Load Classification

The primary objective of a course in mechanics of materials is the development of relationships between the loads applied to a nonrigid body and the internal forces and deformations induced in the body. Ever since the time of Galileo Galilei (1564 ~ 1642) , men of scientific bent have studied the problem of the load-carrying capacity of structural members and machine components, and have developed mathematical and experimental methods of analysis for determining the internal forces and the deformations induced by the applied loads. The experiences and observations of these scientists and engineers of the last three centuries are the heritage of the engineer of today. The fundamental knowledge gained over the last three centuries, together with the theories and analysis techniques developed, permit the modern engineer to design, with complete competence and assurance, structures and machines of unprecedented size and complexity.

材料力学课程的主要目的，是展示施加在非刚性物体上的载荷与物体中由此而引起的内力和变形之间的关系。从伽利略时代(1564 ~1642）以来，科学爱好者已经对结构构件和机器零件的承载能力问题，由外加载荷而引起的内力与变形的数学分析和实验方法进行了大量的研究工作。最近 300年以来，这些科学家和工程师们的经验和研究资料是留给现代工程师们的宝贵遗产。近300年来所获得的基础知识和所发展并建立的理论与分析技术，使得现代工程师们有能力和信心去设计前所未有的，大型的和复杂的结构和机器。

It will frequently be found that the equations of equilibrium (or motion) are not sufficient to determine all the unknown loads or reactions acting on a body. In such cases it is necessary to consider the geometry (the change in size or shape) of the body after the loads are applied. The deformation per unit length in any direction or dimension is called strain. In some instances, the specified maximum deformation and not the specified maximum stress will govern the maximum load that a member may carry.

经常可以发现，为了确定作用在物体上的所有未知载荷或反力，仅仅依据平衡（或运动）方程是不够的。在这种情况下，必须考虑物体在加载后的几何情况（尺寸或形状）的改变。任何一方向或任何一尺寸上单位长度的变形称为应变。在某些情况下，将由规定的最大变形而不是规定的最大应力来决定构件可以承受的最大载荷。

Certain terms are commonly used to describe applied loads; their definitions are given here so that the terminology will be clearly understood.

通常可以用几种术语用来描述外加载荷，这里给出这些术语的定义，使其能被清楚地理解。

 Loads may be classified with respect to time：

(1) A static load is a gradually applied load for which equilibrium is reached in a relatively short time.

(2) A sustained load is a load that is constant over a long period of time, such as the weight of a structure ( called dead load). This type of load is treated in the same manner as a static load; however, for some materials and conditions of temperature and stress, the resistance to failure may be different under short-time loading and under sustained loading.

(3) An impact load is a rapidly applied load ( an energy load). Vibration normally results from an impact load, and equilibrium is not established until the vibration is eliminated, usually by natural damping forces.

(4) A repeated load is a load that is applied and removed many thousands of times. The helical springs that close the valves on automobile engines are subjected to repeated loading. 

载荷可以按时间来分类：

(1)静载荷是一种逐渐施加的载荷，这种载荷在相当短的时间内达到平衡。

(2)持续载荷是一种在相当长的时间内保持不变的载荷，例如结构的重量（称为恒定载荷）。这种类型的载荷按静载荷一样处理；然而，对于某些材料在某些温度和应力条件下，在短期载荷和持续载荷作用下抵抗破坏的强度可能会是不相同的。

(3)冲击载荷是一种快速加上的载荷（能量载荷）。冲击载荷通常引起振动，在振动消失前不会达到平衡，振动通常是由于自然阻尼力的作用而消失的。

(4)重复载荷是一种加载和卸载达到数千次的载荷。关闭汽车发动机阀门的螺旋形弹簧所承受的就是重复载荷。

Loads may also be classified with respect to the area over which the load is applied:

(1) A concentrated load is a load or force applied at a point. Any load applied to a relatively small area compared with the size of the loaded member is assumed to be a concentrated load; for example, a truck wheel load on the longitudinal members of a bridge.

(2) A distributed load is a load distributed along a length or over an area. The distribution may be uniform or nonuniform. The weight of a concrete bridge floor of uniform thickness is an example of a uniformly distributed load. 

载荷还可以按载荷作用的面积分类：

(1)集中载荷是作用在一个点上的载荷或者力。比起承受载荷构件的尺寸来说，作用在相当小的面积上的任何载荷，都可以假设为集中载荷。例如，作用在桥梁纵向构件上的卡车轮子的载荷就可以假设为集中载荷。

(2)分布载荷是沿长度或者是在一个面积上分布的载荷。载荷的分布可以是均匀的或者是非均匀的。厚度均匀的混凝土桥板的重量就是均布载荷的一个例子。

Loads may be classified with respect to the location and method of application:

(1) A centric load is one in which the resultant force passes through the centroid of the resisting section. If the resultant passes through the centroids of all resisting sections, the loading is termed axial.

(2) A torsional load is one that subjects a shaft or some other member to couples that twist the member. If the couples lie in planes transverse to the axis of the member, the member is subjected to pure torsion.

(3) A bending or flexural load is one in which the loads are applied transversely to the longitudinal axis of the member. The applied load may include couples that lie in planes parallel to the axis of the member. A member subjected to bending loads bends or bows along its length.

(4) A combined loading is a combination of two or more of the previously defined types of loading. 

载荷还可以按作用的位置和方式分类：

(1)中心载荷是合力通过受载截面形心的载荷。如果合力通过所有截面的形心，这样的载荷称为轴向载荷。

(2）扭转载荷是轴或其他构件承受力偶尔使构件产生扭转的载荷。如果力偶位于与构件轴线正交的平面内，则该构件承受纯扭转。

(3）弯曲载荷是与构件纵向轴线横着作用的载荷。外加载荷可以包括在平行于构件轴线的平面内的力偶。构件承受弯曲载荷后，沿其长度方向弯曲或变成弓形。

(4)组合载荷是两种或两种以上的上述各种类型载荷的组合。