ASTM B909-17 Standard Guide for Plane Strain Fracture Toughness Testing of Non-Stress Relieved Aluminum Products.pdf

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1、Designation: B909 17Standard Guide forPlane Strain Fracture Toughness Testing of Non-StressRelieved Aluminum Products1This standard is issued under the fixed designation B909; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the yea

2、r 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 guide covers supplementary guidelines for plane-strain fracture toughness testing of aluminum products forwh

3、ich complete stress relief is not practicable. Guidelines forrecognizing when residual stresses may be significantly biasingtest results are presented, as well as methods for minimizingthe effects of residual stress during testing. This guide alsoprovides guidelines for correction and interpretation

4、 of dataproduced during the testing of these products. Test MethodE399 is the standard test method to be used for plane-strainfracture toughness testing of aluminum alloys.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibilit

5、y 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.1.3 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the

6、Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Meta

7、llic MaterialsE561 Test Method forKRCurve DeterminationE1823 Terminology Relating to Fatigue and Fracture Testing2.2 ANSI Standard:ANSI H35.1 Alloy and Temper Designations forAluminum32.3 ISO Standard:ISO 12737 Metallic MaterialsDetermination of PlaneStrain Fracture Toughness43. Terminology3.1 Defin

8、itionsTerminology in Test Method E399 andTerminology E1823 are applicable herein.3.2 Definitions of Terms Specific to This Standard:3.2.1 corrected plane-strain fracture toughness a testresult, designated KQ(corrected), which has been corrected forresidual stress bias by one of the methods outlined

9、in thisguide.3.2.1.1 DiscussionThe corrected result is an estimation ofthe KQor KIcthat would have been obtained in a residual stressfree specimen. The corrected result may be obtained from a testrecord which yielded either an invalid KQor valid KIc, but forwhich there is evidence that significant r

10、esidual stress ispresent in the test coupon.3.2.2 invalid plane-strain fracture toughness a test result,designated KQ, that does not meet one or more validityrequirements in Test Method E399 or ISO 12737 and may ormay not be significantly influenced by residual stress.3.2.3 valid plane-strain fractu

11、re toughness a test result,designated KIc, meeting the validity requirements in TestMethod E399 or ISO 12737 that may or may not be signifi-cantly influenced by residual stress.4. Significance and Use4.1 The property KIc, determined by Test Method E399 orISO 12737, characterizes a materials resistan

12、ce to fracture ina neutral environment and in the presence of a sharp cracksubjected to an applied opening force or moment within a fieldof high constraint to lateral plastic flow (plane strain condi-tion). A KIcvalue is considered to be a lower limiting value offracture toughness associated with th

13、e plane strain state.4.1.1 Thermal quenching processes used with precipitationhardened aluminum alloy products can introduce significant1This guide is under the jurisdiction of ASTM Committee B07 on Light Metalsand Alloys and is the direct responsibility of Subcommittee B07.05 on Testing.Current edi

14、tion approved May 1, 2017. Published June 2017. Originallyapproved in 2000. Last previous edition approved in 2011 as B909 00 (2011).DOI: 10.1520/B0909-17.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTM

15、Standards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.4Available from International Organization for Standardization (ISO), 1 rue deV

16、aremb, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardiza

17、tion established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1residual stresses in the product. Mechanical stress relief pro-cedures (stretching, compression

18、) are commonly used to re-lieve these residual stresses in products with simple shapes.However, in the case of mill products with thick cross-sections(for example, heavy gage plate or large hand forgings) orcomplex shapes (for example, closed die forgings, complexopen die forgings, stepped extrusion

19、s, castings), completemechanical stress relief is not always possible. In otherinstances residual stresses may be unintentionally introducedinto a product during fabrication operations such asstraightening, forming, or welding operations.4.1.2 Specimens taken from such products that containresidual

20、stress will likewise themselves contain residual stress.While the act of specimen extraction in itself partially relievesand redistributes the pattern of original stress, the remainingmagnitude can still be appreciable enough to cause significanterror in the ensuing test result.4.1.3 Residual stress

21、 is superimposed on the applied stressand results in an actual crack-tip stress intensity that is differentfrom that based solely on externally applied forces or displace-ments.4.1.4 Tests that utilize deep edge-notched specimens such asthe compact tension C(T) are particularly sensitive to distorti

22、onduring specimen machining when influential residual stress ispresent. In general, for those cases where such residual stressesare thermal quench induced, the resulting KIcor KQresult istypically biased upward (that is, KQis higher than that whichwould have been achieved in a residual stress free s

23、pecimen).The inflated values result from the combination of specimendistortion and bending moments caused by the redistribution ofresidual stress during specimen machining and excessivefatigue precrack from curvature5.4.2 This guide can serve the following purposes:4.2.1 Provide warning signs that t

24、he measured value of KIchas been biased by residual stresses and may not be a lowerlimit value of fracture toughness.4.2.2 Provide experimental methods by which to minimizethe effect of residual stress on measured fracture toughnessvalues.4.2.3 Suggest methods that can be used to correct residualstr

25、ess influenced values of fracture toughness to values thatapproximate a fracture toughness value representative of a testperformed without residual stress bias.5. Interferences5.1 There are a number of warning signs that test measure-ments are or might be biased by the presence of residual stress.If

26、 any one or more of the following conditions exist, residualstress bias of the ensuing plane strain fracture toughness testresult should be suspected. The likelihood that residual stressesare biasing test results increases as the number of warningsigns increase.5.1.1 A temper designation of a heat t

27、reatable aluminumproduct that does not indicate that it was stress relieved. Stressrelief is indicated by any of the following temper designations:T_51, T_510, T_511, T_52, or T_54, as described in ANSIH35.1.5.1.2 Machining distortion during specimen preparation.Aneffective method to quantify distor

28、tion of a C(T) specimen is tomeasure the specimen height directly above the knife edges(typically at the front face for specimen designs with integralknife edges) prior to and after machining the notch. Experiencehas shown that for an aluminum C(T) specimen with a notchlength to width ratio (ao/W) o

29、f 0.45, a difference in the heightmeasured before and after machining the notch equal to orgreater than 0.003 in. (0.076 mm) is an indicator that theensuing test result will be significantly influenced by residualstress.5.1.3 Excessive fatigue precrack front curvature not meet-ing the crack-front st

30、raightness requirements in Test MethodE399 or ISO 12737.5.1.4 Unusually high loads or number of cycles required forprecracking relative to the same or similar alloy/products.5.1.5 A significant change in fracture toughness that isgreater than that typically observed upon changing specimenconfigurati

31、on (for example, from C(T) to three point bend bar)or upon changing specimens W dimension that cannot beexplained by other means. For example, if residual stress isbiasing fracture toughness tests results, then increasing thespecimens W dimension typically results in increasing KQvalues.NOTE 1Other

32、factors, such as a steeply rising R-curve (Practice E561)in high toughness alloy/products, may also be responsible for KQvaluesincreasing with increasing specimen W dimension.5.1.6 A nonlinear load-COD trace during the initial elasticportion of the test record. This result is indicative of theresidu

33、al stress clamping that is being overcome to open thecrack under the progressively increasing applied load.6. Minimizing Effects of Residual Stress on FractureToughness Measurements6.1 When testing aluminum products that have not beenstress relieved, there are two approaches available to minimizeor

34、eliminate the effects of residual stress on fracture toughnessmeasurements. The first approach involves the use of one ormore experimental methods designed to minimize the residualstress in test specimens. The second approach involves the useof post-test correction methods to estimate the fracture t

35、ough-ness KQor KIcthat would have been obtained had the testspecimen been free of residual stress.7. Experimental Methods to Minimize Effects of ResidualStress7.1 The following considerations can be used to minimizethe magnitude of residual stress in test specimens.7.1.1 To minimize the biasing infl

36、uences of both distortion-induced clamping (or opening) moments and precrack frontcurvature, the specimen thickness (B) should be as small aspossible with respect to the host product thickness, whilemaintaining a specimen W/B ratio of 2. However, this must bedone such that the specimen B and W dimen

37、sions are large5Bucci, R.J., “Effect of Residual Stress on Fatigue Crack Growth RateMeasurement,” Fracture Mechanics: Thirteenth Conference, ASTM STP 743,American Society for Testing and Materials, 1981, pp. 2847.B909 172enough to meet the Test Method E399 or ISO 12737 specimensize requirements for

38、valid KIcmeasurement.7.1.2 In cases where the specimen size required to obtain avalid KIcis too large for the strategy described in 7.1.1 to beeffective, the use of special precracking techniques can pro-duce a straighter fatigue precrack and reduce the residual stressbias. One such technique involv

39、es the use of high stress ratiosfor precracking. Experience has shown that precracking at acyclic stress ratio of 0.7 results in significantly straighter crackfronts than precracks produced at a stress ratio of 0.1.Moreover, the straighter crack fronts that result from precrack-ing at higher R-ratio

40、 have been shown to reduce the error in theensuing fracture toughness measurement by up to 75 %.NOTE 2Test Method E399 requires precracking to be performed atstress ratios between 1 and 0.1 (inclusive). Therefore, specimensprecracked at stress ratios greater than 0.1 and less than or equal to 0.7 wi

41、llresult in KQ, which are invalid in accordance with Test Method E399.However, even though invalid, the KQobtained from a specimen pre-cracked at higher stress ratios but meeting the crack front straightnessrequirements and other validity requirements in Test Method E399 shouldbe a significantly bet

42、ter estimate of the plane-strain fracture toughness,KIc, than an invalid KQobtained from a specimen precracked at a stressratio meeting Test Method E399 requirements but with excessive crackfront curvature.7.1.3 Measurement of the specimen height change, as de-picted in Fig. 1, can be used as a gage

43、 of the severity of thebending moment induced residual stress bias. The measure-ments can also be used as a method to estimate the “true”fracture toughness through a post-test correction described inSection 8.8. Post-Test Residual Stress Correction Methods8.1 Method 1This correction method utilizes

44、the specimenheight change measurement described in Fig. 1 and denoted as. As shown in Fig. 2, the origin of the residual stress biasedload-displacement test record is modified by displacing theorigin by an amount equal to and to the load associated withthat displacement. The test is now analyzed usi

45、ng this neworigin and modified load-displacement record with the standardmethodology described in Test Method E399.NOTE 3Limited experimental evidence6,7indicates that under pre-cracking conditions resulting in excessive crack front curvature (that is,not meeting the crack front straightness require

46、ments in Test MethodE399), KQ(corrected) values obtained by Method 1 are within 15 % of theKIcor KQvalue that would have been obtained in a residual stress freespecimen. Limited experimental evidence also indicates that the accuracyof the correction method decreases when the specimen has been pre-cr

47、acked at higher stress ratios, such as 0.7, to obtain a straighter crackfront. In this case, Method 2 is preferred.8.2 Method 2A second empirical residual correctionmethod involves the use of a modified fatigue precrack lengthin the calculation of KQ. For this correction method, the fatigueprecrack

48、length is calculated as the average of the twospecimen surface precrack lengths. The KQvalue is thencalculated using the standard fracture mechanics equations forthe C(T) specimen. Empirical evidence indicates that thismethod has greater accuracy than that described in 8.1 whenthe specimen has been

49、precracked at higher stress ratios, suchas 0.7.NOTE 4Limited experimental evidence8indicates that KQ(corrected)values obtained by Method 2 are within 10 % of the KIcor KQthat wouldhave been obtained in a residual stress free specimen, regardless of thecrack front straightness for a typical residual stress distribution producedby quenching, which is compression at the surface and tension at thecenter of the specimen. For this typical distribution, the two surfaceprecrack lengths will be smaller than those in the center of the specimen.For non-typica