Introduction to Mechanics of Materials

Introduction to Mechanics of Materials

材料力学引论

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.

在所有的工程构造中，结构的构件必须具有确定的实际尺寸。这些构件必须比例适当，能够承受实际施加或可能施加于其上的力。这就是说，压力容器的壁必须具有足够的强度以承受其内部压力；建筑物的地面对其预期的设计载荷而言必须是足够坚固；机器的轴必须具有适当的尺寸才能传递所要求的扭矩，飞机的机翼必须能够安全地承受在飞行或着陆时加在其上的空气动力载荷。同样地，复合结构中的零件也必须具有足够的刚度，使其在外加载荷下工作时不至于有过大的变形或“下垂”。某一建筑物的地面可能有足够的强度，但是可能会产生过大的变形，这在某些情况下可能会引起生产设备的安装误差或者在其他情况下会导致在此地面下面的天花板的抹灰层开裂。此外，构件也可能太薄或太细，以至于当承受压力载荷时，它会因为弯曲，失去纵向稳定性而塌下来，也就是构件的初始形状会变得不稳定。确定一根细长的柱子在弯曲失稳之前能够承受的最大载荷的能力，或者确定一个容器所能保持真空的安全程度都是具有重要实际意义的。

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.

材料力学是一门相当古老的学科，一般可追溯到17世纪早期伽利略的工作。在他对载荷作用下固体的特性进行研究之前，建筑师是按照先前惯例和经验法则进行工作的。伽利略是第一个试图在理论基础上来解释某些承受载荷构件的特性的人。他研究了许多受拉和受压构件，特别是为意大利海军建造的船只中船体的梁。当然，从那时起本学科已经取得了很大的进步，但是，必须指出的是在发展过程中，本学科的进展要归功于法国研究者，其中最出众的一批人为库伦，泊松，纳维埃，圣维南和哥西，他们在19世纪初所做的工作在本学科中留下了不可磨灭的业绩。

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.

材料力学在所有的工程专业中都得到了广泛的应用。它的方法被土木工程师用于桥梁和建筑物设计中；被采矿工程师和建筑工程师所需要，他们均对结构物感兴趣；被机械和化工工程师所需要，他们依赖这门学科的方法来设计机构和压力容器；被金属学家所需要，他们要用本学科的基本概念去弄懂如何进一步改进现有的材料；最后，被电气工程师所需要，他们需要这门学科的方法是由于机械工程在电气装备的许多部分中具有重要性。材料力学有它本身的特征性方法。它是一门确定了的学科，是工科教学计划中最基本的学科之一，与诸如流体力学，热力学，和电工原理等其他基础课并列。

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.

受力构件的表现不仅依赖于决定力的平衡的牛顿力学基本定律，而且也取决于制造构件的材料的物理特性。关于后者的必要的信息来自于实验室，在那里材料将受到精确的已知力的作用，并且应该特别细心地观察试件的表现，诸如出现断裂、变形等类的现象。确定这类现象是本学科的一个关键部分，但是本学科的这一部分将留给别的书。在这里仅对这些研究的最终结果感兴趣，而这门课只是本学科的分析或数学部分，不同于实验部分。据此可以看出，材料力学是实验与分析力学的牛顿基本原理的混合科学。从后者借来的这一学科的分支称为静力学，本书假定读者对它已经熟悉，而这本书的内容主要是建立在它的基础上。

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. 

这门课程的内容只有通过演算大量的习题才能掌握。对构件和机械零件进行分析和设计所需要的公式数目是很少的，但是，在整个学习过程中，学生们必须提高对问题及正在计算的量的本质的观察和想象能力。对所要解答的问题，完整、认真地画出示意图，在较快和较完全地掌握这门学科的知识方面将得到很大的收获。