1、Designation: B 909 00 (Reapproved 2006)Standard Guide forPlane Strain Fracture Toughness Testing of Non-StressRelieved Aluminum Products1This standard is issued under the fixed designation B 909; the number immediately following the designation indicates the year oforiginal adoption or, in the case
2、of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 al
3、uminum products forwhich 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 correcti
4、on and interpretation of dataproduced during the testing of these products. Test MethodE 399 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.
5、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 Documents2.1 ASTM Standards:2E 399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Meta
6、llic MaterialsE 561 Test Method for K-R Curve DeterminationE 1823 Terminology Relating to Fatigue and Fracture Test-ing2.2 ANSI Standard:ANSI H35.1 Alloy and Temper Designations for Alumi-num32.3 ISO Standard:ISO 12737 Metallic MaterialsDetermination of PlaneStrain Fracture Toughness43. Terminology3
7、.1 DefinitionsTerminology in Test Method E 399 andTerminology E 1823 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
8、 outlined in thisguide. The corrected result is an estimation of the Kqor KIcthatwould have been obtained in a residual stress free specimen.The corrected result may be obtained from a test record whichyielded either an invalid Kqor valid KIc, but for which there isevidence that significant residual
9、 stress is present in the testcoupon.3.2.2 invalid plane-strain fracture toughness a test result,designated Kq, that does not meet one or more validityrequirements in Test Method E 399 or ISO 12737 and may ormay not be significantly influenced by residual stress.3.2.3 valid plane-strain fracture tou
10、ghness a test result,designated KIc, meeting the validity requirements in TestMethod E 399 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 E 399 orISO 12737, characterizes a materials resistance t
11、o 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 the pl
12、ane strain state.4.1.1 Thermal quenching processes used with precipitationhardened aluminum alloy products can introduce significantresidual stresses in the product. Mechanical stress relief pro-cedures (stretching, compression) are commonly used to re-lieve these residual stresses in products with
13、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 extrusions, castings), completemechanical stress relief is not always possible. In
14、 otherinstances residual stresses may be unintentionally introduced1This 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 edition approved Sept. 1, 2006. Published September 2006. Originallyappr
15、oved in 2000. Last previous edition approved in 2000 as B 909 00.2For 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 webs
16、ite.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 deVaremb, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.1Copyright ASTM
17、International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.into a product during fabrication operations such as straight-ening, forming, or welding operations.4.1.2 Specimens taken from such products that containresidual stress will likewise themselves contain
18、 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 is superimposed on the applied stressa
19、nd 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 distortionduring specimen machining when influe
20、ntial 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 specimen).The inflated values result fro
21、m 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 the measured value of KIchas been biased
22、 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 residualstress influenced values of fracture tough
23、ness to values thatapproximate a fracture toughness value representative of a testperformed without residual stress bias.5. Warning Signs5.1 There are a number of warning signs that test measure-ments are or might be biased by the presence of residual stress.If any one or more of the following condi
24、tions 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 treatable aluminumproduct that does not
25、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 AN-SI H35.1.5.1.2 Machining distortion during specimen preparation.Aneffective method to quantify distortion of a C(T) specimen is tomeasure
26、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) of 0.45, a difference in the heightmea
27、sured 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 straightness requirements in Test Metho
28、dE 399 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 specimenconfiguration (for example, from C(T) to three
29、point bend bar)or upon changing specimen plan size that cannot be explainedby other means. For example, if residual stress is biasingfracture toughness tests results, then increasing the specimenplan size typically results in increasing Kqvalues.NOTE 1Other factors, such as a steeply rising R-curve
30、(PracticeE 561) in high toughness alloy/products, may also be responsible for Kqvalues increasing with increasing specimen plan size.5.1.6 A nonlinear load-COD trace during the initial elasticportion of the test record. This result is indicative of theresidual stress clamping that is being overcome
31、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 eliminate the effects of residual stress o
32、n 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 tough-ness Kqor KIcthat would have been obt
33、ained 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 influences of both distortion-induced clamping
34、 (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 dimensions are largeenough to meet the Test Met
35、hod E 399 or ISO 12737 specimensize requirements for 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 redu
36、ce the residual stressbias. One such technique involves 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. More-over, the straighter crack
37、 fronts that result from precracking at5Bucci, 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.B 909 00 (2006)2higher R-ratio have been shown to reduce the er
38、ror in theensuing fracture toughness measurement by up to 75 %.NOTE 2Test Method E 399 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 willresult in Kq, which are invali
39、d in accordance with Test Method E 399.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 E 399 shouldbe a significantly better estimate of the plane-stra
40、in fracture toughness,KIc, than an invalid Kqobtained from a specimen precracked at a stressratio meeting Test Method E 399 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 of the severity of thebendin
41、g 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 the speci-men height change m
42、easurement described in Fig. 1 anddenoted as Dd. As shown in Fig. 2, the origin of the residualstress biased load-displacement test record is modified bydisplacing the origin by an amount equal to Dd and to the loadassociated with that displacement. The test is now analyzedusing this new origin and
43、modified load-displacement recordwith the standard methodology described in Test MethodE 399.NOTE 3Limited experimental evidence6,7indicates that under pre-cracking conditions resulting in excessive crack front curvature (that is,not meeting the crack front straightness requirements in Test MethodE
44、399), 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-cracked at higher stres
45、s 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 length is calculated
46、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 precracked at higher
47、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 p
48、roducedby 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-typical distributions where the residual stresses are in compres-sion at
49、the center and tension at the surface, this method may not beapplicable.NOTE 5A Kq(corrected) value derived from a valid KIcor an invalidKqthat is invalid only due to failure to meet the crack front straightnessrequirements and fatigue precracking stress ratio requirements of TestMethod E 399 or ISO 12737 is an estimate of the plane-strain fracturetoughness, KIc, that would have been obtained in a residual stress freespecimen (see also Note 2).AKq(corrected) value derived from a Kqvalue, which is invalid due to failure to meet other validity requirementssuch as requ
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