外文翻译--机械性能试验-拉伸试验和其他基本测试

原文： Mechanical Testing : tension Test and Other Basic Tests 4.1 INTRODUCTION 4.2 INTRODUCTION TO TENSION TEST 4.3 ENGINEERING STRESS STRAIN PROPERTILES 4.4 TRENDS IN TENSILE BEHAVIOR 4.5 TRUE STRESS STRAIN INTERPRETATION OF TENSION TEST 4.6 CMPRESSION TEST 4.7 HARDNESS TESTS 4.8 NOTCH-IMPACT TESTS 4.9 BENDING AND TORSION TESTS 4.10 SUMMARY OBJECTIVES Become familiar with the basic types of mechanical tests, including tests in tension compression, indentation hardness , notch impact, bending ,and torsion . Analyze date from tension tests to determine materials properties, including both engineering properties and true stress-strain properties . Understand the significance of the properties obtained from basic mechanical tests, and explore some of the major trends in behavior that are seen in these tests. 4.1 INTRODUCTION Samples of engineering materials are subjected to a wide variety of mechanical tests to measure their strength or other properties of interest .Such samples, called specimens, are often broken or grossly deformed in testing. Some of the common forms of test specimen and loading situation are (a). Compression tests (b) are also common. In engineering, hardness is usually defined in terms of resistance of the material to penetration by a hard ball or point, as in (c). Various forms of bending test are also often used ,as is torsion of cylindrical rods or tubes. The simplest test specimens are smooth (unnotched) ones, as illustrated in Fig.4.2(a). More complex geometries can be used to produce conditions resembling those in actual engineering Components, Notches that have a definite radius at the end may be machined into test specimens, as in (b). The term notch is used here in a generic manner to indicate any notch , hole , groove , slot ,etc., that has the effect of a stress raiser . Sharp notches that behave similar to cracks are also used, as well as actual cracks that are introduced into the specimen prior to testing ,as in (c) . Figure 4.1 Geometry and loading situations commonly employed in mechanical testing of materials (a) tension,(b) compression, (c) indentation hardness,(d)cantilever bending,(e) three-point bending,(f) four-point bending, and (g) torsion Figure 4.2 Three classes of test specimen : (a) smooth or unnotched, (b) notched ,and (c) precracked . To understand mechanical testing, it is first necessary to briefly consider materials testing equipment and standard test methods. We will then discuss tests involving tension , compression, indentation, notch impact ,bending, and torsion .Various more specialized tests are discussed in later chapters in connection with such topics as brittle fracture ,fatigue, and creep. 4.2 INERODUCTION TO TENSION TEST A tension test consists of slowly pulling a sample of material with an axial force, as in Fig 4.1(a), until it breaks. This section of the chapter provides an introduction to the methodology for tension tests, as well as some additional comments. Sections that follow discuss tension testing in more detail, after which other types of test are considered. Figure4.5 ensile specimens of metals(left to right): untested specimen with 9 mm diameter test section, and broken specimens of gray cast iron ,aluminum alloy 70575-T651,and hot-rolled AISI 1020 steel.(Photo by R.A simonds.) 4.2.1 Test Methodology The test specimen used may have either a circular or a rectangular cross section, and its ends are usually enlarged to provide extra area for gripping and to avoid having the sample break where it is being gripped. Specimens both before and after testing are shown for several metals and polymers in Fig.4.5 and 4.6. Methods of gripping the ends vary with specimen geometry. A typical arrangement for threaded-end specimens id shown in Fig.4.7.Note that spherical bearings are used at each end to provide a pure tensile force, with no undesirable bending. The usual manner of conducting the test is to deform the specimen at a constant speed. For example, in the universal testing machines of Fig.4.3, the motion between the fixed and moving crossheads can be controlled at a constant speed. Hence, distance h in Fig.4.7 is varied so that The axial force that must be applied to achieve this displacement rate varies as the test proceeds. This force P may be divided by the cross-sectional area Ai to obtain the stress in the specimen at Any time during the test: Figure4.6 Tensile specimens of polymers (left to right): Untested specimen with a 7.6 mm diameter test section, a partially tested specimen of high-density polyethylene (HDPE), and broken specimens of nylon 101 and Teflon (PTFE). (photo by R.A.Simonds.) Displacements in the specimen are measured within a straight central portion of constant cross section over a gage length li, as indicated in Fig.4.7.Strain be computed from the change in this length ,L； 错误 !未找到引用源。 4.2 Stress and strain, based on the initial (undeformed) dimensions, Ai and Li, as just presented, are called engineering stress and strain. Figure 4.7 typical grips for a tension test in a universal testing machine.( adapted from AMST 97 Std.E8；copyright C ASTM； reprinted with permission.) It is sometimes reasonable to assume that all of the grip parts and the specimen ends are nearly rigid .In this case ,virtually all of the change in crosshead motion is due to deformation within the straight section of the test specimen, so that L is approximately the same as h, the change In h ,strain may therefore be estimated as =h/li, However, actual measurement of L is preferable. Strain as calculate from Eq.4.2 is dimensionless. As a convenience, strains are sometimes given as percentages, where %=100.Strain may also be expressed in millionths, called microstrain, where u=100. If strains are given as percentages or as percentages or as microstrain , then, prior to using the value for most calculations, it is necessary to convert to the dimensionless from . The principal result obtained from a tension test is a graph of engineering stress versus engineering strain for the entire test, called a stress-strain curve. With the use of digital computers in the laboratory ,the form of date is a list of numerical values of stress and strain, as sampled at short time intervals during the test. Stress-strain curves vary widely for different materials. Brittle behavior in a tension test is failure without extensive deformation. Gray cast iron, glass, and some polymers, such as PMMA (acrylic), are examples of materials with such behavior. A stress-strain curve for gray iron is shown in Fig.4.8.Othermaterials exhibit ductile behavior, failing in tension only after extensive deformation. Stress-strain curves for ductile behavior in engineering metals and some polymers are similar to Figs. 4.9 and 4.10,respectively. Figure4.8 Stress-strain curve for gray cast iron in tension, showing brittle behavior. Figure4.9 schematic of the engineering stress-strain curve of a typical ductile metal that exhibits necking behavior. Necking begins at the ultimate stress point. 译文 ： 机械性能试验：拉伸试验和其他基本测试 4.1 引言 4.2 拉伸试验介绍 4.3 工程应力应变特性 4.4 拉伸行为趋势 4.5 拉伸试验的真实应力应变解释 4.6 压缩试验 4.7 硬度试验 4.8 抗缺口冲击试验 4.9 点弯曲和扭转试验 4.10 概要 目 标： 熟悉机械试验的基本类型 ，包括拉压、 压痕硬度 、 抗缺口冲击 、 弯曲和扭转试验。 分析拉伸试验的数据来确定材料的性能，包括 工程特性与真实应力 -应变特性。 从基本力学试验中了解性能实验的意义，以及从这些试验中探索 一些行为中的主要趋势 。 4.1 引言 材料的样 品往往要进行各种各样的机械试验，以衡量他们的强度、性能之间的厉害关系。这样的样品，称为标本，这些样本在试验中经常被破坏或者严重变形。一些常见形式的试验样品和载荷情况如（ a）所示。（ b）情况在压缩试验中也很常见。在工程中，硬度通常被定义为材料的抗渗透硬质球或点，如（ c）所示。扭转圆柱棒或管在各种形式的弯曲试验也经常使用。 最简单的试样是光滑的（无缺口），如 图 4.2（ a） 所示 。 更复杂的几何形状可以用来生产类似那些在实际工程元件轴凹口具有一定半径的端部的情况，可被加工成如（ b）所示的试样。凹口这个术语在这里是对所 有切口，孔，槽，缝隙等笼统的表示，这些都是由于应力集中所带来的影响。锋利的凹口表现为类似于裂纹的被使用， 实际的裂纹被引入到测试前 的样品中，如（ c）所示。 图 4.1 图 4.1 中的几何和载荷情况是 在材料力学性能测试 中普遍采用，有（ a）拉伸，（ b）压缩，（ c）压痕硬度，（ d）悬臂弯曲，（ e）三点弯曲， (f)四点弯曲以及（ g）扭转 图 4.2 试验样品 图 4.2 中的三类试验样品为（ a）光滑或无缺口样品，（ b）有缺口样品以及（ c）预制裂纹样品。 要了解机械测试，首先有必要简要地研究材料 试验设备和标准试验方法。然后，我们将讨论包括拉伸，压缩，压痕，缺口冲击，弯曲和扭转测试。各种更专业的测试将在后面的章节中有讨论，这样的课题有脆断裂，疲劳和蠕变。 4.2 拉伸试验介绍 拉伸试验包括在一个轴向力方向慢慢提拉材料试样直到其断裂，如图 4.1（ a）所示。本章节将介绍进行拉伸试验的方法，以及一些附加注解。跟随讨论更详细的拉力试验，之后在考虑其他的测试。 图 4.5 拉伸试样金属 图 4.5 所示为 拉伸试样的金属（左到右） ： 未经 测试 的 9 毫米直径的测试 段试样， 破坏 的灰铸铁标本 ， 70575-T651 铝合金和 AISI1020 热轧钢（图片由R.A.Simonds.提供。） 4.2.1 测试方法 所使用的试样可以是圆形或矩形的横截面，其两端通常需要加粗来为夹持提供额外的面积，以避免样本在被夹持处断裂。图 .4.5 和 4.6 中的多种金属和聚合物所示为测试之前和之后的两个样品。 试样的几何形状不同抓住两端的方式也有所不同。螺纹端的典型布置 id 标本试样如图 4.7 所示。需要注意的是球轴承主要用在每个末端，以提供纯粹拉伸力，没有不需要的弯曲。 通常进行 的 测试 方式 是在一个恒定的速度 下 变形试样 。例如图 4.3 中的万能试验机固定和移动十字头 之间的运动可以以恒定的速度控制。因此，在图 4.7 的距离 h 的变化，满足 必须施加以达到这个位移速率变化的轴向力作为测试所得。这个力 P 可以表示为截面积