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1、Todays materials can be classified as metals and alloys, as polymers or plastics, as ceramics, or as composites; composites, most of which are man-made, actually are combinations of different materials.译文：当今的材料可以分为金属和合金，聚合物或者塑料，陶瓷或复合材料；复合材料，它们大多数是人造的，实际上是不同材料组合而成。Application of these materials depen
2、d on their properties; therefore, we need to know what properties are required by the application and to be able to relate those specification to the material. 译文：这些材料的应用取决于它们的性质；因此，根据应用的场合，我们需要知道什么样的性质是必需的，我们需要能够把这些详细说明同材料联系起来。For example, a ladder must withstand a design load, the weight of a pers
3、on using the ladder. However, the material property that can be measured is strength, which is affected by the load and design dimension. Strength values must therefore be applied to determined the ladder dimensions to ensure safe use. Therefore, in general, the structures of metallic materials have
4、 effects on their properties.译文：比如，一个梯子必须能经受住设计的载荷，也就是使用这个梯子的人的重量。然而，能够测量的材料的性质是强度，它为载荷和设计尺寸所影响。强度值因此被用来判定梯子的尺寸大小以保证安全使用。因此，通常说来，金属材料的结构对它们的性质有影响。In a “tensile test” a sample is gradually elongated to failure and the tensile force required to elongate the sample is measured using a load cell through
5、out the test. The result is a plot of tensile force versus elongation.译文：在一个张力测试实验中，样品缓慢地拉长直到失败，并且拉长样品所必需的拉力在整个测试过程中，用测压元件测量。结果是拉力同伸长度之间的一个图。结果是图表。 （有用的表示方法）The problem is that the load required to elongated the sample by a certain extent depends upon the dimension of the sample. This would be a big
6、 problem if , for example, mechanical property data were to be used to design a bridge, since it is clearly impossible to test an entire bridge. Thus what is clearly needed is to make the data from the tensile test independent of the size of the sample.译文：问题是，拉伸样品到一定程度所必需的载荷取决于样品的尺寸。比如，如果机械性质的数据被用于设
7、计一座桥梁，既然测试整个桥梁是明显不可能的，这样，明显需要的就是从与样品尺寸无关的拉伸实验中获得数据。Thus, what is clearly needed is to make the data from the tensile test independent of the size of the sample. To achieve this end, we use “stress” and “strain”.The “true” stress( ) is defined as : ,where F=force applied to the sample at any given in
8、stant and A= current cross-sectional area of the sample.译文：这样，明显所需要的是从拉力测试实验中得到和样品尺寸无关的数据。为了达到这个目标，我们使用“应力” 和“应变”。真正的应力（ ）可以这样定义：这里 F 等于作用在样品上任何一个给定时刻的拉力，A 等于对应当时样品的哼截面面积。The “true” strain( ) is defined as : ,where is the current gauge length and the origin gauge length of the sample.译文：真正的应力 被定义为 ，
9、这里 是当前测量长度， 是试样的原始测量长度。True stress and true strain provide the most accurate description of what actually happens to the material during testing and so are widely used in materials science. For engineering design, however, there are two problems. 译文：真正的应力和真正的应变提供材料在测试过程中所发生的最精确的描述，并且被广泛使用在材料科学中。然而，对
10、于工程设计来说，存在两个问题。Firstly, true stress requires a knowledge of the value of A throughout the test, whereas in real world applications the designer of structures chooses an initial cross sectional area (A0). Secondly true strain is not very easy to visualize. Consequently for engineering applications an
11、 “engineering” stress (s) and strain (e) are used in place of true stress and true strain:s = F / A0 and e = (l1 - l0) / l0 译文：首先，真实应力需要知道在整个测试过程中 A 的值，然而在实际应用过程中，结构设计者选择初始横截面面积 A0。第二，真实拉力非常不容易看见。相应的对于工程应用来说，工程压力（s）和拉力（e）被用来代替真实的应力和应变： s = F / A0 和 e = (l1 - l0) / l0 Stress has units of Pa (i.e. N m
12、-2) and strain is dimensionless. The concept of a stress is clearly closely related to that of pressure. Using the definitions of stress and strain given above, the load versus elongation curve produced by the tensile test can be converted into true stress - strain or engineering stress - strain cur
13、ves. Using these curves, it is now possible to describe the mechanical properties of metals and alloys.译文：应力有 Pa 的单位（比如 N/m2） ，应变是无量纲的。应力的概念是明显的同应变十分相关的。用上面给出的压力和拉力的定义，借助拉力试验得到的载荷同伸长之间的曲线可以转化为真实应力应变或者工程应力应变曲线。使用这些曲线，就可能描述金属和合金的机械性质。In true and engineering stress-strain relationships for a “typical”
14、metal, linear portion of the stress strain curves the material is deforming elastically at the Initial. 对一个典型的金属来说，在真实和工程应力应变相互关系中，材料压力曲线的线性部分是在最初的弹性形变In other words, if the load were removed the material will return to its initial, undeformed condition. In the linear elastic region, the “stiffness”
15、 or “elastic modulus” is the amount of stress required to produce a given amount of strain. For a tensile test, stiffness is described by Youngs modulus (E), which is given by:E = s / e or E = s / e换句话说，如果载荷被除去，材料将返回到初始的，没有形变的状况。在线性弹性区域，“硬度”和“弹性模量 ”是产生一定量的应力所必须的应变。对一个拉伸测试，硬度借助Youngs 模量来描述，它由下面两式来给出：
16、 E = s / e 或者 E = s / eThe greater the value of the stiffness, the more difficult it will be to produce elastic deformation. Thus, for example, in selecting a material for the springs of a vehicle, stiffness would be a key engineering design criterion.比较级比较级硬度值越大，产生弹性变形将越困难。比如，在选择材料作为交通工具的弹簧时，硬度将是一个重要的工程设计标准。On exceeding a certain stress, known as the “yield stress” or “yield strength” (sy or sy in true a
