1、Designation: E739 10Standard Practice forStatistical Analysis of Linear or Linearized Stress-Life (S-N)and Strain-Life (-N) Fatigue Data1This standard is issued under the fixed designation E739; the number immediately following the designation indicates the year oforiginal adoption or, in the case o
2、f revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers only S-N and -N relationships thatmay be reasonably approximated by a str
3、aight line (on appro-priate coordinates) for a specific interval of stress or strain. Itpresents elementary procedures that presently reflect goodpractice in modeling and analysis. However, because the actualS-N or -N relationship is approximated by a straight line onlywithin a specific interval of
4、stress or strain, and because theactual fatigue life distribution is unknown, it is not recom-mended that (a) the S-N or -N curve be extrapolated outsidethe interval of testing, or (b) the fatigue life at a specific stressor strain amplitude be estimated below approximately the fifthpercentile (P .
5、0.05). As alternative fatigue models andstatistical analyses are continually being developed, laterrevisions of this practice may subsequently present analysesthat permit more complete interpretation of S-N and -N data.2. Referenced Documents2.1 ASTM Standards:2E206 Definitions of Terms Relating to
6、Fatigue Testing andthe Statistical Analysis of Fatigue Data3E468 Practice for Presentation of Constant Amplitude Fa-tigue Test Results for Metallic MaterialsE513 Definitions of Terms Relating to Constant-Amplitude,Low-Cycle Fatigue Testing3E606 Practice for Strain-Controlled Fatigue Testing3. Termin
7、ology3.1 The terms used in this practice shall be used as definedin Definitions E206 and E513. In addition, the followingterminology is used:3.1.1 dependent variablethe fatigue life N (or the loga-rithm of the fatigue life).3.1.1.1 DiscussionLog (N) is denoted Y in this practice.3.1.2 independent va
8、riablethe selected and controlledvariable (namely, stress or strain). It is denoted X in thispractice when plotted on appropriate coordinates.3.1.3 log-normal distributionthe distribution of N whenlog (N) is normally distributed. (Accordingly, it is convenientto analyze log (N) using methods based o
9、n the normaldistribution.)3.1.4 replicate (repeat) testsnominally identical tests ondifferent randomly selected test specimens conducted at thesame nominal value of the independent variable X. Suchreplicate or repeat tests should be conducted independently; forexample, each replicate test should inv
10、olve a separate set of thetest machine and its settings.3.1.5 run outno failure at a specified number of loadcycles (Practice E468).3.1.5.1 DiscussionThe analyses illustrated in this practicedo not apply when the data include either run-outs (orsuspended tests). Moreover, the straight-line approxima
11、tion ofthe S-N or -N relationship may not be appropriate at long liveswhen run-outs are likely.3.1.5.2 DiscussionFor purposes of statistical analysis, arun-out may be viewed as a test specimen that has either beenremoved from the test or is still running at the time of the dataanalysis.4. Significan
12、ce and Use4.1 Materials scientists and engineers are making increaseduse of statistical analyses in interpreting S-N and -N fatiguedata. Statistical analysis applies when the given data can bereasonably assumed to be a random sample of (or representa-tion of) some specific defined population or univ
13、erse ofmaterial of interest (under specific test conditions), and it isdesired either to characterize the material or to predict theperformance of future random samples of the material (undersimilar test conditions), or both.1This practice is under the jurisdiction ofASTM Committee E08 on Fatigue an
14、dFracture and is the direct responsibility of Subcommittee E08.04 on StructuralApplications.Current edition approved Nov. 1, 2010. Published November 2010. Originallyapproved in 1980. Last previous edition approved in 2004 as E739 91 (2004)1.DOI: 10.1520/E0739-10.2For referenced ASTM standards, visi
15、t the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.1C
16、opyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Types of S-N and -N Curves Considered5.1 It is well known that the shape of S-N and -N curvescan depend markedly on the material and test conditions. Thispractice is restricted to line
17、ar or linearized S-N and -Nrelationships, for example,log N 5 A 1 B S! or (1)log N 5 A 1 B ! orlog N 5 A 1 B log S! or (2)log N 5 A 1 B log !in which S and may refer to (a) the maximum value ofconstant-amplitude cyclic stress or strain, given a specificvalue of the stress or strain ratio, or of the
18、minimum cyclicstress or strain, (b) the amplitude or the range of the constant-amplitude cyclic stress or strain, given a specific value of themean stress or strain, or (c) analogous information stated interms of some appropriate independent (controlled) variable.NOTE 1In certain cases, the amplitud
19、e of the stress or strain is notconstant during the entire test for a given specimen. In such cases someeffective (equivalent) value of S or must be established for use inanalysis.5.1.1 The fatigue life N is the dependent (random) variablein S-N and -N tests, whereas S or is the independent(controll
20、ed) variable.NOTE 2In certain cases, the independent variable used in analysis isnot literally the variable controlled during testing. For example, it iscommon practice to analyze low-cycle fatigue data treating the range ofplastic strain as the controlled variable, when in fact the range of totalst
21、rain was actually controlled during testing. Although there may be somequestion regarding the exact nature of the controlled variable in certainS-N and -N tests, there is never any doubt that the fatigue life is thedependent variable.NOTE 3In plotting S-N and -N curves, the independent variables San
22、d are plotted along the ordinate, with life (the dependent variable)plotted along the abscissa. Refer, for example, to Fig. 1.5.1.2 The distribution of fatigue life (in any test) is unknown(and indeed may be quite complex in certain situations). Forthe purposes of simplifying the analysis (while mai
23、ntainingsound statistical procedures), it is assumed in this practice thatthe logarithms of the fatigue lives are normally distributed, thatis, the fatigue life is log-normally distributed, and that thevariance of log life is constant over the entire range of theindependent variable used in testing
24、(that is, the scatter in logNOTE 1The 95 % confidence band for the -N curve as a whole is based on Eq 10. (Note that the dependent variable, fatigue life, is plotted herealong the abscissa to conform to engineering convention.)FIG. 1 Fitted Relationship Between the Fatigue Life N (Y) and the Plastic
25、 Strain Amplitude Dp/2 (X) for the Example Data GivenE739 102N is assumed to be the same at low S and levels as at highlevels of S or ). Accordingly, log N is used as the dependent(random) variable in analysis. It is denoted Y. The independentvariable is denoted X. It may be either S or ,orlogS or l
26、og, respectively, depending on which appears to produce astraight line plot for the interval of S or of interest. Thus Eq1 and Eq 2 may be re-expressed asY 5 A 1 BX (3)Eq 3 is used in subsequent analysis. It may be stated moreprecisely as Y ? X= A + BX, where Y ? Xis the expected valueof Y given X.N
27、OTE 4For testing the adequacy of the linear model, see 8.2.NOTE 5The expected value is the mean of the conceptual populationof all Ys given a specific level of X. (The median and mean are identicalfor the symmetrical normal distribution assumed in this practice for Y.)6. Test Planning6.1 Test planni
28、ng for S-N and -N test programs is discussedin Chapter 3 of Ref (1).4Planned grouping (blocking) andrandomization are essential features of a well-planned testprogram. In particular, good test methodology involves use ofplanned grouping to (a) balance potentially spurious effects ofnuisance variable
29、s (for example, laboratory humidity) and (b)allow for possible test equipment malfunction during the testprogram.7. Sampling7.1 It is vital that sampling procedures be adopted thatassure a random sample of the material being tested.Arandomsample is required to state that the test specimens are repre
30、-sentative of the conceptual universe about which both statisti-cal and engineering inference will be made.NOTE 6A random sampling procedure provides each specimen thatconceivably could be selected (tested) an equal (or known) opportunity ofactually being selected at each stage of the sampling proce
31、ss. Thus, it ispoor practice to use specimens from a single source (plate, heat, supplier)when seeking a random sample of the material being tested unless thatparticular source is of specific interest.NOTE 7Procedures for using random numbers to obtain randomsamples and to assign stress or strain am
32、plitudes to specimens (and toestablish the time order of testing) are given in Chapter 4 of Ref (2).7.1.1 Sample SizeThe minimum number of specimensrequired in S-N (and -N) testing depends on the type of testprogram conducted. The following guidelines given in Chapter3ofRef(1) appear reasonable.Type
33、 of TestMinimum Numberof SpecimensAPreliminary and exploratory (exploratory research anddevelopment tests)6to12Research and development testing of components andspecimens6to12Design allowables data 12 to 24Reliability data 12 to 24AIf the variability is large, a wide confidence band will be obtained
34、 unless a largenumber of specimens are tested (See 8.1.1).7.1.2 ReplicationThe replication guidelines given inChapter 3 of Ref (1) are based on the following definition:% replication = 100 1 (total number of different stress or strain levels usedin testing/total number of specimens tested)Type of Te
35、st Percent ReplicationAPreliminary and exploratory (research and developmenttests)17 to 33 minResearch and development testing of components andspecimens33 to 50 minDesign allowables data 50 to 75 minReliability data 75 to 88 minANote that percent replication indicates the portion of the total numbe
36、r ofspecimens tested that may be used for obtaining an estimate of the variability ofreplicate tests.7.1.2.1 Replication ExamplesGood replication: Supposethat ten specimens are used in research and development forthe testing of a component. If two specimens are tested at eachof five stress or strain
37、 amplitudes, the test program involves50 % replications. This percent replication is considered ad-equate for most research and development applications. Poorreplication: Suppose eight different stress or strain amplitudesare used in testing, with two replicates at each of two stress orstrain amplit
38、udes (and no replication at the other six stress orstrain amplitudes). This test program involves only 20 %replication, which is not generally considered adequate.8. Statistical Analysis (Linear Model Y = A + BX, Log-Normal Fatigue Life Distribution with ConstantVariance Along the Entire Interval of
39、 X Used inTesting, No Runouts or Suspended Tests or Both,Completely Randomized Design Test Program)8.1 For the case where (a) the fatigue life data pertain to arandom sample (all Yiare independent), (b) there are neitherrun-outs nor suspended tests and where, for the entire intervalof X used in test
40、ing, (c) the S-N or -N relationship is describedby the linear model Y=A+BX (more precisely by Y ? X= A+BX), (d) the (two parameter) log-normal distributiondescribes the fatigue life N, and (e) the variance of thelog-normal distribution is constant, the maximum likelihoodestimators of A and B are as
41、follows: 5 Y2 BX(4)B5(i 5 1kXi2 X! Yi2 Y!(i 5 1kXi2 X!2(5)where the symbol “caret”()denotes estimate (estimator),the symbol “overbar”( ) denotes average (for example, Y=( i 5 1kYi/k and X=(i 5 1kXi/k), Yi= log Ni, Xi= Sior i,orlog Sior log i(refer to Eq 1 and Eq 2), and k is the totalnumber of test
42、specimens (the total sample size). The recom-mended expression for estimating the variance of the normaldistribution for log N iss25(i 5 1kYi2 Yi!2k 2 2(6)in which Yi= + BXiand the (k 2) term in the denomi-nator is used instead of k to make s2an unbiased estimator ofthe normal population variance s2
43、.4The boldface numbers in parentheses refer to the list of references appended tothis standard.E739 103NOTE 8An assumption of constant variance is usually reasonable fornotched and joint specimens up to about 106cycles to failure.The varianceof unnotched specimens generally increases with decreasing
44、 stress (strain)level (see Section 9). If the assumption of constant variance appears to bedubious, the reader is referred to Ref (3) for the appropriate statistical test.8.1.1 Confidence Intervals for Parameters A and BTheestimators and Bare normally distributed with expectedvalues A and B, respect
45、ively, (regardless of total sample size k)when conditions (a) through (e)in8.1 are met. Accordingly,confidence intervals for parameters A and B can be establishedusing the t distribution, Table 1. The confidence interval for Ais given by 6 tps,or 6 tpsF1k1X2(i 5 1kXi2 X!2G, (7)and for B is given by
46、B6 tpsB,orB6 tps (i 5 1kXi2 X!2#2(8)in which the value of tpis read from Table 1 for the desiredvalue of P, the confidence level associated with the confidenceinterval. This table has one entry parameter (the statisticaldegrees of freedom, n, for t ). For Eq 7 and Eq 8, n = k 2.NOTE 9The confidence
47、intervals for A and B are exact if conditions(a) through (e)in8.1 are met exactly. However, these intervals are stillreasonably accurate when the actual life distribution differs slightly fromthe (two-parameter) log-normal distribution, that is, when only condition(d) is not met exactly, due to the
48、robustness of the t statistic.NOTE 10Because the actual median S-N or -N relationship is onlyapproximated by a straight line within a specific interval of stress or strain,confidence intervals for A and B that pertain to confidence levels greaterthan approximately 0.95 are not recommended.8.1.1.1 Th
49、e meaning of the confidence interval associatedwith, say, Eq 8 is as follows (Note 11). If the values of tpgivenin Table 1 for, say, P = 95 % are used in a series of analysesinvolving the estimation of B from independent data sets, thenin the long run we may expect 95 % of the computed intervalsto include the value B. If in each instance we were to assert thatB lies within the interval computed, we should expect to becorrect 95 times in 100 and in error 5 times in
