《材料力學性能》大三教學PPT課件,材料力學性能,材料,力學性能,大三,教學,PPT,課件 Fatigue FatigueFatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses(e.g.,bridges,aircraft,and machine components).Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile strength for a static load.The term“fatigue”is used because this type of failure normally occurs after a long period of repeated stress or strain cycling.SignificanceFatigue 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.Furthermore,it is catastrophic and insidious,occurring very suddenly and without warning.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.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.Cyclic StressesThe applied stress may be axial(tension-compression),flexural(bending),or torsional(twisting)in nature.In general,three different fluctuating stress-time modes are possible.Reversed stress cycle;Repeated stress cycle;Random stress cycle.Reversed stress cycleRepeated stress cycleRandom stress cycleMean stressRange of stressStress amplitudeStress ratioThe S-N CurveAs with other mechanical characteristics,the fatigue properties of materials can be determined from laboratory simulation tests.A test apparatus should be designed to duplicate as nearly as possible the service stress conditions(stress level,time frequency,stress pattern,etc.).A schematic diagram of a rotating-bending test apparatusSchematic of a pulsator for fatigue tests in tension-compressionTypes of standard test pieces for fatigue testsA 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.This procedure is repeated on other specimens at progressively decreasing maximum stress amplitudes.Data are plotted as stress S versus the logarithm of the number N of cycles to failure for each of the specimens.Two distinct types of S-N behavior(1)Two distinct types of S-N behavior(2)Fatigue limit/Endurance limitFatigue limit.For fatigue,the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.The fatigue limit represents the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles.For many steels,fatigue limits range between 35 and 60%of the tensile strength.Fatigue StrengthMost 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.Fatigue strength.The maximum stress level that a material can sustain,without failing,for some specified number of cycles(e.g.107 cycles).Fatigue LifeFatigue life.The total number of stress cycles that will cause a fatigue failure at some specified stress amplitude.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.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.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.Fatigue S-N probability of failure curves for a aluminum alloyLow-cycle fatigue and high-cycle fatigueThe 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.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.Problems1.Cite five factors that may lead to scatter in fatigue life data.2.Briefly demonstrate that increasing the value of the stress ratio R produces a decrease in stress amplitude a .FractographThree distinct steps of fatigue failure(1)crack initiation,wherein a small crack forms at some point of stress concentration;(2)crack propagation,during which this crack advances incrementally with each stress cycle;and(3)final failure,which occurs very rapidly once the advancing crack has reached a critical size.Crack initiationCrack associated with fatigue failure almost always initiate(or nucleate)on the surface of a component at some point of stress concentration.Crack nucleation sites include surface scratches,keyways,threads,dents,and the like.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.Crack initiation by slip(P.Neumann)WoodCrack propagationThe region of a fracture surface that formed during crack propagation step may be characterized by two types of markings termed beachmarks and striations.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.Beachmarks(sometimes also called“clamshell marks”)are of macroscopic dimensions,and may be observed with unaided eye.These markings are found for components that experienced interruptions during the crack propagation stage.Each beachmark band represents a period of time over which crack growth occurred.BenchmarksFractographStriationOn the other hand,fatigue striations are microscopic in size and subject to observation with electron microscope(either TEM or SEM).Each striation is thought to represent the advance distance of a crack front during a single load cycle.Striation width depends on,and increases with,increasing stress range.Crack propagation model(Plastic)Crack propagation model(Brittle)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.They may be literally thousands of striations within a single beachmark.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.Beachmarks and striations will not appear on that region over which the rapid failure occurs.Rather,the rapid failure may be either ductile or brittle;evidence of plastic deformation will be present for ductile,and absent for brittle failure.Crack propagation rateThe total durability of a component is Nf=Ni+Np for Np/Nf90%，studies on crack propagation rate is of very important significance.Crack propagation rate(Paris formula)l is the crack length;N is the numbers of cycle;c,n are the constants of a material depending on the coefficient of cycle asymmetry;K is the amplitude of the stress intensity coefficient.(K=Kmax-Kmin)Factors that affect fatigue lifeThe 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.Discussion these factors and measures to be taken to improve the fatigue resistance of structural components.MEAN STRESSThe 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).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.Increasing the mean stress level leads to a decrease in fatigue life.Influence of mean stress m on S-N fatigue behaviorSURFACE EFFECTFor 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.Therefore,it has been observed that fatigue life is especially sensitive to the condition and configuration of the component surface.Design criteria and various surface treatments will lead to an improvement in fatigue life.Design factorsThe 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.Avoiding structural irregularities or making design modifications.How design can reduce stress stress amplification Surface treatmentsDuring 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.One of the most effective methods of increasing fatigue performance is by imposing residual compressive stresses within a thin outer surface layer.Residual compressive stresses are commonly introduced into ductile metals by shot peening.Schematic S-N fatigue curves for normal and shot-peened steelCase hardeningProblems 1.1 Briefly explain difference between fatigue striations and beachmarks both in terms of(a)size and(b)origin.1.2 List four measures that may be taken to increase the resistance to fatigue of a metal alloy.1.3 The fatigue data for a brass alloy are given as follows:Stress Amplitude(MPa)Cycles to failure31022319116815314313412721051106310611073107110831081109(a)Make an S-N plot(stress amplitude versus logarithm cycles to failure)use these data.(b)Determine the fatigue strength at 5105 cycles.(c)Determine the fatigue life for 200MPa.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.Specimen (MPa)(MPa)ABC+450+400+340-350-300-340(a)Rank the fatigue lifetimes of these specimens from the longest to the shortest.(b)Now justify this ranking using a schematic S-N plot1.5 Make a schematic sketch of the fatigue behavior for some metal for which the stress ratio R has a value of+1.