1、Designation: G168 17Standard Practice forMaking and Using Precracked Double Beam StressCorrosion Specimens1This standard is issued under the fixed designation G168; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last r
2、evision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This practice covers procedures for fabricating,preparing, and using precracked double beam stress corrosiontest specimens.
3、 This specimen configuration was formerlydesignated the double cantilever beam (DCB) specimen.Guidelines are given for methods of exposure and inspection.1.2 The precracked double beam specimen, as described inthis practice, is applicable for evaluation of a wide variety ofmetals exposed to corrosiv
4、e environments. It is particularlysuited to evaluation of products having a highly directionalgrain structure, such as rolled plate, forgings, and extrusions,when stressed in the short transverse direction.1.3 The precracked double beam specimen may be stressedin constant displacement by bolt or wed
5、ge loading or inconstant load by use of proof rings or dead weight loading. Theprecracked double beam specimen is amenable to exposure toaqueous or other liquid solutions by specimen immersion or byperiodic dropwise addition of solution to the crack tip, orexposure to the atmosphere.1.4 This practic
6、e is concerned only with precracked doublebeam specimen and not with the detailed environmentalaspects of stress corrosion testing, which are covered inPractices G35, G36, G37, G41, G44, and G50.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use.
7、 It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accor-dance with internationally recognized principles
8、on standard-ization established in the 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:2D1193 Specification for Reagent WaterE
9、8/E8M Test Methods for Tension Testing of Metallic Ma-terialsE399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE1823 Terminology Relating to Fatigue and Fracture TestingG15 Terminology Relating to Corrosion and Corrosion Test-ing (Withdrawn 2010)3G35 Practice
10、 for Determining the Susceptibility of StainlessSteels and Related Nickel-Chromium-Iron Alloys toStress-Corrosion Cracking in Polythionic AcidsG36 Practice for Evaluating Stress-Corrosion-Cracking Re-sistance of Metals and Alloys in a Boiling MagnesiumChloride SolutionG37 Practice for Use of Mattsso
11、ns Solution of pH 7.2 toEvaluate the Stress-Corrosion Cracking Susceptibility ofCopper-Zinc AlloysG41 Practice for Determining Cracking Susceptibility ofMetals Exposed Under Stress to a Hot Salt EnvironmentG44 Practice for Exposure of Metals andAlloys byAlternateImmersion in Neutral 3.5 % Sodium Chl
12、oride SolutionG49 Practice for Preparation and Use of Direct TensionStress-Corrosion Test SpecimensG50 Practice for Conducting Atmospheric Corrosion Testson Metals3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 stress corrosion cracking (SCC) threshold stressintensity, KIsccth
13、e stress intensity level below which stresscorrosion cracking does not occur for a specific combination ofmaterial and environment when plane strain conditions aresatisfied.1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcom
14、mittee G01.06 on Environmen-tally Assisted Cracking.Current edition approved Nov. 1, 2017. Published December 2017. Originallyapproved in 2000. Last previous edition approved in 2013 as G168 00 (2013).DOI: 10.1520/G0168-17.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orconta
15、ct ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.*A Summary of Changes section appears at the end of th
16、is standardCopyright 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 standardization established in the Decision on Principles for theDevelopme
17、nt of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.13.1.1.1 DiscussionTerms relative to this subject mattercan be found in Terminologies G15 and E1823.4. Summary of Practice4.1 This practice covers the preparat
18、ion and testing ofprecracked double beam specimens for investigating the resis-tance to SCC (see Terminology G15) of metallic materials invarious product forms. Precracking by fatigue loading and bymechanical overload are described. Procedures for stressingspecimens in constant displacement with loa
19、ding bolts aredescribed, and expressions are given for specimen stressintensity and crack mouth opening displacement. Guidance isgiven for methods of exposure and inspection of precrackeddouble beam specimens.5. Significance and Use5.1 Precracked specimens offer the opportunity to use theprinciples
20、of linear elastic fracture mechanics (1)4to evaluateresistance to stress corrosion cracking in the presence of apre-existing crack. This type of evaluation is not included inconventional bent beam, C-ring, U-bend, and tension speci-mens. The precracked double beam specimen is particularlyuseful for
21、evaluation of materials that display a strong depen-dence on grain orientation. Since the specimen dimension inthe direction of applied stress is small for the precrackeddouble beam specimen, it can be successfully used to evaluateshort transverse stress corrosion cracking of wrought products,such a
22、s rolled plate or extrusions. The research applicationsand analysis of precracked specimens in general, and theprecracked double beam specimen in particular, are discussedin Appendix X1.5.2 The precracked double beam specimen may be stressedin either constant displacement or constant load. Constantd
23、isplacement specimens stressed by loading bolts or wedgesare compact and self-contained. By comparison, constant loadspecimens stressed with springs (for example, proof rings,discussed in Test Method G49, 7.2.1.2) or by deadweightloading require additional fixtures that remain with the speci-men dur
24、ing exposure.5.3 The recommendations of this practice are based on theresults of interlaboratory programs to evaluate precrackedspecimen test procedures (2, 3) as well as considerableindustrial experience with the precracked double beam speci-men and other precracked specimen geometries (4-8).6. Int
25、erferences6.1 Interferences in Testing:6.1.1 The accumulation of solid corrosion products or oxidefilms on the faces of an advancing stress corrosion crack cangenerate wedge forces that add to the applied load, therebyincreasing the effective stress intensity at the crack tip (6-9).This self-loading
26、 condition caused by corrosion product wedg-ing can accelerate crack growth and can prevent crack arrestfrom being achieved. The effect of corrosion product wedgingon crack growth versus time curve is shown schematically inFig. 1 (9). When wedging forces occur, they can invalidatefurther results and
27、 the test should be ended.6.1.2 Crack-tip blunting or branching out, or both, of theplane of the precrack can invalidate the test. For valid tests, thecrack must remain within 610 of the centerline of thespecimen.6.1.3 Drying or contamination of the corrodent in the crackduring interim measurements
28、of the crack length may affect thecracking behavior during subsequent exposure.NOTE 1Do not allow corrodent in the crack to dry during periodicmeasurements to avoid repassivation at the crack tip and the resultingchange in corrosion conditions. Remove one specimen at a time fromcorrodent. For tests
29、conducted in deaerated test environments or inenvironments that contain readily oxidizable species or corrosionproducts, interim crack length examinations may produce changes in theconditions at the crack tip that can, in turn, affect cracking behavior duringthe subsequent exposure period.6.2 Interf
30、erences in Visual Crack Length Measurements:6.2.1 Corrosion products on the side surfaces of the speci-men can interfere with accurate crack length measurements.Corrosion products on these surfaces may be removed bycareful scrubbing with a nonmetallic abrasive pad. However,for interim measurements,
31、a minimum area of surface shouldbe cleaned to allow for visual crack length measurements ifreexposure is planned.6.2.2 Measurement on side grooved specimens may bedifficult if the advancing crack travels up the side of the groove.This is especially difficult with V-shaped grooves. Adjustmentof the d
32、irection and intensity of the lighting may highlight thelocation of the crack tip.6.2.3 Often the crack length measured at the specimensurface is less than in the interior, due to decreased stress4The boldface numbers in parentheses refer to the list of references at the end ofthis standard.NOTE 1Sc
33、hematic of the influence of corrosion product wedging onSCC growth versus time curves in a decreasing K (constant displacement)test. Solid lines: actually measured curve for case of corrosion productwedging that results in increase in crack growth with time; asterisksindicate temporary crack arrest.
34、 Dashed lines: true crack growth curveexcluding the effect of corrosion product wedging (9).FIG. 1 Effect of Corrosion Product Wedging on Growth CrackVersus Time CurveG168 172triaxiality at the specimen surface. Alternatively, some condi-tions produce an increase in crack length at the surface due t
35、oavailability of the corrodent. Ultrasonic methods can be used toobtain interim crack length measurements at the interior of thespecimen but not near the specimen surface.6.2.4 Transport of species in solution in the through-thickness direction can be important for precracked doublebeam specimens. T
36、his may affect measurement of crack lengthsince it can produce curvature of the crack front (that is,variation in crack length from the edge to the center of thespecimen).7. Specimen Size, Configuration, and Preparation7.1 Specimen Dimensions and Fabrication:7.1.1 Dimensions for the recommended spec
37、imen are givenin Figs. 2 and 3. As a general guideline, specimen dimensionsshould ensure that plane strain conditions are maintained at thecrack tip (1, 10). While there are no established criteria forensuring adequate constraint for a plane strain SCC test, someguidelines are given herein regarding
38、 specimen dimensions(see 7.1.3).7.1.2 Specimen machining shall be in accordance with thestandards outlined in Test Method E399. The principal consid-erations in machining are that the sides, top, and bottom of thespecimen should be parallel; the machined notch should becentered; and the bolt holes s
39、hould be aligned and centered. Atypical bolt loaded specimen is shown in Fig. 4.7.1.3 Recommendations for determining the minimumspecimen thickness, B, which will ensure that plane strainconditions are maintained at the tip of an SCC crack, arediscussed in Brown (1) and Dorward and Helfrich (8). Bas
40、edon a conservative estimate for plane strain conditions, theminimum specimen thickness shall be calculated as B 2.5(KIc/YS)2, where KIcis determined per Test Method E399 andYSis the 0.2 % offset yield strength in tension per TestMethod E8/E8M. For bolt loaded precracked double beamspecimens, the th
41、ickness, B, may also be influenced by the sizeof the loading bolts and the minimum thickness needed tosupport the bolt loading.7.1.4 The specimen half-height, H, may be reduced formaterial under 25 mm (1 in.) thick. The minimum H that canbe used is constrained by the onset of plastic deformation upo
42、nprecracking or stresses in the leg of the specimen since thisinfluences the calculation of K. Outer fiber stresses shall notexceed the yield strength of the test material during precrack-ing or stressing.NOTE 2The effect of notch geometry on specimen compliance andstress intensity solutions, noted
43、in 7.3.4.4, Note 4, 8.1.3, and Note 5,isNOTE 1All dimensions in mm (in.). Top and front views are shown for smooth specimen only; side view is shown for both smooth and side groovedconfiguration.NOTE 2For Chevron notch crack starter, cutter tip angle 90 max.NOTE 3Radius at notch bottom to be 0.25 mm
44、 (0.01 in.) or less.NOTE 4Crack starter to be perpendicular to specimen length and thickness to within 62.NOTE 5Initial COD () may be increased to 12.7 mm (0.5 in.) to accommodate COD gage.NOTE 6All surfaces 32 in. or better, tolerances not specified 60.127 (0.005).NOTE 7For V-shape side groove, con
45、tinue with Chevron cutter on surface to machine grooves. For U-shape side groove, machine groove with radiuscutting tool such as a ball end mill, size equal to notch height.NOTE 8Loading bolt holes shall be perpendicular to specimen center lines within 65.NOTE 9Center line of holes shall be parallel
46、 and perpendicular to specimen surfaces within 62.NOTE 10Center line of holes shall be coincident within 60.127 mm (0.005 in.).NOTE 11The crack length at the start of the exposure test (a0) is achieved by fatigue or mechanical precracking. Precracking length shall extend 2.5to 3.8 mm (0.10 to 0.15 i
47、n.) from the tip of the machine notch at the specimen surface, see 7.3.4.3.FIG. 2 Detailed Machine Drawing for Smooth Face and Side Grooved DCB SpecimenG168 173magnified as H is reduced.7.1.5 The overall length of the specimen, L, can be in-creased to allow for more crack growth. Specimens of SCCsus
48、ceptible material that are loaded in constant deflection tohigh starting stress intensities may require additional crackgrowth to achieve crack arrest as defined in 10.1.7.2 Specimen Configuration:7.2.1 The recommended specimen configuration includes asharp starter notch, which may be either a strai
49、ght through orchevron configuration. The chevron configuration is recom-mended for both the fatigue and the mechanical overloadprecracking operations (see Fig. 2).7.2.2 The use of side grooves is optional. They may behelpful if any difficulty is experienced in keeping the crack inthe center of the specimen. The side groove configuration maybe machined with the chevron V-shaped cutter or with aU-shaped radius cutting tool. The depth of each side grooveshould not exceed 5 % of B, such that the net thickness, Bn, willbe at least 90 % of B.7.
