(again in mm2) of the indentation.
In the U. S. A. , the Rockwell test is favored. In that test the depth of the indentation is measured whilst the load is still being applied (rather than the lateral dimensions). The Rockwell Hardness Number is designated as HR. (4) Fatigue Tests
A very important phenomenon is called fatigue. It has been recognized for many years that static tensile or compressive testing is not adequate for predicting the strength of components subjected to vibration or repeated loading. These can fail at much lower stress levels, and there is a general relationship (due to Goodman) which shows the allowable oscillating stress level for a given mean stress. Fatigue testing needs considerable time, since each point on the final graph of applied stress S against the number N of cycles to failure requires a new specimen and N is usually between 106 and 108. For many non-ferrous alloys the S-N curve falls steadily, but for steels there is often a leveling-off after some 106 to 107 cycles. If the stress does not exceed this endurance limit, the specimen will last indefinitely.
Another very important failure phenomenon is that of high-stress low-cycle fatigue which is potentially dangerous in materials as disparate as animal bone and aerospace components. (5) Impact Testing
Another important subject is that of the behavior of relatively brittle materials such as cast iron, which may fail under even a single impact. Since it may be very important to avoid this type of fracture, impact tests have been devised in which a notched specimen is hit by a heavy pendulum. The energy absorbed is measured from the height of follow-through of the pendulum.
(6) High-Temperature tests
At high temperature the plastic deformation of materials is dominated by diffusion processes which, for metals, become evident above about 2/3 of the absolute melting temperature Tm. Tensile, compression or hardness tests may all be used at elevated temperatures.
(7) Creep Tests
An important feature of the hot tensile deformation of metals and alloys is that, at sufficiently high temperatures, extension will continue at a very slow rate under very low loads. This phenomenon, termed creep, is very important in gas turbines and many other high-temperature components. Creep tests are conducted over long periods, typically from 1000 to 10000 hours. Because of the length of time involved in creep testing, a shorter method is often used in which only approximate measurements of strain are made during the test, the main purpose of which is to determine the time to rupture at a given temperature and stress. These stress-rupture tests can be further speeded up by testing a string of specimens in series in a long furnace. The specimens are all subjected to the same end load but differing temperatures (which must be accurately measured, of course). (8) Fracture Toughness
In recent years much attention has been given to the fracture toughness of certain brittle materials, which is related to the ease with which a crack, once started, will propagate. A simple view of this process is that the opening of a crack releases elastic deformation energy but also requires the supply of surface energy to the two newly created areas of crack surface. If, in a brittle material, the released strain energy U is sufficient for this, the crack will propagate. (9) Plastic Anisotropy
In sheet metal forming in particular it is important to recognize that the properties of rolled sheet
may differ substantially in the rolling and transverse directions? as well as in the \direction. This feature can be measured in terms of the now well-known so-called 7-value, which is the ratio of the transverse to the longitudinal strain in a tensile test on wide, flat strip using techniques described by Hosford and Caddell. Volume is always approximately conserved in plastic deformation, so the thickness strain is also dependent upon the 7-value.
(Selected from; J. M. Alexander and J. S. Gunasekera, Strength of Materials, Vol. 2: Advanced Theory and Applications,Ellis Harwood Ltd, 1991. ) 材料7
机械测试标准
总结前面的讨论，我们知道了解材料的强度是非常重要的，对于最终的使用和使其变形所需要的力的确定都非常重要。因为产品设计和制造后的测试是非常重要的，在以前库存材料都会经过几个简单通用的测试才会生产为最终的产品。
（1）拉力试验
最简单和广泛应用的拉力测试需要一个两端扩大的圆柱棒。拉力试样受到沿其轴线的稳定增加的拉力，拉伸的长度在适当的标准下在应力-应变曲线上精确的表示出来。结果通常需求的最大拉伸应力,屈服应力、断裂伸长率及断面处的收缩率。此外，还要求杨氏弹性模量或者说是杨氏模量。 （2）抗压试验
在金属成形计算中，获得工件在比所受拉力更高的力的情况下的屈服强度是非常重要的。可以使用一个轴向压缩圆筒进行测试，测试前要适当的修正摩擦阻力，但是一个比较正确的结果是横的应变加上轴线所的压缩应变得来的 （3）硬度测试
拉力和抗压测试是破坏样本的，但是在没有破坏的情况下测试库存材料或是完成件的力学性能是非常重要的。在联合国最原始和最为人知的硬度测试是布氏硬度测试和维氏硬度测试，布氏硬度测试是在试验中标准棒（通常直径是10毫米）在规定搞得压力下被压入金属，典型的是3000kgf(等于29.42KN或者是6615lbf)。布氏硬度数值（BHN或者HB）的测试是在2毫米压痕时的力除以被压球表面面积。同样的，维氏硬度值得测量是在2毫米压痕时的力除以被压锥面面积。
在美国，洛克威尔测试是受人追捧的。在这个试验中，压痕的深度是要测量的，同时载荷是要应用的。HR特指洛克威尔硬度值。
（4）疲劳测试
一个非常重要的现象就是疲劳。静力拉压测试不能够充分的预测承受震动或是反复载荷作用的组件的强度的事已经被认可多年了。这种可以在很低的应力水平下失效，对于在给定的平均应力下允许的振动应力水平的一般关系。疲劳试验需要尝试很多的次数，因为每个点N都承受应力S循环破坏N次直到最后实效，需要很多新的试样做试验。通常N是在106和108之间。对于许多无铁的合金S-N曲线稳定地下降。如果是钢，则在约N为106到107个循环之后常有个平衡点。认为应力作用不允许超过这一个疲劳限制，否则试样可能不会正
常工作。
另外一个很重要地事实是，在高载荷低循环下，如动物骨骼运动和太空用品，在这种情况下工件有很大潜在危险。 （5）碰撞试验
另外的重要测试是对铸铁那样的脆性材料的测试，在很小的碰撞下铸铁会被破坏，这种测试能避免试样被破坏，它用锥形压件压入试样，锤击的大小用锤下落的高度来测试。 （6）高温测试
在高温下，材料容易发生塑性变形，对于金属，温度高于熔化温度Tm的2/3就变的容易伸展了。拉伸、压缩或者硬度试验可能要全部在高温下重复测试。 （7）蠕变试验
金属和合金受热变形的重要特征是，在足够高温度下，伸长将会在非常低的负载下以一个非常慢的速度连续发生变化。这个现象在力学上称为蠕变，在气轮机和其它的许多在高温下工作的工件是非常重要的。蠕变测试需要很长时间，一般需要1000到10000小时。 由于蠕变时间过长，我们采用一个更简单的方法，用一个设定的应力来测试，主要目的是测定试样在一定温度和应力下裂开所用的时间，那些应力测试在一个炉子里完成的，试样受同样的应力。（当然，这点必须被正确的测量） （8）断裂韧性
近年来我们更关注脆性材料，它伴随强烈的松弛，一旦开始，将会一直蔓延，我们看一个简单的例子，裂开时试样将释放弹性能。在脆性材料中，如果释放的应变能U足够，裂纹就会蔓延。
（9）塑性的向导性
在金属片中，认为钢板性能在旋转和横向移动时和穿过厚度的方向上的金属性能时不一样时很重要的。这一特点可以从现在人所共知的被测量的γ－型值知道，这是一个拉伸试验在纵向应变中的定量，平面裂纹被Hosford和Caddell用技术性的语言描述出来，大量的断裂通常被认为是塑性变形，所以厚度的应变也是根据γ－型值决定的。
（选自：J.M.Alexander and J.S.Gunasekera，材料强度，第二卷：先进的理论和应用，埃利斯
哈伍德有限公司，1991。）
Reading Material 8
Examples of Manufacturing Processes (continued)
Casting Casting can be characterized as: mass conserving, fluid state of material, mechanical primary basic process—filling of the die cavity. Casting is one of the oldest manufacturing methods and one of the best known processes. The material is melted and poured into a die cavity corresponding to the desired geometry. The fluid material takes the shape of the die cavity and this geometry is finally stabilized by the solidification of the material.
The stages or steps in a casting process are the making of a suitable mold, the melting of the material, the filling or pouring of the material into the cavity, and the solidification. Depending on the mold material, different properties and dimensional accuracies are obtained. Equipment used in a casting process includes furnaces, mold-making machinery, and casting machines.
Turning Turning can be characterized as : mass reducing, solid state of work material, mechanical
primary basic process—fracture. The turning process, which is the best known and most widely used mass-reducing process, is employed to manufacture all types of cylindrical shapes by removing material in the form of chips from the work material with a cutting tool. The work material rotates and the cutting tool is fed longitudinally. The cutting tool is much harder and more wear resistant than the work material. A variety of types of lathes are employed, some of which are automatic in operation. The lathes are usually powered by electric motors which, through various gears, supply the necessary torque to the work material and provide the feed motion to the tool.
A wide variety of machining operations or processes based on the same metal-cutting principle are available; among the most common are milling and drilling carried out on various machine tools. By varying the tool shape and the pattern of relative work-tool motions, many different shapes can be produced.
Electrical Discharge Machining Electrical Discharge Machining (EDM) can be characterized as: mass reducing, solid state of work material, thermal primary basic process—melting and evaporation. In EDM, material is removed by the erosive action of numerous small electrical discharges (sparks) between the work material and the tool (electrode), the latter having the inverse shape of the desired geometry. Each discharge occurs when the potential difference between the work material and the tool is large enough to cause a breakdown in the fluid medium, fed into the gap between the tool and workpiece under pressure, producing a conductive spark channel. The fluid medium, which is normally mineral oil or kerosene, has several functions. It serves as a dielectric fluid and coolant, maintains a uniform resistance to the flow of current, and removes the eroded material. The sparking, which occurs at rate of thousands of times per second, always occurs at the point where the gap between the tool and workpiece is smallest and develops so much heat that a small amount of material is evaporated and dispersed into the fluid. The material surface has a characteristic appearance composed numerous small craters.
Electrochemical Machining Electrochemical Machining (ECM) can be characterized as: mass reducing, solid state of work material, chemical primary basic process—electrolytic dissolution. Electrolytic dissolution of the workpiece is established through an electric circuit, where the work material is made of the anode, and the tool* which is approximately the inverse shape of the desired geometry, is made of the cathode. The electrolytes normally used are water-based saline solutions (sodium chloride and sodium nitrate in 10%^ 30% solutions ). The voltage, which usually is in the range 5 -20 V, maintains high current densities, 0. 5-^2 A/mm2, giving a relatively high removal rate, 0. 5—6 cm3/min, depending on the work material.
Flame Cutting Flame Cutting can be characterized as ; mass reducing, solid state of work material, chemical primary basic process—combustion. In flame cutting, the material (a ferrous metal) is heated to a temperature where combustion by the oxygen supply can start. Theoretically, the heat liberated should be sufficient to maintain the reaction once started, but because of heat losses to the atmosphere and the material, a certain amount of heat must be supplied continuously. A torch is designed to provide heat both for starting and maintaining the reaction. Most widely used is the oxyacetylene cutting torch, where heat is created by the combustion of acetylene and oxygen. The oxygen for cutting is normally supplied through a center hole in the tip of the torch.
The flame cutting process can only be used for easily combustible materials. For other materials,
cutting processes based on the thermal basic process—melting—have been developed (arc cutting, arc plasma cutting, etc. ). This is the reason cutting is listed in the table, which is at the beginning of Unit 8, under both thermal and chemical basic processes.
(Selected from: Stainless Steels* Materials Park, ASM International, 1994. )
材料8
制造过程的举例（续）
铸造 铸造的特性可以描绘如下：大量保存，流动形态，机械的最初过程-填满型腔。铸造师最老的制造方法之一，最为人知的过程之一。材料被熔解然后倒入型腔得到相应的几何形状。流动的液体由于材料的凝固而形成型腔的形状。
铸造的各步骤是制造相适合的模子，熔化金属，将熔融的金属液体倒入型腔之中，然后凝固。不同的性能和尺寸精度还取决于模子的材料。在铸造中使用的设备有熔炉，模具制造机械和铸造机械。
车削 车削可以被描述为：大量的减少，工作材料的固体情形，机械最初的过程—破裂。车削过程是最为人们所知的和最广泛利用的减少材料的过程，制造业中应用于各种圆柱类型，通过切削工具以小木片的形式减少材料。工作材料旋转，切削工具进行切削。切削工具比一般的工作材料要坚硬和抗磨损。应用车床多种多样，有些是自动的。车床通过电动机带动，电动机带动齿轮为工作材料提供必要的扭矩对工作材料进行切削加工。
多种广泛应用的加工操作或者过程基于相类似的金属切削原则，最普遍的磨和钻依靠多种机械进行。通过各种工具造型，于工作工具的相对关系可以制造出非常多的形状。
放电机 放电机（简写EDM）可以被描述为：材料大量的减少，工作材料的固体形态，热能的最初过程—溶解和蒸发。在发电机里，材料通过侵蚀行的操作和大量的在工作材料和工具间移动的电子进行移动的，后者可以得到相反的几何形状。工作材料和工具的电势不同产生电流大到足以在流动体中产生故障，在工具和受压工件之间建立一个导电通道。流动介质，通常是矿物油或者是煤油有好几个功能。它是作为绝缘流体，有一定的阻力，排除有腐蚀的材料。火花发生在千分之一秒的几率，通常是发生在工具和工件的之间，很小但是产生含高的温度使得一小部分材料浓缩蒸。材料的表面典型的面貌是许多的小小的坑洞。 电解加工 电解加工（简称ECM）可以被描述为：材料的大量减少，工作材料的固体形态，化学的最初过程—电解。工件的电解是建立在导电的回路，在回路中，工作材料在阳极，接近与相反的几何尺寸的工具作为阴极。通常使用的是水性的电解质盐溶液（氯化钠 硝酸钠10%-30%）电压通常在5-20V，高密度，0.5-2A每平方毫米，给一个相对较高的去除率，0.5-6立方厘米每分，依据其工作材料。
气割 气割可以被描述为：材料大量减少，工作材料的固体状态，化学的最初的过程—燃烧。气割，材料（亚铁）通过氧气供给加热到燃烧温度。理论上，热解放一旦开始应该充分保持反应，但是热损耗到空气中，热量必须要保持连续。最广泛的应用是氧乙炔切割机，热量由氧和乙炔的燃烧供应。用于切割的氧通常是通过火炬的尖口。
气割的过程只能用于简单的燃烧材料。对于其他的材料，切割过程基于热过程—融合-已经发展。切割的热量和化学过程原理已经列述在下第八单元的表格。
（选自不锈钢，Materials Park，美国金属协会，1994）