LESSON 2
Properties of Materials材料的特性
The properties of a material are those characteristics that help modify and distinguish one material from another.
材料的特性是变更和区分一种材料和另一种材料的特征。
All properties are observable and most can be measured quantitatively.
所有的特性都是可见的，而且大多数是可以定量测量的。
Properties are classified into two main groups, physical and chemical properties
材料特性可分为物理特性和化学特性两大类。
Physical properties involve no change in the composition of the material .
物理特性指的是材料成分不发生改变的特性。
Chemical properties are associated with the transformation of one material into another
化学特性指的是能够结合一种材料变成另外一种材料的特性。
.Physical properties are, in turn, arbitrarily subdivided into many categories.
物理特性，可以被任意地分为许多种类。
These subdivisions bear names such as mechanical, metallurgical , fabrication ,general ,magnetic ,electrical thermal ,optical , thermonuclear ,and electro-optical.
这些细类名称包括机械特性，冶金特性，装配特性，通用特性，磁性特性，电子特性
Regardless of the name of subdivision ,physical properties result from the response of the materials to some environmental variable ,such as a mechanical force, a temperature change, or an electro-magnetic field. In the following, the mechanical property of materials will be discussed.   
这些特性是由于材料在环境变化下形成的反应而产生，比如机械力，温度的改变，电磁场。
接下来，我们将要讨论材料的机械特性
Mechanical Property of Materials
In selecting a material for a product such as a piston in an internal combustion engine, a designer is interested in properties such as strength, ductility, hardness, or fatigue strength.
在为一个产品选择材料上，譬如内燃机上的一个活塞，一个设计者感兴趣的特性是材料的长度，延展性，硬度或者疲劳强度等。
Mechanical properties are defined as a measure of a material’s ability to carry or resist mechanical forces or stresses.
机械特性是用来衡量材料抵抗机械载荷或者压力性能的指标。
When any matter is at rest, the atomic or molecular structure is in equilibrium. The bonding forces in this structure resist any attempt to disrupt this equilibrium. 当任何材料在静止状态时，原子或分子结构保持着平衡，它们之间的结合力阻止任何破坏这种平衡的意图。
One such attempt may be an external force or load. Stress results from forces such as tens
ion, compression, or shear that pull, push, twist, cut, or in some way deform or change the shape of a piece of material. 这个意图也许是一个外力或外载荷。压力产生于拉伸，压缩，或者拖，推，扭，削之类的剪切，或者某种变形以及材料的部分形状改变。
Stress and strain
Stress is defined as the resistance offered by a material to external forces or loads.
压力是用来定义抵抗材料面对外力和外载荷时的性能指标。
It is measured in terms of the force exerted per area.
它是根据外在受力的每一份面积来衡量的。
Normal stress is that applied perpendicular to the surface to which it is applied, i.e., tension or compression.
正应力是垂直作用在材料表面上的，也就是指的拉伸和压缩。
Another way of defining it is to say that stress is the amount of force divided by the area over which it acts.
另外一种定义是说压力是根据力的作用面积来计算它的数值的。
An assumption is made that the stress is the same on each particle of area making up the total area.
有一种猜想声称，压力是作用在每个粒子大小的面积上的力综合起来构成的整个作用面上的综合力。
If  this is so, the stress in uniformly distributed. 如果确实如此的话，压力就是均衡分布的。
The force and the area over which the force acts can be used to calculate the stress produced in the material. 
力和力作用的面积可以用来计算出材料内部产生的压力。
With the use of polarized light and models made of photoelastic plastic, it is possible to det
ect the concentration of stress.
随着分极光和光弹性塑料模型的应用，很有可能测算出压力的密度。
Strain, or unit deformation, is defined as the unit change in the size or shape of material as a result of force on the material.
values翻译
应变或者单位变形是材料上的压力造成材料大小和形状的单位改变量。
Many times, we assume that a solid body is rigid; that is, when the body is loaded with some force, the body keeps its same size and shape.
很多时候，我们猜想固体是硬的。也就是说，当受到外在力时，材料机体保持它的大小和形状。
This is far from correct.
其实事实并非如此。
Regardless of how small the force ,a body will alter its shape when subjected to a force.
无论力有多小，一旦受到压力，机体都会改变它的形状。
The change in a physical dimension is called deformation.
物理尺寸上的改变就叫做变形。
When a piece of material is subjected to a load, it will not only deform in the direction of the load(axial deformation),but it will also deform in a lateral direction.
当材料的一部分受到载荷时，它不只是在轴向有变形，而且还存在横向变形。
The ratio of the lateral unit deformation or strain to the unit longitudinal deformation or strain is known as Poisson’s ratio.横向变形或应变与纵向变形或应变的比值称为泊松比。
tress-strain Diagrams应力-应变图

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