故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用--中英文翻譯

1、<p>　　外 文 翻 譯</p><p>　　故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用</p><p>　　Failure Analysis，Dimensional Determination And Analysis，Applications Of Cams</p><p>　　Abstract：It is absolutely esse

2、ntial that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designed；Cams are among the most versatile mechanisms available．A cam is a simple two-member devi

3、ce．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．</p><

4、;p>　　Key words: failure high-speed cams design properties</p><p>　　INTRODUCTION</p><p>　　It is absolutely essential that a design engineer know how and why parts fail so that reliable machi

5、nes 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

6、 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 </p><p>　　The type of load

7、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 concer

8、n is whether the material is ductile or brittle．For example，brittle materials are considered to be unacceptable where fatigue is involved．</p><p>　　Many people mistakingly interpret the word failure to mean

9、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 deform

10、ation，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 fu

11、n</p><p>　　In general，the design engineer must consider all possible modes of failure，which include the following．</p><p><b>　　——Stress</b></p><p>　　——Deformation</p&

12、gt;<p><b>　　——Wear</b></p><p>　　——Corrosion</p><p>　　——Vibration</p><p>　　——Environmental damage</p><p>　　——Loosening of fastening devices</p>

13、<p>　　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 contour

14、s，and the application of interference fit joints．</p><p>　　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 membe

15、r 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</p><p>　　——Camshaf

16、t and distributor shaft of automotive engine </p><p>　　——Production machine tools</p><p>　　——Automatic record players</p><p>　　——Printing machines</p><p>　　——Automatic

17、washing machines</p><p>　　——Automatic dishwashers</p><p>　　The contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematically．However，the vast majority of cams op

18、erate 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 c

19、am contour is machined．</p><p>　　DESIGN PROPERTIES OF MATERIALS</p><p>　　The following design properties of materials are defined as they relate to the tensile test．</p><p>　　Static

20、 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 pr

21、oportional limit，since no plastic deformation takes place and no damage theoretically is done to the material．</p><p>　　Stiffness． Stiffness is the deformation-resisting property of a material．The slope of t

22、he modulus line and，hence，the modulus of elasticity are measures of the stiffness of a material．</p><p>　　Resilience． Resilience is the property of a material that permits it to absorb energy without permane

23、nt deformation．The amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic region．</p><p>　　Toughness． Resilience and toughness are similar properties．How

24、ever，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 br

25、ittle materials are very low and are approximately equal．</p><p>　　Brittleness． A brittle material is one that ruptures before any appreciable plastic deformation takes place．Brittle materials are generally

26、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．</p><p>　　Duc

27、tility． 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 co

28、nsidered to be the dividing line between ductile and brittle materials．</p><p>　　Malleability． Malleability is essentially a measure of the compressive ductility of a material and，as such，is an important cha

29、racteristic of metals that are to be rolled into sheets．</p><p>　　Hardness． The hardness of a material is its ability to resist indentation or scratching．Generally speaking，the harder a material，the more bri

30、ttle it is and，hence，the less resilient．Also，the ultimate strength of a material is roughly proportional to its hardness．</p><p>　　Machinability． Machinability is a measure of the relative ease with which a

31、material can be machined．In general，the harder the material，the more difficult it is to machine． </p><p>　　COMPRESSION AND SHEAR STATIC STRENGTH</p><p>　　In addition to the tensile tests，there a

32、re other types of static load testing that provide valuable information．</p><p>　　Compression Testing． Most ductile materials have approximately the same properties in compression as in tension．The ultimate

33、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

34、 fails in compression as a result of deformation，not stress．</p><p>　　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 ul

35、timate 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 strengt

36、h is approximately 75% of the ultimate strength in tension．This difference must be taken into account when shear stresses are encountered in ma</p><p>　　DYNAMIC LOADS</p><p>　　An applied force t

37、hat 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 the

38、refore a static load．</p><p>　　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．</p>

39、<p>　　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．</p><p>　　Reversing Loa

40、d． 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．</p><p>　　Shock Loa

41、d． 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 sam

42、e type of shock load occurs in automobile springs when a tire hits a bump or hole in the road．</p><p>　　FATIGUE FAILURE-THE ENDURANCE LIMIT DIAGRAM</p><p>　　The test specimen in Figure 2.10a．，af

43、ter 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 usuall

44、y 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

45、 shaft．The conception o</p><p>　　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.</

46、p><p>　　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 autom

47、obile parts have failed through fatigue．</p><p>　　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 t

48、he 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．</p><p>　　Let us return our attention to the fatigue te

49、sting 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 bend

50、ing 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 repea

51、ted until failu</p><p>　　The relationship depicted in Figure 2.14 is typical for steel，because the curve becomes horizontal as Napproaches a very large number．Thus the endurance limit equals the stress level

52、 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 readil

53、y detected by the horizontal straight line．For steel，the endurance limit equals approximately 50% </p><p>　　The most common type of fatigue is that due to bending．The next most frequent is torsion failure，wh

54、ereas 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．</p>&l

55、t;p>　　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 f

56、ailure 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．</p>&l

57、t;p>　　EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITY</p><p>　　Generally speaking，when stating that a material possesses specified values of properties such as modulus of elasticity and

58、 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 temperatu

59、res．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</p><p>　　Figure 2.24 shows the reduction in the

60、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 th

61、at a part loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperatures．</p><p>　　CREEP: A PLASTIC PHENOMENON</p><p>　　Temperat

62、ure 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 r

63、arely 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

64、material is its ability to resist creep，and creep st</p><p>　　Since creep is a plastic deformation phenomenon，the dimensions of a part experiencing creep are permanently altered．Thus，if a part operates with

65、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． </p><p>　　Creep also

66、 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 wil

67、l 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．</p><p>　　Figure 2.25 shows t

68、ypical 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

69、) may represent a period of 10 years，the anticipated life of the product．</p><p><b>　　SUMMARY</b></p><p>　　The machine designer must understand the purpose of the static tensile stre

70、ngth 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 ductilit

71、y define properties that can be determined from the tensile test．</p><p>　　Dynamic loads are those which vary in magnitude and direction and may require an investigation of the machine part’s resistance to f

72、ailure．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．</p><p>　　Stress concentration occu

73、rs 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

74、 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 gra

75、dual．</p><p>　　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 var

76、iations 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 determ

77、ine the allowable stress．</p><p>　　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

78、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 me

79、ans of interference fits．A hub or ring has an inside diameter slightly smaller than the mating shaft or post</p><p>　　故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用</p><p>　　摘要：作為一名設(shè)計(jì)工程師有必要知道零件如何發(fā)生和為什么會(huì)發(fā)生故障，以便通過進(jìn)行最低限度的維

80、修以保證機(jī)器的可靠性；凸輪是被應(yīng)用的最廣泛的機(jī)械結(jié)構(gòu)之一，是一種僅僅有兩個(gè)組件構(gòu)成的設(shè)備。主動(dòng)件本身就是凸輪，而輸出件被稱為從動(dòng)件。通過使用凸輪，一個(gè)簡(jiǎn)單的輸入動(dòng)作可以被修改成幾乎可以想像得到的任何輸出運(yùn)動(dòng)。</p><p>　　關(guān)鍵詞：故障 高速凸輪 設(shè)計(jì)屬性</p><p><b>　　前言介紹：</b></p><p>　　作為一名設(shè)計(jì)工

81、程師有必要知道零件如何發(fā)生和為什么會(huì)發(fā)生故障，以便通過進(jìn)行最低限度的維修以保證機(jī)器的可靠性。有時(shí)一次零件的故障或者失效可能是很嚴(yán)重的一件事情，比如，當(dāng)一輛汽車正在高速行駛的時(shí)候，突然汽車的輪胎發(fā)生爆炸等。另一方面，一個(gè)零件發(fā)生故障也可能只是一件微不足道的小事，只是給你造成了一點(diǎn)小麻煩。一個(gè)例子是在一個(gè)汽車?yán)鋮s系統(tǒng)里的暖氣裝置軟管的松動(dòng)。后者發(fā)生的這次故障造成的結(jié)果通常只不過是一些暖氣裝置里冷卻劑的損失，是一種很容易被發(fā)現(xiàn)并且被改正的情況

82、。</p><p>　　能夠被零件進(jìn)行吸收的載荷是相當(dāng)重要的。一般說來，與靜載重相比較，有兩個(gè)相反方向的動(dòng)載荷將會(huì)引起更大的問題，因此，疲勞強(qiáng)度必須被考慮。另一個(gè)關(guān)鍵是材料是可延展性的還是脆性的。例如，脆的材料被認(rèn)為在存在疲勞的地方是不能夠被使用的。</p><p>　　很多人錯(cuò)誤的把一個(gè)零件發(fā)生故障或者失效理解成這樣就意味著一個(gè)零件遭到了實(shí)際的物理破損。無論如何，一名設(shè)計(jì)工程師必須從一個(gè)

83、更廣泛的范圍來考慮和理解變形是究竟如何發(fā)生的。一種具有延展性的材料，在破裂之前必將發(fā)生很大程度的變形。發(fā)生了過度的變形，但并沒有產(chǎn)生裂縫，也可能會(huì)引起一臺(tái)機(jī)器出毛病，因?yàn)榘l(fā)生畸變的零件會(huì)干擾下一個(gè)零件的移動(dòng)。因此，每當(dāng)它不能夠再履行它要求達(dá)到的性能的時(shí)候，一個(gè)零件就都算是被毀壞了（即使它的表面沒有被損毀）。有時(shí)故障可能是由于兩個(gè)兩個(gè)相互搭配的零件之間的不正常的磨擦或者異常的振動(dòng)引起的。</p><p>　　故障也

84、可能是由一種叫蠕變的現(xiàn)象引起的，這種現(xiàn)象是指金屬在高溫下時(shí)一種材料的塑性流動(dòng)。此外，一個(gè)零件的實(shí)際形狀可能會(huì)引起故障的發(fā)生。例如，應(yīng)力的集中可能就是由于輪廓的突然變化引起的，這一點(diǎn)也需要被考慮到。當(dāng)有用兩個(gè)相反方向的動(dòng)載荷，材料不具有很好的可延展性時(shí)，對(duì)應(yīng)力考慮的評(píng)估就特別重要。 </p><p>　　一般說來，設(shè)計(jì)工程師必須考慮故障可能發(fā)生的全部方式，包括如下一些方面：</p><p>

85、<b>　　——壓力</b></p><p><b>　　——變形</b></p><p><b>　　——磨損</b></p><p><b>　　——腐蝕</b></p><p><b>　　——振動(dòng)</b></p>

86、<p><b>　　——環(huán)境破壞</b></p><p><b>　　——固定設(shè)備松動(dòng)</b></p><p>　　在選擇零件的大小與形狀的時(shí)候，也必須考慮到一些可能會(huì)產(chǎn)生外部負(fù)載影響的空間因素，例如幾何學(xué)間斷性，為了達(dá)到要求的外形輪廓及使用相關(guān)的連接件，也會(huì)產(chǎn)生相應(yīng)的殘余應(yīng)力。</p><p>　　凸輪是被應(yīng)

87、用的最廣泛的機(jī)械結(jié)構(gòu)之一，是一種僅僅有兩個(gè)組件構(gòu)成的設(shè)備。主動(dòng)件本身就是凸輪，而輸出件被稱為從動(dòng)件。通過使用凸輪，一個(gè)簡(jiǎn)單的輸入動(dòng)作可以被修改成幾乎可以想象得到的任何輸出運(yùn)動(dòng)。常見的一些關(guān)于凸輪應(yīng)用的例子有：</p><p>　　——凸輪軸和汽車發(fā)動(dòng)機(jī)工程的裝配</p><p><b>　　——專用機(jī)床</b></p><p><b>

88、;　　——自動(dòng)電唱機(jī)</b></p><p><b>　　——印刷機(jī)</b></p><p><b>　　——自動(dòng)的洗衣機(jī)</b></p><p><b>　　——自動(dòng)的洗碗機(jī)</b></p><p>　　高速凸輪(凸輪超過1000 rpm的速度)的輪廓必須從數(shù)學(xué)意

89、義上來定義。無論如何，大多數(shù)凸輪以低速(少于500 rpm)運(yùn)行而中速的凸輪可以通過一個(gè)大比例的圖形表示出來。一般說來，凸輪的速度和輸出負(fù)載越大，凸輪的輪廓在被床上被加工時(shí)就一定要更加精密。</p><p><b>　　材料的設(shè)計(jì)屬性</b></p><p>　　當(dāng)他們與抗拉的試驗(yàn)有關(guān)時(shí)，材料的下列設(shè)計(jì)特性被定義如下。</p><p>　　靜強(qiáng)

90、度：一個(gè)零件的強(qiáng)度是指零件在不會(huì)失去它被要求的能力的前提下能夠承受的最大應(yīng)力。因此靜強(qiáng)度可以被認(rèn)為是大約等于比例極限，從理論上來說，我們可以認(rèn)為在這種情況下，材料沒有發(fā)生塑性變形和物理破壞。</p><p>　　剛度：剛度是指材料抵抗變形的一種屬性。這條斜的模數(shù)線以及彈性模數(shù)是一種衡量材料的剛度的一種方法。</p><p>　　彈性：彈性是指零件能夠吸收能量但并沒有發(fā)生永久變形的一種材料的

91、屬性。吸收的能量的多少可以通過下面彈性區(qū)域內(nèi)的應(yīng)力圖表來描述出來。</p><p>　　韌性：韌性和彈性是兩種相似的特性。無論如何，韌性是一種可以吸收能量并且不會(huì)發(fā)生破裂的能力。因此可以通過應(yīng)力圖里面的總面積來描述韌性，就像用圖2.8 b 描繪的那樣。顯而易見，脆性材料的韌性和彈性非常低，并且大約相等。</p><p>　　脆性：一種脆性的材料就是指在任何可以被看出來的塑性變形之前就發(fā)生破

92、裂的材料。脆性的材料一般被認(rèn)為不適合用來做機(jī)床的零部件，因?yàn)楫?dāng)遇到由軸肩，孔，槽，或者鍵槽等幾何應(yīng)力集中源引起的高的應(yīng)力時(shí)，脆性材料是無法來產(chǎn)生局部屈服的現(xiàn)象以適應(yīng)高的應(yīng)力環(huán)境的。</p><p>　　延展性：一種延展性材料會(huì)在破裂之前表現(xiàn)出很大程度上的塑性變形現(xiàn)象。延展性是通過可延展的零件在發(fā)生破裂前后的面積和長(zhǎng)度的百分比來測(cè)量的。一個(gè)在發(fā)生破裂的零件，其伸長(zhǎng)量如果為5%，則認(rèn)為該伸長(zhǎng)量就是可延展性和脆性材料分

93、界線。</p><p>　　可鍛性：從根本上來說是指材料的一種在承受擠壓或壓縮是可以發(fā)生塑性變形的能力，同時(shí)，它也是一種在金屬被滾壓成鋼板時(shí)所需金屬的重要性能。</p><p>　　硬度：一種材料的硬度是指它抵抗擠壓或者拉伸它的能力。一般說來，材料越硬，它的脆性也越大，因此，彈性越小。同樣，一種材料的極限強(qiáng)度粗略與它的硬度成正比。</p><p>　　機(jī)械加工性能（

94、或切削性）：機(jī)械加工性能是指材料的一種容易被加工的性能。通常，材料越硬，越難以加工。</p><p>　　壓應(yīng)力和剪應(yīng)力:除抗拉的試驗(yàn)之外，還有其它一些可以提供有用信息的靜載荷的實(shí)驗(yàn)類型。</p><p>　　壓縮測(cè)試：大多數(shù)可延展材料大約有相同特性，當(dāng)它們處于受壓狀態(tài)的緊張狀態(tài)時(shí)。極限強(qiáng)度，無論如何，不能夠被用于評(píng)價(jià)壓力狀態(tài)。當(dāng)一件具有可延展性的樣品受壓發(fā)生塑性變形時(shí)，材料的其它部分會(huì)凸

95、出來，但是在這種緊張的狀態(tài)下，材料通常不會(huì)發(fā)生物理上的破裂。因此，一種可延展的材料通常是由于變形受壓而損壞的，并不是壓力的原因。 </p><p>　　剪應(yīng)力測(cè)試：軸，螺釘，鉚釘和焊接件被用這樣一種方式定位以致于生產(chǎn)了剪應(yīng)力。一張抗拉試驗(yàn)的試驗(yàn)圖紙就可以說明問題。當(dāng)壓力大到可以使材料發(fā)生永久變形或發(fā)生破壞時(shí)，這時(shí)的壓力就被定義為極限剪切強(qiáng)度。極限剪切強(qiáng)度，無論如何，不等于處于緊張狀態(tài)的極限強(qiáng)度。例如，以鋼的材料為

96、例，最后的剪切強(qiáng)度是處于緊張狀態(tài)大約極限強(qiáng)度的75%。當(dāng)在機(jī)器零部件里遇到剪應(yīng)力時(shí)，這個(gè)差別就一定要考慮到了。</p><p>　　動(dòng)力載荷:不會(huì)在各種不同的形式的力之間不停發(fā)生變化的作用力被叫作靜載荷或者穩(wěn)定載荷。此外，我們通常也把很少發(fā)生變化的作用力叫作靜載荷。在拉伸實(shí)驗(yàn)中，被分次、逐漸的加載的作用力也被叫作靜載荷。</p><p>　　另一方面，在大小和方向上經(jīng)常發(fā)生變化的力則被稱為

97、動(dòng)載荷。動(dòng)載荷可以被再細(xì)分為以下的3種類型。</p><p>　　變載荷：所謂變載荷，就是說載荷的大小在變，但是方向不變的載荷。比如說，變載荷會(huì)產(chǎn)生忽大忽小的張應(yīng)力，但不會(huì)產(chǎn)生壓應(yīng)力。</p><p>　　周期性載荷：像這樣的話，如果大小和方向同時(shí)改變，則就是說這種載荷會(huì)反復(fù)周期性的產(chǎn)生變化的拉應(yīng)力和壓應(yīng)力，這種現(xiàn)象往往就伴隨著應(yīng)力在方向和大小上的周期性變化。</p><

98、;p>　　沖擊載荷：這類載荷是由于沖擊作用產(chǎn)生的。一個(gè)例子就是一臺(tái)升降機(jī)墜落到位于通道底部的一套彈簧裝置上，這套裝置產(chǎn)生的力會(huì)比升降機(jī)本身的重量大上好幾倍。當(dāng)汽車的一個(gè)輪胎碰撞到道路上的一個(gè)突起或者路上的一個(gè)洞時(shí)，相同的沖擊荷載的類型也會(huì)在汽車的減震器彈簧上發(fā)生。</p><p>　　疲勞失效－疲勞極限線圖</p><p>　　如果材料的某處經(jīng)常會(huì)產(chǎn)生大量的周期性作用力，那么在材料的

99、表面就很可能會(huì)出現(xiàn)裂縫。裂縫最初是在應(yīng)力超過它極限壓力的地方開始出現(xiàn)的，而通常這往往是有微小的表面缺陷的地方，例如有一處材料出現(xiàn)瑕疵或者一道極小的劃痕。當(dāng)循環(huán)的次數(shù)增加時(shí)，最初的裂縫開始在軸的周圍的逐漸產(chǎn)生許多類似的裂縫。所以說，第一道裂縫的意義就是指應(yīng)力集中的地方，它會(huì)加速其它裂縫的產(chǎn)生。一旦整個(gè)的外圍斗出現(xiàn)了裂縫，裂縫就會(huì)開始向軸的中心轉(zhuǎn)移。最后，當(dāng)剩下的固體的內(nèi)部地區(qū)變得足夠小，且當(dāng)壓力超過極限強(qiáng)度時(shí)，軸就會(huì)突然發(fā)生斷裂。對(duì)斷面的

100、檢查可以發(fā)現(xiàn)一種非常有趣的圖案，如圖2.13中所示。外部的一個(gè)環(huán)形部分相對(duì)光滑一些，因?yàn)樵瓉肀砻嫔舷嗷ソ诲e(cuò)的裂縫之間不斷地發(fā)生磨擦導(dǎo)致了這種現(xiàn)象的產(chǎn)生。無論如何，中心部分是粗糙的，表明中心是突然發(fā)生了斷裂，類似于脆性材料斷裂時(shí)的現(xiàn)象。</p><p>　　這就表明了一個(gè)有趣的事實(shí)。當(dāng)正在使用的機(jī)器零件由于靜載荷的原因出現(xiàn)問題時(shí)，由于材料具有的延展性，他們通常會(huì)發(fā)生一定程度的變形。</p><p