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.

材料力學課程的主要目的，是展示施加在非剛性物體上的載荷與物體中由此而引起的內力和變形之間的關系。從伽利略時代(1564 ~1642）以來，科學愛好者已經對結構構件和機器零件的承載能力問題，由外加載荷而引起的內力與變形的數學分析和實驗方法進行了大量的研究工作。最近 300年以來，這些科學家和工程師們的經驗和研究資料是留給現代工程師們的寶貴遺產。近300年來所獲得的基礎知識和所發(fā)展并建立的理論與分析技術，使得現代工程師們有能力和信心去設計前所未有的，大型的和復雜的結構和機器。

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.

經?？梢园l(fā)現，為了確定作用在物體上的所有未知載荷或反力，僅僅依據平衡（或運動）方程是不夠的。在這種情況下，必須考慮物體在加載后的幾何情況（尺寸或形狀）的改變。任何一方向或任何一尺寸上單位長度的變形稱為應變。在某些情況下，將由規(guī)定的最大變形而不是規(guī)定的最大應力來決定構件可以承受的最大載荷。

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

通常可以用幾種術語用來描述外加載荷，這里給出這些術語的定義，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.?

載荷可以按時間來分類：

(1)靜載荷是一種逐漸施加的載荷，這種載荷在相當短的時間內達到平衡。

(2)持續(xù)載荷是一種在相當長的時間內保持不變的載荷，例如結構的重量（稱為恒定載荷）。這種類型的載荷按靜載荷一樣處理；然而，對于某些材料在某些溫度和應力條件下，在短期載荷和持續(xù)載荷作用下抵抗破壞的強度可能會是不相同的。

(3)沖擊載荷是一種快速加上的載荷（能量載荷）。沖擊載荷通常引起振動，在振動消失前不會達到平衡，振動通常是由于自然阻尼力的作用而消失的。

(4)重復載荷是一種加載和卸載達到數千次的載荷。關閉汽車發(fā)動機閥門的螺旋形彈簧所承受的就是重復載荷。

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.?

載荷還可以按載荷作用的面積分類：

(1)集中載荷是作用在一個點上的載荷或者力。比起承受載荷構件的尺寸來說，作用在相當小的面積上的任何載荷，都可以假設為集中載荷。例如，作用在橋梁縱向構件上的卡車輪子的載荷就可以假設為集中載荷。

(2)分布載荷是沿長度或者是在一個面積上分布的載荷。載荷的分布可以是均勻的或者是非均勻的。厚度均勻的混凝土橋板的重量就是均布載荷的一個例子。

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.?

載荷還可以按作用的位置和方式分類：

(1)中心載荷是合力通過受載截面形心的載荷。如果合力通過所有截面的形心，這樣的載荷稱為軸向載荷。

(2）扭轉載荷是軸或其他構件承受力偶爾使構件產生扭轉的載荷。如果力偶位于與構件軸線正交的平面內，則該構件承受純扭轉。

(3）彎曲載荷是與構件縱向軸線橫著作用的載荷。外加載荷可以包括在平行于構件軸線的平面內的力偶。構件承受彎曲載荷后，沿其長度方向彎曲或變成弓形。

(4)組合載荷是兩種或兩種以上的上述各種類型載荷的組合。