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 簡介 1.1 介紹 1.2 材料的失效類型 1.3 設計和選材 1.4 技術要求 1.5 斷裂的經濟價值 1.6 總結 （ 小結 ） 目標 總結了影響機械結構設計的材料失效類型。 了解在工程設計上一般是如何處理限制材料強度和延展性因素的。 開發(fā)一個有價值的新技術，需要新的材料以及可以評估材料力學行為的新方法。 知道斷裂將會造成巨大的經濟損失。 1.1 介紹 設計師所設計的機器、車輛和結構的性能和經濟必須達到可接受的水平 ， 同時力求保證參與項目既可靠又耐用。 為了 保證性能、安全性和耐用性 ， 有必要避免過量 的 變形 即 ， 機器 、車輛或結構部件的彎曲 ， 扭轉 ， 或拉伸。此外 ， 由于 部件中的開裂必須完全避免，或嚴格限制，所以，它并不會進展到完全斷裂點。 材料的變形與斷裂的研究被稱為材料的力學行為。這方面的知識 為 避免這些類型在工程應用中的故障提供了基礎。 主題的一方面 是通過施加力和變形來進行材料試樣的力學試驗。一旦對給定材料的性能進行已知的定量測試 ， 或對公布的數(shù)據(jù)進行測試 ， 其在一個特定的工程設計中正確的概率是可以被預測的。 在設計中，避免結構失效最應該 關注 的是 ，在部件中的應力必須不超過材料的強度 ， 強度則是應力所導致的變形或斷裂 失效。額外的復雜性或特殊的原因所引起的失效，往往需要作進一步的分析 ， 如以下 ： 1、 應力往往呈現(xiàn)出多個方向 ， 即應力狀態(tài)是雙向或三向的。 2、真實的部件中可能包含缺陷甚至裂縫 ，所以 必須特別考慮。 10 3、應力可以持續(xù)很長時間。 4、應力可能會被反復地施 加和去除，或應力的方向會反復逆轉。 在剩下的章節(jié)中 ， 我們將定義并簡要討論材料失效的各種類型 ， 并會考慮材料的力學性能對工程設計 、 新技術和經濟之間的影響關系。 1.2 材料的失效類型 變形失效就是部件在實際尺寸或形狀方面的改變 ，即 足 以讓其功能喪失或減弱。裂縫在某種程度上 ，就是 一個部件被分離成兩個或多個塊，也被稱為開裂。腐蝕是由于化學作用而導致的材料損失 ， 磨損是由于磨擦或固體表面 之間的 接觸粘輥而導致的表面材料的去除。如果磨損是由于流體 (氣體或液體 )所導致的，則它被稱為侵蝕 ， 如果流體中包含硬顆粒，那這就更加有可能了。盡管腐蝕和磨損也很重要 ，但是 這本書中主要考慮的還是變形和斷裂。 圖 1.1中 展現(xiàn)了材料失效的基本類型，其可分為變形和斷裂。 由于存在幾個不同的因素 ， 因此對于一個給定的設計，正確地識別那些可能的原因是很重要的 ，所以 ， 可以選擇 適當?shù)姆治龇椒?，來預測可能發(fā)生的失效。 有了這樣的分類以后就需要記住，對各種不同類型的變形和斷裂下面作了簡要的定義和描述。 圖 1.1 變形和斷裂的基本類型 11 圖 1.2中，圖（ a）是軸向加載、卸載 (b)圖 顯示了彈性變形， (c)圖 是彈塑性變形 1.2.1 彈塑性變形 變形是按照標準來定量的，而剪切應變則是按照基礎材料力學來定量的。在部件中應力的累積效應所導致的變形 有 彎曲、扭轉、和拉伸。變形有時是有用的，如彈簧的變形。而過度變形 ， 特別是永久性的 ， 往往是有害的。 按加載后是否出現(xiàn)迅速變形可以分為彈性變形和塑性變形 ,見 圖 1.2。彈性變形是卸載后可以立即恢復的。這是目前唯一的一 種應力和應變是成比例的變形。對于軸向加載 ,比例常數(shù)就是圖 1.2(b)中所定義的彈性模量 E。彈性變形所引起的失效的一個例子，就好比是一個高大的建筑，在風中擺動，使居住者感到不舒服 ,盡管崩塌的可能性還很小。彈性變形是通過彈性和結構分析的方法來分析的。 塑性變形卸載后是不可恢復的 ,因此是永久的變形。彈性和塑性變形之間的差異，見圖 1.2(c)。一旦出現(xiàn)塑性變形 ,則一個微小應力的增加都會引起一個很大的變形。這個過程相當容易發(fā)生進一步的變形稱為屈服。對于一個給定的材料，一旦開始發(fā)生這種變形時，所對應的的應力值 是非常有意義的，這個值被稱之為屈服強度o。 材料產生大的塑性變形表現(xiàn)了它的塑性 ,而那些沒有太大塑性 變形則表現(xiàn)了它的脆性。許多金屬具有塑性 ,如低強度鋼、銅和鉛 ,還有一些塑料 ,如聚乙烯。 12 脆性行為常發(fā)生在玻璃、石材、丙烯酸塑料 ,和一些金屬中 ,如高強度鋼經常用于建病例。 (注意 ,這個詞用于塑料是高分子材料的公共名稱和用于確定塑性變形 ,它可以用在任何類型的材料 )。 圖 1.3 拉伸試驗表明了材料的脆性和塑性行為。對于脆性材料幾乎沒有塑性變形 ,但對于塑性材料有大量的塑性變形。 拉伸試驗常被用來評估材料的強度和延展性 ,見圖 1.3。這種測試是通過緩慢拉伸棒料直至斷裂。極限抗拉強度 u , 即斷裂前所達到的最大應力 ,是伴隨著屈服強度和斷裂時的應變 f 所得到的。后者是衡量延展性的 ,通常 用一個百分數(shù)表示 ,被稱為比例延伸率。材料的 u 和 f 有較大的值，則是高強度的。高強度的材料用于設計中通常是比較理想的。 較大的