ASTM E1681-2003(2008) Standard Test Method for Determining a Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials.pdf

1、Designation: E 1681 03 (Reapproved 2008)Standard Test Method forDetermining Threshold Stress Intensity Factor forEnvironment-Assisted Cracking of Metallic Materials1This standard is issued under the fixed designation E 1681; the number immediately following the designation indicates the year oforigi

2、nal 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.1. Scope1.1 This test method covers the determination of theenvironment-assi

3、sted cracking threshold stress intensity factorparameters, KIEACand KEAC, for metallic materials fromconstant-force testing of fatigue precracked beam or compactfracture specimens and from constant-displacement testing offatigue precracked bolt-load compact fracture specimens.1.2 This test method is

4、 applicable to environment-assistedcracking in aqueous or other aggressive environments.1.3 Materials that can be tested by this test method are notlimited by thickness or by strength as long as specimens are ofsufficient thickness and planar size to meet the size require-ments of this test method.1

5、.4 A range of specimen sizes with proportional planardimensions is provided, but size may be variable and adjustedfor yield strength and applied force. Specimen thickness is avariable independent of planar size.1.5 Specimen configurations other than those contained inthis test method may be used, pr

6、ovided that well-establishedstress intensity calibrations are available and that specimendimensions are of sufficient size to meet the size requirementsof this test method during testing.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is t

7、heresponsibility 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 Documents2.1 ASTM Standards:2D 1141 Practice for the Preparation of Substitute OceanWaterE 8/E 8M Test Methods for

8、 Tension Testing of MetallicMaterialsE 399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE 647 Test Method for Measurement of Fatigue CrackGrowth RatesE 1823 Terminology Relating to Fatigue and Fracture Test-ingG1 Practice for Preparing, Cleaning, and Evaluati

9、ng Cor-rosion Test SpecimensG5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG15 Terminology Relating to Corrosion and CorrosionTesting3. Terminology3.1 Definitions:3.1.1 For definitions of terms relating to fracture testingused in this test metho

10、d, refer to Terminology E 1823.3.1.2 For definitions of terms relating to corrosion testingused in this test method, refer to Terminology G15.3.1.3 stress-corrosion cracking (SCC)a cracking processthat requires the simultaneous action of a corrodent andsustained tensile stress.3.1.4 stress intensity

11、 factor threshold for plane strainenvironment-assisted cracking (KIEACFL3/2)the highestvalue of the stress intensity factor (K) at which crack growth isnot observed for a specified combination of material andenvironment and where the specimen size is sufficient to meetrequirements for plane strain a

12、s described in Test MethodE 399.3.1.5 stress intensity factor threshold for environment-assisted cracking (KEACFL3/2)the highest value of thestress intensity factor (K) at which crack growth is notobserved for a specified combination of material and environ-ment and where the measured value may depe

13、nd on specimenthickness.3.1.6 physical crack size (apL)the distance from areference plane to the observed crack front. This distance mayrepresent an average of several measurements along the crackfront. The reference plane depends on the specimen form, andit is normally taken to be either the bounda

14、ry or a plane1This 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 Nov. 1, 2008. Published February 2009. Originallyapproved in 1995. Last previous edition appr

15、oved in 2003 as E 1681 - 03.2For 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 International, 10

16、0 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.containing either the loadline or the centerline of a specimen orplate. The reference plane is defined prior to specimen defor-mation.3.1.7 original crack size (aoL)the physical crack size atthe start of testing.3.1.8

17、original uncracked ligament (boL)distance fromthe original crack front to the back edge of the specimen (bo=Wao).3.1.9 specimen thickness (BL)the side-to-side dimen-sion of the specimen being tested.3.1.10 tensile strength (sTSFL2)the maximum tensilestress that a material is capable of sustaining. T

18、ensile strengthis calculated from the maximum force during a tension testcarried to rupture and the original cross-section area of thespecimen.3.2 Definitions of Terms Specific to This Standard:3.2.1 environment-assisted cracking (EAC)a crackingprocess in which the environment promotes crack growth

19、orhigher crack growth rates than would occur without thepresence of the environment.3.2.2 normalized crack size (a/W)the ratio of crack size,a, to specimen width, W. Specimen width is measured from areference position such as the front edge in a bend specimen orthe loadline in the compact specimen t

20、o the back edge of thespecimen.3.2.3 yield strength (sYSFL2)the stress at which amaterial exhibits a specific limiting deviation from the propor-tionality of stress to strain. This deviation is expressed in termsof strain.NOTE 1In this test method, the yield strength determined by the 0.2 %offset me

21、thod is used.3.2.4 effective yield strength (sYFL2)an assumed valueof uniaxial yield strength that represents the influences ofplastic yielding upon fracture test parameters. For use in thismethod, it is calculated as the average of the 0.2 % offset yieldstrength sYS, and the ultimate tensile streng

22、th, sTS,orsY5 sYS1sTS! / 2 (1)3.2.5 notch length (an(L)the distance from a referenceplane to the front of the machined notch. The reference planedepends on the specimen form and normally is taken to beeither the boundary or a plane containing either the loadline orthe centerline of a specimen or pla

23、te. The reference plane isdefined prior to specimen deformation.4. Summary of Test Method4.1 This test method involves testing of single-edge notchedSE(B) specimens, compact C(T) specimens, or bolt-loadcompact MC(W) specimens, precracked in fatigue. Thesingle-edge notched beam specimen is tested by

24、dead weightloading. An environmental chamber is either attached to thespecimen, or the specimen is contained within the chamber.The chamber must enclose the portion of the specimen wherethe crack tip is located. Prescribed environmental conditionsmust be established and maintained within the chamber

25、 at alltimes during the test.4.1.1 Specimens shall be deadweight loaded or otherwiseheld under constant force or held under constant displacement(defined in 6.2) for a prescribed length of time, during whichfailure by crack growth leading to fracture may or may notoccur. KIEACand KEACare defined as

26、the highest value of stressintensity factor at which neither failure nor crack growthoccurs. The stress intensity factor (K) is calculated from anexpression based on linear elastic stress analysis. To establisha suitable crack-tip condition for constant force tests, thestress-intensity level at whic

27、h the fatigue precracking of thespecimen is conducted is limited to a value substantially lessthan the measured KIEACor KEACvalues. For constant dis-placement tests, the stress-intensity level at which the fatigueprecracking of the specimen is conducted is limited to therequirements of Test Method E

28、 399. The validity of the KIEACvalue determined by this test method depends on meeting thesize requirements to ensure plane strain conditions, as stated inTest Method E 399. The validity of the KEACvalue depends onmeeting the size requirements for linear elastic behavior, asstated in the Test Method

29、 E 647.4.1.2 This test method can produce information on the onsetof environment-assisted crack growth. Crack growth rateinformation can be obtained after crack nucleation, but themethod for obtaining this information is not part of this testmethod (1).34.2 The mechanisms of environment-assisted cra

30、cking arevaried and complex. Measurement of a KEACor KIEACvaluefor a given combination of material and environmental pro-vides no insight into the particular cracking mechanism thatwas either operative or dominant. Two prominent theories ofenvironment-assisted cracking are anodic reaction and hydro-

31、gen embrittlement (2). The data obtained from this test methodmay be interpreted by either theory of environment-assistedcracking.4.3 Specimen thickness governs the proportions of planestrain and plane stress deformation local to the crack tip, alongwith the environmental contribution to cracking. S

32、ince thesechemical and mechanical influences cannot be separated insome material/environment combinations, thickness must betreated as a variable. In this test method, however, the stress inthe specimen must remain elastic. For these reasons, twothreshold values of EAC are defined by this test metho

33、d. Themeasurement of KIEACrequires that the thickness requirementsof plane strain constraint are met. The less restrictive require-ments of KEACare intended for those conditions in which theresults are a strong function of the thickness of the specimenand the application requires the testing of spec

34、imens withthickness representative of the application.4.4 A variety of environmental (temperature, environmentcomposition, and electrode potential, for example) and metal-lurgical (yield strength, alloy composition, and specimenorientation) variables affect KEACand KIEAC.5. Significance and Use5.1 T

35、he parameters KEACor KIEACdetermined by this testmethod characterize the resistance to crack growth of amaterial with a sharp crack in specific environments under3The boldface numbers in parentheses refer to the list of references at the end ofthis standard.E 1681 03 (2008)2loading conditions in whi

36、ch the crack-tip plastic region is smallcompared with the crack depth and the uncracked ligament.The less restrictive thickness requirements of KEACare in-tended for those conditions in which the results are a strongfunction of the thickness of the specimen and the applicationrequires the testing of

37、 specimens with thickness representativeof the application. Since the chemical and mechanical influ-ences cannot be separated, in some material/environmentcombinations, the thickness must be treated as a variable. AKEACor KIEACvalue is believed to represent a characteristicmeasurement of environment

38、-assisted cracking resistance in aprecracked specimen exposed to an environment under sus-tained tensile loading. A KEACor KIEACvalue may be used toestimate the relationship between failure stress and defect sizefor a material under any service condition, where the combi-nation of crack-like defects

39、, sustained tensile loading and thesame specific environment would be expected to occur. (Back-ground information concerning the development of this testmethod can be found in Refs (3-18).5.1.1 The apparent KEACor KIEACof a material under agiven set of chemical and electrochemical environmentalcondi

40、tions is a function of the test duration. It is difficult tofurnish a rigorous and scientific proof for the existence of athreshold (4, 5). Therefore, application of KEACor KIEACdatain the design of service components should be made withawareness of the uncertainty inherent in the concept of a truet

41、hreshold for environment-assisted cracking in metallic mate-rials (6, 18). A measured KEACor KIEACvalue for a particularcombination of material and environment may, in fact, repre-sent an acceptably low rate of crack growth rather than anabsolute upper limit for crack stability. Care should be exer-

42、cised when service times are substantially longer than testtimes.5.1.2 The degree to which force deviations from statictensile stress will influence the apparent KEACor KIEACof amaterial is largely unknown. Small-amplitude cyclic loading,well below that needed to produce fatigue crack growth,superim

43、posed on sustained tensile loading was observed tosignificantly lower the apparent threshold for stress corrosioncracking in certain instances (7, 8). Therefore, caution shouldbe used in applying KEACor KIEACdata to service situationsinvolving cyclic loading. In addition, since this standard is fors

44、tatic loading, small-amplitude cyclic loading should beavoided during testing.5.1.3 In some material/environment combinations, thesmaller the specimen, the lower the measured KEACvalue,while in other material/environment combinations the mea-sured KIEACvalue will be the lowest value (5, 9, 10, 11, 1

45、2). If,for the material/environment combination of interest, it is notknown which specimen size will result in the lower measuredvalue, then it is suggested that the use of both specimen sizesshould be considered; that is, specimens with thicknessesrepresentative of the application and specimens in

46、which thethickness meets the requirements (see 7.2.1)ofaKIEACvalue.5.1.3.1 The user may optionally determine and report aKEACvalue or a KIEACvalue. The specimen size validityrequirements for a KEACvalue meet the size requirementsdeveloped for Test Method E 647 to achieve predominatelyelastic behavio

47、r in the specimen. Test Method E 647 sizerequirements for compact specimens should be applied to boththe compact specimen and the beam specimen. The specimensize validity requirements for a KIEACvalue meet the sizerequirements developed for plane strain conditions for TestMethod E 399.5.1.4 Evidence

48、 of environment-assisted crack growth underconditions that do not meet the validity requirements of 7.2may provide an important indication of susceptibility toenvironmental cracking but cannot be used to determine a validKEACvalue (14).5.1.5 Environment-assisted cracking is influenced by bothmechani

49、cal and electrochemical driving forces. The latter canvary with crack depth, opening, or shape and may not beuniquely described by the fracture mechanics stress intensityfactor. As an illustrative example, note the strong decreasereported in KISCC4with decreasing crack size below 5 mm forsteels in 3 % NaCl in water solution (15). Geometry effects onK similitude should be experimentally assessed for specificmaterial/environment systems. Application modeling based onKEACsimilitude should be conducted with caution whensubstantial differences