Introduction to Mechanics of Materials

材料力學(xué)引論

In all engineering construction the component parts of a structure must be?assigned definite physical sizes. Such parts must be properly?proportioned to resist the actual?or probable forces that may be imposed upon them. Thus, the wall of a pressure vessel must be?of adequate strength to withstand the internal pressure; the floors?of a building must be sufficiently strong for their intend purpose; the shalt f a machine must be a?adequate size to carry the the required?torque; a wing of an airplane must safely withstand the aerodynamic loads which may come upon it in flight or landing. Likewise, the parts of a composite structure must be rigid enough so as not to deflect or?"sag" excessively when in operation under the imposed loads. A floor of a building may be strong enough but yet may deflect excessively, which in some instances may cause misalignment of manufacturing equipment, or in other cases result in the cracking of a plaster ceiling attached underneath. Also a member?may be so thin or slender?that, upon being subjected to compressive loading, it will collapse through

buckling; i. e., the initial configuration of a member may become?unstable. Ability to determine the maximum load that a slender column can carry before buckling occurs or determination of the safe level of vacuum that can be maintained by a vessel is of great practical?importance.

在所有的工程構(gòu)造中，結(jié)構(gòu)的構(gòu)件必須具有確定的實(shí)際尺寸。這些構(gòu)件必須比例適當(dāng)，能夠承受實(shí)際施加或可能施加于其上的力。這就是說，壓力容器的壁必須具有足夠的強(qiáng)度以承受其內(nèi)部壓力；建筑物的地面對(duì)其預(yù)期的設(shè)計(jì)載荷而言必須是足夠堅(jiān)固；機(jī)器的軸必須具有適當(dāng)?shù)某叽绮拍軅鬟f所要求的扭矩，飛機(jī)的機(jī)翼必須能夠安全地承受在飛行或著陸時(shí)加在其上的空氣動(dòng)力載荷。同樣地，復(fù)合結(jié)構(gòu)中的零件也必須具有足夠的剛度，使其在外加載荷下工作時(shí)不至于有過大的變形或“下垂”。某一建筑物的地面可能有足夠的強(qiáng)度，但是可能會(huì)產(chǎn)生過大的變形，這在某些情況下可能會(huì)引起生產(chǎn)設(shè)備的安裝誤差或者在其他情況下會(huì)導(dǎo)致在此地面下面的天花板的抹灰層開裂。此外，構(gòu)件也可能太薄或太細(xì)，以至于當(dāng)承受壓力載荷時(shí)，它會(huì)因?yàn)閺澢?/span>，失去縱向穩(wěn)定性而塌下來，也就是構(gòu)件的初始形狀會(huì)變得不穩(wěn)定。確定一根細(xì)長的柱子在彎曲失穩(wěn)之前能夠承受的最大載荷的能力，或者確定一個(gè)容器所能保持真空的安全程度都是具有重要實(shí)際意義的。

Mechanics of materials is a fairly old subject, generally dated?from the work of Galileo in the early part of the seventeenth century.?Prior to his investigations into the behavior of solid bodies under?loads, constructors followed precedents and empirical rules. Galileo?was the first to attempt?to explain?the behavior?of some of the members?under load on a rational?basis. He studied members in tension and?compression, and notably?beams used in the construction of hulls of ships for the Italian navy. Of course much progress has been made since that time, but it must be noted in passing that much is owed in the development of this subject?to?the French investigators, among?whom a group of outstanding men such as Coulomb, Poisson, Navier, St. Venant, and Cauchy, who worked at the break of the nineteenth?century, has left an indelible impression on this subject.

材料力學(xué)是一門相當(dāng)古老的學(xué)科，一般可追溯到17世紀(jì)早期伽利略的工作。在他對(duì)載荷作用下固體的特性進(jìn)行研究之前，建筑師是按照先前慣例和經(jīng)驗(yàn)法則進(jìn)行工作的。伽利略是第一個(gè)試圖在理論基礎(chǔ)上來解釋某些承受載荷構(gòu)件的特性的人。他研究了許多受拉和受壓構(gòu)件，特別是為意大利海軍建造的船只中船體的梁。當(dāng)然，從那時(shí)起本學(xué)科已經(jīng)取得了很大的進(jìn)步，但是，必須指出的是在發(fā)展過程中，本學(xué)科的進(jìn)展要?dú)w功于法國研究者，其中最出眾的一批人為庫倫，泊松，納維埃，圣維南和哥西，他們?cè)?9世紀(jì)初所做的工作在本學(xué)科中留下了不可磨滅的業(yè)績。

The subject of mechanics of materials cuts broadly?across all?branches of the engineering profession with remarkably many applications. Its methods are needed by civil engineers, in the design?of bridges and buildings; by mining?engineers and architectural?engineers, each of whom is interested in structures; by mechanical and chemical engineers, who rely upon?the methods of this subject?for the design of machinery and pressure vessels; by metallurgists, who need the fundamental concepts of this subject in order to understand how to improve existing?materials further; finally,?by electrical engineers, who need the methods of this subject because of the?importance of the?mechanical engineering phases of many portion of?electrical equipment. Mechanics of materials has characteristic methods all its own. It is a define discipline and one of the most fundamental subjects of an engineering curriculum, standing alongside such other basic subjects as fluid mechanics, thermodynamics, and?basic electricity.

材料力學(xué)在所有的工程專業(yè)中都得到了廣泛的應(yīng)用。它的方法被土木工程師用于橋梁和建筑物設(shè)計(jì)中；被采礦工程師和建筑工程師所需要，他們均對(duì)結(jié)構(gòu)物感興趣；被機(jī)械和化工工程師所需要，他們依賴這門學(xué)科的方法來設(shè)計(jì)機(jī)構(gòu)和壓力容器；被金屬學(xué)家所需要，他們要用本學(xué)科的基本概念去弄懂如何進(jìn)一步改進(jìn)現(xiàn)有的材料；最后，被電氣工程師所需要，他們需要這門學(xué)科的方法是由于機(jī)械工程在電氣裝備的許多部分中具有重要性。材料力學(xué)有它本身的特征性方法。它是一門確定了的學(xué)科，是工科教學(xué)計(jì)劃中最基本的學(xué)科之一，與諸如流體力學(xué)，熱力學(xué)，和電工原理等其他基礎(chǔ)課并列。

The behavior,?of a?member subjected to forces depends not only?on the fundamental laws of Newtonian mechanics that govern the equilibrium of the forces but also on the physical characteristics of the?materials of which the member is?fabricated. The necessary?information regarding the latter comes from the laboratory where materials are subjected to the action of accurately known forces and the behavior of test specimens is observed with particular regard to such phenomena as the occurrence of?breaks,?deformations, etc.?Determination of such phenomena is a vital part of the subject, but this branch of the subject is left to other books. Here the end results of such investigations are of interest, and this course is concerned with the analytical or mathematical part of the subject in contradistinction to experimentation. For the above reasons, it is seen that mechanics of materials is a blended science of experiment and Newtonian postulates of analytical mechanics. From the latter is borrowed the branch of the science called statics?subject with which the reader of this book is presumed to be familiar, and on which the subject of this book primarily depends.

受力構(gòu)件的表現(xiàn)不僅依賴于決定力的平衡的牛頓力學(xué)基本定律，而且也取決于制造構(gòu)件的材料的物理特性。關(guān)于后者的必要的信息來自于實(shí)驗(yàn)室，在那里材料將受到精確的已知力的作用，并且應(yīng)該特別細(xì)心地觀察試件的表現(xiàn)，諸如出現(xiàn)斷裂、變形等類的現(xiàn)象。確定這類現(xiàn)象是本學(xué)科的一個(gè)關(guān)鍵部分，但是本學(xué)科的這一部分將留給別的書。在這里僅對(duì)這些研究的最終結(jié)果感興趣，而這門課只是本學(xué)科的分析或數(shù)學(xué)部分，不同于實(shí)驗(yàn)部分。據(jù)此可以看出，材料力學(xué)是實(shí)驗(yàn)與分析力學(xué)的牛頓基本原理的混合科學(xué)。從后者借來的這一學(xué)科的分支稱為靜力學(xué)，本書假定讀者對(duì)它已經(jīng)熟悉，而這本書的內(nèi)容主要是建立在它的基礎(chǔ)上。

The subject matter can be mastered best by solving numerous?problems. The number of formulas necessary for the analysis and design of structural and machine members by the methods of mechanics of materials is remarkably small; however, throughout this study the student must develop an ability to visualize a problem and the nature of the quantities being?computed.?Complete, carefully?drawn diagrammatic sketches of problems to be solved will pay large dividends in a quicker and more complete mastery of this subject.?

這門課程的內(nèi)容只有通過演算大量的習(xí)題才能掌握。對(duì)構(gòu)件和機(jī)械零件進(jìn)行分析和設(shè)計(jì)所需要的公式數(shù)目是很少的，但是，在整個(gè)學(xué)習(xí)過程中，學(xué)生們必須提高對(duì)問題及正在計(jì)算的量的本質(zhì)的觀察和想象能力。對(duì)所要解答的問題，完整、認(rèn)真地畫出示意圖，在較快和較完全地掌握這門學(xué)科的知識(shí)方面將得到很大的收獲。