# ASTM E647-15e1

Designation: E647 − 15´1Standard Test Method forMeasurement of Fatigue Crack Growth Rates1This standard is issued under the fixed designation E647; the number immediately following the designation indicates the year oforiginal adoption or, in the case of 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.ε1NOTE—Table X1.1 was editorially corrected in July 2016.1. Scope1.1 This test method2covers the determination of fatiguecrack growth rates from near-threshold to Kmaxcontrolledinstability. Results are expressed in terms of the crack-tipstress-intensity factor range (∆K), 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 be 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 size 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. Specimenconfigurations other than those contained in this method maybe used provided that well-established stress-intensity factorcalibrations are available and that specimens are of sufficientplanar size to remain predominantly elastic during testing.1.6 Residual stress/crack closure may significantly influencethe fatigue crack growth rate data, particularly at low stress-intensity factors and low stress ratios, although such variablesare not incorporated into the computation of ∆K.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 firstpart 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, andprocedures 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 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 A4Guidelines 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 for Measuring the Growth Rates Of Small FatigueCracksAppendix X3Recommended Practice for Determination Of ACR-BasedStress-Intensity Factor RangeAppendix X41.9 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 TensionSpecimenAnnex A31.10 This 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.1This test method is under the jurisdiction of ASTM Committee E08 on Fatigueand Fracture and is the direct responsibility of Subcommittee E08.06 on CrackGrowth Behavior.Current edition approved May 1, 2015. Published July 2015. Originally approvedin 1978. Last previous approved in 2013 as E647 – 13aε1. DOI: 10.1520/E0647-15E01.2For additional information on this test method see RR: E24 – 1001. Availablefrom ASTM Headquarters, 100 Barr Harbor Drive, West Conshohocken, PA 19428.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States12. Referenced Documents2.1 ASTM Standards:3E4 Practices for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical TestingE8/E8M Test Methods for Tension Testing of Metallic Ma-terialsE338 Test Method of Sharp-Notch Tension Testing of High-Strength Sheet Materials (Withdrawn 2010)4E399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE467 Practice for Verification of Constant Amplitude Dy-namic Forces in an Axial Fatigue Testing SystemE561 Test Method forKRCurve DeterminationE1012 Practice for Verification of Testing Frame and Speci-men Alignment Under Tensile and Compressive AxialForce ApplicationE1820 Test Method for Measurement of Fracture ToughnessE1823 Terminology Relating to Fatigue and Fracture Testing3. Terminology3.1 The terms used in this test method are given in Termi-nology E6, and Terminology E1823. Wherever these terms arenot in agreement with one another, use the definitions given inTerminology E1823 which are applicable to this test method.3.2 Definitions:3.2.1 crack size, a[L],n—a linear measure of a principalplanar dimension of a crack. This measure is commonly usedin the calculation of quantities descriptive of the stress anddisplacement fields and is often also termed crack length ordepth.3.2.1.1 Discussion—In fatigue testing, crack length is thephysical crack size. See physical crack size in TerminologyE1823.3.2.2 cycle—in fatigue, under constant amplitude loading,the force variation from the minimum to the maximum andthen to the minimum force.3.2.2.1 Discussion—In spectrum loading, the definition ofcycle varies with the counting method used.3.2.2.2 Discussion—In this test method, the symbol N isused to represent the number of cycles.3.2.3 fatigue-crack-growth rate, da/dN, [L/cycle]—the rateof crack extension under fatigue loading, expressed in terms ofcrack extension per cycle .3.2.4 fatigue cycle—See cycle.3.2.5 force cycle—See cycle.3.2.6 force range, ∆ P [ F]—in fatigue, the algebraicdifference between the maximum and minimum forces in acycle expressed as:∆P 5 Pmax2 Pmin(1)3.2.7 force ratio (also called stress ratio), R—in fatigue, thealgebraic ratio of the minimum to maximum force (stress) in acycle, that is, R = Pmin/Pmax.3.2.8 maximum force, Pmax[F]—in 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, Kmax[FL−3/2]—infatigue, the maximum value of the stress-intensity factor in acycle. This value corresponds to Pmax.3.2.10 minimum force, Pmin[F]—in 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, Kmin[FL−3/2]—infatigue, the minimum value of the stress-intensity factor in acycle. This value corresponds to PminwhenR0andistakento be zero when R ≤ 0.3.2.12 stress cycle—See cycle in Terminology E1823.3.2.13 stress-intensity factor, K, K1,K2,K3[FL−3/2]—SeeTerminology E1823.3.2.13.1 Discussion—In this test method, mode 1 is as-sumed and the subscript 1 is everywhere implied.3.2.14 stress-intensity factor range, ∆K [FL−3/2]—infatigue, the variation in the stress-intensity factor in a cycle,that is∆K 5 Kmax2 Kmin(2)3.2.14.1 Discussion—The loading variables R, ∆K, andKmaxare related in accordance with the following relation-ships:∆K 5 ~1 2 R!Kmaxfor R$0, and (3)∆K 5 Kmaxfor R#0.3.2.14.2 Discussion—These operational stress-intensity fac-tor definitions do not include local crack-tip effects; forexample, crack closure, residual stress, and blunting.3.2.14.3 Discussion—While the operational definition of∆K states that ∆K 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 ∆K does not influencethe material’s response since this response (da/dN) is indepen-dent of the operational definition of ∆K. For predictingcrack-growth lives generated under various R conditions, thelife prediction methodology must be consistent with the datareporting methodology.3.2.14.4 Discussion—An alternative definition for thestress-intensity factor range, which utilizes the full range of R,is ∆Kfr= 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 developedunder 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 curve—a curve (a fixed-force or fixed-displacement crack-extension-force curve) obtained from a3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.4The last approved version of this historical standard is referenced onwww.astm.org.E647 − 15´12fracture mechanics analysis for a specific specimen configura-tion. The curve relates the stress-intensity factor to crack sizeand either applied force or displacement.3.3.1.1 Discussion—The resulting analytical expression issometimes called a K calibration and is frequently available inhandbooks for stress-intensity factors.3.3.2 fatigue crack growth threshold, ∆Kth[FL−3/2]—thatasymptotic value of ∆K at which da/dN approaches zero. Formost materials an operational, though arbitrary, definition of∆Kthis given as that ∆K which corresponds to a fatigue crackgrowth rate of 10−10m/cycle. The procedure for determiningthis operational ∆Kthis given in 9.4.3.3.2.1 Discussion—The intent of this definition is not todefine a true threshold, but rather to provide a practical meansof characterizing a material’s 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 growth rate, da/dN or ∆a/∆N, [L]—infatigue, 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 [L–1]—thefractional rate of change of K with increasing crack size.3.3.4.1 Discussion—When C is held constant the percentagechange in K is constant for equal increments of crack size. Thefollowing identity is true for the normalized K-gradient in aconstant force ratio test:1K·dKda51Kmax·dKmaxda51Kmin·dKminda51∆K·d∆Kda(4)3.3.5 K-decreasing test—a test in which the value of C isnominally negative. In this test 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 test—a test in which the value of C isnominally positive. For the standard specimens in this methodthe constant-force-amplitude test will result in 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 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, ∆K, which is calculated fromexpressions based on linear elastic stress analysis.5. Significance and Use5.1 Fatigue crack growth rate expressed as a function ofcrack-tip stress-intensity factor range, d a/dN versus ∆K,characterizes a material’s resistance to stable crack extensionunder cyclic loading. Background information on the ration-alefor employing linear elastic fracture mechanics to analyzefatigue crack growth rate data is given in Refs (1)5and (2).5.1.1 In innocuous (inert) environments fatigue crackgrowth rates are primarily a function of ∆K and force ratio, R,or Kmaxand R (Note 1). Temperature and aggressive environ-ments can significantly affect da/ dN versus ∆K, and in manycases accentuate R-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 1—∆K, Kmax, and R are not independent of each other. Specifi-cation of any two of these variables is sufficient to define the loadingcondition. It is customary to specify one of the stress-intensity parameters(∆K or Kmax) along with the force ratio, R.5.1.2 Expressing da/dN as a function of ∆K provides resultsthat are independent of planar geometry, thus enabling ex-change and comparison of data obtained from a variety ofspecimen configurations and loading conditions. Moreover,this feature enables d a/dN versus ∆K data to be utilized in thedesign and evaluation of engineering structures. The concept ofsimilitude is assumed, which implies that cracks of differinglengths subjected to the same nominal ∆K 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 growth rate are mixed. Fatigue crack growthrates over a wide range of ∆K 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. Forexample, materials may exhibit thickness effects over theterminal range of da/ dN versus ∆K, which are associated witheither nominal yielding (Note 2)orasKmaxapproaches thematerial fracture toughness. The potential influence of speci-men thickness should be considered when generating data forresearch or design.NOTE 2—This 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 besignific