机械类毕业设计外文翻译

本科毕业论文(设计)

外文翻译

学院：机电工程学院

专业：机械工程及自动化

姓名：高峰

指导教师：李延胜

2011年05 月10日

教育部办公厅

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．R esilience 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．

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．M achinability 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．

Figure 2.8

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 replacing 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 ( μin．)，the percentage drops to about 40%．For rough surfaces (300μin．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 1000o F．

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 1000o F．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 300o C 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 part’s 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 F T 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 α，the pressure angle，measured from the line representing the direction of motion of the follower．Therefore，the force F T has a horizontal component F H and a vertical component F V．The vertical component is the one that drives the follower upward and，therefore，neglecting guide friction，equals the follower F load．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 F H．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．

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。

（图2.7）

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。

（图2.8）

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。

（图2.13）

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（μin ）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（300μin，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1000o C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300o C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。

（图2.24）

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显示了三种承受了恒定的张紧力的低碳钢零件的典型的蠕变曲线。从中，我们可以注意到在高温条件下，蠕变发生的速度逐渐加速，直到零件失效。从图表里的时间轴上(x轴)，我们可以描述在10年的时间里，这种产品的预期寿命。

（图2.25）

总结

机器设计者必须理解进行抗拉的静止强度的测试目的。这种试验可以确定被在设计方程式过程中使用的许多金属的机械特性。像弹性模数，比例极限，屈服强度，弹性，以及延展性等等可以根据抗拉试验来决定它们的特性。

动载荷是指那些，在大小和方向上发生变化并且可能需要对机器零件在抵抗失效能力上的研究。由于应力的反复作用，允许使用的安全应力是基于材料的持久极限而不是基于屈服强度或者是极限强度。

压力集中在机器零件改变尺寸的位置发生，例如在一块平的金属板上的一个孔或者一块平板、一个沟槽、一个圆轴上的皮带在宽度方向上的突然变化。尤其是在一块平板上或一块条板上有一个孔的情况下，当孔的大小减少时，最大应力的值相对于平均应力变得大得多。减少的压力集中影响的方法通常就是使在形状上的变化更有规律性。

被设计出来的机械零件被用于在低于屈服强度或者极限强度的一些允许的环境下使用。这种方法可以用来照顾到在加工期间像材料属性的变化和残余应力的产生这样的未知因素，以及用来做近似而不是精确计算的方程式。根据屈服强度或者极限强度来确定安全系数以决定安全应力的大小。

温度能影响金属的机械特性。温度的增加可能会引起金属的热胀和蠕变，并且还