材料科学概论复习题答案不完整版

科学概论复习题不完整版 一、名词解释 Reaction bonding 反应粘合 A ceramic processing technique, by which a shape is made using one material that is later converted into a ceramic material by chemical reaction and sintering. Materials science and engineering 材料科学与工程 is an interdisciplinary field concerned with inventing new materials and improving previously known materials by developing a deeper understanding of the microstructure-composition-synthesis-processing relationships. Structure 结构 means a description of the arrangements of atoms or ions in a material Composition 成分 means the chemical make-up of a material. Trans-granular (晶内的，穿晶的) Fracture 穿晶断裂 fracture in which cracks would go through the grains. Ductile Fracture 韧性断裂 In metals that have good ductility and toughness, ductile fracture normally occurs in a trans-granular method.Often, a considerable amount of deformation-including necking is observed in the failed component Glass Intermediates 玻璃中间体 Oxides that, when added to a glass, help to extend the glassy network; although the oxides normally do not form a glass themselves. shot peening 喷丸加工 A process in which metal spheres are shot at a component.This leads to a residual compressive stress at the surface of a component and this enhances fatigue life. Flexural modulus 弯曲模量 The modulus of elasticity calculated from the results of a bend test, giving the slope of the stress-deflection(偏转) curve Tempering 回火 A low –temperature heat treatment used to reduce the hardness of martensite (马氏体)by permitting the martensite to begin to decompose to the equilibrium phase. Stress 压力 Force or load per unit area of cross-section (横界面) over which the force or load is acting Strain 张力 Elongation (延长) change in dimension per unit length. Young’s modulus 杨氏模量 The slope (斜率) of the linear (线性的) part of the stress-strain curve in the elastic (弹性 的) region (区域的), same as modulus of elasticity (弹性 系数). Synthesis 合成 is the process by which materials are made from naturally occurring or other chemical reaction Processing 加工 means different ways for shaping materials into useful components or changing their properties. Aspect ratio 长宽比 The length of a fiber divided by its diameter. Delamination 分层 Separation of individual plies of a fiber-reinforced composite. Ductile to brittle transition temperature (DBTT) 延脆性过渡温度 The temperature below which a material behaves in a brittle manner in an impact test Creep rate 蠕变速率 The rate at which a material deforms when a stress is applied at a high temperature Rupture time 断裂时间 The time required for a specimen to fail by creep at a particular temperature and stress Fatigue life 疲劳寿命 The number of cycles permitted at a particular stress before a material fails by fatigue Endurance limit 疲劳极限 An older concept that defined a stress below which a material will not fail in a fatigue test. Tape casting 流延成型 A process for making thin sheets of ceramics using a ceramic slurry consisting of binders (粘结剂), plasticizers(可塑剂), etc. The slurry is cast with the help of a blade(刀片) onto a plastic substrate. True stress 实际应力 Fatigue strength 疲劳强度 The stress required to cause failure by fatigue in a given number of cycles, such as 500 million cycles. Notch sensitivity 切口灵敏度 Measures the effect of a notch, scratch (剐擦敏感度)-, or other imperfection(不完整性, 非理想性)on a material’s properties, such as toughness or fatigue life Linear polymer 线型高分子 Any polymer in which molecules are in the form of spaghetti-like chains(类似与意大利细面条). Brittle stress-rupture failures 脆性应力断裂失效 usually show little necking and occur more often at smaller creep rates and high temperatures. Creep 蠕变 a time dependent, permanent deformation at high temperature, occurring at constant load or constant stress Stress-corrosion 应力腐蚀 A phenomenon in which materials react with corrosive chemicals in the environment leading to the formation of cracks and lowering of strength Rule of mixtures 混合定律 The statement that the properties of a composite material are a function of the volume fraction of each material in the composite True porosity 真实孔隙率 The percentage of a ceramic body that is composed of both closed and interconnected porosity Electrical Contacts 电触头 Materials, used for electrical contacts in switches and relays(继电器) must have a good combination of wear resistance and electrical conductivity. Polymers 高聚物 these materials are made up of many (poly) identical chemical units (mers) that are joined together to construct giant molecules (巨大的分子). Tensile toughness 断裂韧性 the energy absorbed by a material prior to fracturing(断裂) Ductility 延展性 the amount of deformation that a material can withstand without breaking True strain \真实形变 The strain calculated using actual and not original dimensions, given by et = ln(l/l0) =ln(A/A0) Glass-ceramics 微晶玻璃 are crystalline materials that are derived from amorphous glasses. Usually, glass-ceramics have a substantial level of crystallinity (~>70-99%). Addition Polymerization 加成聚合反应 Process by which polymer chains are built up by adding monomers (单体) together without creating a byproduct(副产品). Condensation Polymerization 缩聚反应 A polymerization mechanism in which a small molecule (e.g., water, methanol, etc.) is condensed out as a byproduct Deflection temperature 变形温度 The tamperture at which a polymer will deform a given amount under a standard load. Degree of Polymerization 聚合度 The average molecular weight of the polymer divided by the molecular weight of the monomer. Injection molding 喷射造型法 A processing technique inwhich a thermoplastic mass (loaded with ceramic powder) is mixed in an extruder-like setup and then injected into a die to form complex parts. In the case of ceramics, the thermoplastic is burnt off. Conchoidal fracture (贝壳状的断裂) Fracture surface containing a very smooth mirror zone near the origin of the fracture, with tear lines comprising the remainder of the surface. Poisson’s ratio 泊松比 longitudinal elastic deformation (纵向变形) to the lateral deformation (横向变形) occurring at the same time by a tensile or compressive stress. Bulk density 体积密度 The mass of a ceramic body per unit volume, including closed and interconnected porosity Powder metallurgy 粉末冶金 Powder processing routes used for converting metal and alloy powders into useful shapes Tempered glass 钢化玻璃 is produced by quenching(淬火) the surface of plate glass with air causing the surface layers to cool and contract. When the center cools, its contraction is restrained by the already rigid surface, which is placed in compression Glass temperature 玻璃化温度 A temperature below which an otherwise ductile(延展性) material behaves as if it is brittle Glass formers 玻璃形成体 Oxides with a high-bond strength that easily produce a glass during processing Intermediates 中间体 Oxides that, when added to a glass, help to extend the glassy network; although the oxides normally do not form a glass themselves. stiffness 刚度 A qualitative measure of the elastic deformation produced in a material . Fracture mechanics 断裂力学 The study of a material’s ability to withstand stress in the presence of a flaw Synthesis 合成 is the process by which materials are made from naturally occurring or other chemicals Apparent porosity 虚表孔率 The percentage of a ceramic body that is composed of interconnected porosity True porosity 真实气孔率 The percentage of a ceramic body that is composed of both closed and interconnected porosity Vulcanization 橡胶的硫化 Cross-linking elastomer chains by introducing sulfur or other chemicals. Degradation temperature 降解温度 The temperature above which a polymer burns, chars, or decomposes Thermoplastic elastomers 热塑性弹性体 These are polymers (thermoplastics or lightly cross-linked thermosets) that have an elastic(弹性形变) deformation > 200%. Unsaturated bond 不饱和键 The double- or even triple-covalent bond joining two atoms together in an organic molecule. Functionality The number of sites on a monomer at which polymerization(聚合作用) can occur Branched polymer 支链型高分子 In a branched polymer, there are primary polymer chains and secondary offshoots(分支) of smaller chains that stem from these main chains. Crystallinity 结晶度 refers to the degree of structural order in a solid Tacticity 立构规整度 Describes the location in the polymer chain of atoms or atom groups in non-symmetrical monomers(非对称性单体). Necking 颈缩 Localized deformation(局部变形) of a ductile material during a tensile test produces a necked region(收缩区). Elastic limit 弹性极限 the critical stress(临界应力) value needed to initiate plastic deformation(开始塑性变形) or dislocation motion (位错运动). Tensile strength 抗张强度 the stress at which Necking begins in ductile materials. Stress-corrosion应力腐蚀 A phenomenon in which materials react with corrosive chemicals in the environment leading to the formation of cracks and lowering of strength Chevron pattern (回纹状图案)- A common fracture feature produced by separate crack fronts propagating at different levels in the material Climb 爬移 Movement of a dislocation perpendicular(垂直的) to its slip plane by the diffusion of atoms to or from the dislocation line Impact energy 冲击能量 The energy required to fracture a standard specimen when the load is applied suddenly Impact toughness 冲击韧性 Energy absorbed by a material, usually notched(有凹槽的), during fracture, under the conditions of impact test Particulate composites Rule of mixtures 混合物的颗粒混合法则 The statement that the properties of a composite material are a function of the volume fraction of each material in the composite Relaxation time 松弛时间 A property of a polymer that is related to the rate at which stress relaxation occurs. Crack Growth Rate 裂纹扩展速率 the rate of propagation of a crack Copolymer 共聚物 Macrohardness 宏观硬度 Overall bulk hardness of materials measured using loads >2 N. Microhardness 微观硬度 Hardness of materials typically measured using loads less than 2 N using such test as Knoop (HK 努普硬度). Nano-hardness 纳米硬度 Hardness of materials measured at 1-10 nm length scale using extremely small (~100 µN) forces Shear modulus 剪切模量 (G，剪切模量) - The slope of the linear part of the shear stress-shear strain curve Viscosity(h 粘度) – Measure of resistance to flow, defined as the ratio of shear stress to shear strain rate (units Poise or Pa?s). Strain rate 应变速率 the rate at which strain develops Load. 载荷 The force applied to a material during testing. Thermoplastics 热塑性塑料 Linear or branched polymers in which chains of molecules are not interconnected to one another. Engineering stress 工程应力 The applied load, or force, divided by the original(原始的) cross-sectional(交叉的) area of the material. Engineering strain 工程应变 The amount that a material deforms per unit length in a tensile test Geometric isomer (几何学异构体)- A molecule that has the same composition as, but a structure different from, a second molecule. Cross-linking 交联 Attaching chains of polymers together to produce a three-dimensional network polymer. The Rule of Mixtures in Fiber-Reinforced Composites纤维增强复合材料的混合法则 As for particulate composites, the rule of mixtures always predicts the density of fiber-reinforced composites: where the subscripts m and f refer to the matrix and the fiber. Note that fm = 1 -ff. 二、英译汉 1. Ceramic materials are generally composed of at least two elements: one (or more) a metallic element and one non-metallic. Metal oxides (AlO, ZrO, FeO, etc.) are common examples of ceramics, but other compounds 232 such as carbides and nitrides are also included. The most common elements in the earth‘s crust are silicon and oxygen, and silica (SiO) is the most common 2 component of ceramics and glasses even though for most purposes we would not consider Si a metallic element. The type of chemical bonds : covalent bond, ionic bond and van der waals bond. 陶瓷材料一般是由至少两个元素:一个(或多个)金属元素和非金属。金属氧化物(氧化铝、氧化锆、FeO说等等)是常见的陶瓷的例子,但其 他化合物碳化物和氮化物等也包括在内。 硅和氧是地壳中最常见的元素,石英(二氧化硅)是最常见的陶瓷和玻璃的成分,尽管在大多数情况下我们不会考虑硅是一种金属元素。化学 键的类型:共价键、离子键和范德瓦耳斯键。 2 The role of materials in the advance of civilization and culture is powerfully summarized by the fact that it is the name of each dominant new material that has been used to describe the culture - the Stone Age, the Bronze Age, the Iron Age, and so forth. 材料在文明和文化的进步中的作用可以有力地被概括以这个的事实:每一个占主导地位的新材料的名称已经被用于描述当时的文化——石 器时代,青铜时代,铁器时代,等等。 3. Metallic fibers, glass fibers, and polymer fibers can be formed by drawing processes, Boron, carbon, and ceramics are too brittle and reactive to be worked by conventional drawing processes. Boron fiber is produced by chemical vapor deposition. Carbon fiber is made by carbonizing, or pyrolizing an organic filament, which is more easily drawn or spun into, continuous lengths. The organic filament, known as a precursor is often rayon, polyacrylonitrile, or a pitch. 金属纤维，玻璃纤维和聚合物纤维可以通过回火加工来形成，硼，碳和陶瓷由于高脆性和化学活性而 不能在传统的回火加工中工作。硼纤维是通过化学气相沉积法来生产。 碳纤维是通过碳化和热解有机纤维来生产的，这种更容易回火或者纺丝成连续的长度。这种有机 纤维就是人们熟知的前驱体，通常是人造纤维，聚丙烯腈，或者是沥青。 4 Materials Science and Engineering forms the bedrock for the engineering disciplines because the structures, components, and devices that engineers design and use must be made out of something, and that is a material. The properties of the materials that are available define and limit the capabilities that the device or structure an have, and the techniques that can be used to fabricate it. c 材料科学与工程构成了工程学科的根基，因为结构，成分，和由工程师设计和使用的设备必须由某些 东西所组成，这个就是材料。 材料的性能可以定义和限定为设备或结构拥有的性能，以及可以运用于制造的技术。 5 The mechanical behavior of materials is described by their mechanical properties, which are measured with idealized, simple tests. These tests are designed to represent different types of loading conditions. The properties of a material reported in various handbooks are the results of these tests. Consequently, we should always remember that handbook values are average results obtained from idealized tests and, therefore, must be used with care. 材料的力学特性用它们的机械性质来描述，而机械性质是用理想化的，简单的测试来测量的。这些测 试被设计成代表不同荷载条件的类型。被各种各样的手册记录的材料性质就是这些测试的结果。总之，我 们必须记住，这些手册上的数值是从理想化的测试得出来的平均值，因此，我们必须小心使用它们。 6 Microstructure includes structure at dimensions ranging from the atoms in the material and the order (or lack of it) of their arrangement, up to the tiny grains of individual crystals that pack together to form most solids, and even up to the nearly macroscopic level of fibers in paper, sand in concrete, and the thin, multiple layers of plastic, metal, and paper in a microwave popcorn bag. 微观结构包括结构尺寸从材料的原子层面和它们排列的顺序(或者没有这个)到结合在一起来构成大多数 固体材料的单晶的微小晶粒，甚至到纸张上的纤维，混凝土上的沙粒的接近宏观层面，以及在微波爆米花 袋上的塑料，金属和纸张的薄的多层的层面。 7 Toughness refers to the ability of materials to absorb energy before they fracture. Tensile toughness is equal to the area under the true stress-true strain curve. The impact toughness is measured using the impact test. This could be very different from the tensile toughness. Fracture toughness describes how easily a crack or flaw in a material propagates. The plane strain fracture toughness K is a common result of these tests. IC 韧性指的是在材料断裂之前，材料所吸收能量的能力。拉伸韧性等于在真实应力与真实应变曲线下的区域。 冲击韧性是用冲击实验来测量的。这和拉伸韧性十分不同。断裂韧性描述裂纹或者缺陷在材料中扩散的难 易程度。平面应变断裂韧性K是这些实验的普遍结果。 IC 8 Materials may be fabricated into objects in a variety of ways, including casting into a mold, machining to remove extra material, joining parts (e.g., by soldering or welding), forming (forging, rolling, bending, etc.), or compacting particles which are then fused together (sintering, used for both metal powders and most ceramics). These operations also modify the final microstructure and must be taken into account. 材料可能通过很多方法来制造成物体，包括铸到模具中，加工去掉多余的材料，通过焊接连接部件，通过 锻造，旋转，弯曲等成型，或者压实微粒成熔融状态，例如烧结，可用在金属粉末和大多数的陶瓷。这些 操作也会修饰最终的微观结构，必须加以考虑。 9 When a material is designed for a given application, a number of factors must be considered. The material must acquire the desired physical and mechanical properties, must be capable of being processed or manufactured into the desired shape, and must provide an economical solution to the design problem. Satisfying these requirements in a manner that protects the environment – perhaps by encouraging recycling of the materials – is also essential. 当一种材料要设计成给定的用途时，很多因素必须要考虑到。这种材料必须获得想要的物理和力学性能， 必须可以加工或者生产成需要的形状，和必须提供一个经济性的来解决设计上的困难。以保护环境的 形式满足这些要求或者是通过鼓励这种材料的循环利用也是必要的。 10 The properties of these various classes of materials are usually rather distinct. For instance, metals are required to light and reflective. They are (usually) ductile, meaning that they can be bent before they break. They are electrically and thermally conducting. On the other hand, ceramics and glasses are usually brittle, can be transparent to light, and are good insulators. They are particularly useful at high temperatures or in corrosive environments, since they retain their properties. Most polymers, on the other hand, cannot withstand high temperatures. Most are insulators, and many are highly deformable (which is the real meaning of the word, "plastic"), and some have unique elastic properties (rubber bands). Semiconductors, of course, are distinguished by their electrical behavior. All of these property characteristics, and the reasons they exist, are discussed in some detail in the chapters that follow. 这些各种各样材料的种类的性质通常是相当明显的，例如，金属能够吸收光和反射光，它们通常是有延性 的，意味着它们在断裂之前可以弯曲。它们能够导电导热。 另一方面，陶瓷和玻璃通常是脆性的，可以对光透明，是良好的绝缘体。它们特别在高温或者腐蚀性的环 境中使用，因为它们可以保留它们的性质。 另外的，大多数的聚合物不能承受高温。大多数是绝缘体，很多是可以高度变形的(这就是“塑料”这个 词的真实含义)，和一些有独特的弹性性能(橡皮筋)。 当然，半导体可以从它们的电学行为来区别出来。它们全部的性能特点和它们存在的原因将被在下面的章 节中详细讨论。 11 Elastomers are thermoplastics or lightly cross-linked thermosets that exhibit greater than 200% elastic deformation. Chains are eventually cross-linked using vulcanization. The cross-linking makes it possible to obtain very large elastic deformations without permanent plastic deformation. Increasing the number of cross-links increases the stiffness and reduces the amount of elastic deformation of the elastomers. 弹性体是热塑性塑料或轻度交联热固性材料,表现出弹性变形大于200%。链最终通过硫化交联。交联可以使它在没有永久塑性变形的条件下 获得非常大的弹性变形。增加交叉连接的数量提高刚度和降低弹性体的弹性变形量。 12 The factors that affect a material’s resistance to crack propagation : A The size of a flaw Larger flaws reduce the permitted stress. Special manufacturing techniques, such as filtering impurities from liquid metals and hot pressing or hot iso-static pressing of powder particles to produce ceramic or superalloy components reduce flaw size and improve fracture toughness. B eformation ability The ability of a material to deform is critical. In ductile metals, the material near the tip of the flaw can deform, causing the tip of any crack to become blunt, reducing the stress intensity factor, and preventing growth of the crack. Increasing the strength of a given metal usually decreases ductility and gives a lower fracture toughness. Brittle materials such as ceramics and many polymers have much lower fracture toughness than metals. C thickness or rigidity of a sample Thicker, more rigid pieces of a given material have a lower racture toughness than thin materials. D strain rate Increasing the rate of application of the load, such as in an impact test, typically educes the fracture toughness of the material. E service temperature Increasing the temperature normally increases the fracture toughness, just as in the impact test. F grain size and defect density A small grain size normally improves fracture toughness, whereas more point defects and dislocations reduce fracture toughness. Thus, a fine-grained ceramic material may provide improved resistance to crack growth. G tensile or compressive stress In certain ceramic materials we can also take advantage of stress-induced transformations that lead to compressive stresses that cause increased fracture toughness. 以下是影响材料抵抗裂纹扩展能力的因素: A 缺陷的尺寸 大的缺陷会降低许用应力。特殊的生产技术，例如从液态金属中过滤杂质，粉体颗粒通 过热压或者热等静压来生产陶瓷和超耐热不锈钢的部件来减小裂纹尺寸并提高断裂韧性。 B热变形温度的能力 材料的变形能力是至关重要的。在韧性金属中，缺陷附近的材料可以变形，因为 任何裂纹的尖端会变钝从而减小了应力强度因子，阻碍了裂纹生长。增加一个给定的金属的强度通常降低 它的延展性和断裂韧性。脆性材料比如陶瓷和许多高聚物拥有比金属低得多的断裂韧性。 C样品的厚度和刚度 给定材料越厚，越硬，比薄的材料拥有更低的断裂韧性。 D应变速率 增加施加负载的速率，比如在一次冲击测试中通常降低材料的断裂韧性。 E使用温度 就像在冲击测试中，提高温度通常提高材料的断裂韧性。 F晶粒尺寸和缺陷密度 通常小的晶粒尺寸提升断裂韧性，而更多的点缺陷和位错减少断裂韧性。因此， 细粒度的陶瓷材料材料可以增加断裂韧性。 G 拉伸或压缩应力 在某些陶瓷材料中我们也可以利用提高断裂韧性的压应力所诱发的转变的优 点。 13 Alumina (A1O) is used to contain molten metal or in applications where a material must operate at high 23 temperatures, but where high strength is also required. Alumina is also used as a low dielectric constant substrate for electronic packaging that houses silicon chips. One classical application is for insulators in spark plugs. Some unique applications are also being found in dental and medical use 氧化铝被用来容纳熔融金属或是材料必须在高温下工作的应用，但是高强度也是必需的。氧化铝也通 常用来当做一种用于硅芯片里面的电子封装的低介电常数的基底。一种传统的运用是火花塞的绝缘体。一 些独特的应用也被发现在牙科和医学运用。 14 Many factors must be considered when designing a fiber-reinforced composite, including the length, diameter, orientation, amount, and properties of the fibers; the properties of the matrix; and the bonding between the fibers and the matrix. Fiber can be short, long or even continuous. Their dimensions are often characterized by the aspect ratio I/d, -4where I is the fiber length and d is the diameter. Typical fibers have diameters varying from10 ,m (10x10cm) to -4150 ,m (150x 10 cm). 当设计纤维增强复合材料时，许多因素都必须要考虑到，这包括纤维的长度、直径、取向、数量和性能; 基底的性能;以及纤维和基底之间的键合作用。 纤维可以很短，很长甚至是连续的。他们的尺寸通常以I/d来表征，I指纤维的长度，d指直径。典型的纤 维有直径范围从10微米((10 x10-4cm)到150微米(150 x10*4)。 15. The impact test describes the response of a material to a rapidly applied load. The Charpy and Izod tests are typical. The energy required to fracture the specimen is measured and can be used as the basis for comparison of various materials tested under the same conditions. In addition, a transition temperature above which the material fails in a ductile, rather than a brittle, manner can be determined. 材料的冲击试验描述了迅速外加负载材料的反应。简支梁和悬臂梁测试是经典的。断裂试样所需的能量被 测量并且被用来当做在相同条件下各种材料测试比较的基础。此外，转变温度高于材料的韧性失效,而不 是脆的,可以确定。 16 Examination of the fracture surface at a high magnification-perhaps using a scanning electron microscope-reveals a dimpled(酒窝状的) surface. The dimples are traces of the microvoids(微孔) produced during fracture. Normally, these microvoids are round, or equiaxed(各向等大的), when a normally tensile stress produces the failure, However, on the shear lip the dimples are oval- shaped 椭圆形的, or elongated细长的, with the ovals, pointing toward the origin of the fracture. 17. The strength of ceramics and glass depends upon the probability of finding a flaw that exceeds a certain critical size. For large components or larger fibers this probability increases. As a result, the strength of larger components and fibers is likely to be lower than that of smaller components or shorter fibers. 18. In materials science, the emphasis is on the underlying(根本的) relationships between the synthesis合 成 and processing, structure, and properties of materials. In materials engineering, the focus is on how to translate转化 or transform转变 materials into a useful device or structure. 19 Fracture in fiber-reinforced composite materials is more complex, Typically, these composites contain strong, brittle fibers surrounded by a soft, ductile matrix, as in boron - reinforced aluminum. When a tensile stress is applied along the fibers, the soft Aluminum deforms in a ductile manner, with void formation and coalescence合并 eventually producing a dimpled fracture surface. As the aluminum deforms, the load is no longer transmitted effectively to tile fibers; the fibers break in a brittle manner until there are too few of them left intact完整 to support the final load. 20 One way is to define ceramics based on their class of chemical compounds (e.g., oxides, carbides, nitride, sulfides, etc.). Another way which we will use here is to classify ceramics by their major function (Table4-1).Ceramics are used in a wide range of technologies such as refractories耐火材料, spark plugs , dielectrics in capacitors电容器中的电介质 , sensors , abrasives研磨剂 , magnetic recording media磁记录介 质, etc. The space shuttle航天飞机 makes use of ~25,000 reusable , light-weight, highly porous ceramic tiles砖 that protect the aluminum frame铝制框架 from the heat generated during re-entry into the Earth’s atmosphere. 21 Fiber length and diameter Fiber can be short, long or even continuous. Their dimensions are often characterized by the aspect ratio I/d, where I is the fiber length and d is the diameter. -4-4 Typical fibers have diameters varying from10 ,m (10x10cm) to150,m (150x 10 cm). The strength of a composite improves when the aspect ratio is large. Fibers often fracture because of surface imperfections. Making the diameter as small as possible gives the fiber less surface area, and consequently, fewer flaws might propagate. Amount of Fiber A greater volume fraction体积分数 of fibers increases the strength and stiffness刚度 of the composite, as we would expect from the rule of mixtures混合定律. However, the maximum volume fraction is about 80%, beyond which fibers can no longer be completely surrounded by the matrix. Orientation of Fibers The reinforcing fibers may be introduced into the matrix in a number of orientations. Short randomly oriented fiber, having a small aspect ratio such as glass fiber, are easily introduced into the matrix and give relatively isotropic各向同性 behavior in the composite. Long, or even continuous, unidirectional 单向 arrangements of fibers produce anisotropic properties, with particularly good strength and stiffness parallel to平 行于 the fibers. Fiber properties In most fiber-reinforced composites, the fibers are strong, stiff, and lightweight. If the composite is to be used at elevated temperatures, the fiber should also have a high melting temperature. Thus the specific strength and specific modulus of the fiber are important characteristics Matrix Properties The matrix supports the fiber and keeps them in the proper position and to transfer the load to the strong fibers, protects the fibers from damage during manufacture and use of the composite, and prevent cracks in the fiber from propagating throughout the entire composite. 基底性能 基底首先应当支持纤维的附着，使纤维维持在一个合适的位置，以将载荷转移到强度高的纤维 上;其次，在复合材料的制造和使用过程中，基底要保护纤维免受破坏;最后，基底可以阻止纤维中的缺 陷在整体材料中的扩展。 The matrix usually provides the major control over electrical properties, chemical behavior, and elevated-temperature use of the composite. Bonding and Failure Particularly in polymer and metal-matrix composites, good bonding must be obtained between the various constituent. The fibers must be firmly bonded to the matrix material if the load is to be properly transmitted from the matrix to the fibers. In addition, the fibers may pull out of 拉出the matrix during loading, reducing the strength and fracture resistance of the composite if bonding is poor. 22 In general, for a given type of thermoplastic热塑性塑料 (e.g., polyethylene聚乙烯) the tensile strength, creep resistance抗蠕变性, impact toughness, wear resistance耐磨性, and melting temperature all increase with increasing average molecular weight平均分子量 or degree of polymerization聚合度. The increases in these properties are not linear. As the average molecular weight increases, the melting temperature increases and this makes the processing more difficult. 23 The fracture mechanics断裂力学 approach allows us to design and select materials while taking into account the inevitable不可避免 presence of flaws. There are three variables to consider: the property of the material (KC or KIC), the stress , that the material must withstand, and the size of the flaw a. If we know two of these variables, the third can be determined. 24. Silicon carbide (SiC) provides outstanding oxidation resistance at temperatures even above the melting point of steel. SiC often is used as a coating涂层 for metals, carbon-carbon composites, and other ceramics to provide protection at these extreme temperatures. SiC is also used as an abrasive in grinding wheels砂轮 and as particulate微粒 and fibrous reinforcement in both metal matrix and ceramic matrix composites. It is also used to make heating elements发热元件 for furnaces炉子. 25 Compaction and sintering- one of the most cost-effectively ways to produce thousands of relatively small pieces (<6 inches) of simple shapes is compaction and sintering. The driving force of 驱动力 is the reduction in the surface area of a powder . Fine powders can be spray dried喷雾干燥, forming soft agglomerates团聚体 that flow and compact well .The different steps of uniaxial单轴 compaction, in which the compacting force is applied in one direction, are shown in Figure 4-3(a). The microstructure of a barium magnesium tantalate 钽酸 钡镁ceramic prepared using compaction and sintering is shown in Figure 4-3(b). 26 (We also discussed the concepts of short-versus long-range order长程有序 in terms of atomic or ionic arrangements in non-crystalline materials. The most important of the non-crystalline materials are glasses, especially those based on silica. Of course, there are glasses based on other compounds (eg. Sulfides, fluorides氟 化物). A glass is a meta-stable亚稳态 material, in some ways resembles类似 an undercooled过冷的 liquid. Below the glass temperature (Tg). The rate of volume contraction on cooling is reduced and the material can be considered a ‘glass’ rather than an ‘under-cooled liquid’. 27. In ductile metallic materials, the engineering stress-strain curve typically goes through a maximum; this maximum stress is the tensile strength of the material. Failure occurs at a lower stress after necking has reduced the cross-sectional area横截面积 supporting the load. In more brittle materials, failure occurs at the maximum load, where the tensile strength and breaking strength are the same. In brittle materials, including many ceramics, yield strength, tensile strength and breaking strength are all the same. 28 Sintering involves different mass transport mechanisms质量运输机制 [Figure 4-3(c)]. With sintering the grain boundary and bulk (volume) diffusion contribute to densification致密化 (increase in density) , Surface diffusion and evaporation condensation蒸发凝聚 can cause grain growth, but they do not cause densification The compaction process can be completed within one minute for smaller parts; thus, uniaxial单轴的 compaction is well suited for making a large number of smaller and simple shapes. Compaction is used to create what we call ‘green ceramics’; these have respectable strengths and can be handled触摸 and machined. 29 . In a simple tensile test, ductile fracture韧性断裂 begins with the nucleation成核, growth, and coalescence of microvoids at the center of the test bar. Microvoids form when a high stress causes separation of the metal at grain boundaries or interfaces between the metal and small impurity particles杂质粒子 (inclusions 包含物). As the local局部 stress increases, the microvoids grow and coalesce into larger cavities空洞. Eventually, the metal- to metal contact area接触面积 is too small to support the load and fracture occurs. 30 Glasses are manufactured into useful articles有用物品 at a high temperature with viscosity(粘性) controlled so that the glass can be shaped without breaking Figure 4-9 helps us understand the processing in terms of the viscosity ranges. 1 Liquid range Sheet薄片 and plate glass平板玻璃 are produced when the glass is in the molten state熔 融状态. Techniques include rolling旋转 the molten glass through water-cooled rolls or floating the molten glass over a pool of liquid tin(锡) (Figure 4-10). The liquid-tin process produces an exceptionally smooth surface on the glass. The development of the float-glass process浮法玻璃生产工艺 was a genuine breakthrough in the area of glass Processing. Basic float-glass composition has been essentially unchanged for many years (Table 4-6). 2. Working range Shapes such as those of containers or light bulbs can be formed by pressing, drawing, or blowing glass into molds . A hot gob凝块 of liquid glass may be preformed into a crude粗糙的 shape , then pressed or blown into a heated die模具 to produce the final shape. The glass is heated to the working range so that the glass is formable, but not ‘runny .’ 3 . Annealing range热处理范围 Some ceramic parts may be annealed to reduce residual stresses 残余应 力introduced during foxing. Large glass castings铸件, for example, are often annealed and slowly cooled to prevent cracking. Some glasses may be beat treated to cause devitrification不透明 or the precipitation沉淀 of a crystalline phase from the glass. 31. Metals and alloys have good strength, good ductility, and good formability. Metals have good electrical and thermal conductivity. Metals and alloys play an indispensable 不可缺少的role in many applications such as automotives, buildings, bridges, aerospace航空宇宙, and the like. Polymers are classified in several ways: by how the molecules are synthesized, by their molecular structure, or by their chemical family. One way to classify polymers is to state if the polymer is a linear polymer or a branched分枝的 32 polymer. Linear polymer - Any polymer in which molecules are in the form of spaghetti-like chains 意大利面条状 的链条. Branched polymer – In a branched polymer, there are primary polymer chains and secondary offshoots分支 of smaller chains that stem from 起源于these main chains. Thermoplastics - Linear or branched polymers in which chains of molecules are not interconnected to one another. Elastomers弹性体 - These are polymers (thermoplastics or lightly cross-linked交联 thermosets) that have an elastic deformation > 200%. Thermosetting polymers热固性聚合物 - Polymers that are heavily cross-linked to produce a strong three dimensional network structure. 33(The techniques used to form the polymers depend to a large extent很大程度上 on the nature of the polymer – in particular, whether it is thermoplastic or thermosetting. The greatest variety of techniques are used to form the thermoplastics. The polymer is heated to near or above the melting temperature so that it becomes rubbery橡胶似的 or liquid. The polymer is then formed in a mold or die to produce the required shape. Silica (SiO) is probably the most widely used ceramic material. silica is an essential ingredient(成分) in 2 glasses and many glass ceramics. silica-based materials are used in thermal insulation, refractories, abrasives, fiber-reinforced composites, laboratory glassware玻璃器皿 etc. In the form of long continuous fibers, silica is used to make optical fibers光导纤维 for communications. Powders made using fine particles of silica are used in tires(轮胎), paints绘画颜料, and many other applications. 34. The steps of converting转变 a ceramic powder (or mixture of powders) into a useful shape are known as powder processing. We begin with a ceramic powder and gel胶体 it ready for shaping by crushing压碎, grinding磨, separating impurities, blending 混合different powders, and spray drying, to form soil agglomerates 凝聚体. Different techniques such as compaction, tape casting流延成型, extrusion挤出成型, and slip casting注 浆成型 are then used to convert properly processed powders into a desired shape to form what is known as a green ceramic. 35 A special group of dispersion-strengthened分散加强 nano-composite materials containing particles l0 to 250 nm in diameter is classified as particulate composites颗粒复合材料. These dispersoids 弥散体, usually a metallic oxide, are introduced into the matrix by means other than不同于 traditional phase transformations . Even though the small particles are not coherent黏在 with the matrix, they block阻止 the movement of dislocations and produce a pronounced显著的 strengthening effect. 36 We also discussed the concepts of short-versus long-range order in terms of atomic or ionic arrangements in non-crystalline materials. The most important of the non-crystalline materials are glasses, especially those based on silica. Of course, there are glasses based on other compounds (eg. Sulfides, fluorides). A glass is a meta-stable material, in some ways resemblesan undercooled liquid. Below the glass temperature (Tg). The rate of volume contraction on cooling is reduced and the material can be considered a ‘glass’ rather than an ‘under-cooled liquid’. 37 Titanium Dioxide (TiO) is used to make electronic ceramics such as BaTiO3. The largest use, though, 2 is as a white pigment颜料 to make paints绘画颜料. Titania二氧化钛 is used in certain某种 glass-ceramics as a nucleating agent成核剂. Fine particles of TiO are sed to make suntan lotions防晒油 that provide 2 protection against ultraviolet rays紫外线. 38. These two aspects of ductile fracture give the failed surface characteristic features性能特征. In thick metal sections截面, we expect to find evidence of necking, with a significant portion部分 of the fracture surface having a flat face where microvoids first nucleated and coalesced, and a small shear lip, where the fracture 0surface is at a 45 angle to the applied stress. The shear lip, indicating that slip滑移occurred, gives the fracture a cup and cone圆锥体 appearance. Simple macroscopic宏观 observation of this fracture may be sufficient to identify the ductile fracture mode. 39. Fatigue is the lowering of strength or failure of a material due to repetitive stress重复应力 which may be above or below the yield strength. The possibility of a fatigue failure is the main reason why aircraft components have a finite life有限寿命. Fatigue is an interesting phenomenon in that load-bearing承载 components can fail while the overall stress applied may not exceed the yield stress. Fatigue can occur even if the components are subjected to承受 stress above the yield strength. A component is often subjected to the repeated application of stress below the yield strength of the material. 40. One of the most important functions of materials scientists and engineers is to establish the relationships between a material or a device’s properties and performance and the microstructure of that material, its composition, and the way the material or the device was synthesized and processed. 41 The properties for which ceramics are most often selected include: high melting temperatures (the ability to be used at elevated temperatures) high electrical resistivity (although some ceramics are super conductors); a broad range of thermal conductivity (some ceramics are excellent insulators); high hardness (although many ceramics are brittle); good resistance to chemical attack and other environmental conditions; 42 (In general, for a given type of thermoplastic (e.g., polyethylene) the tensile strength, creep resistance, impact toughness, wear resistance, and melting temperature all increase with increasing average molecular weight or degree of polymerization. The increases in these properties are not linear. As the average molecular weight increases, the melting temperature increases and this makes the processing more difficult. 43. Sintering involves different mass transport mechanisms. With sintering the grain boundary and bulk (volume) diffusion contribute to densification (increase in density), Surface diffusion and evaporation condensation can cause grain growth, but they do not cause densification 44 Most fiber-reinforced composites provide improved strength, fatigue resistance, Young's modulus, and strength-to-weight ratio by incorporating strong, stiff, but brittle fibers into a softer, more ductile matrix. The matrix material transmits the force to the fibers, which carry most of the applied force. The matrix also provides protection for the fiber surface and minimizes diffusion of species such as oxygen or moisture that can degrade the mechanical properties of fibers. The strength of the composite may be high at both room temperature and elevated temperatures. 45 We begin with a ceramic powder and get it ready for shaping by crushing , grinding, separating impurities, blending different powders, and spray drying to form soil agglomerates . Different techniques such as compaction , tape casting , extrusion , and slip casting are then used to convert properly processed powders into a desired shape to form what is known as a green ceramic. 46 The impact test describes the response of a material to a rapidly applied load. The Charpy and Izod tests are typical. The energy required to fracture the specimen is measured and can be used as the basis for comparison of various materials tested under the same conditions. In addition, a transition temperature above which the material fails in a ductile, rather than a brittle, manner can be determined. 47 In ductile metallic materials, the engineering stress-strain curve typically goes through a maximum; this maximum stress is the tensile strength of the material. Failure occurs at a lower stress after necking has reduced the cross-sectional area supporting the load. In more brittle materials, failure occurs at the maximum load, where the tensile strength and breaking strength are the same. In brittle materials, including many ceramics, yield strength, tensile strength and breaking strength are all the same. 48. When a material is designed for a given application, a number of factors must be considered. The material must acquire the desired physical and mechanical properties, must be capable of being processed or manufactured into the desired shape, and must provide an economical solution to the design problem. Satisfying these requirements in a manner that protects the environment – perhaps by encouraging recycling of the materials – is also essential. For sintered ceramics, the average grain size, grain size distribution, and the level and type of porosity are important. Similarly, depending upon the application, second phases in the microstructure auld occur as separate grains of components dissolved in solid solutions溶解到固溶体 of the matrix, so second phases at grain boundaries also become important, In the rose of extruded ceramics , orientation effects also can be important. 49 Grains and Grain Boundaries the average grain size is often closely related to the primary particle size . An exception例外 to this is if there is grain growth due to long sintering times or exaggerated扩大 or abnormal grain growth . Typically, ceramics with a small grain size are stronger than coarse-grained粗晶 ceramics. Finer grain sizes help reduce stresses that develop at grain boundaries due to anisotropic expansion 膨胀and contraction 收缩. 50 Thermosetting polymers often begin as linear chains. Depending on the type of repeat units and the degree of polymerization, the initial polymer may be either a solid or a liquid resin树脂; in some cases, a two- or three-part liquid resin is used. Heat, pressure, mixing of the various resin, or other methods initiate开始 the cross-linking process. Cross-linking is not reversible可逆的; once formed, the thermosets cannot be reused or recycled conveniently. 51 Composites are produced when two or more materials or phases are used together to give a combination of properties that cannot be attained otherwise在其他方面 . Composite materials may be selected to give unusual combinations of strength, weight, high-temperature performance, corrosion resistance, hardness, or conductivity. Composites highlight突出 how different materials can work in synergy协同. Abalone shell石决明 , wood, bone, and teeth are examples of naturally occuring composites. 52 Most fiber-reinforced composites provide improved strength, fatigue resistance, Young's modulus, and strength-to-weight ratio by incorporating strong, stiff, but brittle fibers into a softer, more ductile matrix. The matrix material transmits the force to the fibers, which carry most of the applied force. The matrix also provides protection for the fiber surface and minimizes diffusion of species such as oxygen or moisture that candegrade the mechanical properties of fibers. 绝大部分的增强纤维复合材料是通过将强度高，硬度大，但脆性较大的纤维混合到延展性较好的软基底 上，从而改善材料的强度、抗断裂性、杨氏模量和强度-质量比等性能指标。基底材料将负载转移到纤维 上，使得纤维承载了更大一部分外加载荷。同时，基底也为纤维表面提供保护，并且减少材料内部杂质(例 如氧气或水汽)的扩散，从而避免纤维力学性能的下降。这类复合材料在室温或者是高温条件下，都可以 保持较高的强度。 47 In ductile metallic materials, the engineering stress-strain curve typically goes through a maximum; this maximum stress is the tensile strength of the material. Failure occurs at a lower stress after necking has reduced the cross-sectional area supporting the load. In more brittle materials, failure occurs at the maximum load, where the tensile strength and breaking strength are the same. In brittle materials, including many ceramics, yield strength, tensile strength and breaking strength are all the same. 在韧性的金属材料中，工程应力应变曲线通常有一个最大值，这个最大应力就是材料的抗张强度。 48. When a material is designed for a given application, a number of factors must be considered. The material must acquire the desired physical and mechanical properties, must be capable of being processed or manufactured into the desired shape, and must provide an economical solution to the design problem. Satisfying these requirements in a manner that protects the environment – perhaps by encouraging recycling of the materials – is also essential. For sintered ceramics, the average grain size, grain size distribution, and the level and type of porosity are important. Similarly, depending upon the application, second phases in the microstructure auld occur as separate grains of components dissolved in solid solutions of the matrix, so second phases at grain boundaries also become important, In the rose of extruded ceramics , orientation effects also can be important. 当一种材料要设计成给定的用途时，很多因素必须要考虑到。这种材料必须获得想要的物理和力学性能， 必须可以加工或者生产成需要的形状，和必须提供一个经济性的方案来解决设计上的困难。以保护环境的 形式满足这些要求或者是通过鼓励这种材料的循环利用也是必要的。 对于陶瓷的烧结来说，平均晶粒尺寸，晶粒的粒度分布以及孔隙度的水平和类型是重要的。相似的，取 决于应用，微观结构的第二相就像组分中分开的晶粒融入到基底中，因此晶界的第二相也很重要。在挤压 陶瓷的过程中，定向效应也很重要。 49 Grains and Grain Boundaries the average grain size is often closely related to the primary particle size . An exception to this is if there is grain growth due to long sintering times or exaggerated or abnormal grain growth . Typically, ceramics with a small grain size are stronger than coarse-grained ceramics. Finer grain sizes help reduce stresses that develop at grain boundaries due to anisotropic expansion and contraction . 晶粒和晶界 晶粒的平均尺寸与一次粒径紧密相连。 一个例外是由于长的烧结时间导致晶粒生长或者是晶粒的反常生长。 典型的，小的晶粒尺寸的陶瓷比粗粒度的陶瓷拥有更高的强度。细尺寸的颗粒帮助减小由于各向异性的扩 张和收缩所增加的在晶界处的压力 50 Thermosetting polymers often begin as linear chains. Depending on the type of repeat units and the degree of polymerization, the initial polymer may be either a solid or a liquid resin; in some cases, a two- or three-part liquid resin is used. Heat, pressure, mixing of the various resin, or other methods initiate the cross-linking process. Cross-linking is not reversible; once formed, the thermosets cannot be reused or recycled conveniently. 51 Composites are produced when two or more materials or phases are used together to give a combination of properties that cannot be attained otherwise . Composite materials may be selected to give unusual combinations of strength, weight, high-temperature performance, corrosion resistance, hardness, or conductivity. Composites highlight how different materials can work in synergy. Abalone shell , wood, bone, and teeth are examples of naturally occuring composites. 复合材料的生产是指当两种或两种以上的材料或相一起将属性组合起来,否则是不可能达到的。 复合材料可以被选择来得到不同强度，重量，耐高温性，抗腐蚀性，硬度或者是导电性的组合。复合材 料突出不同的材料可以是如何协同工作的。石决明、木、骨和牙齿的自然发生复合材料的例子。 52 Most fiber-reinforced composites provide improved strength, fatigue resistance, Young's modulus, and strength-to-weight ratio by incorporating strong, stiff, but brittle fibers into a softer, more ductile matrix. The matrix material transmits the force to the fibers, which carry most of the applied force. The matrix also provides protection for the fiber surface and minimizes diffusion of species such as oxygen or moisture that can degrade the mechanical properties of fibers. 绝大部分的增强纤维复合材料是通过将强度高，硬度大，但脆性较大的纤维混合到延展性较好的软基底 上，从而改善材料的强度、抗疲劳强度、杨氏模量和强度-质量比等性能指标。 基底材料将负载转移到纤维上，使得纤维承载了更大一部分外加载荷。同时，基底也为纤维表面提供保 护，并且减少材料内部的可以降低纤维力学性能的杂质(例如氧气或水汽)的扩散。 三、分析或计算 1. Silicon carbide particles are compacted and fired at a high temperature to produce a strong ceramic shape. 3The specific gravity of SiC is 3.2 g/cm. The ceramic shape subsequently is weighed when dry (360g), after soaking in water (385 g), and while suspended in water (224 g). Calculate the apparent porosity, the true porosity, and the fraction of the pore volume that is closed. The closed-pore percentage is the true porosity minus the apparent porosity, or 30 - 15.5 = 14.5%. Thus 2 The flexural strength (弯曲强度) of a composite material reinforced with glass fibers is 45,000 psi and the flexural modulus is 18 ， 106 psi. A sample, which is 0.5 in. wide, 0.375 in. high, and 8 in. long, is supported between two rods 5 in. apart. Determine the force required to fracture the material and the deflection of the sample at fracture, assuming that no plastic deformation(塑性变形) occurs. 3. Assume that an advanced ceramic, Sialon (acronym for silicon aluminum oxy-nitride), has a tensile strength of 60,000 psi. Let us assume that this value is for a flaw-free ceramic. (in practice, it is almost impossible to produce flaw-free ceramics.) A thin crack 0.01 in. deep is observed before a Sialon part is tested. The part unexpectedly fails at a stress of 500 psi by propagation of the crack. Estimate the radius of the crack tip. 4 A chain used to hoist heavy loads fails (提升重物的链断裂失效). Examination of the failed link indicates considerable deformation and necking prior to failure. List some of the possible reasons for failure. Solution 1. The load exceeded the hoisting capacity of the chain. Thus, the stress due to the load exceeded the yield strength of the chain, permitting failure. Comparison of the load to the manufacturer’s specifications will indicate that the chain was not intended for such a heavy load. This is the fault of the user! 2. The chain was of the wrong composition or was improperly heattreated. Consequently, the yield strength was lower than intended by the manufacturer and could not support the load. This may be the fault of the manufacturer! 5. . A titanium pipe used to transport a corrosive material at 400? is found to fail after several months. How would you determine the cause for the failure? Since a period of time at a high temperature was required before failure occurred, we might first suspect a creep or stress-corrosion mechanism for failure. Microscopic examination of the material near the fracture surface would be advisable. If many tiny, branched cracks leading away from the surface are noted, stress-corrosion is a strong possibility. However, if the grains near the fracture surface are elongated, with many voids between the grains, creep is a more likely culprit. 6 An aluminum rod is to withstand(经受) an applied force of 45,000 pounds. To assure a sufficient safety(足 够的安全), the maximum allowable stress on the rod is limited to 25,000 psi. The rod must be at least 150 in. long but must deform elastically no more than 0.25 in. when the force is applied. Design an appropriate rod. 7 We produce good chemical resistance in a glass when we introduce BO into silica. To assure that we have 23 good glass-forming tendencies, we wish the O:Si ratio to be no more than 2.5, but we also want the glassware to have a low-melting temperature to make the glass-forming process easier and more economical. Design such a glass. Let us determine the amount of B2O3 we must add to obtain exactly an O:Si ratio of 2.5. Let fB be the mole fraction of B2O3 added to the glass, and (1 – fB) be the mole fraction of SiO2: Therefore, we must produce a glass containing no more than 14.3 mol% B2O3. In weight percent: 8 Design a supporting 3-in.-wide plate made of sialon, which has a fracture toughness of 9,000 inpsi , that will withstand a tensile load of 40,000 lb. The part is to be nondestructively tested (非破 坏性的实验)to assure that no flaws are present that might cause failure. 9 An engineer investigating the cause of an automobile accident finds that the right rear wheel has broken off at the axle. The axle is bent. The fracture surface reveals a Chevron pattern pointing toward the surface of the axle. Suggest a possible cause for the failure. The Chevron pattern indicates that the wheel was subjected to an intense impact blow, which was transmitted to the axle, causing failure. Further examination of the fracture surface, microstructure, composition, and properties may verify that the axle was manufactured properly. 10 The flexural strength of a composite material reinforced with glass fibers is 45,000 psi and the flexural modulus is 18 ， 106 psi. A sample, which is 0.5 in. wide, 0.375 in. high, and 8 in. long, is supported between two rods 5 in. apart. Determine the force required to fracture the material and the deflection of the sample at fracture, assuming that no plastic deformation occurs. (计算部分同第二题) Design requirements to be met by the sledgehammer. A partial list 1. The handle should be light in weight, yet tough enough that it will not catastrophically break. 2. The head must not break or chip during use, even in subzero temperatures. 3. The head must not deform during continued use. 4. The head must be large enough to assure that the user doesn’t miss the fence post, and it should not include sharp notches that might cause chipping. 5. The sledgehammer should be inexpensive. 11 A large steel plate used in a nuclear reactor has a plane strain fracture tough ness of 80,000 psi and is in. exposed to a stress of 45,000 psi during service. Design a testing or inspection procedure capable of detecting a crack at the edge of the plate before the crack is likely to grow a t a catastrophic rate (Assuming f = 1.12). 12 Suppose 2 wt% ThO is added to nickel. Each ThO particle has a diameter of 1000 Å. How many particles 22 3, respectively) are present in each cubic centimeter?( The densities of ThO and nickel are 9.69 and 8.9 g/cm2 The volume fraction is: Therefore, there is 0.0184 cm3 of ThO2 per cm3 of composite. The volume of each ThO2 sphere is: 13 Design a unidirectional fiber-reinforced epoxy-matrix strut having a round cross-section. The strut is 10 ft long and, when a force of 500 pounds is applied, it should stretch no more than 0.10 in. We want to assure that the stress acting on the strut is less than the yield strength of the epoxy matrix, 12,000 psi. If the fibers should happen to break, the strut will stretch an extra amount but may not catastrophically fracture. Epoxy costs about $0.80/lb and has a modulus of elasticity of 500,000 psi. For high modulus carbon fibers, E = 77 ， 106 psi; the density is 1.9 g/cm3 =0.0686 lb/in.3, and the cost is about $30/lb. The minimum volume fraction of carbon fibers needed to give a composite modulus of 14.5 ， 106 psi is: The volume fraction of epoxy remaining is 0.817. An area of 0.817 times the total cross-sectional area of the strut must support a 500-lb load with no more than 12,000 psi if all of the fibers should fail: Although the carbon fibers are the most expensive, they permit the lightest weight and the lowest material cost strut. (This calculation does not, however, take into consideration the costs of manufacturing the strut.) Our design, therefore, is to use a 0.255-in.-diameter strut containing 0.183 volume fraction high modulus carbon fiber. 14 A high-strength steel plate (Figure 7-19), which has a plane strain fracture toughness of 80 MPa , is alternately loaded in tension to 500 MPa and in compression to 60 MPa. The plate is to survive for 10 years, with the stress being applied at a frequency of once every 5 minutes. Design a manufacturing and testing procedure that assures that the component will serve as intended. Solution 15 A cemented carbide cutting tool used for machining contains 75 wt% WC, 15 wt% TiC, 5 wt% TaC, and 5 wt% Co. Estimate the density of the composite First, we must convert the weight percentages to volume fractions. The densities of the components of the composite are: From the rule of mixtures, the density of the composite is 16. A silver-tungsten composite for an electrical contact is produced by first making a porous tungsten powder metallurgy compact, then infiltrating pure silver into the pores. The density of the tungsten compact before 3infiltration is 14.5 g/cm. Calculate the volume fraction of porosity and the final weight percent of silver in the compact after infiltration. From the rule of mixtures: After infiltration, the volume fraction of silver equals the volume fraction of pores: 17 Describe the difference in fracture mechanism between a boron-reinforced aluminum composite and a glass fiber-reinforced epoxy composite. In the boron-aluminum composite, the aluminum matrix is soft and ductile; thus we expect the matrix to fail in a ductile manner. Boron fibers, in contrast, fail in a brittle manner. Both glass fibers and epoxy are brittle; thus the composite as a whole should display little evidence of ductile fracture. 18 Compare engineering stress and strain with true stress and strain for the aluminum alloy in Example 6.1 at (a) the maximum load and (b) fracture. The diameter at maximum load is 0.497 in. and at fracture is 0.398 in. 19 An advanced engineered ceramic has a Weibull modulus m = 9. The flexural strength is 250 MPa at a probability of failure F = 0.4. What is the level of flexural strength if the probability of failure has to be 0.1? solution Now, we want to determine the value of for F = 0.1. We know that m = 9 and 0 = 269.4 MPa, so we need to get the value of . We substitute these values into Equation 6-29: 20 Seven silicon carbide specimens were tested and the following fracture strengths were obtained: 23, 49, 34, 30, 55, 43, and 40 MPa. Estimate the Weibull modulus for the data by fitting the data to Equation 7-11. Discuss the reliability of the ceramic. 21 A crankshaft in a diesel engine fails. Examination of the crankshaft reveals no plastic deformation. The fracture surface is smooth. In addition, several other cracks appear at other locations in the crankshaft. What type of failure mechanism would you expect? SOLUTION Since the crankshaft is a rotating part, the surface experiences cyclical loading. We should immediately suspect fatigue. The absence of plastic deformation supports our suspicion. Furthermore, the presence of other cracks is consistent with fatigue; the other cracks didn’t have time to grow to the size that produced catastrophic failure. Examination of the fracture surface will probably reveal beach marks or fatigue striations. 22 A solid shaft for a cement kiln produced from the tool steel in Figure 6.50 must be 96 in. long and must survive continuous operation for one year with an applied load of 12,500 lb. The shaft makes one revolution per minute during operation. Design a shaft that will satisfy these requirements. Solution 23 We want to make 1000 kilograms of BaTiO3 ceramic from BaCO3 and TiO2. How much barium carbonate and titanium dioxide should be ball milled and calcined? Even though the actual formation of BaTiO3 involves many different intermediate phases, we can write the final reaction as: BaCO3(s) + TiO2(s) ,BaTiO3(s) + CO2(g) Molecular weights are: BaCO3: 137(Ba) + 12 (C) + (3 *16(O)) = 197 g/mol TiO2: 48(Ti) + (2 *16) = 80 g/mol BaTiO3: 137(Ba) + 48(Ti) + (3 *16(O)) = 233 g/mol , CO2: 12(C) + 2 *16(O) = 44 g/mol Thus, 197 g of BaCO3 reacts with 80 g of TiO2 to form 233 g of BaTiO3 and 44 g of CO2. Since 233 g of BaTiO3 is produced using 197 g of BaCO3, for 1000 kilograms of BaTiO3, we will need 845.5 kilograms of BaCO3. Similarly, since 233 g of BaTiO3 is produced using 80 grams of TiO2, for 1000 kilograms of BaTiO3, we will need 343.3 kilograms of TiO2. 24 Derive the rule of mixtures (Equation 6.5) for the modulus of elasticity of a fiber-reinforced composite when a stress ( ) is applied along the axis of the fibers. We use the symbol ‘‘ ’’ for stress to distinguish it from the symbol used for conductivity. The total force acting on the composite is the sum of the forces carried by each constituent: Fc = Fm + Ff Since F = σA: If the fibers have a uniform cross-section, the area fraction equals the volume fraction f : From Hooke’s law, σ = εE. Therefore: If the fibers are rigidly bonded to the matrix, both the fibers and the matrix must stretch equal amounts (iso-strain conditions): 25 A manufacturer of barium zinc tantalate (Ba(Zn1/3Ta2/3)O3 or BZT ceramics) produces cylindrical, puck-shaped devices known as dielectric resonators. The particular pucks made in this case are 2 inches in diameter and 1/2 inch tall and they have a 1/2 inch hole at the center. The sintering of these pieces is conducted at 1500oC, using additives. The manufacturing process used to make these pieces is well established. Suddenly, one day, a batch of the dielectric resonators comes out of the furnace with cracks and must be rejected. The plant manager determines that the material has reached 98% of the theoretical target level of densification. What could be the reason for the cracks? Problems such as these have to be traced back through the manufacturing process starting with the raw materials, blending, compaction, and the use of binders. In a quality manufacturing operation (such as the one having a ISO 9000 certification), all raw materials, manufacturing steps, procedures, and suppliers must be traceable. Discussions with the furnace operator revealed that he had increased the speed of the furnace conveyer belt so that he could get off work an hour early, causing the parts to cool too quickly. The resultant dielectric resonators developed micro cracks because of rapid cooling from the sintering temperature. The problem, in this case, turned out to be due to a human error. 26 We are now using a 7075-T6 aluminum alloy (modulus of elasticity of 10 ， 106 psi) to make a 500-pound panel on a commercial aircraft. Experience has shown that each pound reduction in weight on the aircraft reduces the fuel consumption by 500 gallons each year. Design a material for the panel that will reduce weight, yet maintain the same specific modulus, and will be economical over a 10-year lifetime of the aircraft. let’s consider using a boron fiber-reinforced Al-Li alloy in the T6 condition. The specific modulus of the current 7075-T6 alloy is: If we use 0.6 volume fraction boron fibers in the composite, then the density, modulus of elasticity, and specific modulus of the composite are: If the specific modulus is the only factor influencing the design of the component, the thickness of the part might be reduced by 75%, giving a component weight of 125 pounds rather than 500 pounds. The weight savings would then be 375 pounds, or (500 gal/lb)(375 lb) = 187,500 gal per year. At about $2.00 per gallon, about $375,000 in fuel savings could be realized. eachyear, or $3.75 million over the 10-year aircraft lifetime