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Atlas Of Fatigue Curves __FULL__

Contains more than 500 fatigue curves for industrial ferrous and nonferrous alloys. Also includes a thorough explanation of fatigue testing and interpretation of test results. Each curve is presented independently and includes an explanation of its particular importance. The curves are titled by standard industrial designations (AISI, CDA, AA, etc.) of the metals, and a complete reference is given to the original source to facilitate further research.

Atlas of Fatigue Curves

The collection includes standard S-N curves, curves showing effect of surface hardening on fatigue strength, crack growth-rate curves, curves comparing the fatigue strengths of various alloys, effect of variables (i.e. temperature, humidity, frequency, aging, environment, etc.) and much, much more.

data on a particular metal or alloy. In the past, the first stepto locating thisdata was an expensive and time-consuming searchthrough the technical liter-ature. Now, many ofthe important andfrequently referenced curves are pre-sented together in this onevolume. They are arranged by standard alloy des-ignationsand areaccompanied by a textual explanation offatigue testingandinterpretation of test results. In each case, the individualcurve is thoroughlyreferenced to the original source.

Having these important curves compiled in a single book willalso facili-tate the computerization of these data. Plans arecurrently under way also tomake the data presented in this bookavailable in ASCII files for analysis bycomputer programs.

The Atlas of Fatigue Curves is obviously not complete, in thatmanymore curves could be included. Persons wishing to contributecurves to thiscompilation for inclusion in future revisions shouldcontact the Editors,Technical Books, American Society for Metals,Metals Park, Ohio 44073.

nent structural change that occurs in materialssubjected tofluctuating stresses and strains thatmay result in cracks orfracture after a sufficientnumber of fluctuations. Fatiguefractures arecaused by the simultaneous action of cyclicstress,tensile stress and plastic strain. If anyoneof these three is notpresent, fatigue cracking willnot initiate and propagate. Thecyclic stressstarts the crack; the tensile stress producescrackgrowth (propagation). Although compressivestress will notcause fatigue, compression loadmay do so.

Fatigue cracking normally results from cyclicstresses that arewell below the static yieldstrength of the material. (In low-cyclefatigue,however, or if the material has anappreciablework-hardening rate, the stresses also may beabove thestatic yield strength.)

Fatigue cracks initiate and propagate in re-gions where thestrain is most severe. Becausemost engineering materials containdefects andthus regions of stress concentration thatintensifystrain, most fatigue cracks initiate and growfromstructural defects. Under the action of cy-clic loading, a plasticzone (or region of deforma-tion) develops at the defect tip. Thiszone of highdeformation becomes an initiation site for a fa-tiguecrack. The crack propagates under the ap-plied stress through thematerial until completefracture results. On the microscopic scale,themost important feature of the fatigue process isnucleation ofone or more cracks under the influ-

the number of stress (strain) cycles required tocause failure.This number is a function of manyvariables, including stress level,stress state, cy-clic wave form, fatigue environment, andthemetallurgical condition of the material. Smallchanges in thespecimen or test conditions cansignificantly affect fatiguebehavior, making ana-lytical prediction of fatigue life difficult.There-fore, the designer may rely on experience withsimilarcomponents in service rather than onlaboratory evaluation ofmechanical test speci-mens. Laboratory tests, however, areessential inunderstanding fatigue behavior, and currentstudies withfracture mechanics test specimensare beginning to providesatisfactory designcriteria.

Laboratory fatigue tests can be classified ascrack initiation orcrack propagation. In crackinitiation testing, specimens or partsare sub-jected to the number of stress cycles required fora fatiguecrack to initiate and to subsequentlygrow large enough to producefailure.

Fatigue notch sensitivity, q, for a material isdetermined bycomparing the fatigue notch fac-tor, KJ, and thestress-concentration factor, K"for a specimen of a given sizecontaining a stressconcentrator of a given shape and size. Acom-mon definition of fatigue notch sensitivity is q =(KJ - l)f(K,- 1), in which q may vary betweenzero (where KJ= 1) and 1 (whereKJ= K,). Thisvalue may be stated as percentage.

of a crack does not necessarily imply imminentfailure of thepart. Significant structural life mayremain in the cyclic growth ofthe crack to a sizeat which a critical failure occurs. Theobjective offatigue crack propagation testing is to determinetherates at which subcritical cracks grow undercyclic loadings priorto reaching a size critical forfracture.

The growth or extension of a fatigue crackunder cyclic loadingis principally controlled bymaximum load and stress ratio. However,as incrack initiation, there are a number of additionalfactors thatmay exert a strong influence, includ-ing environment, frequency,temperature, andgrain direction. Fatigue crack propagation test-ingusually involves constant-load-amplitude cy-

a specimen or part is subjected to cyclic loadingto failure. Alarge portion of the total number ofcycles in these tests is spentinitiating the crack.Although crack initiation tests conductedonsmall specimens do not precisely establish the fa-tigue life of alarge part, such tests do providedata on the intrinsic fatiguecrack initiation be-havior of a metal or alloy. As a result, suchdatacan be utilized to develop criteria to prevent fa-tiguefailures in engineering design. Examples ofthe use ofsmall-specimen fatigue test data can befound in the basis of thefatigue design codes forboilers and pressure vessels, complexwelded, riv-eted, or bolted structures, and automotive andaerospacecomponents.

stresses that are below the yield strength of themetal. Inlow-cycle fatigue, however, the cyclicstress may be above thestatic yield strength, es-pecially in a material with anappreciable work-hardening rate. Generally, a fatigue crack isin-itiated at a highly stressed region of a componentsubjected tocyclic loading of sufficient magni-tude. The crack then propagatesin progressivecyclic growth through the cross section of thepartuntil the maximum load cannot be carried,and complete fractureresults.

Crack Nucleation. A variety of crystallo-graphic features havebeen observed to nucleatefatigue cracks. In pure metals, tubularholes thatdevelop in persistent slip bands, slip bandextru-sion-intrusion pairs at free surfaces, and twinboundaries arecommon nucleation sites. Grainboundaries in polycrystalline metals,even in theabsence of inherent grain boundary weakness,are cracknucleation sites. At high strain rates,this appears to be thepreferred site. Nucleationat grain boundaries appears to be ageometricaleffect, whereas nucleation at twin boundariesisassociated with active slip on crystallographicplanes immediatelyadjacent and parallel to thetwin boundary.

of these phenomena have a significant influenceon the cracknucleation process. In general, al-loying that (1) enhances crossslip, (2) enhancestwinning, or (3) increases the rate of workhard-ening will stimulate crack nucleation. On theother hand,alloying usually raises the flow stressof a metal, thus offsettingits potentially detri-mental effect on fatigue cracknucleation.

Crack Initiation. Fatigue cracks initiate atpoints of maximumlocal stress and minimumlocal strength. The local stress pattern isdeter-mined by the shape of the part and by the typeand magnitudeof the loading. In addition to thegeometric features of a part,features such as sur-face and metallurgical imperfections can acttoconcentrate stress locally. Surface imperfectionssuch asscratches, dents, burrs, cuts, and othermanufacturing flaws are themost obvious sitesat which fatigue cracks initiate. Except forin-stances where internal defects or special surface-hardeningtreatments are involved, fatigue cracksinitiate at the surface.

Relation to Environment. Corrosion fatiguedescribes thedegradation of the fatigue strengthof a metal by the initiation andgrowth of cracksunder the combined action of cyclic loading andacorrosive environment. Because it is a synergis-tic effect offatigue and corrosion, corrosion fa-tigue can produce a far greaterdegradation instrength than either effect acting alone or bysu-perposition of the singular effects. An unlimitednumber ofgaseous and liquid mediums may af-fect fatigue crack initiation ina given material.Fretting corrosion, which occurs fromrelativemotion between joints, may also accelerate fa-tigue crackinitiation.

clically loaded component frequently is mea-sured by the amountof overstress-that is, theamount by which the nominal stressexceeds thefatigue limit or the long-life fatigue strength ofthematerial used in the component. The numberof load cycles that acomponent under low over-stress can endure is high; thus, the termhigh-cycle fatigue is often applied.

As the magnitude of the nominal stress in-creases, initiation ofmultiple cracks is morelikely. Also, spacing between fatiguestriations,which indicate the progressive growth of thecrack front,is increased, and the region of finalfast fracture is increased insize.

Low-cycle fatigue is the regime characterizedby high overstress.The arbitrary, but commonlyaccepted, dividing line betweenhigh-cycle andlow-cycle fatigue is considered to be about 104 to105cycles. In practice, this distinction is made bydetermining whetherthe dominant componentof the strain imposed during cyclic loadingiselastic (high cycle) or plastic (low cycle), which inturn dependson the properties of the metal aswell as the magnitude of thenominal stress.

Presentation of Fatigue Data. High-cycle fa-tigue data arepresented graphically as stress (S)versus cycles-to-failure (N) inS-N diagrams orS-N curves. These are described in the Introduc-tionto this Section along with the symbols andnomenclature commonlyapplied in fatigue test-ing. Because the stress in high-cyclefatigue testsis usually within the elastic range, the calculationofstress amplitude, stress range, or maximumstress on the S-axis ismade using simple equa-tions from mechanics ofmaterials; i.e.,stress cal-culated using the specimen dimensions and thecontrolledload or deflection applied axially, inflexure, or in torsion. 041b061a72


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