凸轮的分析和应用外文翻译@中英文翻译@外文文献翻译

本科生毕业设计（论文）外文翻译 毕业设计（论文）题目： XXXXXXXXXXXXXXXXXXXXXXXXX 外文题目： Failure Analysis， Dimensional Determination And Analysis，Applications Of Cams 译文题目： 故障的分析、尺寸的决定以及凸轮的分析和应用 学 生 姓 名： 专 业： 指导教师姓名： 评 阅 日 期： 原文 Failure Analysis， Dimensional Determination And Analysis， Applications Of Cams INTRODUCTION It is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designed Sometimes a failure can be serious， such as when a tire blows out on an automobile traveling at high speed On the other hand， a failure may be no more than a nuisance An example is the loosening of the radiator hose in an automobile cooling system The consequence of this latter failure is usually the loss of some radiator coolant， a condition that is readily detected and corrected The type of load a part absorbs is just as significant as the magnitude Generally speaking，dynamic loads with direction reversals cause greater difficulty than static loads， and therefore，fatigue strength must be considered Another concern is whether the material is ductile or brittle For example， brittle materials are considered to be unacceptable where fatigue is involved Many people mistakingly interpret the word failure to mean the actual breakage of a part However， a design engineer must consider a broader understanding of what appreciable deformation occurs A ductile material， however will deform a large amount prior to rupture Excessive deformation， without fracture， may cause a machine to fail because the deformed part interferes with a moving second part Therefore， a part fails(even if it has not physically broken)whenever it no longer fulfills its required function Sometimes failure may be due to abnormal friction or vibration between two mating parts Failure also may be due to a phenomenon called creep， which is the plastic flow of a material under load at elevated temperatures In addition， the actual shape of a part may be responsible for failure For example，stress concentrations due to sudden changes in contour must be taken into account Evaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductile In general， the design engineer must consider all possible modes of failure， which include the following Stress Deformation Wear Corrosion Vibration Environmental damage Loosening of fastening devices The part sizes and shapes selected also must take into account many dimensional factors that produce external load effects， such as geometric discontinuities， residual stresses due to forming of desired contours， and the application of interference fit joints Cams are among the most versatile mechanisms available A cam is a simple two-member device The input member is the cam itself ， while the output member is called the follower Through the use of cams， a simple input motion can be modified into almost any conceivable output motion that is desired Some of the common applications of cams are Camshaft and distributor shaft of automotive engine Production machine tools Automatic record players Printing machines Automatic washing machines Automatic dishwashers The contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematically However， the vast majority of cams operate at low speeds(less than 500 rpm) or medium-speed cams can be determined graphically using a large-scale layout In general， the greater the cam speed and output load， the greater must be the precision with which the cam contour is machined DESIGN PROPERTIES OF MATERIALS The following design properties of materials are defined as they relate to the tensile test Figure 2.7 Static Strength The strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required function Thus the static strength may be considered to be approximately equal to the proportional limit， since no plastic deformation takes place and no damage theoretically is done to the material Stiffness Stiffness is the deformation-resisting property of a material The slope of the modulus line and， hence， the modulus of elasticity are measures of the stiffness of a material Resilience Resilience is the property of a material that permits it to absorb energy without permanent deformation The amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic region Toughness Resilience and toughness are similar properties However， toughness is the ability to absorb energy without rupture Thus toughness is represented by the total area underneath the stress-strain diagram， as depicted in Figure 2 8b Obviously， the toughness and resilience of brittle materials are very low and are approximately equal Brittleness A brittle material is one that ruptures before any appreciable plastic deformation takes place Brittle materials are generally considered undesirable for machine components because they are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders， holes， notches， or keyways Ductility A ductility material exhibits a large amount of plastic deformation prior to rupture Ductility is measured by the percent of area and percent elongation of a part loaded to rupture A 5%elongation at rupture is considered to be the dividing line between ductile and brittle materials Malleability Malleability is essentially a measure of the compressive ductility of a material and， as such， is an important characteristic of metals that are to be rolled into sheets Figure 2.8 Hardness The hardness of a material is its ability to resist indentation or scratching Generally speaking， the harder a material， the more brittle it is and， hence， the less resilient Also， the ultimate strength of a material is roughly proportional to its hardness Machinability Machinability is a measure of the relative ease with which a material can be machined In general， the harder the material， the more difficult it is to machine COMPRESSION AND SHEAR STATIC STRENGTH In addition to the tensile tests， there are other types of static load testing that provide valuable information Compression Testing Most ductile materials have approximately the same properties in compression as in tension The ultimate strength， however， can not be evaluated for compression As a ductile specimen flows plastically in compression， the material bulges out，but there is no physical rupture as is the case in tension Therefore， a ductile material fails in compression as a result of deformation， not stress Shear Testing Shafts， bolts， rivets， and welds are located in such a way that shear stresses are produced A plot of the tensile test The ultimate shearing strength is defined as the stress at which failure occurs The ultimate strength in shear， however， does not equal the ultimate strength in tension For example， in the case of steel， the ultimate shear strength is approximately 75% of the ultimate strength in tension This difference must be taken into account when shear stresses are encountered in machine components DYNAMIC LOADS An applied force that does not vary in any manner is called a static or steady load It is also common practice to consider applied forces that seldom vary to be static loads The force that is gradually applied during a tensile test is therefore a static load On the other hand， forces that vary frequently in magnitude and direction are called dynamic loads Dynamic loads can be subdivided to the following three categories Varying Load With varying loads， the magnitude changes， but the direction does not For example， the load may produce high and low tensile stresses but no compressive stresses Reversing Load In this case， both the magnitude and direction change These load reversals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversals Shock Load This type of load is due to impact One example is an elevator dropping on a nest of springs at the bottom of a chute The resulting maximum spring force can be many times greater than the weight of the elevator， The same type of shock load occurs in automobile springs when a tire hits a bump or hole in the road FATIGUE FAILURE-THE ENDURANCE LIMIT DIAGRAM The test specimen in Figure 2.10a， after a given number of stress reversals will experience a crack at the outer surface where the stress is greatest The initial crack starts where the stress exceeds the strength of the grain on which it acts This is usually where there is a small surface defect， such as a material flaw or a tiny scratch As the number of cycles increases， the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaft The conception of the initial crack is itself a stress concentration that accelerates the crack propagation phenomenon Once the entire periphery becomes cracked， the cracks start to move toward the center of the shaft Finally， when the remaining solid inner area becomes small enough， the stress exceeds the ultimate strength and the shaft suddenly breaks Inspection of the break reveals a very interesting pattern， as shown in Figure 2.13 The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each other However， the center portion is rough， indicating a sudden rupture similar to that experienced with the fracture of brittle materials This brings out an interesting fact When actual machine parts fail as a result of static loads，they normally deform appreciably because of the ductility of the material Figure 2.13 Thus many static failures can be avoided by making frequent visual observations and replac ing all deformed parts However， fatigue failures give to warning Fatigue fail mated that over 90% of broken automobile parts have failed through fatigue The fatigue strength of a material is its ability to resist the propagation of cracks under stress reversals Endurance limit is a parameter used to measure the fatigue strength of a material By definition， the endurance limit is the stress value below which an infinite number of cycles will not cause failure Let us return our attention to the fatigue testing machine in Figure 2.9 The test is run as follows： A small weight is inserted and the motor is turned on At failure of the test specimen， the counter registers the number of cycles N， and the corresponding maximum bending stress is calculated from Equation 2.5 The broken specimen is then replaced by an identical one， and an additional weight is inserted to increase the load A new value of stress is calculated， and the procedure is repeated until failure requires only one complete cycle A plot is then made of stress versus number of cycles to failure Figure 2.14a shows the plot， which is called the endurance limit or S-N curve Since it would take forever to achieve an infinite number of cycles， 1 million cycles is used as a reference Hence the endurance limit can be found from Figure 2.14a by noting that it is the stress level below which the material can sustain 1 million cycles without failure The relationship depicted in Figure 2.14 is typical for steel， because the curve becomes horizontal as N approaches a very large number Thus the endurance limit equals the stress level where the curve approaches a horizontal tangent Owing to the large number of cycles involved，N is usually plotted on a logarithmic scale， as shown in Figure 2.14b When this is done， the endurance limit value can be readily detected by the horizontal straight line For steel， the endurance limit equals approximately 50% of the ultimate strength However， if the surface finish is not of polished equality， the value of the endurance limit will be lower For example， for steel parts with a machined surface finish of 63 microinches ( in )， the percentage drops to about 40% For rough surfaces (300in or greater)， the percentage may be as low as 25% The most common type of fatigue is that due to bending The next most frequent is torsion failure， whereas fatigue due to axial loads occurs very seldom Spring materials are usually tested by applying variable shear stresses that alternate from zero to a maximum value， simulating the actual stress patterns In the case of some nonferrous metals， the fatigue curve does not level off as the number of cycles becomes very large This continuing toward zero stress means that a large number of stress reversals will cause failure regardless of how small the value of stress is Such a material is said to have no endurance limit For most nonferrous metals having an endurance limit， the value is about 25% of the ultimate strength EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITY Generally speaking， when stating that a material possesses specified values of properties such as modulus of elasticity and yield strength， it is implied that these values exist at room temperature At low or elevated temperatures， the properties of materials may be drastically different For example， many metals are more brittle at low temperatures In addition， the modulus of elasticity and yield strength deteriorate as the temperature increases Figure 2.23 shows that the yield strength for mild steel is reduced by about 70% in going from room temperature to 1000oF Figure 2.24 shows the reduction in the modulus of elasticity E for mild steel as the temperature increases As can be seen from the graph， a 30% reduction in modulus of elasticity occurs in going from room temperature to 1000oF In this figure， we also can see that a part loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperatures Figure 2.24 CREEP: A PLASTIC PHENOMENON Temperature effects bring us to a phenomenon called creep， which is the increasing plastic deformation of a part under constant load as a function of time Creep also occurs at room temperature， but the process is so slow that it rarely becomes significant during the expected life of the temperature is raised to 300oC or more， the increasing plastic deformation can become significant within a relatively short period of time The creep strength of a material is its ability to resist creep， and creep strength data can be obtained by conducting long-time creep tests simulating actual part operating conditions During the test， the plastic strain is monitored for given material at specified temperatures Since creep is a plastic deformation phenomenon， the dimensions of a part experiencing creep are permanently altered Thus， if a part operates with tight clearances， the design engineer must accurately predict the amount of creep that will occur during the life of the machine Otherwise， problems such binding or interference can occur Creep also can be a problem in the case where bolts are used to clamp tow parts together at elevated temperatures The bolts， under tension， will creep as a function of time Since the deformation is plastic， loss of clamping force will result in an undesirable loosening of the bolted joint The extent of this particular phenomenon， called relaxation， can be determined by running appropriate creep strength tests Figure 2.25 shows typical creep curves for three samples of a mild steel part under a constant tensile load Notice that for the high-temperature case the creep tends to accelerate until the part fails The time line in the graph (the x-axis) may represent a period of 10 years， the anticipated life of the product Figure 2.25 SUMMARY The machine designer must understand the purpose of the static tensile strength test This test determines a number of mechanical properties of metals that are used in design equations Such terms as modulus of elasticity， proportional limit， yield strength， ultimate strength， resilience，and ductility define properties that can be determined from the tensile test Dynamic loads are those which vary in magnitude and direction and may require an investigation of the machine parts resistance to failure Stress reversals may require that the allowable design stress be based on the endurance limit of the material rather than on the yield strength or ultimate strength Stress concentration occurs at locations where a machine part changes size， such as a hole in a flat plate or a sudden change in width of a flat plate or a groove or fillet on a circular shaft Note that for the case of a hole in a flat or bar， the value of the maximum stress becomes much larger in relation to the average stress as the size of the hole decreases Methods of reducing the effect of stress concentration usually involve making the shape change more gradual Machine parts are designed to operate at some allowable stress below the yield strength or ultimate strength This approach is used to take care of such unknown factors as material property variations and residual stresses produced during manufacture and the fact that the equations used may be approximate rather that exact The factor of safety is applied to the yield strength or the ultimate strength to determine the allowable stress Temperature can affect the mechanical properties of metals Increases in temperature may cause a metal to expand and creep and may reduce its yield strength and its modulus of elasticity If most metals are not allowed to expand or contract with a change in temperature， then stresses are set up that may be added to the stresses from the load This phenomenon is useful in assembling parts by means of interference fits A hub or ring has an inside diameter slightly smaller than the mating shaft or post The hub is then heated so that it expands enough to slip over the shaft When it cools， it exerts a pressure on the shaft resulting in a strong frictional force that prevents loosening TYPES OF CAM CONFIGURATIONS Plate Cams This type of cam is the most popular type because it is easy to design and manufacture Figure 6 1 shows a plate cam Notice that the follower moves perpendicular to the axis of rotation of the camshaft All cams operate on the principle that no two objects can occupy the same space at the same time Thus， as the cam rotates ( in this case， counterclockwise )， the follower must either move upward or bind inside the guide We will focus our attention on the prevention of binding and attainment of the desired output follower motion The spring is required to maintain contact between the roller of the follower and the cam contour when the follower is moving downward The roller is used to reduce friction and hence wear at the contact surface For each revolution of the cam， the follower moves through two strokes-bottom dead center to top dead center (BDC to TDC) and TDC to BDC Figure 6.2 illustrates a plate cam with a pointed follower Complex motions can be produced with this type of follower because the point can follow precisely any sudden changes in cam contour However， this design is limited to applications in which the loads are very light；otherwise the contact point of both members will wear prematurely， with subsequent failure Two additional variations of the plate cam are the pivoted follower and the offset sliding follower， which are illustrated in Figure 6.3 A pivoted follower is used when rotary output motion is desired Referring to the offset follower， note that the amount of offset used depends on such parameters as pressure angle and cam profile flatness， which will be covered later A follower that has no offset is called an in-line follower Figure 6.3 Translation Cams Figure 6.4 depicts a translation cam The follower slides up and down as the cam translates motion in the horizontal direction Note that a pivoted follower can be used as well as a sliding-type follower This type of action is used in certain production machines in which the pattern of the product is used as the cam A variation on this design would be a three-dimensional cam that rotates as well as translates For example， a hand-constructed rifle stock is placed in a special lathe This stock is the pattern， and it performs the function of a cam As it rotates and translates， the follower controls a tool bit that machines the production stock from a block of wood Figure 6.4 Positive-Motion Cams In the foregoing cam designs， the contact between the cam and the follower is ensured by the action of the spring forces during the return stroke However， in high-speed cams， the spring force required to maintain contact may become excessive when added to the dynamic forces generated as a result of accelerations This situation can result in unacceptably large stress at the contact surface， which in turn can result in premature wear Positive-motion cams require no spring because the follower is forced to contact the cam in two directions There are four basic types of positive-motion cams: the cylindrical cam， the grooved-plate cam ( also called a face cam ) ， the matched-plate cam， and the scotch yoke cam Cylindrical Cam The cylindrical cam shown in Figure 6.5 produces reciprocating follower motion， whereas the one shown in Figure 6.6 illustrates the application of a pivoted follower The cam groove can be designed such that several camshaft revolutions are required to produce one complete follower cycle Grooved-plate Cam In Figure 6.8 we see a matched-plate cam with a pivoted follower， although the design also can be used with a translation follower Cams E and F rotate together about the camshaft B Cam E is always in contact with roller C， while cam F maintains contact with roller D Rollers C and D are mounted on a bell-crank lever， which is the follower oscillating about point A Cam E is designed to provide the desired motion of roller C， while cam F provides the desired motion of roller D Scotch Yoke Cam This type of cam， which is depicted in Figure 6.9， consists of a circular cam mounted eccentrically on its camshaft The stroke of the follower equals two times the eccentricity e of the cam This cam produces simple harmonic motion with no dwell times Refer to Section 6.8 for further discussion CAM TERMINOLOGY Before we become involved with the design of cams， it is desirable to know the various terms used to identify important cam design parameters The following terms refer to Figure 6.11 The descriptions will be more understandable if you visualize the cam as stationary and the follower as moving around the cam Trace Point The end point of a knife-edge follower or the center of the roller of a roller-type follower Cam Contour The actual shape of the cam Base Circle The smallest circle that can be drawn tangent to the cam contour Its center is also the center of the camshaft The smallest radial size of the cam stars at the base circle Pitch Curve The path of the trace point， assuming the cam is stationary and the follower rotates about the cam Prime Circle The smallest circle that can be drawn tangent to the pitch curve Its center is also the center of the camshaft Pressure Angle The angle between the direction of motion of the follower and the normal to the pitch curve at the point where the center of the roller lies Cam Profile Same as cam contour BDC Bottom Dead Center， the position of the follower at its closest point to the cam hub Stroke The displacement of the follower in its travel between BDC and TDC Rise The displacement of the follower as it travels from BDC to TDC Return The displacement of the follower as it travels from TDC or BDC Ewell The action of the follower when it remains at a constant distance from the cam hub while the cam turns A clearer understanding of the significance of the pressure angle can be gained by referring to Figure 6.12 Here FT is the total force acting on the roller It must be normal to the surfaces at the contact point Its direction is obviously not parallel to the direction of motion of the follower Instead， it is indicated by the angle ， the pressure angle， measured from the line representing the direction of motion of the follower Therefore， the force FT has a horizontal component FH and a vertical component FV The vertical component is the one that drives the follower upward and， therefore， neglecting guide friction， equals the follower Fload The horizontal component has no useful purpose but it is unavoidable In fact， it attempts to bend the follower about its guide This can damage the follower or cause it to bind inside its guide Obviously， we want the pressure angle to be as possible to minimize the side thrust FH A practical rule of thumb is to design the cam contour so that the pressure angle does not exceed 30o The pressure angle，in general， depends on the following four parameters: Size of base circle Amount of offset of follower Size of roller Flatness of cam contour ( which depends on follower stroke and type of follower motion used ) Some of the preceding parameters cannot be changed without altering the cam requirements，such as space limitations After we have learned how to design a cam， we will discuss the various methods available to reduce the pressure angle 译文： 故障 的 分析 、尺寸的决定以及凸轮的分析和应用 前言介绍： 作为 一 名 设计工程师 有 必要知道 零件如何发生 和为什么 会发生故障， 以便 通过进行 最 低限度的 维修 以保证机器的 可靠 性。 有时一 次零件的故障或者失效可能是很严重的一件事情，比如，当一辆汽车 正在高速行驶的时候，突然汽车的轮胎发生爆炸等。 另一方面 ，一个零件发生故障也 可能 只 是一件 微不足道的小事，只是给你造成了一点小麻烦。 一个例子是在一个汽车冷却系统里的暖气装置软管的松 动。后者发生的这次故障造成 的结果通常 只不过 是一些暖气装置 里 冷却剂的损失 ，是一种很 容易被发现并且 被 改正的 情况。 能够被零件进行吸收的载荷是相当重要的。一般说来， 与静载重相比较，有 两个相反方向 的动载荷 将会 引起更大的 问题， 因此 ， 疲劳强度必须被考虑 。 另一 个关键 是材料是可延展性的 还是脆 性 的 。例如， 脆的材料被认为在 存在 疲劳的地方是 不能够被使用 的 。 很多人 错误的把一个零件发生故障或者失效理解成这样就 意味着一个 零件遭到了实际的物理 破损 。无论如何， 一 名 设计工程师必须 从一个更广泛的范围来 考虑 和 理解变形 是究竟如何 发生 的。 一种 具有 延展 性 的材料 ， 在破裂之前 必 将 发生很大程度 的 变形。发生了 过度的变形 ，但并 没有 产生 裂缝 ，也 可能 会 引起一台机器出毛病，因为 发生 畸 变 的 零件会 干扰 下 一个 零件 的移动 。因此， 每当它不 能够 再履行它要求 达到 的 性 能的时候，一个 零件就都算是被毁坏了（ 即使它 的表面没有被损毁）。 有时 故障 可能 是由于 两个 两个相互搭配 的 零件 之间 的不正 常的磨擦或者 异常的 振动 引起的。 故障 也可能是 由一种 叫 蠕变 的现象 引起的， 这 种现象是指金属 在高温下 时 一种材料的塑 性 流动 。此外， 一个 零件 的实际形状可能 会引起故障的发生。例如，应力的 集中 可能就是 由于轮廓的突然变化 引起的，这一点也需要 被考虑到 。 当有用 两个相 反方向的动载荷 ， 材料不 具有很好的 可延展 性 时，对 应力 考虑的评估 就 特别重要 。 一般说来， 设计工程师必须考虑 故障 可能 发生 的全部方式 ， 包括如下 一些方面： 压力 变形 磨损 腐蚀 振动 环境破坏 固定设备 松动 在 选择 零件的 大小与形状 的时候， 也必须考虑到 一些可能会 产 生 外部负 载 影响的 空间 因素 ， 例如几何学间断性 ，为了达到 要求的 外形 轮廓 及使用相关的连接件，也会产生相应 的残余应力 。 凸轮是被应用的最广泛的机械结构之一。凸轮是一种仅仅有两个组件构成的设备。主动件本身就是凸轮，而输出件被称为从动件。通过使用凸轮，一个简单的输入动作可以被修改成几乎可以想像得到的任何输出运动。常见的一些关于凸轮应用的例子有： 凸轮轴和汽车发动机工程的装配 专用机床 自动电唱机 印刷机 自动的洗衣机 自动的洗碗机 高速凸轮 (凸轮超过 1000 rpm 的速度 )的轮廓必须从数学意义上来定义 。无论如何，大多数凸轮以低速 (少于 500 rpm)运行而中速的凸轮可以通过一个大比例的图形表示出来。一般说来，凸轮的速度和输出负载越大，凸轮的轮廓在被床上被加工时就一定要更加精密。 材料的设计属性 当他们与抗拉的试验有关时，材料的下列设计特性被定义 如下。 静强度： 一个 零件 的 强度 是 指零件在不会 失去它 被 要求的能 力的前提下 能 够承受的 最大应力 。 因此静 强度 可 以 被认为 是 大约等于比例极限 ，从理论上来说，我们可以认为在这种情况下，材料没有发生塑性变形和物理破坏。 刚度 ： 刚度是 指 材料抵抗变形的一种 属性。 这条斜 的 模数线 以 及 弹性模数是一种 衡量 材料的刚度的 一种方法。 弹性： 弹性是 指零件能够 吸收能量 但并 没有 发生 永久变形的一种材料的 属性。 吸收的能量的 多少可以通过下面弹性区域内的应力图表来描述出来。 韧性： 韧 性 和弹性是 两种 相似的特性 。无论如何， 韧 性 是 一种可以 吸收能量 并且不会发生 破裂的能力 。 因此 可以通过应力 图 里面的 总面积 来 描述韧 性，就像 用图 2.8 b 描绘的那样 。显而易见， 脆 性 材料的 韧性 和弹性非常低，并且大约相等 。 脆性： 一种脆 性 的材料 就 是 指 在任何 可以被看出来 的塑性变形之前 就发生 破裂 的材料。脆性 的材料一般被 认为不适合用来做 机床的零部件 ，因为 当遇到由轴肩，孔，槽，或者键槽等几何应力集中源引起的高的应力时，脆性材料是无法来产生局部屈服的现象以适应高的应力环境的。 延展性： 一种延展性材料 会 在破裂之前 表现出很大程度上的 塑性变形 现象。 延展性 是通过可延展的零件在发生破裂前后的面积和长度的百分比来测量的。 一 个在发生破裂的零件，其伸长量如果为 5%，则认为该伸长量就是 可延展 性 和脆 性 材料 分界 线 。 可锻性 ： 可锻性 从根本上来说是指材料的一种在承受挤压或压缩是可以发生塑性变形的能力，同时，它也 是一种 在金属被滚压成钢板时所需金属的重要性能。 硬 度： 一种材料的硬度是 指它 抵抗 挤压 或者 拉伸 它的能力 。一般说来，材料越硬，它的脆性也越大，因此，弹性越小。同样， 一种材料的极限强度粗略与它的硬度成正比 。 机械加工性能（或切削性）： 机械加工性能是指材料的一种容易被加工的性能。通常，材料越硬，越难以加工。 压应力和剪应力 除抗拉的试验之外 ，还 有 其它一些可以提供有用信息的静载荷的实验类型。 压缩测试 ： 大多数可延展材料大约有相同特性 ，当它们 处于受压 状态的 紧张状态 时。 极限强度 ，无论如何， 不能 够 被用于 评价 压 力状态。 当一件 具有 可延展 性 的样品受压 发生塑性变形 时 ， 材料的其它部分会凸出来，但是在这种紧张的状态下，材料通常不会发生物理上的破裂。因此，一种可延展的材料 通常是 由于变形受压 而 损坏 的，并不是压力的原因。 剪 应力 测试 ： 轴，螺钉，铆钉和焊接件被用这样一种方式定 位以致于生产 了 剪应力 。 一 张 抗拉试验的试验图纸就可以说明问题。当压力大到可以使材料发生永久变形或发生破坏时，这时的压力就被定义为极限剪切强度。极限剪切强度，无论如何， 不等于处于紧张状态的极限强度 。例如，以钢的材料为例， 最后的剪切强度是处于紧张状态大约极限强度的 75%。 当 在机器零部件里遇到 剪应力 时， 这个差别 就 一 定 要 考虑到 了。 动力载荷 不 会在各种不同的形式的力之间不停发生 变化的 作 用 力被叫作静载荷或者稳定载荷。此外，我们通常也把很少发生变化的作用力叫作静载荷。在拉伸实验中，被分次、 逐渐 的加载的作用力也被叫作静载荷。 另一方面， 在大小 和方向上 经常 发生 变化的力 则被称 为动载荷 。 动载荷可以被再 细 分 为以下的 3 种类 型。 变载荷： 所谓变载荷，就是说载荷的大小在变，但是方向不变的载荷。比如说，变载荷会产生忽大忽小的张应力，但不会产生压应力。 周期性载荷： 像 这样的话 ，如果 大小和方向 同时 改变 ，则就是说这种载荷会反复周期性 的产生变化的拉应力和压应力，这种现象往往就伴随着应力在方向和大小上的周期性变化。 冲击载荷： 这类 载荷是由于冲击作用产生的。 一个例子 就 是一 台 升降机 坠落到位于通道底部的一套弹簧装置上，这套装置产生的力会比升降机本身的重量大上好几倍。当汽车的一个轮胎碰撞到道路上的一个突起或者路上的一个洞时， 相同的冲击荷载的类型 也会 在汽车 的减震器弹簧上 发生 。 疲劳失效 疲劳极限线图 正如图 2.10a 所示，如果材料的某处经常会产生大量的周期性作用力，那么在材料的表面就很可能会出现裂缝。 裂缝最初 是 在 应 力超过它 极限压力 的地方开始 出现的，而 通常 这往往 是有 微 小的表面缺陷的地方 ， 例如 有 一处 材料出现 瑕疵或者一道极小的 划 痕 。 当循环的 次数增加时 ， 最初 的 裂缝开始在轴的周围的 逐渐产生许多类似的裂缝。所以说，第一道裂缝的意义就是指应力集中的地方，它会加速其它裂缝的产生。 一旦整个 的外 围 斗出现了裂缝， 裂缝 就会 开始向轴的中心 转移。最后， 当剩下的固体的内部地区变得足够小 ，且当 压力超过极限强度 时 ，轴 就会 突然 发生 断 裂。 对 断面 的检查 可以发现 一种非常有趣的图案 ， 如图 2.13中所示 。 外部 的一个 环形 部分 相对光滑 一些 ，因为 原来表面上相互交错的裂缝之间不断地发生磨擦导致了 这种现象的产生。无论如何， 中心部分是粗 糙 的 ， 表明 中心是突然发生了断裂，类似于脆 性 材料 断裂时的现象。 这 就表明了 一个有趣的事实 。 当 正在使用的 机器零件由于静 载荷的原因出现问题时，由于 材料 具有 的延展性，他们通常 会发生一定程度的 变形 。 尽管许多地由于静压力导致的零件故障可以通过 频繁的 做实际的观察 并且替换全部 发生变 形的 零件来 避免 。 不管怎样， 疲劳失效 有助于起到 警告 的作用。汽车中发生故障的零件中的 90%的原因都是因为疲劳的作用。 一种材料的疲劳强度是 指 在压力 的 反 复作用 下 的 抵抗 产生 裂缝的能力 。 持久极限是用来评价 一种材料 的疲劳强度的一个 重要 参数 。进一步说明就是， 持久极限 就 是 指在 无限循环的作用力下 不引起 失效 的压力值 。 让我们 回头来看 在图 2.9 所示的 疲劳试验机器的 。 试验 是这样被进行的： 一件小的重物被插入，电动机被 启动。 在试样的 失效过程中，由 计算寄存器 记录下 循环 的次数 N， 并且弯曲压力的相应最大量由第 2.5 方程式计算 。然后用一个新的样品替换掉 被毁坏的样品 ， 并且将另一个 重物插入 以 增加负荷 量。 压力的新 的数值再次 被计算 ， 并且 相同的 程序 再次 被重复进行 ，直到 零件的失效 只需要一个完整周期 时为止。然后根据压力值和所需的循环的次数来绘制一个 图。正如图表 2.14a 所 示 图形，该图 被 称 为持久极限 曲线 或者 S-N 曲线 。由于这需要的前提是要进行无限次 的循环 ，所以我们可以以 100 万个循环 用来 作 循环 参考 单位。 因此，持久极限可 以 从 图表 2.14a那里 看到，该材料是在承受了 100 万个循环 后而没有发生失效的。 用图 2.14 描绘的关系对于钢 的材料来说更为 典型 ， 因为当 N 接近非常大的 数字 时，曲线 就会 变 得 水平 。 因此持久极限等于曲线接近一条水平的切线 时 的压力水平 。 由于包含 了 大量的循环 ，在绘图时， N 通常 会 被 按照 对数 标度来画， 如图 2.14 b 中所示 。当采用这样的方法做时， 水平的直线 就 可 以更 容易发现 材料的持久 极限值 。 对于钢 的材料 来说 ， 持久极限 值大约等于极限强度的 50%。无论如何，已经加工 完成 的 表面 如果 不是 一样的光滑， 持久极限的值 就会被降低。例如， 对于钢 材料的零件 来说 ， 63 微英寸（ in ） 的机械加工的表面 ，零件的持久极限占理论的持久极限的 百分比降低到 了 大约 40%。而 对于粗糙的表面来说 （ 300in，甚至更多）， 百分比可能 降低到 25%左右的水平。 最 常见 的疲劳 损坏的 类型 通常是 由于弯曲 应力所引起的。 其次 就 是扭 应力导致的 失 效，而 由于轴向负载 引起的 疲劳 失效却 极少发生 。弹性 材料 通常使用从零到最大 值之间变化 的剪应力 值来做实验，以此来 模拟 材料 实际 的 受力 方式。 就一些有色金属而论 ， 当循环的 次 数变得非常大时，疲劳曲线不 会随着循环次数的增大而变得水平。，而疲劳曲线的继续变小，表明不管作用力有多么的小 ， 多次的应力反复作用都会引起零件的失效。 这样的一种材料据说没有持久极限 。 对于大多数有色金属来说 ，它们都 有一个持久极限 ，数值大小 大约 是 极限强度的 25%。 温度对屈服强度和弹性模数的影响 一般说来， 当 在 说明一种拥有 特殊的属性 的材料时，如弹性模数和屈服强度 ， 表示这些性能 在室温 环境下就可以 存在 。 在 低 的 或者 较 高的温度 下， 材料的特性可能 会有很大的 不同 。例如， 很多金属在低温 时会变得 更脆 。此外， 当温度 升高 时， 材料的 弹性模数和屈服强度 都会变差。 图 2.23 显示 了低碳 钢的屈服强度在从室温 升高 到 1000o C 过程中被降低 了 大约70%。 当温度 升高 时，图 2.24 显示 了低碳 钢在弹性模数 E 方面的削减 。正如 从图 上 可以看见的那样 ， 弹性模数在从室温 升高 到 1000oC 过程中 大约降低了 30%。从这张图表中， 我们也能看 到 在室温 下承受了一定载荷而不会发生变形的零件却 可能 在 高温 时承受相同载荷时发生 永久 变形。 蠕变 ： 一 种塑性变形的 现 象 由于 温度效应 的影响，金属中产生了一种被称为蠕变的 现象 ，一个承受了一定的载荷的零件的塑性变形是按照一个 时间函数 来逐渐增加的。蠕变现象 在室温 的条件下也是 存在 的，但它发生的 过程 是 如此 之 慢 ， 以致于很少变得 像在 预期寿命 中温度被升高到 300oC 或更多 时那样显著，逐渐 增加的塑性变形 可能在一段短的 时期内变得 很明显。 材料的 抗蠕变强度是指材料抵抗蠕变的属性， 并且 抗蠕变强度的 数据可以通过处理长期的蠕变试验 (模拟实际 零件的 操作条件 )来 获得 。在试验的过程中，给定的材料在规定的温度下的 塑性应变被 被进行了实时 监控 。 由于蠕变 是一 种 塑性变形现象 ，发生了蠕变 的 零件的 尺寸 可能就会被永久的改变。因此，如果一个 零件是在很强的强度下运转的话，那么 设计工程师必须 精确地 预言将在机器的 使用寿命 期间 可能发生的蠕变的次数。否则，与此伴随的或者相关的问题就可能 发生 。 在高温下，当 螺栓 被用来紧固零件时，蠕变就可能变成一个必须解决的问题。处在压力状态下的螺钉，蠕变是按照 一个时间函数 来发生的。 因为变形是塑 性的， 夹 紧 力的损失将 可能 导致 螺纹连接件的意外松动。像这种特殊的 现象 ，通常被称为 松弛 ，我们 可以 通过进行适当的 蠕变强度时测试 来 确定 是不是发生了蠕变。 图 2.25 显示 了三种承