《材料力學(xué)性能》中英文全套PPT課件

《材料力學(xué)性能》中英文全套PPT課件,材料力學(xué)性能,材料,力學(xué)性能,中英文,全套,PPT,課件 Fatigue疲勞 FatigueFatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses(e.g.,bridges,aircraft,and machine components).疲勞是受變動(dòng)載荷結(jié)夠而發(fā)生的一種失效形式。Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile strength for a static load.疲勞時(shí)交變失效應(yīng)力可能比靜載荷失效應(yīng)力小很多。The term“fatigue”is used because this type of failure normally occurs after a long period of repeated stress or strain cycling.這種失效形式常常被稱為“疲勞”，因?yàn)樗３０l(fā)生在長(zhǎng)期重復(fù)循環(huán)應(yīng)力應(yīng)變下。Significance特點(diǎn)Fatigue is important inasmuch as it is the single largest cause of failure in metals,estimated to comprise approximately 90%of all metallic failure;polymers and ceramics(except for glasses)are also susceptible to this type of failure.金屬90%失效是因疲勞斷裂的原因。高分子聚合物，陶瓷（除玻璃外）也易受疲勞的影響。Furthermore,it is catastrophic and insidious,occurring very suddenly and without warning.另外，它是災(zāi)難性的，并有潛伏性。常常在沒有預(yù)兆的情況下突然發(fā)生。Fatigue failure is brittlelike in nature even in normal ductile metals,in that there is very little,if any,gross plastic deformation associated with failure.即使在延性很好的金屬中，疲勞失效類似脆性，也很少觀察到塑性變形。The process occurs by the initiation and propagation of cracks,and ordinarily the fracture surface is perpendicular to the direction of an applied tensile stress.斷裂過程是裂紋的形成、擴(kuò)展過程，斷裂面通常垂直于拉力方向。Cyclic Stresses循環(huán)應(yīng)力The applied stress may be axial(tension-compression),flexural(bending),or torsional(twisting)in nature.In general,three different fluctuating stress-time modes are possible.載荷可能是軸向的（拉壓），彎曲力或扭轉(zhuǎn)力。通?？煞譃槿N波動(dòng)應(yīng)力時(shí)間模型。Reversed stress cycle;交變應(yīng)力循環(huán)Repeated stress cycle;重復(fù)應(yīng)力循環(huán)Random stress cycle.隨機(jī)應(yīng)力循環(huán)Reversed stress cycle交變應(yīng)力循環(huán)Repeated stress cycle重復(fù)應(yīng)力循環(huán)Random stress cycle隨機(jī)應(yīng)力循環(huán)Mean stress平均應(yīng)力Range of stress應(yīng)力范圍Stress amplitude應(yīng)力幅Stress ratio應(yīng)力比（應(yīng)力循環(huán)對(duì)稱系數(shù)）The S-N Curve疲勞曲線As with other mechanical characteristics,the fatigue properties of materials can be determined from laboratory simulation tests.和其它力學(xué)性能一樣，材料疲勞性質(zhì)也可由實(shí)驗(yàn)?zāi)M實(shí)驗(yàn)得到。A test apparatus should be designed to duplicate as nearly as possible the service stress conditions(stress level,time frequency,stress pattern,etc.).實(shí)驗(yàn)儀器要盡可能地去模擬真實(shí)受載應(yīng)力情況（應(yīng)力大小，頻率，應(yīng)力樣式等）A schematic diagram of a rotating-bending test apparatus旋轉(zhuǎn)-彎曲試驗(yàn)儀器原理圖Schematic of a pulsator for fatigue tests in tension-compression拉壓疲勞實(shí)驗(yàn)的震動(dòng)器原理圖Types of standard test pieces for fatigue tests疲勞實(shí)驗(yàn)標(biāo)準(zhǔn)件A series of tests are commenced by subjecting a specimen to the stress cycling at a relatively large maximum stress amplitude(max),usually on the order of two thirds of the static tensile strength;the number of cycles to failure is counted.通常加載一個(gè)最大應(yīng)力max為最大靜載荷2/3的循環(huán)應(yīng)力，做一系列實(shí)驗(yàn)，并統(tǒng)計(jì)出使試樣斷裂的周期數(shù)。This procedure is repeated on other specimens at progressively decreasing maximum stress amplitudes.逐步減小最大應(yīng)力幅，在其它試樣上重復(fù)此步驟。Data are plotted as stress S versus the logarithm of the number N of cycles to failure for each of the specimens.將每個(gè)試樣的實(shí)驗(yàn)數(shù)據(jù)繪制在“應(yīng)力-失效循環(huán)周期”曲線上（S-N曲線）。Two distinct types of S-N behavior(1)兩種截然不同的S-N曲線（1）Two distinct types of S-N behavior(2)兩種截然不同的曲線（2）Fatigue limit/Endurance limit疲勞極限/耐久強(qiáng)度Fatigue limit.For fatigue,the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.疲勞極限。材料能承受無限次應(yīng)力循環(huán)而不斷裂的最大應(yīng)力幅。The fatigue limit represents the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles.疲勞極限是使材料承受無限次應(yīng)力循環(huán)而不發(fā)生斷裂的最大變應(yīng)力值。For many steels,fatigue limits range between 35 and 60%of the tensile strength.許多鋼的疲勞極限范圍是靜載荷最大拉應(yīng)力 的35 and 60%。Fatigue Strength疲勞強(qiáng)度Most nonferrous alloys(e.g.,aluminum,copper,magnesium)do not have a fatigue limit,in that the SN curve continues its downward trend at increasingly greater N values.Thus,fatigue will ultimately occur regardless of the magnitude of the stress.大多數(shù)非鐵合金沒有疲勞極限，SN曲線隨N的增加繼續(xù)它下滑的趨勢(shì)。因此，不管應(yīng)力大小，疲勞最終還是會(huì)發(fā)生。Fatigue strength.The maximum stress level that a material can sustain,without failing,for some specified number of cycles(e.g.107 cycles).疲勞強(qiáng)度。材料能經(jīng)受一個(gè)特定的循環(huán)周期數(shù)（N=107）而不發(fā)生疲勞斷裂的最大應(yīng)力水平。Fatigue Life疲勞壽命Fatigue life.The total number of stress cycles that will cause a fatigue failure at some specified stress amplitude.疲勞壽命。在某個(gè)特定的應(yīng)力幅上導(dǎo)致材料疲勞斷裂的總的應(yīng)力循環(huán)周期數(shù)。Unfortunately,there always exists considerable scatter in fatigue data,that is,a variation in the measured N value for a number of specimens tested at the same stress level.This may lead to significant design uncertainties when fatigue life and/or fatigue limit(or strength)are being considered.不幸的是，疲勞實(shí)驗(yàn)數(shù)據(jù)始終是分散的，一系列試樣在同樣的應(yīng)力水平上得到了不同的N值。這會(huì)導(dǎo)致疲勞壽命/疲勞極限設(shè)計(jì)上的不確定性。The scatter of results is a consequence of the fatigue sensitivity to a number of test and material parameters that are impossible to control precisely.These parameters include specimen fabrication and surface preparation,metallurgical variables,specimen alignment in the apparatus,mean stress,and test frequency.數(shù)據(jù)的分散性是一系列無法控制精度的實(shí)驗(yàn)和材料參數(shù)對(duì)疲勞敏感性的結(jié)果。Fatigue S-N curves similar to those shown before represent“best fit”curves which have been drawn through average-value data points.It should be remembered that S-N curves represented in literature are normally average values,unless noted otherwise.疲勞S-N曲線與用平均值繪制的最優(yōu)曲線相似。除特別說明外，文獻(xiàn)中的 S-N曲線 都是由平均值表示的。Fatigue S-N probability of failure curves for a aluminum alloy鋁合金中可能的S-N疲勞曲線Low-cycle fatigue and high-cycle fatigue低周疲勞和高周疲勞The fatigue behaviors may be classified into two domains.One is associated with relatively high loads that produce not only elastic strain but also some plastic strain during each cycle.Consequently,fatigue lives are relatively short;this domain is termed low-cycle fatigue and occurs at less than about 104 to 105 cycles.疲勞特性可分為兩類。一種是相對(duì)較高的載荷，它在每個(gè)周期中不僅產(chǎn)生彈性應(yīng)變也有塑性應(yīng)變。因此，疲勞壽命相對(duì)較短；這種疲勞稱為低周疲勞，其周期一般少于104 到105 次。For lower stress levels wherein deformations are total elastic,longer lives result.This is called high-cycle fatigue inasmuch as relatively large numbers of cycles are required to produce fatigue failure.High-cycle fatigue is associated with fatigue lives greater than about 104 to 105 cycles.對(duì)于低應(yīng)力水平，所有的變形都是彈性的，其壽命較長(zhǎng)。由于造成疲勞斷裂需要一個(gè)相對(duì)大的循環(huán)周期數(shù)，所以稱為高周疲勞。其疲勞壽命一般大于104到105次。Problems1.Cite five factors that may lead to scatter in fatigue life data.列舉五個(gè)導(dǎo)致疲勞壽命數(shù)據(jù)分散的因素。2.Briefly demonstrate that increasing the value of the stress ratio R produces a decrease in stress amplitude a .簡(jiǎn)略證明應(yīng)力比R的增加使應(yīng)力幅a下降。Fractograph斷口形貌Three distinct steps of fatigue failure疲勞斷裂的三個(gè)步驟(1)crack initiation,wherein a small crack forms at some point of stress concentration;裂紋形成：在某些應(yīng)力集中區(qū)形成小裂紋。(2)crack propagation,during which this crack advances incrementally with each stress cycle;and 裂紋擴(kuò)展：每個(gè)應(yīng)力周期下裂紋向前推進(jìn)。(3)final failure,which occurs very rapidly once the advancing crack has reached a critical size.最終斷裂：當(dāng)裂紋迅速擴(kuò)展達(dá)到一個(gè)極限尺寸時(shí)將發(fā)生斷裂。Crack initiation裂紋形成Crack associated with fatigue failure almost always initiate(or nucleate)on the surface of a component at some point of stress concentration.導(dǎo)致疲勞斷裂的裂紋常在元件表面的某些應(yīng)力集中點(diǎn)出形成（或形核）。Crack nucleation sites include surface scratches,keyways,threads,dents,and the like.裂紋形核區(qū)包括表面劃痕，鍵槽，螺紋，壓痕等類似地方。In addition,cyclic loading can produce microscopic surface discontinuities resulting from dislocation slip steps which may also act as stress raisers,and therefore as crack initiation sites.另外，周期載荷由于位錯(cuò)滑移而導(dǎo)致微觀表面的不連續(xù)性（即裂紋）。這些位錯(cuò)滑移會(huì)使應(yīng)力上升，并成為應(yīng)力形成區(qū)。Crack initiation by slip(P.Neumann)滑移形成裂紋Wood木材Crack propagation裂紋擴(kuò)展The region of a fracture surface that formed during crack propagation step may be characterized by two types of markings termed beachmarks and striations.因裂紋擴(kuò)展而形成的斷裂表面有兩種形態(tài)特征：貝殼狀紋理和疲勞輝紋。Both of these features indicate the position of the crack tip at some point in time and appear as concentric ridges that expand away from the crack initiation site(s),frequently in a circular or semicircular pattern.這兩種特征表征了某時(shí)刻裂紋尖端所處的位置?？雌饋砭拖袷且粋€(gè)中心脊從裂痕形成區(qū)以圓或半圓的形式連續(xù)向外擴(kuò)展。Beachmarks(sometimes also called “clamshell marks”)are of macroscopic dimensions,and may be observed with unaided eye.貝殼狀紋理是斷裂口的宏觀特征，裸眼可視。These markings are found for components that experienced interruptions during the crack propagation stage.這些記號(hào)是由于元件在裂紋擴(kuò)展階段中受到阻礙而留下的。Each beachmark band represents a period of time over which crack growth occurred.每一條貝殼狀紋理帶代表了裂紋增長(zhǎng)的一段時(shí)期。Beachmarks貝殼狀紋理貝殼狀紋理Fractograph斷口形貌學(xué)Striation疲勞輝紋/細(xì)槽On the other hand,fatigue striations are microscopic in size and subject to observation with electron microscope(either TEM or SEM).另一方面，疲勞條痕是斷裂口的微觀特征，只能借助于電子顯微鏡（TEM 或 SEM）來觀察到。Each striation is thought to represent the advance distance of a crack front during a single load cycle.每一個(gè)輝紋代表著一個(gè)周期載荷下裂紋前進(jìn)的距離。Striation width depends on,and increases with,increasing stress range.輝紋寬度依賴于應(yīng)力變化范圍，并隨之增長(zhǎng)而變寬。Crack propagation model(Plastic)裂紋擴(kuò)展模型（塑性）Crack propagation model(Brittle)裂紋擴(kuò)展模型（脆性）It should be emphasized that although both beachmarks and striations are fatigue fracture surface features having similar appearances,they are nevertheless different,both in origin and size.必須強(qiáng)調(diào)：盡管貝殼狀紋理和輝紋都是疲勞斷裂表面特征且外觀極其相似，但它們?cè)趤碓春痛笮∩线€是不同的。They may be literally thousands of striations within a single beachmark.一條貝殼狀紋理中可能有上千條疲勞輝紋。The presence of beachmark and/or striations on a fracture surface confirms that the cause of failure was fatigue.Nevertheless,the absence of either or both does not exclude fatigue as the cause of failure.表面斷裂出現(xiàn)貝殼狀紋理和/或輝紋證明斷裂原因是疲勞。然而，其中一個(gè)的缺失或全部缺失并不能排除斷裂的原因是疲勞。Beachmarks and striations will not appear on that region over which the rapid failure occurs.瞬時(shí)斷裂區(qū)并沒有出現(xiàn)貝殼狀紋理和輝紋。Rather,the rapid failure may be either ductile or brittle;evidence of plastic deformation will be present for ductile,and absent for brittle failure.另外，瞬時(shí)斷裂可能是延性的，也可能是脆性的；塑性變形的出現(xiàn)證明是延性的，反之則是脆性斷裂。Crack propagation rate裂紋擴(kuò)展速率The total durability of a component is Nf=Ni+Np for Np/Nf90%，studies on crack propagation rate is of very important significance.元件總耐用度：Nf=Ni+Np 對(duì)Np/Nf90%，裂紋擴(kuò)展速率的研究非常有意義。Crack propagation rate(Paris formula)裂紋擴(kuò)展速率（帕里斯公式）l is the crack length;l是裂紋長(zhǎng)度N is the numbers of cycle;N是周期數(shù)c,n are the constants of a material depending on the coefficient of cycle asymmetry;c,n是由載荷周期不對(duì)稱性系數(shù)決定的材料常數(shù) K is the amplitude of the stress intensity coefficient.(K=Kmax-Kmin)K是應(yīng)力場(chǎng)強(qiáng)度因子幅。(K=Kmax-Kmin)Factors that affect fatigue life影響疲勞壽命的因素The fatigue behavior of engineering materials is highly sensitive to a number of factors,e.g.mean stress level,geometrical design,surface effects,and metallurgical variables,as well as the environment.工程材料的疲勞行為對(duì)某些因素很敏感。例如：平均應(yīng)力，幾何設(shè)計(jì)，表面效應(yīng)，金屬成分和環(huán)境。Discussion these factors and measures to be taken to improve the fatigue resistance of structural components.討論這些因素，采取措施來增加結(jié)構(gòu)零件的抗疲勞性。MEAN STRESS平均應(yīng)力The dependence of fatigue life on stress amplitude is represented on the S-N plot.Such data are taken from for a constant mean stress(m),often for the reversed cycle situation(m=0).S-N圖中可看到疲勞壽命對(duì)應(yīng)力幅的依賴。當(dāng)平均應(yīng)力(m)為常數(shù)時(shí)，交變載荷(m=0)的情況下?？紤]這個(gè)量。Mean stress,however,will also affect fatigue life,which influence may be represented by a series of S-N curves,each measured at a different m;this is depicted schematically in following diagram.但平均應(yīng)力會(huì)影響疲勞壽命，這些影響可在一系列S-N曲線上看出，每一個(gè)都由一個(gè)不同的m測(cè)定。簡(jiǎn)單圖解描述如下：Increasing the mean stress level leads to a decrease in fatigue life.平均應(yīng)力水平的增加導(dǎo)致疲勞壽命的縮短。Influence of mean stress m on S-N fatigue behavior平均應(yīng)力m對(duì)S-N 疲勞行為的影響SURFACE EFFECT表面效應(yīng)For many common loading situations,the maximum stress within a component or structure occurs at its surface.Consequently,most cracks leading to fatigue failure originated at surface positions,specifically in stress amplification sites.在一些通常的載荷情況下，最大應(yīng)力常發(fā)生在元件或結(jié)構(gòu)的表面。因此，大多數(shù)導(dǎo)致疲勞斷裂的裂紋常在表面形成，尤其是在應(yīng)力放大區(qū)。Therefore,it has been observed that fatigue life is especially sensitive to the condition and configuration of the component surface.因此，可觀察到疲勞壽命對(duì)元件表面的情況和結(jié)構(gòu)特別敏感。Design criteria and various surface treatments will lead to an improvement in fatigue life.合理的設(shè)計(jì)準(zhǔn)則和一些表面處理將會(huì)延長(zhǎng)疲勞壽命。Design factors設(shè)計(jì)因素The design of a component can have a significant influence on its fatigue characteristics.Any notch or geometrical discontinuity can act as stress raiser and fatigue crack initiation site;these design features include grooves,holes,keyways,threads,and so on.The sharper the discontinuity,the more severe the stress concentration.元件設(shè)計(jì)對(duì)它的疲勞特性有明顯的影響。任何壓痕或幾何缺陷能使應(yīng)力上升而成為疲勞裂紋形成區(qū)；這些設(shè)計(jì)特征包括：溝槽，孔洞，鍵槽，螺紋等。缺陷越鋒利，應(yīng)力集中越嚴(yán)重。Avoiding structural irregularities or making design modifications.防止結(jié)構(gòu)缺陷或修改設(shè)計(jì)。How design can reduce stress stress amplification 能減小應(yīng)力、應(yīng)力放大的設(shè)計(jì)Surface treatments表面處理During machining operations,small scratches and grooves are invariably introduced into the workpiece surface by cutting tool action.These surface markings can limit the fatigue life.It has been observed that improving the surface finish by polishing will enhance fatigue life significantly.在機(jī)械加工中，微小的劃痕和溝槽總會(huì)由于加工工具的運(yùn)動(dòng)而留在工件表面上。這些表面痕跡能減小疲勞壽命。表面最后加工時(shí)拋光會(huì)顯著延長(zhǎng)疲勞壽命。One of the most effective methods of increasing fatigue performance is by imposing residual compressive stresses within a thin outer surface layer.一種有效延長(zhǎng)疲勞壽命的方法是在薄外表面層施加殘余壓應(yīng)力。Residual compressive stresses are commonly introduced into ductile metals by shot peening.殘余壓應(yīng)力通常用在塑性金屬噴丸加工上。Schematic S-N fatigue curves for normal and shot-peened steel普通鋼和噴丸處理鋼的S-N疲勞曲線簡(jiǎn)圖Case hardening表層硬化Problems 1.1 Briefly explain difference between fatigue striations and beachmarks both in terms of(a)size and(b)origin.從尺寸和來源上簡(jiǎn)述疲勞輝紋和貝殼狀紋理的區(qū)別。1.2 List four measures that may be taken to increase the resistance to fatigue of a metal alloy.舉出增加合金的抗疲勞性的方法。1.3 The fatigue data for a brass alloy are given as follows:黃銅的疲勞數(shù)據(jù)如下：Stress Amplitude(MPa)Cycles to failure31022319116815314313412721051106310611073107110831081109(a)Make an S-N plot(stress amplitude versus logarithm cycles to failure)use these data.繪制S-N曲線圖(b)Determine the fatigue strength at 5105 cycles.當(dāng)周期為5105時(shí)的疲勞強(qiáng)度。(c)Determine the fatigue life for 200MPa.確定疲勞強(qiáng)度為200MPa時(shí)的疲勞壽命。1.4 Three identical fatigue specimens (denoted A,B,and C)are fabricated from a nonferrous alloy.Each is subjected to one of the maxium-minimum stress cycles listed bellow.The frequency is the same for three tests.三種典型的疲勞試樣由合金組成。每一個(gè)都加載如下表所示的最大、最小應(yīng)力循環(huán)。三個(gè)載荷的頻率一樣。Specimen (MPa)(MPa)ABC+450+400+340-350-300-340(a)Rank the fatigue lifetimes of these specimens from the longest to the shortest.把疲勞壽命由長(zhǎng)到短排序。(b)Now justify this ranking using a schematic S-N plot 用S-N圖證明(a)1.5 Make a schematic sketch of the fatigue behavior for some metal for which the stress ratio R has a value of+1.當(dāng)應(yīng)力比為+1時(shí)，為某些金屬作出疲勞行為圖。