凸輪的分析和應(yīng)用外文翻譯@中英文翻譯@外文文獻(xiàn)翻譯

本科生畢業(yè)設(shè)計(jì)（論文）外文翻譯 畢業(yè)設(shè)計(jì)（論文）題目： XXXXXXXXXXXXXXXXXXXXXXXXX 外文題目： Failure Analysis， Dimensional Determination And Analysis，Applications Of Cams 譯文題目： 故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用 學(xué) 生 姓 名： 專 業(yè)： 指導(dǎo)教師姓名： 評(píng) 閱 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 譯文： 故障 的 分析 、尺寸的決定以及凸輪的分析和應(yīng)用 前言介紹： 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(guò)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(guò) 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。 能夠被零件進(jìn)行吸收的載荷是相當(dāng)重要的。一般說(shuō)來(lái)， 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(wèn)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 。 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(wú)論如何， 一 名 設(shè)計(jì)工程師必須 從一個(gè)更廣泛的范圍來(lái) 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(guò)度的變形 ，但并 沒(méi)有 產(chǎn)生 裂縫 ，也 可能 會(huì) 引起一臺(tái)機(jī)器出毛病，因?yàn)?發(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(méi)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。 故障 也可能是 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 。 一般說(shuō)來(lái)， 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： 壓力 變形 磨損 腐蝕 振動(dòng) 環(huán)境破壞 固定設(shè)備 松動(dòng) 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 。 凸輪是被應(yīng)用的最廣泛的機(jī)械結(jié)構(gòu)之一。凸輪是一種僅僅有兩個(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(guò)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)。常見(jiàn)的一些關(guān)于凸輪應(yīng)用的例子有： 凸輪軸和汽車發(fā)動(dòng)機(jī)工程的裝配 專用機(jī)床 自動(dòng)電唱機(jī) 印刷機(jī) 自動(dòng)的洗衣機(jī) 自動(dòng)的洗碗機(jī) 高速凸輪 (凸輪超過(guò) 1000 rpm 的速度 )的輪廓必須從數(shù)學(xué)意義上來(lái)定義 。無(wú)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(guò)一個(gè)大比例的圖形表示出來(lái)。一般說(shuō)來(lái)，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。 材料的設(shè)計(jì)屬性 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。 靜強(qiáng)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(lái)說(shuō)，我們可以認(rèn)為在這種情況下，材料沒(méi)有發(fā)生塑性變形和物理破壞。 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。 彈性： 彈性是 指零件能夠 吸收能量 但并 沒(méi)有 發(fā)生 永久變形的一種材料的 屬性。 吸收的能量的 多少可以通過(guò)下面彈性區(qū)域內(nèi)的應(yīng)力圖表來(lái)描述出來(lái)。 韌性： 韌 性 和彈性是 兩種 相似的特性 。無(wú)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(guò)應(yīng)力 圖 里面的 總面積 來(lái) 描述韌 性，就像 用圖 2.8 b 描繪的那樣 。顯而易見(jiàn)， 脆 性 材料的 韌性 和彈性非常低，并且大約相等 。 脆性： 一種脆 性 的材料 就 是 指 在任何 可以被看出來(lái) 的塑性變形之前 就發(fā)生 破裂 的材料。脆性 的材料一般被 認(rèn)為不適合用來(lái)做 機(jī)床的零部件 ，因?yàn)?當(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(wú)法來(lái)產(chǎn)生局部屈服的現(xiàn)象以適應(yīng)高的應(yīng)力環(huán)境的。 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(guò)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(lái)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 分界 線 。 可鍛性 ： 可鍛性 從根本上來(lái)說(shuō)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。 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(shuō)來(lái)，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 。 機(jī)械加工性能（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。 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。 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(wú)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)凸出來(lái)，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。 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(shuō)明問(wèn)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(wú)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。 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。 另一方面， 在大小 和方向上 經(jīng)常 發(fā)生 變化的力 則被稱 為動(dòng)載荷 。 動(dòng)載荷可以被再 細(xì) 分 為以下的 3 種類 型。 變載荷： 所謂變載荷，就是說(shuō)載荷的大小在變，但是方向不變的載荷。比如說(shuō)，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。 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(shuō)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。 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 。 疲勞失效 疲勞極限線圖 正如圖 2.10a 所示，如果材料的某處經(jīng)常會(huì)產(chǎn)生大量的周期性作用力，那么在材料的表面就很可能會(huì)出現(xiàn)裂縫。 裂縫最初 是 在 應(yīng) 力超過(guò)它 極限壓力 的地方開(kāi)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(kāi)始在軸的周圍的 逐漸產(chǎn)生許多類似的裂縫。所以說(shuō)，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(kāi)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(guò)極限強(qiáng)度 時(shí) ，軸 就會(huì) 突然 發(fā)生 斷 裂。 對(duì) 斷面 的檢查 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)?原來(lái)表面上相互交錯(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(wú)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。 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(wèn)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 。 盡管許多地由于靜壓力導(dǎo)致的零件故障可以通過(guò) 頻繁的 做實(shí)際的觀察 并且替換全部 發(fā)生變 形的 零件來(lái) 避免 。 不管怎樣， 疲勞失效 有助于起到 警告 的作用。汽車中發(fā)生故障的零件中的 90%的原因都是因?yàn)槠诘淖饔谩?一種材料的疲勞強(qiáng)度是 指 在壓力 的 反 復(fù)作用 下 的 抵抗 產(chǎn)生 裂縫的能力 。 持久極限是用來(lái)評(píng)價(jià) 一種材料 的疲勞強(qiáng)度的一個(gè) 重要 參數(shù) 。進(jìn)一步說(shuō)明就是， 持久極限 就 是 指在 無(wú)限循環(huán)的作用力下 不引起 失效 的壓力值 。 讓我們 回頭來(lái)看 在圖 2.9 所示的 疲勞試驗(yàn)機(jī)器的 。 試驗(yàn) 是這樣被進(jìn)行的： 一件小的重物被插入，電動(dòng)機(jī)被 啟動(dòng)。 在試樣的 失效過(guò)程中，由 計(jì)算寄存器 記錄下 循環(huán) 的次數(shù) N， 并且彎曲壓力的相應(yīng)最大量由第 2.5 方程式計(jì)算 。然后用一個(gè)新的樣品替換掉 被毀壞的樣品 ， 并且將另一個(gè) 重物插入 以 增加負(fù)荷 量。 壓力的新 的數(shù)值再次 被計(jì)算 ， 并且 相同的 程序 再次 被重復(fù)進(jìn)行 ，直到 零件的失效 只需要一個(gè)完整周期 時(shí)為止。然后根據(jù)壓力值和所需的循環(huán)的次數(shù)來(lái)繪制一個(gè) 圖。正如圖表 2.14a 所 示 圖形，該圖 被 稱 為持久極限 曲線 或者 S-N 曲線 。由于這需要的前提是要進(jìn)行無(wú)限次 的循環(huán) ，所以我們可以以 100 萬(wàn)個(gè)循環(huán) 用來(lái) 作 循環(huán) 參考 單位。 因此，持久極限可 以 從 圖表 2.14a那里 看到，該材料是在承受了 100 萬(wàn)個(gè)循環(huán) 后而沒(méi)有發(fā)生失效的。 用圖 2.14 描繪的關(guān)系對(duì)于鋼 的材料來(lái)說(shuō)更為 典型 ， 因?yàn)楫?dāng) N 接近非常大的 數(shù)字 時(shí)，曲線 就會(huì) 變 得 水平 。 因此持久極限等于曲線接近一條水平的切線 時(shí) 的壓力水平 。 由于包含 了 大量的循環(huán) ，在繪圖時(shí)， N 通常 會(huì) 被 按照 對(duì)數(shù) 標(biāo)度來(lái)畫， 如圖 2.14 b 中所示 。當(dāng)采用這樣的方法做時(shí)， 水平的直線 就 可 以更 容易發(fā)現(xiàn) 材料的持久 極限值 。 對(duì)于鋼 的材料 來(lái)說(shuō) ， 持久極限 值大約等于極限強(qiáng)度的 50%。無(wú)論如何，已經(jīng)加工 完成 的 表面 如果 不是 一樣的光滑， 持久極限的值 就會(huì)被降低。例如， 對(duì)于鋼 材料的零件 來(lái)說(shuō) ， 63 微英寸（ in ） 的機(jī)械加工的表面 ，零件的持久極限占理論的持久極限的 百分比降低到 了 大約 40%。而 對(duì)于粗糙的表面來(lái)說(shuō) （ 300in，甚至更多）， 百分比可能 降低到 25%左右的水平。 最 常見(jiàn) 的疲勞 損壞的 類型 通常是 由于彎曲 應(yīng)力所引起的。 其次 就 是扭 應(yīng)力導(dǎo)致的 失 效，而 由于軸向負(fù)載 引起的 疲勞 失效卻 極少發(fā)生 。彈性 材料 通常使用從零到最大 值之間變化 的剪應(yīng)力 值來(lái)做實(shí)驗(yàn)，以此來(lái) 模擬 材料 實(shí)際 的 受力 方式。 就一些有色金屬而論 ， 當(dāng)循環(huán)的 次 數(shù)變得非常大時(shí)，疲勞曲線不 會(huì)隨著循環(huán)次數(shù)的增大而變得水平。，而疲勞曲線的繼續(xù)變小，表明不管作用力有多么的小 ， 多次的應(yīng)力反復(fù)作用都會(huì)引起零件的失效。 這樣的一種材料據(jù)說(shuō)沒(méi)有持久極限 。 對(duì)于大多數(shù)有色金屬來(lái)說(shuō) ，它們都 有一個(gè)持久極限 ，數(shù)值大小 大約 是 極限強(qiáng)度的 25%。 溫度對(duì)屈服強(qiáng)度和彈性模數(shù)的影響 一般說(shuō)來(lái)， 當(dāng) 在 說(shuō)明一種擁有 特殊的屬性 的材料時(shí)，如彈性模數(shù)和屈服強(qiáng)度 ， 表示這些性能 在室溫 環(huán)境下就可以 存在 。 在 低 的 或者 較 高的溫度 下， 材料的特性可能 會(huì)有很大的 不同 。例如， 很多金屬在低溫 時(shí)會(huì)變得 更脆 。此外， 當(dāng)溫度 升高 時(shí)， 材料的 彈性模數(shù)和屈服強(qiáng)度 都會(huì)變差。 圖 2.23 顯示 了低碳 鋼的屈服強(qiáng)度在從室溫 升高 到 1000o C 過(guò)程中被降低 了 大約70%。 當(dāng)溫度 升高 時(shí)，圖 2.24 顯示 了低碳 鋼在彈性模數(shù) E 方面的削減 。正如 從圖 上 可以看見(jiàn)的那樣 ， 彈性模數(shù)在從室溫 升高 到 1000oC 過(guò)程中 大約降低了 30%。從這張圖表中， 我們也能看 到 在室溫 下承受了一定載荷而不會(huì)發(fā)生變形的零件卻 可能 在 高溫 時(shí)承受相同載荷時(shí)發(fā)生 永久 變形。 蠕變 ： 一 種塑性變形的 現(xiàn) 象 由于 溫度效應(yīng) 的影響，金屬中產(chǎn)生了一種被稱為蠕變的 現(xiàn)象 ，一個(gè)承受了一定的載荷的零件的塑性變形是按照一個(gè) 時(shí)間函數(shù) 來(lái)逐漸增加的。蠕變現(xiàn)象 在室溫 的條件下也是 存在 的，但它發(fā)生的 過(guò)程 是 如此 之 慢 ， 以致于很少變得 像在 預(yù)期壽命 中溫度被升高到 300oC 或更多 時(shí)那樣顯著，逐漸 增加的塑性變形 可能在一段短的 時(shí)期內(nèi)變得 很明顯。 材料的 抗蠕變強(qiáng)度是指材料抵抗蠕變的屬性， 并且 抗蠕變強(qiáng)度的 數(shù)據(jù)可以通過(guò)處理長(zhǎng)期的蠕變?cè)囼?yàn) (模擬實(shí)際 零件的 操作條件 )來(lái) 獲得 。在試驗(yàn)的過(guò)程中，給定的材料在規(guī)定的溫度下的 塑性應(yīng)變被 被進(jìn)行了實(shí)時(shí) 監(jiān)控 。 由于蠕變 是一 種 塑性變形現(xiàn)象 ，發(fā)生了蠕變 的 零件的 尺寸 可能就會(huì)被永久的改變。因此，如果一個(gè) 零件是在很強(qiáng)的強(qiáng)度下運(yùn)轉(zhuǎn)的話，那么 設(shè)計(jì)工程師必須 精確地 預(yù)言將在機(jī)器的 使用壽命 期間 可能發(fā)生的蠕變的次數(shù)。否則，與此伴隨的或者相關(guān)的問(wèn)題就可能 發(fā)生 。 在高溫下，當(dāng) 螺栓 被用來(lái)緊固零件時(shí)，蠕變就可能變成一個(gè)必須解決的問(wèn)題。處在壓力狀態(tài)下的螺釘，蠕變是按照 一個(gè)時(shí)間函數(shù) 來(lái)發(fā)生的。 因?yàn)樽冃问撬?性的， 夾 緊 力的損失將 可能 導(dǎo)致 螺紋連接件的意外松動(dòng)。像這種特殊的 現(xiàn)象 ，通常被稱為 松弛 ，我們 可以 通過(guò)進(jìn)行適當(dāng)?shù)?蠕變強(qiáng)度時(shí)測(cè)試 來(lái) 確定 是不是發(fā)生了蠕變。 圖 2.25 顯示 了三種承