four valence clectrons with the four oxygen atoms. Chains are extremely long and join together in three dimenions. The chains are held together by ionic bonds,whereas individual silica tetrahedrals bond together covalently. Silica is combined with metals such aluminum, magnesium, and other elements to form a variety of ceramic materials.Compostites A composite is a material containing two or more integrated materials, with each material keeping its won identity. Normally combining of the materials serves to rectify weaknesses possessed by each constituent when it exists alone.Most of the groups of the family of materials could be classed as composites because of the way they are placed in service; the composite classification commnly refers to materials developed to meet the demands in building, electronic, aerospace, and auto industries. With an even increasing use of composites, they are truly the material of today and the near future because composites can be designed to be stronger, lighter, stiffer, and more heat resistant than natural materials or to possess properties required by technology are not available in a single material.The subgroups of composites include polymer based, metallic based, ceramic based, cermets, and others. It is also possible to classify composites by their structure. Composite structures include layers, fibers, particles, and any combination of the three. Layered composites consist of lamination alike a sandwich. The laminations are usually bonded together by adhesives, but other forces could be used, such as those provided by welding. Fibers and particles are integrated into composites by suspending them in a matrix or by the use of cohesive forces. The matrix is the material component, such as plastic, epoxy cement, rubber, or metal, that surrounds the fibers or particles. Cohesive forces involve the molecular attraction of one constituent to the other.Other MaterialsThis group is used to include materials that do not fit well into the groups discussed previously. Each major group of our family of materials has some materials from a newly evolving subgroup known as intelligent material systems.Intelligent materials systems, smart materials, or smart structures are materials and material systems designed to mimic biological organisms and offer the ultimate material system that can place control and feedback into a material structure. These materials take their cue from biological elements such as muscles, nerves, and bodily control systems that adapt to environmental changes. Just as alchemists long yearned to turn lead into gold, so too, materials scientists and engineer seek to endow materials with abilities such as: 1) making controlled adaptations to changes in stress or heat, 2) making self repairs, and 3) providing feedback information on conditions that may have caused a materials failure.Biological structures have evolved over the millennia, and we can learn from their traits and have begun to imitate nature with designs of synthetic materials. Some of the design goals for smart materials and intelligent structures follow: Cost-efficient, durable structures whose performances match demands on the structure.Change properties, color, shape, and manner to handle external physical loads to repair damage or make repair from damage.Possess the five senses of smell, taste, hearing, sight, and touch.Allow structures to learn, grow, survive, and age with grace and simplicity.Incorporate adaptive features and intelligence to reduce mass and energy needs.Allow for specification of materials and structural requirements to arrive at designs that are affordable while fulfilling design objectives.工程材料的分类材料是是一个世界性的问题。这些物质有个在结构，机械，设备，产品和系统有用的特点。这个术语是用来描述材料的行为当受到外界情况时。工程材料是一个通常用来不确切的定义大多数关于产品和系统的材料的术语。工程材料也有很多确定的意思是指按照所给要求来设计的形成了特定特性的结构的材料。材料几乎有无限多种。要把它们当作材料大家庭的每一个成员。在材料家簇中按照各自所拥用的共同特性来对它们进行分门归类。仅管是互相交叠存在，但当各自的身份确认后， 是很容易识别的。四组，金属，聚合物，陶瓷和复合材料组成这主要部分；其它材料的五分之一不能很好地容入这四组中。每一组又分成子组，比如运输，通信，或建筑及大量其它现存的材料。这些系统必须提取材料从材料家簇中以满足它们各自的需求。由于材料的相容性使使用者识别这可用的然后以形成一个材料系统以满足一个特别的需求。这些分组系统要严格按照归组系统将之归于工程。在其它组别中会稍存在一点差异。例如，一个系统可能将材料分成金属，聚合物，陶瓷，玻璃，木料和混凝土。有些系统将材料分成三组：有机的，金属的和陶瓷的。然后又区别为自然的和合成的。另外还分成晶体和非晶。至于这系统有一些不一致由于近乎无限种的材料。确实，分组超出了这种限制。金属金属或金属材料包括金属合金。严格来说，金属仅指像铁、金、铝和铅等元素。这些被用作金属的定义差别取决于研究领域。化学家会为金属作不同的定义而不是物理学家。金属元是按照它们的特性来区分，如：可段性，韧性，可延展性，电导性和热传导性及热澎涨性等。金属是一个很大的集合。金属通常只有少于四个电子，正好与有四到七个电子的非金属相反，金属原子通常要比非金属原子要大。合金是由金属和其它非金属元素组成的。钢是由铁、碳和其它元素组成的铁碳合金。我们所用的东西中四分之三是金属，很少有纯铁的。不用纯铁有以下几个原因。纯金属不是太硬就是太软，或者因为缺乏而太贵，但关键因素还是希望金属和元素的融合在工程上。因此，合成是最有用的。所以，金属和金属物就成了可以相互交换的术语。金属还有铸铁和非铸铁之分。含铁的 含铁金属包括铁和至少50%的合金，如铸铁，锻铁，钢和不锈钢。每一种合金属都是取决于含碳量。钢是我们用得最多的合金。板钢组成车身，桌身，冰箱厨，电炉和洗衣机。它还被用在门上，罐盖，架子和成千上万种其它产品。耐用钢产品，比如碟，梁架，三角架，管和杆，用来构成建筑框架，如桥，船，汽车，高速公路和许多其它结构。非铁的 不含铁金属元而不是铁通常被称作不含铁金属。这些不含铁组包括常见的超轻金属，比如钛和铍和常见的重金属比如铜，铅，锡和锌等。在这重金属中有一种白色金属，包括锡，铅和镉；它们有很低的熔点，大约在230度到330度。在这高温下不含铁金属是钼，铌，钽和钨。钨在金属中有最高的融点：3400度。非铁金属合金被称作非铁合金。粉未金属 合金与这主要成份融在一起以便于达到冷却，这些金属通常是一些无孔的固体。粉未金属经常会被替代因为它不能通过合金或铸造零部件或其它成形技术来合在一起。粉未金属有时候叫做熔结金属。这个过程由生产小微粒，接触和消熔几步组成。粉未金属可能是含铁，不含铁的或含铁与不含铁混合元的混合物。聚合物聚合材料基本上是含有许多其它部分的金属。一个聚合物是一个由很多小分子组的链状分子。这个单体由绑在一起形成共用电子对的原子组成因此通常用一个碳基。成千上万个聚合物结合在一起形成了塑料。塑料 塑料这个术语通常是用来定义那些将碳原子与其它元素共用价电子而结合在一起的人造聚合树的，除有机的和无机的东西除外。塑料这个单词也意味着可塑性与可行性，比如面团或湿粘土。塑料在处理时既易液化的也易碎的在处理的过程中。在处理之后有些塑料不能再转化成塑料或可塑状态。它们是热固性塑料。常见的热固性塑料环氧树脂，苯酚的和聚氨基甲酸酯。其它塑料能反复地被加热回到塑料态。它们是热塑性塑料。例如尼龙，丙烯酸纤维和聚乙烯。木料 所有的木都可用在工业上，木料是这最熟悉和最有用的。木料是一种自然的聚合物用同一种方式将乙烯聚合形成聚乙烯，葡萄糖单体聚合形成了纤维素。葡萄糖是由C、H、O组成的。纤维素聚合物就像胶一样一层层结合成另外的聚合物。合成橡胶 合成橡胶被定义为在室温下至少能伸长到它的两倍和当外力撤消后又能回到原状态的一个物质。橡胶能储藏能量，以便于能反复回到它们原始长度和原始形状。橡胶有一个类似于其它聚合材料的分子和晶体结构。这无定形的或无规是的结构包含巨大的彼此缠绕的卷链。相邻的聚合物结合不牢固。当施加了张力后，这些卷链就能被拉直和和折断，而当撤去外力时又能回到原始状态。其它的自然聚合物 一个最令人惊奇的是人造革，没有其它的代用品。动物皮或以兽皮和皮革已经限制了工业的使用因为聚合材料已经被研究出来了，并提供更多优于这些天然聚合物的聚合物。医学继续研诸如自然聚合物像骨，指甲，和人类和动物的组织结构来结这些材料来替代当它们是受到破坏或受伤时。生物力学和生物链是一个更新的领域将工程和医学融合在一起来解决材料问题目在治疗人类时。陶瓷陶是透明的化合物并含有金属和非金属的元素。玻璃被归为陶瓷类因为它有很多类似的特性，但大多数玻璃都是非晶体。包含在瓷中的如陶器，磨料如金刚砂是被用在砂纸上的，难熔的如碳化物，熔点温度大约在3870度左右，结构粘土如砖。包括玻璃在内的陶瓷是硬的，脆的，疆直的和有很高的熔点。陶瓷主要有爱奥尼亚术柱式的架，但也是显露的。氧化硅是许多陶瓷中的基本单元。这氧化硅结构有一个锥体占用这空间在氧原子和共用四个价电子与四个氧原子。链是非常长的和联合成了三维。这链是被容在一起通过捆绑，然而单独的氧化硅四面体通过共价结合在一起。氧化硅与金属联合如铝，镁和其它元素以形成各种陶瓷材料。复合材料一个复合材料就是一个包含有两种或更多种材料，每一种材料都保持有自己的特怀。一般地讲，材料融合可以调整单个材料所具有的不足之处。大多数组材料可以归为复合材料因为这在设备中被替代的方式。复合材料归类通常指的是取决于满足在建筑，电子，航空和汽车工业的要求来定的。随着复合材使用的增加因为复合材料能被设计得更强，更轻，更硬和更具有耐热性比自然材料或拥用单独材料所不拥有的特性。复合材料包括聚合物基, 金属基, 陶瓷基, 金属陶瓷和其他。它们还能根据它们的结构来对它们进行分类。复合材料结构包括层数、纤维、微粒, 或三个中两两组合的任一个。层状复合材料由片状构成就像一个三明治。片层通过胶粘剂粘在一起,除了由焊接所提供的应力。纤维和微粒是悬浮在铸模内部或通过粘合力而在复合材料内部的。铸模是物质组分, 譬如塑料, 环氧水泥, 橡胶, 或金属,在周围的纤维或微粒。粘合力是需要分子之间一个成分与另一个成分之间的吸引力。其它材料这些材料不适合我们先前所讨论过的材料。每个主组材料都有一些涉及最新的子组中的众所周知的智能材料系统。各个主要小组材料我们的家庭有一些材料从最近演变的小群以聪明的物质系统著名。聪明的材料系统、灵敏材料, 或灵敏的结构是材料和被设计的材料系统来仿造生物有机体和提供这最好的材料系统以控制和反馈这材料结构。这些材料能治愈从生物成分譬如肌肉、神经, 和身体控制系统以适应环境的变化。炼金术士长期盼望变成金子, 那么, 材料科学家和工程师寻求资助多性能的材料譬如: 1) 受控以适应力和热的变化, 2) 进行自我修复, 和3) 提供材料无用的反馈信息。生物结构演变了在千年期间, 并且我们能学会从他们的特征和开始仿效自然以综合性材料设计。一些灵敏材料的设计目标和智能结构如下：成本效率，结构的耐用性的性能符合结构要求。改变物产、颜色、形状, 和方式处理外在物理装载修理损伤或由损伤进行修理。拥有五种感觉，口味、听力、视力, 和接触。附译文二Properties of MaterialsThe 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 the 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.Mechanical Property of MaterialsIn 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 materials 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.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 is 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 concentrations of stress.Strain, or unit deformation, is defined as the unit change in the size or shape of material as a result of force of 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.When a piece of material is subjected to a load, it will not only deform in the direction of the load, 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 Poissons ratio.Ultimate Strength or 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 to much smaller values than that accompanying the maximum stress.Yield Strength Many materials do not have a yield point. This poses a problem in deciding when plastic deformation begins 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.Shear Stress A second family of stresses is known as shear stress of 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 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.Ductility A material that can undergo large plastic deformation without fracture is called a 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.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 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.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.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 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.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 stress 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.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 clasped under constant load, the creep strain will increase and some materials will rupture. This rupture, or fracture, is known as creep rupture. Creep occurs at any temperature. 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.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.Hardness Hardness is a measure of a materials resistance to penetration 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. Binell hardness numbers (HB) are a measure of the size of the penetration made by a 10 mm 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.材料的特性这些材料的特性就是那些能修改和区分一个与另一个材料的特性。所有的都是可见和大多数是可定量的测量的。特性被分成成两大类物理类和化学类特性。物理特性不涉及材料的内部结构的变化。化学特性与一种材料转化成另一种材料有关。特理特性又可分成很多种类。如机械学的，冶金学的，制造的，专用的，磁性的，电子的，热学的，光学的，热核的和光电的。至于这子分组的名字，物理特性从这里得到答案的即材料对一些环境的可变性，比如机械应力，温度变化或一个电磁场。在以下讨论材料的机械应力。材料的机械特性在选择材料用来生产内燃机内部的活塞，一个设计者对强度可延展性，硬度或疲劳强度这些特性是非常感兴趣的。材料性能是被定义为用来衡量材料在受到机械外力或应力时的能力。当任何物质都静止时，这原子和分子结构是处于平衡状态。结构中的力将抵抗试图破坏这种平衡的力。这可能是外力或负载。应力来自于如张力，压力或弯曲，那拉力，推力，扭转，剪切力或用某种方式使其变形或改变这块材料的外形。压应力或拉应力 压应力被认为是由外力或负载提供的。是用每平方单位面积的受力来衡量的。通常应力用垂直于平面上的力来算，如张力或压力。另一种定义方法就是将所受的外力平均分到这个面积上。一个假设就是在一个单位面积组成了总面积。如果就是这样，那么应力就是这样被分配。这个力就是用来计算材料所产生的外力。当使用由光弹性塑料组成的模型时，它可能察出这应力的压缩。拉压力是被定义为当材料受外力时它的大小和外形的每单元的变化。我们假设固体是刚性的，那么当物体受负载时，这物体仍然估持原来的大小和形状。事实上，这是完全不正确的。不论这个力是多么小，当受外力时物体总会发生变化。在物理上这种变化就叫变形。当材料受到负荷时，它不仅在负载方向发生变化而且在侧面也会发生变化。这侧面的应力或变形与经向的应力或变形的比值就叫做泊松比。极限强度和拉伸强度 极限强度和拉伸强度是材料在测试中所受的最大的应力时所产生的。它是呈现金属材料的结构的最好的指示剂，但塑料用得不是很多因为当它受到外力时多半是要变形了。在很多应用中大多数塑料变形肯定是被限制得更小比这最大值。屈服强度 许多材料没有屈服点。这问题是在于判断中当塑性变形开始时。按惯例，一个真实的近似弹性极限称为补偿屈服极限通常被用。它就是一材料所显出的特别的塑性应变。大多情况下，塑性变形只能耐0.002应变所产生的应力。它屈服强度是被测出和画在一根直线上，在与水平轴平行的直线部分的应力应变曲线上的线偏移。在与应力应变曲线相交的线上表明这屈服强度。切应力 第二个应力家簇就是切应力。剪切力在材料上将产生剪切应力，接着就会产生剪切变形。应力应变图可能就描绘成剪切应力与剪切变形。这样的图就会显示确定的直线部分在剪切应力与剪切变形。像这通常的应力应变比率，这两个剪切量的比值就是弹性剪切模数。延展性 一种经受了大塑性变形而没有裂缝的就叫做延展性。相反脆性材料正缺少了这点延展性。因此一个脆性材料显示了小迹象当到达屈服点，正如具有延展性材料将表现的那样。一个可延展性材料超过屈服点一点时，将会产生裂缝。这个屈服点能完成而没减少其它的强度。韧性 在材料发生塑性变形时吸收能量的能力就称为韧性。韧性的系数等于在这整个面积上应力应变到破裂点描述在静态情况下材料折断在单位面积上所需做的功。韧性也能用来表述增加碎裂这一方面的难易程度。它也能按照材料在单位面积上破裂所吸收的能量来计算。韧材料没有缺陷在它的微观组织方面。冲击则定义为一个负载突然间施加到材料的局部面积上。用锤子的敲击来举例，与静止的或慢速的相反，以相当快的速度施加一个力到材料上引起相当大的破坏且不能再重新分配在材料上。延展性材料能够很好的存在是因为它们的允许滑移产生的微型结构。大多数的金属都有一个很好的韧性因此都有一个好的抵抗冲击性。由于它们的天生的金属与非金属的混合，陶瓷没有重新分配应力与塑性变形的能力，因此，它们的韧性，抗冲击性和破裂韧性是很差的。可塑性 可塑性，可加工性，可成形性是指可延展性的术语，即当材料经受各种机械加工之后而没有发生塑性变形的能力。正如可焊性，可钎焊性，和机械性能，尽管按照加工性能更适合分等类，它们被记载在这儿作为术语的补充例子，即这些通常用来描述这些材料在经过各各生产和制造过程之后的的反应。疲劳强度 疲劳极限就是假定情况下材料经受无限次的应力循环后的最大应力。疲劳强度就是在一个特定值的多次循环下而没有产生裂缝的最大应力。换句话说，疲劳强度可能是应力循环图内的任何一个值。大多设计时应力应比疲劳强度与屈服强度更低因为对材料表面情况有不利的影响。另外一个描述疲劳的词就是耐用性，就是对拉伸强度的耐用性。对于许多含铁的合金，这持久性金属拉伸强度的一半。蠕变（蠕变强度） 蠕动是发生塑料变形的一个缓慢的过程当材料有待于装货重音的恒定的条件在它的正常出产量之下。在某一时间扣紧了在恒定的装载之下之后, 蠕动张力将增加并且一些材料将爆裂。这破裂, 或破裂, 为人所知当蠕动破裂。蠕动发生在任一个温度。但是, 在低温, 滑动由杂质原子和晶界妨碍。在高温, 原子扩散和空位允许脱臼行动在杂质原子附近和在晶界之外, 导致更高的蠕动率。不同的类型材料有不同的蠕动特征, 依赖于材料结构。扭转强度 扭力描述扭转的过程。扭转力重音是抗剪应力导致在材料由应用的扭矩和被计算使用扭力惯例。扭转力出产量大致对应于出产量在剪。破裂的最后扭转力力量或模数表达材料的能力的措施承受扭转的装载。它与最后切变强度是大致等效的。扭转力弹性模量与剪模数或弹性模量是近似地相等的在剪。硬度 坚硬是材料的抵抗的措施对渗透或抓的。最旧和最共同的坚硬测试的当中一个, 根据测量程度材料的渗透作为坚硬的征兆, Brinell 。Binell 坚硬数字(HB) 是渗透的大小的措施由用不同的装载10 毫米钢或碳化钨球形做, 根据材料在测试之下。凹进大小被测量使用显微镜包含一个视觉标度。Vickers 坚硬numbers(HV) 使用金刚石金字塔indenter 。罗克韦尔坚硬测试器, 使用各种各样的indenters 和装载与对应的标度, 是直接读书仪器。附译文二Selecting MaterialsWe are surrounded by materials and we rarely think about how these materials are selected. Why was your desk made of solid wood, plywood, or plastic-laminated particleboard? Why have so many plastics replaced steel and zinc in automobiles? What is the controversy about using foamed polystyrene plastic to package fast food?While you might take granted the materials that make up your products, you can be sure that the designers did not. People who design homes, cars, aircraft, clothing, furniture, and other products or systems devote a lot of attention to the selection of the materials they use. Material selection might make or break a company. But how do the designers make that selection to arrive at the best material? What selection criteria are most important?The Ideal MaterialWhat is an ideal material? Among other characteristics, we can list the following for the ideal material:1. Endless and readily available source of supply2. Cheap to refine and produce3. Energy efficient4. strong, stiff, and dimensionally stable at all temperatures5. Lighweight6. Corrosion resistant7. No harmful effects on the environment or people8. Biodegradable9. Numerous secondary usesIt is a very complex process for the designer to find the ideal material for a specific product.Obstacles to ChangeSwitching from traditional materials such as steel and concrete to newer materials such as plastic-based composites seem a simple, straightforward approach for the contemporary designer. The newer materials are often superior, but sometimes there are complications. Often, lack of experience with new materials causes hesitation by designers. Departures from tried-and-true materials may be costly. It requires time before both designers and fabricators gain sufficient experience to make products or systems. This problem is exacerbated when human life might be in jeopardy, such as when designing for aircraft. Consequently, new materials and processes are usually slower to enter the marketplace than might be expected. These issues are all a part of engineering problem solving. Materials selection is a problem-solving issue that requires an algorithm for its solution.Algorithm for Materials SelectionEngineering requires clearly stated, unambiguous steps for problem solving. Algorithms are well-defined methods for solving specific problems. Computer programs are written after an algorithm has been developed to lie out clearly the steps that the program is to solve. For example, you could write a simple algorithm to calculate the strength required of a light pole to withstand the pushing forces from a light fixture. A much more complex algorithm would be required to select a piston connecting rod for an internal-combustion engine. The first problem requires only the selection of a material of suitable size/strength to hold up the light fixture, and almost any material would suffice as long as it was sufficiently strong and pleasing to the user. On the other hand, a connecting rod will undergo many types of mechanical stress, ranging from compressive to tensile to torsional to gravity forces, in addition to thermal stress from the combustion chamber. How does the designer match component requirements with available materials?Selection Tools To aid in the creation of materials selection algorithms, databases must be available to answer questions on material suitability. A materials database involves tables listing properties of materials, such as tensile strength, hardness, corrosion resistance, and the ability to withstand heat. Thousands of reference books are available with such data. Much of these data are computerized to allow easier access. Certain graphical techniques aid the designer in materials selection.Properties of Materials Periodicals can provide current data and performance criteria that involve that involve structural materials:1. Strength (tensile, compressive, flexural, shear, and torsional)2. Resistance to elevated temperatures3. Fatigue resistance (repeated loading and unloading)4. Toughness (resistance to impact)5. Wear resistance (harddness)6. Corrosion resistanceSuch publications present values for the performance criteria (properties) for metals, polymers, and ceramics, with updates on newer materials such as aramid fibers, zinc aluminum alloys, and super alloys. Various periodicals have annual materials selectors that provide general information on properties for a long list of materials.The many tables, covering representative materials, provide general data on properties for a simple comparison. Selection of specific materials requires many more detailed specifications. General databases from handbooks will provide much detail, but the final selection often requires that material manufacturers supply their own properties database for their product lines. While databases are imperative in the initial selection steps, there are other factors that complicate materials selection.Materials Systems Materials rarely exist in isolation without interacting with other materials. Rather, a combination of materials is selected to complement one another. In a successful materials system, each component is compatible with the others while contributing its distinctive properties to the overall characteristics of the system of which it is a part. A state-of-the art telephone is a good example. The casing might be a tough ABS plastic, which houses a microchip (a solid-state ceramic device) that provides memory and sound-transmission capabilities. Copper leads join the circuitry together. There might be a battery and a ceramic must light-emitting diode to show when the battery is low. The acid in the battery must be isolated to prevent corrosion, and the copper leads must be insulated so that they do not short out. Each component is made of materials that meet the demands of the physical and chemical environment normally encountered when using the system.Additional Selection Criteria Existing specifications have a lot of influence on the choice of material. These specifications or “standards” are used when redesigning an improved model of the product. When the materials-selection algorithm results in selection of a new material, I might not be covered by current specifications from standardization agencies. The conditions of safety must be met by those involved in the manufacture or use of goods and services. It might take the new material or they might not approve its use.Availability is another concern of the designer. Will the material be easily available in the quantities and sizes required by the production demands? In addition, will it be available in the shapes required? Aluminum extrusions, for example, are available in many varieties of standard shapes, such as round, oval, and square tubing. In the past, designers were limited to existing materials such as metal alloys, woods, or concrete. Now, it is possible to start from scratch at the synthesis stage to have materials engineers design a materials system to provide properties to meet the expected needs.Processibility, the ease with which raw materials can be transformed into a finished product, is of paramount concern. Much of the current focus is on low-energy processing. Companies may have difficulty processing the new material on existing equipment. Can they afford to invest in new equipment? Today, the reverse question is usually asked: Can we afford to use the new material and process? If we do not, the competition might make the change and run us out of business with their superior product. Many new technologies are now available.Quality and performance are two aspects that achieve consumer satisfaction. The high cost of most durable goods and the competition for customer acceptance has resulted in extended warranties. Materials selection must ensure that parts will not rust, break under repeated stress, or fail to perform in any other way for the predicted service life of the product.Consumer acceptance includes many factors beyond excellent quality and high performance; there are also societal aspects. Society as a whole as well as governmental agency is requiring a closer look at manufactured products. Any product has to be considered in terms of its total life cycle. What are the results of the processing methods? Are polluting gases being released into the environment, or are toxic metals and chemicals being flushed into our rivers and streams? During use, does the product safeguard our health? At the end of the products useful life, how can it be disposed of safely? Municipal solid waste is a hidden product cost that we pay in the form of higher taxes and poorer quality of life. Fast-food restaurant chains moved away from polystyrene packages because the public felt these plastic containers were more harmful to the environment than paper packaging. Soft-drink manufacturers are moving toward reusable plastic bottles.Design for disassembly has become a theme in much of product design by major corporations. Europe, which has a higher degree of ecological concern, has led the way. With the desire to facilitate recycling, manufacturers of small appliances and durable goods are establishing procedures to ensure that products can be broken into components for easy sorting prior to recycling. Among the procedures are reducing the variety of plastics, adding labels to plastics for easy identification of plastic type, and eliminating screws and adhesives so that parts will disassemble easily.One of the latest software programs is designed specifically to make products easier to fix. Known as Design for Service, this program takes its place alongside previous software programs called Design for Assembly and Design for Manufacturability. This new program helps product designers consider repair issues early in the design stage. Objectives of the program include making repairs less costly and extending the functioning life of products. Environmental issues such as recycling are directly addressed by this new computer software, which may have customers fixing products rather than tossing them out. In addition, this software augments previous software that addresses the need for disassembly of a product for whateve
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