建筑专业英语21511.docx

1、建筑专业英语21511UNIT ONEText Introduction to Mechanics of Materials1.Mechanics of materials is a branch of applied mechanics that deals with the behavior of solid bodies subjected to various types of loading。It is a field of study that is known by a variety of names, including“ strength of materials” and

2、 “mechanics of deformable bodies。”Thesolid bodies considered in this book include axially-loaded bars, shafts, beams, and columns,as well as structures that are assemblies of these components。 Usually the objective of our analysis will be the determination of the stresses ,strains ,and deformations

3、produced by the loads: if these quantities can be found for all values of load up to the failure load, then we will have obtained a complete picture of the mechanical behavior of the body.2 Theoretical analyses and experimental results have equally important roles in the studyof mechanics of materia

4、ls。 On many occasions we will make logical derivations to obtain formulas and equations for predicting mechanical behavior, but at the same time we must recognize that these formulas cannot be used in a realistic way unless certain properties of the material are known. These properties are available

5、 to us only after suitable experiments have been made in the laboratory。 Also, many problems of importance in engineering can not be handled efficiently by theoretical means ,and experimental measurements become a practical necessity. The historical development of mechanics of materials is a fascina

6、ting blend of both theory and experiment, with experi1nents pointing the way to useful results in some instances and with theory doing so in others. Such famous men as Leonardo da Vinci(14521519) and Galileo (15641642) made experiments to determine the strength of wires, bars ,and beams,although the

7、y did not develop any adequate theories(by todays standards ) to explain theirtest results. By contrast, the famous mathematician Leonhard Euler(17071783) developed the mathematical theory of columns and calculated the critical load of a column in 1744, long before any experimental evidence existed

8、to show the significance of his results. Thus, Eulers theoretical results remained unused for many years, although today they form the basis of column theory.3 The importance of combining theoretical derivations with experimentally determinedproperties of materials will be evident as we proceed with

9、 our study of the subject。 In this article we will begin by discussing some fundamental concepts, such as stress and strain, and then we will investigate the behavior of simple structural elements subjected to tension, compression, and shear.4 The concepts of stress and strain can be illustrated in

10、an elementary way by consideringthe extension of a prismatic bar(see Fig1-1a). A prismatic bar is one that has constant crosssection throughout its length and a straight axis。 In this illustration the bar is assumed to be1oaded at its ends by axial forces P that produce a uniform stretching, or tens

11、ion, of the bar,By making an artificial cut through the bar at right angle to its axis, we can isolate part of the bar as a free body, At the right-hand end the tensile force P is applied, and at the other there are forces representing the action of the removed portion of the bar upon the part that

12、remains. These forces w l be continuously distributed over the cross section, analogous to the continuous distribution of hydrostatic pressure over a submerged surface。 The intensity of force, that is, the force per unit area, is called the stress and is commonly denoted by the Greek letter 。Assumin

13、g that the stress has a uniform distribution over the cross section(see Fig.1-1b), we can readily see that its resultant is equal to the intensity times the cross-sectional areaA of the bar。 Furthermore, from the equilibrium of the body shown in Fig.1-1b,we can also see that this resultant must be e

14、qual in magnitude and opposite in direction to the force P, Hence, we obtain as the equation for the uniform stress in a prismatic bar, This equation shows that stress has units of force divided by area for example, pounds per square inch(psi) or kips per square inch(ksi). When the bar is being stre

15、tched by the force P, as shown in the figure, the resulting stress is a tensile stress; if the forces are reversed in direction, causing the bar to be compressed, they are called compressive stresses。5 A necessary condition for Eq (1-1)to be valid is that the stress must be uniform over the cross se

16、ction of the bar , This condition will be realized if the axial force P acts through the centroid of the cross section, as can be demonstrated by statics. When the load P does not act at the centroid, bending of the bar will result, and a more complicated analysis is necessary。 Throughout this book,

17、 however, it is assumed that all axial forces are applied at the centroid of the cross section unless specifically stated to the contrary, Also, unless stated otherwise, it is generally assumed that the weight of the object itself is neglected, as was done when discussing the bar in Fig。 1-1。6. The

18、total elongation of a bar carrying an axial force will be denoted by the Greek letter (see Fig.1-1a), and the elongation per unit length, or strain, is then deter 1ined by the equation where L is the total length of the bar。Note that the strain C is a nondimensional quantity, It can be obtained accu

19、rately from Eq。 (12) as Iong as the strain is uniform throughout the length of the bar。 If the bar is in tension, the strain is a tensile strain, representing an elongation or stretching of the material; if the bar is in compression, the strain is a compressive strain, which means that adjacent cros

20、s sections of the bar move closer to one another。New Words and Expressions(be)subjected to 承受,经受 deformable 可变形的 axially 轴向地 shaft 轴,杆状物 derivation 推导 realistic现实的,实际的fascinate 迷住,强烈吸引 blend混合,融合 prismatic等截面的tensile拉力的,拉伸的 sectional截面的,部分的 hydrostatic静力学的analogous类似的 analogous to类似于 submerged浸在水中的u

21、niform 均匀的 denote 指示,表示 equilibrium平衡resu1tant合力 magnitude大小,尺寸 equation方程kip千磅 tensile拉力的 compressive 压力的,压缩的centroid矩心,形心 specifically具体地,特定地 elongation伸长,拉长nondimensional无量纲的 adjacent相邻的UNIT TWOText The Tensile Test1 The relationship between stress and strain in a particular material is determine

22、d by means of a tensile test. A specimen of the material, usually in the form of a round bar, is placed in a testing1nachine and subjected to tension。 he force on the bar and the elongation othe bar are measured as the load is increased . The stress in the bar is found by dividing the force by the c

23、ross-sectional area, and the strain is found by dividing the elongation by the length along which the elongation occurs。 In this1nanner a Complete stress-strain diagran1can be obtained for the material。2The typical shape of the stress-strain diagram for structural steel is shown in Fig.2-1 (a), wher

24、e the axial strains are plotted on the horizontal axis and the corresponding stresses are given by the ordinates to the Curve OABCDE. From O to A the stress and strain are directly proportional to one another and the diagram is linear. Beyond point the1inear relationship between stress and train no

25、longer exists; hence the stress at z is called the proportional limit. For low-carbon (structural) steels, this limit is usually between 30,000psi, and 36,000 psi, but for high-strength steels it may be much greater。 With an increase in loading, the strain increases more rapidly than the stress, unt

26、il at point B a considerable elongation begins to occur with no appreciable increase in the tensile force。This phenomenon is known as yielding of the material, and the stress at point B is cal1ed the yield point or yield stress。In the region BC the material is said to have become plastic, and the ba

27、r may actually elongate plastically by an amount which is to 10 or 15 times the elongation which occurs up to the proportional limit。At point C the material begins to strain harden and to offer additional resistance to increase in load。Thus, with further elongation the stress increases, and it reach

28、es its maximum value, or ultimate stress, at point D。Beyond this point further stretching of the bar is accompanied by a reduction in the load, and fracture of the specimen finally occurs at point E on the diagram。 3 During elongation of the bar a lateral contraction occurs, resulting in a decrease

29、in the cross-sectional area of the bar。 This phenomenon has no effect on the stress-strain diagram upto about point C, but beyond that point the decrease in area will have a noticeable effect uponthe calculated value of stress。 A pronounced necking of the bar occurs(see Fig。2-2),and if theactual cro

30、ss-sectional area at the narrow part of the neck is used in calculating , it will be found that the true stress-strain Curve follows the ashed line CE 。Whereas the total load the bar can carry does indeed diminish after the ultimate stress is reached ( ne DE), this reduction is due to the decrease i

31、n area and not to a loss in strength of the material itself。The material actually withstands an increase in stress up to the point of failure。For most practical purposes, however, the conventional stress-strain curve o BCDE, based upon the original cross-sectional area of the specimen, provides sati

32、sfactory in formation for design purposes。4 The diagram in Fig。2-1(a)has been drawn to show the general characteristics of the stress-strain curve for steel, but its proportions are not realistic because, as already mentioned,the strain which occurs from B to C may be 15 times as great as the strain occurring from O toA. Also, the strains from C to E are even greater than those from B to C. A diagram drawn in proper proportions is shown in Fig。2-1(b)。 In this figure the strains from C to A are soSmall in comparison to the strains from A to E that they cannot be seen, and the l