1、Designation: G168 00 (Reapproved 2013)Standard 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,
2、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 practice covers procedures for fabricating,preparing, and using precracked double beam stress corrosi
3、ontest specimens. 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 ex
4、posed 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 displaceme
5、nt 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 atmosphere
6、.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, associa
7、ted 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:2D1193 Specification for Reagent WaterE399 Test Method for Li
8、near-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Terminology Relating to Fatigue and Fracture TestingG15 Terminology Relating to Corrosion and Corrosion Test-ing (Withdrawn 2010)3G35 Practice for Determining the Susceptibility of StainlessSteels and Related Nickel-Chromium-I
9、ron 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 Mattssons Solution of pH 7.2 toEvaluate the Stress-Corrosion Cracking Susceptibility ofCopp
10、er-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 Chloride SolutionG49 Practice for Preparation and Use of Direct TensionStress-Corrosion
11、 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, KIsccthe stress intensity level below which stresscorrosion cracking does not occur for a s
12、pecific combination ofmaterial and environment when plane strain conditions aresatisfied.3.1.1.1 DiscussionTerms relative to this subject mattercan be found in Terminologies G15 and E1823.4. Summary of Practice4.1 This practice covers the preparation and testing ofprecracked double beam specimens fo
13、r 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 stressing1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the
14、direct responsibility of Subcommittee G01.06 on Environmen-tally Assisted Cracking.Current edition approved May 1, 2013. Published July 2013. Originally approvedin 2000. Last previous edition approved in 2006 as G168 00 (2006). DOI:10.1520/G0168-00R13.2For referenced ASTM standards, visit the ASTM w
15、ebsite, 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.3The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM Internatio
16、nal, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1specimens in constant displacement with loading bolts aredescribed, and expressions are given for specimen stressintensity and crack mouth opening displacement. Guidance isgiven for methods of exposure and inspe
17、ction of precrackeddouble beam specimens.5. Significance and Use5.1 Precracked specimens offer the opportunity to use theprinciples 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 inclu
18、ded inconventional bent beam, C-ring, U-bend, and tension speci-mens. The precracked double beam specimen is particularlyuseful 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 precrack
19、eddouble beam specimen, it can be successfully used to evaluateshort transverse stress corrosion cracking of wrought products,such as rolled plate or extrusions. The research applicationsand analysis of precracked specimens in general, and theprecracked double beam specimen in particular, are discus
20、sedin Appendix X1.5.2 The precracked double beam specimen may be stressedin either constant displacement or constant load. Constantdisplacement specimens stressed by loading bolts or wedgesare compact and self-contained. By comparison, constant loadspecimens stressed with springs (for example, proof
21、 rings,discussed in Test Method G49, 7.2.1.2) or by deadweightloading require additional fixtures that remain with the speci-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 a
22、s considerable indus-trial experience with the precracked double beam specimen andother precracked specimen geometries (4-8).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 w
23、edge forces that add to the applied load, therebyincreasing the effective stress intensity at the crack tip (6-9).This self-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 cr
24、ack growth versus time curve is shown schematically inFig. 1 (9). When wedging forces occur, they can invalidatefurther results 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
25、within 610 of the centerline of thespecimen.6.1.3 Drying or contamination of the corrodent in the crackduring interim measurements 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 repa
26、ssivation at the crack tip and the resultingchange in corrosion conditions. Remove one specimen at a time fromcorrodent. For tests conducted in deaerated test environments or inenvironments that contain readily oxidizable species or corrosionproducts, interim crack length examinations may produce ch
27、anges in theconditions at the crack tip that can, in turn, affect cracking behavior duringthe subsequent exposure period.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.Corrosio
28、n products on these surfaces may be removed bycareful scrubbing with a nonmetallic abrasive pad. However,for interim measurements, 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 bediffic
29、ult if the advancing crack travels up the side of the groove.This is especially difficult with V-shaped grooves. Adjustmentof 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 interi
30、or, due to decreased stresstriaxiality at the specimen surface. Alternatively, some condi-tions produce an increase in 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 nea
31、r the specimen surface.6.2.4 Transport of species in solution in the through-thickness direction can be important for precracked doublebeam specimens. This 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
32、center of thespecimen).4The 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 displacement)test. Solid lines: actually measured curve for
33、 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 Product Wedging on Growth CrackVersus Time CurveG168 0
34、0 (2013)27. Specimen Size, Configuration, and Preparation7.1 Specimen Dimensions and Fabrication:7.1.1 Dimensions for 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
35、 there are no established criteria forensuring adequate constraint for a plane strain SCC test, someguidelines are given herein regarding 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 ma
36、chining are that the sides, top, and bottom of thespecimen should be parallel; the machined notch should becentered; 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
37、ensure that plane strainconditions are maintained at the tip of an SCC crack, arediscussed in Brown (1) and Dorward and Helfrich (8).Aconservative estimate for the specimen thickness shall be madeby adopting the thickness criteria for plane strain fracturetoughness testing, as described in Test Meth
38、od E399. For boltloaded precracked double beam specimens, the thickness, B,may also be influenced by the size of the 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 c
39、anbe used is constrained by the onset of plastic deformation uponprecracking 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 geome
40、try on specimen compliance andstress intensity solutions, noted 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, cu
41、tter tip angle 90 max.NOTE 3Radius at notch bottom to be 0.25 mm (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 accomodate COD gage.NOTE 6All surfaces 32 in. or better, tolerances
42、not specified 60.127 (0.005).NOTE 7Continue with Chevron cutter on surface to machine grooves.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 s
43、hall be coincident within 60.127 mm (0.005 in.).FIG. 2 Detailed Machine Drawing for Smooth and Face Grooved DCB SpecimenG168 00 (2013)3magnified 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 ar
44、e 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 straight through orchevron con
45、figuration. 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
46、configuration maybe machined with the chevron V-shaped cutter or with aU-shaped cutter. The depth of each side groove 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 Me
47、thod E399) relativeto the direction of loading and the direction of crackpropagation, namely, S-L, S-T, T-L, T-S, L-T, and L-S. Inwrought products, the S-L orientation is usually the mostcritical and is the most frequently used to avoid crack branch-ing7.2.4 More detailed discussions of the factors
48、described inthis section are given in Brown (1), Sprowls et al (6), andSprowls (9).7.3 Specimen Preparation:7.3.1 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 testin
49、g. Successive ultrasonic cleaning inacetone and methyl alcohol is suggested. Specimens shall notbe recleaned after precracking to prevent contamination of thecrack with cleaning or degreasing chemicals. If cleaning of theside surfaces of the specimen following precracking isnecessary, then this should be performed by lightly wipingthese surfaces and not by immersion of the specimen into thecleaning or degreasing media.NOTE 3Only chemicals appropriate for the metal or alloy of interestshall be used. All chemicals shall be of reagent grade purity.7.3.3 Specim
