ASTM E647-2008 459 Standard Test Method for Measurement of Fatigue Crack Growth Rates.pdf

1、Designation: E 647 08Standard Test Method forMeasurement of Fatigue Crack Growth Rates1This standard is issued under the fixed designation E 647; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number i

2、n parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method2covers the determination of fatiguecrack growth rates from near-threshold to Kmaxcontrolledinstability. Results are expressed

3、 in terms of the crack-tipstress-intensity factor range (DK), defined by the theory oflinear elasticity.1.2 Several different test procedures are provided, the opti-mum test procedure being primarily dependent on the magni-tude of the fatigue crack growth rate to be measured.1.3 Materials that can b

4、e tested by this test method are notlimited by thickness or by strength so long as specimens are ofsufficient thickness to preclude buckling and of sufficientplanar size to remain predominantly elastic during testing.1.4 A range of specimen sizes with proportional planardimensions is provided, but s

5、ize is variable to be adjusted foryield strength and applied force. Specimen thickness may bevaried independent of planar size.1.5 The details of the various specimens and test configu-rations are shown in Annex A1-Annex A3. Specimen configu-rations other than those contained in this method may be u

6、sedprovided that well-established stress-intensity factor calibra-tions are available and that specimens are of sufficient planarsize to remain predominantly elastic during testing.1.6 Residual stress/crack closure may significantly influencethe fatigue crack growth rate data, particularly at low st

7、ress-intensity factors and low stress ratios, although such variablesare not incorporated into the computation of DK.1.7 Values stated in SI units are to be regarded as thestandard. Values given in parentheses are for information only.1.8 This test method is divided into two main parts. The firstpar

8、t gives general information concerning the recommenda-tions and requirements for fatigue crack growth rate testing.The second part is composed of annexes that describe thespecial requirements for various specimen configurations, spe-cial requirements for testing in aqueous environments, andprocedure

9、s for non-visual crack size determination. In addition,there are appendices that cover techniques for calculatingda/dN, determining fatigue crack opening force, and guidelinesfor measuring the growth of small fatigue cracks. Generalinformation and requirements common to all specimen typesare listed

10、as follows:SectionReferenced Documents 2Terminology 3Summary of Use 4Significance and Use 5Apparatus 6Specimen Configuration, Size, and Preparation 7Procedure 8Calculations and Interpretation of Results 9Report 10Precision and Bias 11Special Requirements for Testing in Aqueous Environments Annex A4G

11、uidelines for Use of Compliance to Determine Crack Size Annex A5Guidelines for Electric Potential Difference Determination ofCrack SizeAnnex A6Recommended Data Reduction Techniques Appendix X1Recommended Practice for Determination of Fatigue CrackOpening Force From ComplianceAppendix X2Guidelines fo

12、r Measuring the Growth Rates Of Small FatigueCracksAppendix X31.9 Special requirements for the various specimen configu-rations appear in the following order:The Compact Specimen Annex A1The Middle Tension Specimen Annex A2The Eccentrically-Loaded Single Edge Crack Tension Speci-menAnnex A31.10 This

13、 standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents

14、2.1 ASTM Standards:3E4 Practices for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical Test-ingE 8/E 8M Test Methods for Tension Testing of MetallicMaterials1This test method is under the jurisdiction of ASTM Committee E08 on Fatigueand Fracture and is the direct

15、 responsibility of Subcommittee E08.06 on CrackGrowth Behavior.Current edition approved April 1, 2008. Published May 2008. Originallyapproved in 1978. Last previous approved in 2005 as E 647 05.2For additional information on this test method see RR: E 24 1001. Availablefrom ASTM Headquarters, 100 Ba

16、rr Harbor Drive, West Conshohocken, PA 19428.3For referenced ASTM standards, visit 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.1Copyright ASTM

17、International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.E 337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)E 338 Test Method of Sharp-Notch Tension Testing ofHigh-Strength Sheet MaterialsE 399 T

18、est Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE 467 Practice for Verification of Constant Amplitude Dy-namic Forces in an Axial Fatigue Testing SystemE 561 Test Method for K-R Curve DeterminationE 1012 Practice for Verification of Test Frame and Speci-men Align

19、ment Under Tensile and Compressive AxialForce ApplicationE 1820 Test Method for Measurement of Fracture Tough-nessE 1823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 The terms used in this test method are given in Termi-nology E6, and Terminology E 1823. Wherever these term

20、sare not in agreement with one another, use the definitions givenin Terminology E 1823 which are applicable to this testmethod.3.2 Definitions:3.2.1 crack size, aL, na linear measure of a principalplanar dimension of a crack. This measure is commonly usedin the calculation of quantities descriptive

21、of the stress anddisplacement fields and is often also termed crack length ordepth.3.2.1.1 DiscussionIn fatigue testing, crack length is thephysical crack size. See physical crack size in TerminologyE 1823.3.2.2 cyclein fatigue, under constant amplitude loading,the force variation from the minimum t

22、o the maximum andthen to the minimum force.3.2.2.1 DiscussionIn spectrum loading, the definition ofcycle varies with the counting method used.3.2.2.2 DiscussionIn this test method, the symbol N isused to represent the number of cycles.3.2.3 fatigue-crack-growth rate, da/dN, Lcrack exten-sion per cyc

23、le of loading.3.2.4 fatigue cycleSee cycle.3.2.5 force cycleSee cycle.3.2.6 force range, D P Fin fatigue, the algebraic differ-ence between the maximum and minimum forces in a cycleexpressed as:DP 5 Pmax2 Pmin(1)3.2.7 force ratio (also called stress ratio), Rin fatigue, thealgebraic ratio of the min

24、imum to maximum force (stress) in acycle, that is, R = Pmin/Pmax.3.2.8 maximum force, PmaxFin fatigue, the highestalgebraic value of applied force in a cycle. Tensile forces areconsidered positive and compressive forces negative.3.2.9 maximum stress-intensity factor, KmaxFL3/2infatigue, the maximum

25、value of the stress-intensity factor in acycle. This value corresponds to Pmax.3.2.10 minimum force, PminFin fatigue, the lowestalgebraic value of applied force in a cycle. Tensile forces areconsidered positive and compressive forces negative.3.2.11 minimum stress-intensity factor, KminFL3/2infatigu

26、e, the minimum value of the stress-intensity factor in acycle. This value corresponds to PminwhenR0andistakento be zero when R # 0.3.2.12 stress cycleSee cycle in Terminology E 1823.3.2.13 stress-intensity factor, K, K1, K2, K3FL3/2SeeTerminology E 1823.3.2.13.1 DiscussionIn this test method, mode 1

27、 is as-sumed and the subscript 1 is everywhere implied.3.2.14 stress-intensity factor range, DK FL3/2in fa-tigue, the variation in the stress-intensity factor in a cycle, thatisDK 5 Kmax2 Kmin(2)3.2.14.1 DiscussionThe loading variables R, DK, andKmaxare related in accordance with the following relat

28、ion-ships:DK 5 1 2 R!Kmaxfor R$0, and (3)DK 5 Kmaxfor R#0.3.2.14.2 DiscussionThese operational stress-intensity fac-tor definitions do not include local crack-tip effects; forexample, crack closure, residual stress, and blunting.3.2.14.3 DiscussionWhile the operational definition ofDK states that DK

29、 does not change for a constant value of Kmaxwhen R # 0, increases in fatigue crack growth rates can beobserved when R becomes more negative. Excluding thecompressive forces in the calculation of DK does not influencethe materials response since this response (da/dN) is indepen-dent of the operation

30、al definition of DK. For predictingcrack-growth lives generated under various R conditions, thelife prediction methodology must be consistent with the datareporting methodology.3.2.14.4 DiscussionAn alternative definition for thestress-intensity factor range, which utilizes the full range of R,is DK

31、fr= Kmax Kmin. (In this case, Kminis the minimum valueof stress-intensity factor in a cycle, regardless of R.) If usingthis definition, in addition to the requirements of 10.1.13, thevalue of R for the test should also be tabulated. If comparingdata developed under R # 0 conditions with data develop

32、edunder R 0 conditions, it may be beneficial to plot the da/dNdata versus Kmax.3.3 Definitions of Terms Specific to This Standard:3.3.1 applied-K curvea curve (a fixed-force or fixed-displacement crack-extension-force curve) obtained from afracture mechanics analysis for a specific specimen configur

33、a-tion. The curve relates the stress-intensity factor to crack sizeand either applied force or displacement.3.3.1.1 DiscussionThe resulting analytical expression issometimes called a K calibration and is frequently available inhandbooks for stress-intensity factors.3.3.2 fatigue crack growth thresho

34、ld, DKthFL3/2thatasymptotic value of DK at which da/dN approaches zero. Formost materials an operational, though arbitrary, definition ofE647082DKthis given as that DK which corresponds to a fatigue crackgrowth rate of 1010m/cycle. The procedure for determiningthis operational DKthis given in 9.4.3.

35、3.2.1 DiscussionThe intent of this definition is not todefine a true threshold, but rather to provide a practical meansof characterizing a materials fatigue crack growth resistance inthe near-threshold regime. Caution is required in extending thisconcept to design (see 5.1.5).3.3.3 fatigue crack gro

36、wth rate, da/dN or Da/DN, Linfatigue, the rate of crack extension caused by fatigue loadingand expressed in terms of average crack extension per cycle.3.3.4 normalized K-gradient, C = (1/K). dK/da L1thefractional rate of change of K with increasing crack size.3.3.4.1 DiscussionWhen C is held constan

37、t the percent-age change in K is constant for equal increments of crack size.The following identity is true for the normalized K-gradient ina constant force ratio test:1KdKda51KmaxdKmaxda51KmindKminda51DKdDKda(4)3.3.5 K-decreasing testa test in which the value of C isnominally negative. In this test

38、 method K-decreasing tests areconducted by shedding force, either continuously or by a seriesof decremental steps, as the crack grows.3.3.6 K-increasing testa test in which the value of C isnominally positive. For the standard specimens in this methodthe constant-force-amplitude test will result in

39、a K-increasingtest where the C value increases but is always positive.4. Summary of Test Method4.1 This test method involves cyclic loading of notchedspecimens which have been acceptably precracked in fatigue.Crack size is measured, either visually or by an equivalentmethod, as a function of elapsed

40、 fatigue cycles and these dataare subjected to numerical analysis to establish the rate of crackgrowth. Crack growth rates are expressed as a function of thestress-intensity factor range, DK, which is calculated fromexpressions based on linear elastic stress analysis.5. Significance and Use5.1 Fatig

41、ue crack growth rate expressed as a function ofcrack-tip stress-intensity factor range, da/dN versus DK, char-acterizes a materials resistance to stable crack extension undercyclic loading. Background information on the ration-ale foremploying linear elastic fracture mechanics to analyze fatiguecrac

42、k growth rate data is given in Refs (1)4and (2).5.1.1 In innocuous (inert) environments fatigue crackgrowth rates are primarily a function of DK and force ratio, R,or Kmaxand R (Note 1). Temperature and aggressive environ-ments can significantly affect da/dN versus DK, and in manycases accentuate R-

43、effects and introduce effects of otherloading variables such as cycle frequency and waveform.Attention needs to be given to the proper selection and controlof these variables in research studies and in the generation ofdesign data.NOTE 1DK, Kmax, and R are not independent of each other. Specifi-cati

44、on of any two of these variables is sufficient to define the loadingcondition. It is customary to specify one of the stress-intensity parameters(DK or Kmax) along with the force ratio, R.5.1.2 Expressing da/dN as a function of DK provides resultsthat are independent of planar geometry, thus enabling

45、 ex-change and comparison of data obtained from a variety ofspecimen configurations and loading conditions. Moreover,this feature enables da/dN versus DK data to be utilized in thedesign and evaluation of engineering structures. The concept ofsimilitude is assumed, which implies that cracks of diffe

46、ringlengths subjected to the same nominal DK will advance byequal increments of crack extension per cycle.5.1.3 Fatigue crack growth rate data are not alwaysgeometry-independent in the strict sense since thickness effectssometimes occur. However, data on the influence of thicknesson fatigue crack gr

47、owth rate are mixed. Fatigue crack growthrates over a wide range of DK have been reported to eitherincrease, decrease, or remain unaffected as specimen thicknessis increased. Thickness effects can also interact with othervariables such as environment and heat treatment. For ex-ample, materials may e

48、xhibit thickness effects over the termi-nal range of da/dN versus DK, which are associated with eithernominal yielding (Note 2)orasKmaxapproaches the materialfracture toughness. The potential influence of specimen thick-ness should be considered when generating data for research ordesign.NOTE 2This

49、condition should be avoided in tests that conform to thespecimen size requirements listed in the appropriate specimen annex.5.1.4 Residual stresses can influence fatigue crack growthrates, the measurement of such growth rates and the predict-ability of fatigue crack growth performance. The effect can besignificant when test specimens are removed from materialsthat embody residual stress fields; for example weldments orcomplex shape forged, extruded, cast or machined thicksections, where full stress relief is no