材料力学专业英语1.ppt

* * Chapter1 Tension and Compression 第一章 拉伸与压缩 Axially Loaded Bar The simplest case to consider at the start is that of an initially straight metal bar of constant cross section, loaded at its ends by a pair of oppositely directed collinear forces coinciding with the longitudinal axis of the bar and acting through the centroid of each cross section. For static equilibrium the magnitudes of the forces must be equal. If the forces are directed away from the bar, the bar is said to be in tension; if they are directed toward the bar, a state of compression exists. These two conditions are illustrated in Fig. 1-1. 轴向受载杆件 同线的 质心 平衡 大小 不变的 首先我们研究最简单情况：一等截面金属直杆在其两端承受一对共线、反向力的作用。这两个力的作用线与各截面形心组成的纵向轴线重合，为了满足静力学平衡条件，这两个力的数值必须相等。如果这两个力的方向是离开此杆的，杆将承受拉伸；如果这两个力是指向此杆的，杆将承受压缩，如图1-1所示。 Fig. 1-1 图1-1 Under the action of this pair of applied forces, internal resisting forces are set up within the bar and their characteristics may be studied by imagining a plane to be passed through the bar anywhere along its length and oriented perpendicular to the longitudinal axis of the bar. Such a plane is designated as a-a in Fig. 1-2(a). Fig.1-2 图1-2 垂直的， 正交的 在这样两个力的作用之下，杆的内部将产生抗力。我们可以用位于杆轴某处、且与杆轴垂直的假想截面来研究杆的内部抗力，这样的截面如图1-2（a）中的a-a所示。 If for purposes of analysis the portion of the bar to the right of this plane is considered to be removed, as in Fig. 1-2(b), then it must be replaced by whatever effect it exerts upon the left portion. By this technique of introducing a cutting plane, the originally internal forces now become external with respect to the remaining portion of the body. For equilibrium of the portion to the left this “effect” must be a horizontal force of magnitude P. However, this force P acting normal to the cross-section a-a is actually the resultant of distributed forces acting over this cross section in a direction normal to it. 施加（力） 水平的 法线 合力 分布式的 为了分析计算，可考虑将此截面右侧的杆段除去，如图1-2（b）所示。因而，必须补充右侧杆段对左侧杆段的作用。用此处引入的截面法，初始的内力便成为保留杆段的外力。为使左侧杆段平衡，这种效应在