Lesson 1土木工程专业英语.ppt

1/22 Lesson 1 Stress and Strain The concepts of stress and strain can be illustrated in an elementary way by considering the extension of a prismatic bar（see Fig.1-1a）. 应力和应变的概念可以通过考虑等截面杆延伸的 基本方法来说明（参见图1-1a）。 A prismatic bar is one that has constant cross section throughout its length and a straight axis. 等截面杆沿整个长度和直线轴线方向具有恒定的截面。 In this illustration the bar is assumed to be loaded at its ends by axial forces P that produce a uniform stretching, or tension, of the bar. 此图中，假定杆两端施加轴向力P，在轴向力P的作用下（沿杆长）产生均匀拉伸或（在杆截面）产生均匀拉力。 By making an artificial cut (section m-m) through the bar at right angles to its axis, we can isolate part of the bar as a free body (see Fig.1-1b). 垂直于轴线做一个截面（截面m-m），取杆其中一部分作为隔离体（如图1-1b）。 At the left-hand end the tensile force P is applied, and at the other end there are forces representing the action of the removed portion of the bar upon the part that remains. 在杆左端作用拉力P，在杆另一端移动部分对保留部分存在力的作用。 These forces will be continuously distributed over the cross section, analogous to the continuous distribution of hydrostatic pressure over a submerged surface. 这些力连续分布在杆整个横截面上，类似于浸没面上液体静压力的连续分布。 The intensity of force, that is , the force per unit area, is called the stress and is commonly denoted by the Greek letter σ. 力的集度，即单位面积上的力叫应力，通常用希腊字母σ表示。 Assuming 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 σ times the cross-sectional area A of the bar. 假定应力沿横截面均匀分布（如图1-1b），我们很容易得到截面上的合力等于强度σ乘以杆截面积A。 Furthermore, from the equilibrium of the body shown in Fig.1b, we can also see that this resultant must be equal in magnitude and opposite in direction to the force P. Hence, we obtain 此外，从图Fig.1b中由力的平衡我们也可以看出截面上合力必须与力P大小相等方向相反，我们得到方程（1） Eq. (1) can be regarded as the equation for the uniform stress in a prismatic bar. 方程（1）即为等截面杆均匀应力方程。 This equation shows that stress has units of force divided by area——for example, pounds per square inch (psi) or kips per sq