1、Designation: G 168 00 (Reapproved 2006)Standard Practice forMaking and Using Precracked Double Beam StressCorrosion Specimens1This standard is issued under the fixed designation G 168; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision
2、, 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 practice covers procedures for fabricating, prepar-ing, and using precracked double beam stress co
3、rrosion testspecimens. This specimen configuration was formerly desig-nated the double cantilever beam (DCB) specimen. Guidelinesare 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 ofme
4、tals exposed to corrosive 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 dis
5、placement by bolt or wedge 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 atm
6、osphere.1.4 This practice 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,
7、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.2. Referenced Documents2.1 ASTM Standards:2D 1193 Specification for Reagent WaterE 399 Test Meth
8、od for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE 1823 Terminology Relating to Fatigue and Fracture Test-ingG15 Terminology Relating to Corrosion and CorrosionTestingG35 Practice for Determining the Susceptibility of Stain-less Steels and Related Nickel-Chromium-Iron All
9、oys toStress-Corrosion Cracking in Polythionic AcidsG36 Practice for Evaluating Stress-Corrosion-CrackingResistance of Metals and Alloys in a Boiling MagnesiumChloride SolutionG37 Practice for Use of Mattssons Solution of pH toEvaluate the Stress-Corrosion Cracking Susceptibility ofCopper-Zinc Alloy
10、sG41 Practice for Determining Cracking Susceptibility ofMetals Exposed Under Stress to a Hot Salt EnvironmentG44 Practice for Exposure of Metals and Alloys by Alter-nate Immersion in Neutral 3.5 % Sodium Chloride Solu-tionG49 Practice for Preparation and Use of Direct TensionStress-Corrosion Test Sp
11、ecimensG50 Practice for Conducting Atmospheric Corrosion Testson Metals3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 stress corrosion cracking (SCC) threshold stress in-tensity, KIsccthe stress intensity level below which stresscorrosion cracking does not occur for a specifi
12、c combination ofmaterial and environment when plane strain conditions aresatisfied.3.1.1.1 DiscussionTerms relative to this subject mattercan be found in Terminologies G15and E 1823.4. Summary of Practice4.1 This practice covers the preparation and testing ofprecracked double beam specimens for inve
13、stigating 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 loading bolts are1This guide is under the jurisdiction of ASTM
14、 Committee G01 on Corrosion ofMetals and is the direct responsibility of Subcommittee G01.06 on EnvironmentallyAssisted Cracking.Current edition approved Nov. 1, 2006. Published January 2007. Originallyapproved in 2000. Last previous edition approved in 2000 as G 168 00.2For referenced ASTM standard
15、s, 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, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 1
16、9428-2959, United States.described, 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 the
17、principles of linear elastic fracture mechanics (1)3to 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 particularly
18、useful for 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 prod
19、ucts,such as 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
20、. Constantdisplacement 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 sp
21、eci-men during 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 considerable indus-trial experience with the precracked double beam specimen andother precracked specimen geometries (4
22、-8).6. Interferences6.1 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 se
23、lf-loading 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 r
24、esults and 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 mea
25、surements 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.
26、For tests conducted in deaerated test environments or inenvironments that contain readily oxidizable species or corrosion prod-ucts, interim crack length examinations may produce changes in theconditions at the crack tip that can, in turn, affect cracking behavior duringthe subsequent exposure perio
27、d.6.2 Interferences 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 me
28、asurements, 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. Adjust
29、mentof the direction 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 stresstriaxiality at the specimen surface. Alternatively, some condi-tions produce an increase in
30、crack length at the surface due toavailability 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 p
31、recracked doublebeam specimens. This may affect measurement of crack length3The boldface numbers in parentheses refer to the list of references at the end ofthis standard.NOTE 1Schematic of the influence of corrosion product wedging onSCC growth versus time curves in a decreasing K (constant displac
32、ement)test. Solid lines: actually measured curve for case of corrosion productwedging that results in increase in crack growth with time; asterisksindicate temporary crack arrest. Dashed lines: true crack growth curveexcluding the effect of corrosion product wedging (9).FIG. 1 Effect of Corrosion Pr
33、oduct Wedging on Growth CrackVersus Time CurveG 168 00 (2006)2since 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
34、the recommended specimen 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 giv
35、en herein regarding specimen dimensions(see 7.1.3).7.1.2 Specimen machining shall be in accordance with thestandards outlined in Test Method E 399. The principal con-siderations in machining are that the sides, top, and bottom ofthe specimen should be parallel; the machined notch should becentered;
36、and the bolt holes should 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 a
37、nd Helfrich (8).Aconservative estimate for the specimen thickness shall be madeby adopting the thickness criteria for plane strain fracturetoughness testing, as described in Test Method E 399. For boltloaded precracked double beam specimens, the thickness, B,may also be influenced by the size of the
38、 loading bolts and theminimum thickness needed to support 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 uponprecracking or stresses in the leg of the specimen
39、since thisNOTE 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 (0.01 in.) or less.NOTE 4Crac
40、k starter to be perpendicular to specimen length and thickness to within 62.NOTE 5Initial COD (D) may be increased to 12.7 mm (0.5 in.) to accomodate COD gage.NOTE 6All surfaces 32 in. or better, tolerances not specified 60.127 (0.005).NOTE 7Continue with Chevron cutter on surface to machine grooves
41、.NOTE 8Loading bolt holes shall be perpendicular to specimen center lines within 65.NOTE 9Center line of holes shall be parallel and perpendicular to specimen surfaces within 62.NOTE 10Center line of holes shall be coincident within 60.127 mm (0.005 in.).FIG. 2 Detailed Machine Drawing for Smooth an
42、d Face Grooved DCB SpecimenG 168 00 (2006)3influences 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 in 7.3.4.4, Note 4,
43、 8.1.3, and Note 5,ismagnified 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 SCCsusceptible material that are loaded in constant deflection tohigh starting stress intensities may require additional crackgrowth to achieve c
44、rack 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 straight through orchevron configuration. The chevron configuration is recom-mended for both the fatigue and the mechanical overloadprecracking
45、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 cutter. The depth of each side gro
46、ove should notexceed 5 % of B, such that the net thickness, Bn, will be at least90 % of B.7.2.3 Specimens machined from rectangular product canhave six possible orientations (see Test Method E 399) relativeto the direction of loading and the direction of crack propaga-tion, namely, S-L, S-T, T-L, T-
47、S, L-T, and L-S. In wroughtproducts, the S-L orientation is usually the most critical and isthe most frequently used to avoid crack branching7.2.4 More detailed discussions of the factors described inthis section are given in Brown (1), Sprowls et al (6), andSprowls (9).7.3 Specimen Preparation:7.3.
48、1 Specimen surfaces along the path of expected crackpropagation may be polished to assist in crack measurement.7.3.2 Specimens shall be cleaned and degreased prior toprecracking and testing. Successive ultrasonic cleaning inacetone and methyl alcohol is suggested. Specimens shall notbe recleaned aft
49、er precracking to prevent contamination of thecrack with cleaning or degreasing chemicals. If cleaning of theside surfaces of the specimen following precracking is neces-sary, then this should be performed by lightly wiping thesesurfaces and not by immersion of the specimen into thecleaning or degreasing media.NOTE 1All dimensions in mm (in). Tolerances not specified 60.127(60.005).NOTE 2Suggested material: Strong enough not to fail in tensionduring loading or mechanical precracking.NOTE 3Bolt head design optional. Commercial stainless steel sockethead cap
