外文翻译--材料的失效类型

1 原文 Chapter 1 Introduction 1.1 INTRODUCTION 1.2 TYPES OF MATERIAL FAILURE 1.3 DESIGN AND MATERIALS SELECTION 1.4 TECHNOLOGICAL CHALLENGE 1.5 ECONOMIC IMPORTANCE OF FRACTURE 1.6 SUMMARY OBJECTIVES Gain an overview of the types of material failure that affect mechanical and structural design. Understand in general how the limitations on strengths and ductility of materials are dealt with in engineering design. Develop an appreciation of how the development of new technology requires new materials and new methods of evaluating the mechanical behavior of materials. Learn of the surprisingly large costs of fracture to the economy. 1.1 INTRODUCTION Designers of machines, vehicles, and structures must achieve acceptable levels of performance and economy, while at the same time striving to guarantee that the item is both safe and durable. To assure performance, safety, and durability, it is necessary to avoid excess deformation- that is, bending, twisting, or stretching-of the components (parts) of the machine, vehicle, or structure. IN addition, cracking in components must be avoided entirely, or strictly limited, so that it does not progress to the point of complete fracture. The study of deformation and fracture in materials is called mechanical behavior of materials. Knowledge of this area provides the basis for avoiding these types of failure in engineering 2 applications. One aspect of the subject is the physical testing of samples of materials by applying forces and deformations. Once the behavior of a given material is quantitatively known from testing, or from published test data, its chances of success in a particular engineering design can be evaluated. The most basic concern in design to avoid structural failure is that the stress in a component must not exceed the strength of the material, where the strength is simply the stress that causes a deformation or fracture failure. Additional complexities or particular causes of failure often require further analysis, such as the following: 1. Stresses are often present that act in more than one direction； that is, the state of stress is biaxial or triaxial. 2. Real components may contain flaws or even cracks that must be specifically considered. 3. Stresses may be applied for long periods of time. 4. Stresses may be repeatedly applied and removed, or the direction of stress repeatedly reversed. In the remainder of this introductory chapter, we will define and briefly discuss various types of material failure, and we will consider the relationships of mechanical behavior of materials to engineering design, to new technology, and to the economy. 1.2 TYPES OF MATERIAL FAILURE A deformation failure is a change in the physical dimensions or shape of a component that is sufficient for its function to be lost or impaired. Cracking to the extent that a component is separated into two or more pieces is termed fracture. Corrosion is the loss of material due to chemical action, and wear is surface removal due to abrasion or sticking between solid surfaces that touch. IF wear is caused by a fluid (gas or liquid), it is called erosion, which is especially likely if the fluid contains hard particles. Although corrosion and wear also of great importance, this book primarily considers deformation and fracture. The basis types of material failure that are classified as either deformation or fracture are indicated in Fig. 1.1. Since several different causes exist, it is important to correctly identify the ones that may apply to a given design, so that the appropriate analysis methods can be chosen to predict the behavior. With such a need for classification in mind, the various types of deformation 3 and fracture are defined and briefly described next. Figure 1.1 Basic types of deformation and fracture. Figure 1.2 Axial member (a) subject to loading and unloading, showing elastic deformation (b) and both elastic and plastic deformation (c). 1.2.1 Elastic and Plastic Deformation Deformations are quantified in terms of normal and shear strain in elementary mechanics of materials. The cumulative effect of the strains in a component is a deformation, such as a bend, twist, or stretch. Deformations are sometimes essential for function, as in a spring. Excessive deformation, especially if permanent, is often harmful. Deformation that appears quickly upon loading can be classed as either elastic deformation or plastic deformation, as illustrated in Fig. 1.2. Elastic deformation is recovered immediately upon unloading. Where this is the only deformation present, stress and strain are usually 4 proportional. For axial loading, the constant of proportionality is the modulus of elasticity, E, as defined in Fig.1.2 (b). An example of failure by elastic deformation is a tall building that sways in the wind and causes discomfort to the occupants, although there may be only remote chance of collapse. Elastic deformations are analyzed by the methods of elasticity and structural analysis. Plastic deformation is not recovered upon unloading and is therefore permanent. The difference between elastic and plastic deformation is illustrated in Fig.1.2(c). Once plastic deformation begins, only a small increase in stress usually causes a relatively large additional deformation. This process of relatively easy further deformation is called yielding, and the value of stress where this behavior begins to be important for a given material is called the yield strength, o. Materials capable of sustaining large amounts of plastic deformation are said to behave in a ductile manner, and those that fracture without very much plastic deformation behave in a brittle manner. Ductile behavior occurs for many metals, such as low-strength steels copper, and lead, and for some plastic, such as polyethylene. Brittle behavior occurs for glass, stone, acrylic plastic, and some metals, such as the high-strength steel used to make a file. (Note that the word plastic is used both as the common name for polymeric materials and in identifying plastic deformation, which can occur in any type of material.) Figure 1.3 Tension test showing brittle and ductile behavior. There is little plastic deformation for brittle behavior, but a considerable amount for ductile behavior. Tension tests are often employed to assess the strength and ductility of materials, as illustrated in Fig. 1.3. Such a test is done by slowly stretching a bar of the material in tension until 5 it breaks (fractures). The ultimate tensile strength,u, which is the highest stress reached before fracture, is obtained along with the yield strength and the strain at fracture,f. The latter is a measure of ductility and is usually expressed as a percentage, then being called the percent elongation. Materials having high values of both u andf are said to be tough, and tough materials are generally desirable for use in design. Large plastic deformations virtually always constitute failure. For example, collapse of a steel bridge or building during an earthquake could occur due to plastic deformation. However, plastic deformation can be relatively small, but still cause malfunction of a component. For example, in a rotating shaft, a slight permanent bend results in unbalanced rotation, which in turn may cause vibration and perhaps early failure of the bearings supporting the shaft. Buckling is deformation due to compressive stress that causes large changes in alignment of columns or plates, perhaps to the extent of folding or collapse. Either elastic or plastic deformation, or a combination of both, can dominate the behavior. Buckling is generally considered in books on elementary mechanics of materials and structural analysis. 1.2.2 Creep Deformation Creep is deformation that accumulates with time. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function. Plastic and low-melting-temperature metals may creep at room temperature, and virtually any material will creep upon approaching its melting temperature. Creep is thus often an important problem where high temperature is encountered, as in gas-turbine aircraft engines. Buckling can occur in a time-dependent manner due to creep deformation. 6 Figure 1.4 A tungsten lightbulb filament sagging under its own weight. The deflection increases with time due to creep and lead to touching of adjacent coils, which causes bulb failure. An example of an application involving creep deformation is the design of tungsten lightbulb filaments. The situation is illustrated in Fig.1.4. Sagging of the filament coil between its supports increases with time due to creep deformation caused by the weight of the filament itself. If too much deformation occurs, the adjacent turns of the coil touch one another, causing an electrical short and local overheating, which quickly leads to failure of the filament. The coil geometry and supports are therefore designed to limit the stresses caused by the weight of the filament, and a special tungsten alloy that creeps less than pure tungsten is used. 1.2.3 Fracture under Static and Impact Loading Rapid fracture can occur under loading that does not vary with time or that changes only slowly, called static loading. If such a fracture is accompanied by little plastic deformation, it is called a brittle fracture. This is the normal mode of failure of glass and other materials that are resistant to plastic deformation. If the loading is applied very rapidly, called impact loading, brittle fracture is more likely to occur. If a crack or other sharp flaw is present, brittle fracture can occur even in ductile steels or aluminum alloys, or in other materials that are normally capable of deforming plastically by large amounts. Such situations are analyzed by the special technology called fracture mechanics, which is the study of cracks in solids. Resistance to brittle fracture in the presence of a crack is measured by a material property called the fracture toughness,lc, as illustrated in Fig. 1.5. Materials with 7 high strength generally have fracture toughness, and vice versa. This trend is illustrated for several classes of high-strength steel in Fig. 1.6. Figure 1.5 Fracture toughness test. K is a measure of the severity of the combination of crack size, geometry, and load.lc is the particular value, called the fracture toughness, where the material fails. Figure 1.6 Decreased fracture toughness, as yield strength is increases by heat treatment, for various classes of high-strength steel. (Adapted from 【 Knott 79】 ； used with permission.) 8 Ductile fracture can also occur. This type of fracture is accompanied by significant plastic deformation and is sometimes a gradual tearing process. Fracture mechanics and brittle or ductile fracture are especially important in the design of pressure vessels and large welded structures, such as bridges and ships. Fracture may occur as a result of a combination of stress and chemical effects, and this is called environmental cracking. Problems of this type are a particular concern in the chemical industry, but also occur widely elsewhere. For example, some low-strength steels are susceptible to cracking in caustic (basic or high pH) chemicals such as NaOH, and high-strength steels may crack in the presence of hydrogen sulfide gas. The term stress-corrosion cracking is also used to describe such behavior. This latter term is especially appropriate where material removal by corrosive action is also involved, which is not the case for all types of environmental cracking. Photographs of cracking caused by a hostile environment are shown in Fig. 1.7. Creep deformation may proceed to the point that separation into two pieces occurs. This is called creep rupture and is similar to ductile fracture, except that the process is time dependent. Figure 1.7 Stainless steel wires broken as a result of environmental attack. These were employed in a filter exposed at 300 to a complex organic environment that included molten nylon. Cracking occurred along the boundaries of the crystal grains of the material. (Photos by W. G. Halley； courtesy of R. E. Swanson.) 9 第 1章 简介 1.1 介绍 1.2 材料的失效类型 1.3 设计和选材 1.4 技术要求 1.5 断裂的经济价值 1.6 总结 （ 小结 ） 目标 总结了影响机械结构设计的材料失效类型。 了解在工程设计上一般是如何处理限制材料强度和延展性因素的。 开发一个有价值的新技术，需要新的材料以及可以评估材料力学行为的新方法。 知道断裂将会造成巨大的经济损失。 1.1 介绍 设计师所设计的机器、车辆和结构的性能和经济必须达到可接受的水平 ， 同时力求保证参与项目既可靠又耐用。 为了 保证性能、安全性和耐用性 ， 有必要避免过量 的 变形 即 ， 机器 、车辆或结构部件的弯曲 ， 扭转 ， 或拉伸。此外 ， 由于 部件中的开裂必须完全避免，或严格限制，所以，它并不会进展到完全断裂点。 材料的变形与断裂的研究被称为材料的力学行为。这方面的知识 为 避免这些类型在工程应用中的故障提供了基础。 主题的一方面 是通过施加力和变形来进行材料试样的力学试验。一旦对给定材料的性能进行已知的定量测试 ， 或对公布的数据进行测试 ， 其在一个特定的工程设计中正确的概率是可以被预测的。 在设计中，避免结构失效最应该 关注 的是 ，在部件中的应力必须不超过材料的强度 ， 强度则是应力所导致的变形或断裂 失效。额外的复杂性或特殊的原因所引起的失效，往往需要作进一步的分析 ， 如以下 ： 1、 应力往往呈现出多个方向 ， 即应力状态是双向或三向的。 2、真实的部件中可能包含缺陷甚至裂缝 ，所以 必须特别考虑。 10 3、应力可以持续很长时间。 4、应力可能会被反复地施 加和去除，或应力的方向会反复逆转。 在剩下的章节中 ， 我们将定义并简要讨论材料失效的各种类型 ， 并会考虑材料的力学性能对工程设计 、 新技术和经济之间的影响关系。 1.2 材料的失效类型 变形失效就是部件在实际尺寸或形状方面的改变 ，即 足 以让其功能丧失或减弱。裂缝在某种程度上 ，就是 一个部件被分离成两个或多个块，也被称为开裂。腐蚀是由于化学作用而导致的材料损失 ， 磨损是由于磨擦或固体表面 之间的 接触粘辊而导致的表面材料的去除。如果磨损是由于流体 (气体或液体 )所导致的，则它被称为侵蚀 ， 如果流体中包含硬颗粒，那这就更加有可能了。尽管腐蚀和磨损也很重要 ，但是 这本书中主要考虑的还是变形和断裂。 图 1.1中 展现了材料失效的基本类型，其可分为变形和断裂。 由于存在几个不同的因素 ， 因此对于一个给定的设计，正确地识别那些可能的原因是很重要的 ，所以 ， 可以选择 适当的分析方法，来预测可能发生的失效。 有了这样的分类以后就需要记住，对各种不同类型的变形和断裂下面作了简要的定义和描述。 图 1.1 变形和断裂的基本类型 11 图 1.2中，图（ a）是轴向加载、卸载 (b)图 显示了弹性变形， (c)图 是弹塑性变形 1.2.1 弹塑性变形 变形是按照标准来定量的，而剪切应变则是按照基础材料力学来定量的。在部件中应力的累积效应所导致的变形 有 弯曲、扭转、和拉伸。变形有时是有用的，如弹簧的变形。而过度变形 ， 特别是永久性的 ， 往往是有害的。 按加载后是否出现迅速变形可以分为弹性变形和塑性变形 ,见 图 1.2。弹性变形是卸载后可以立即恢复的。这是目前唯一的一 种应力和应变是成比例的变形。对于轴向加载 ,比例常数就是图 1.2(b)中所定义的弹性模量 E。弹性变形所引起的失效的一个例子，就好比是一个高大的建筑，在风中摆动，使居住者感到不舒服 ,尽管崩塌的可能性还很小。弹性变形是通过弹性和结构分析的方法来分析的。 塑性变形卸载后是不可恢复的 ,因此是永久的变形。弹性和塑性变形之间的差异，见图 1.2(c)。一旦出现塑性变形 ,则一个微小应力的增加都会引起一个很大的变形。这个过程相当容易发生进一步的变形称为屈服。对于一个给定的材料，一旦开始发生这种变形时，所对应的的应力值 是非常有意义的，这个值被称之为屈服强度o。 材料产生大的塑性变形表现了它的塑性 ,而那些没有太大塑性 变形则表现了它的脆性。许多金属具有塑性 ,如低强度钢、铜和铅 ,还有一些塑料 ,如聚乙烯。 12 脆性行为常发生在玻璃、石材、丙烯酸塑料 ,和一些金属中 ,如高强度钢经常用于建病例。 (注意 ,这个词用于塑料是高分子材料的公共名称和用于确定塑性变形 ,它可以用在任何类型的材料 )。 图 1.3 拉伸试验表明了材料的脆性和塑性行为。对于脆性材料几乎没有塑性变形 ,但对于塑性材料有大量的塑性变形。 拉伸试验常被用来评估材料的强度和延展性 ,见图 1.3。这种测试是通过缓慢拉伸棒料直至断裂。极限抗拉强度 u , 即断裂前所达到的最大应力 ,是伴随着屈服强度和断裂时的应变 f 所得到的。后者是衡量延展性的 ,通常 用一个百分数表示 ,被称为比例延伸率。材料的 u 和 f 有较大的值，则是高强度的。高强度的材料用于设计中通常是比较理想的。 较大的