1、Designation: G168 00 (Reapproved 2013)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 rev
2、ision, 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. Scope Scope*1.1 This practice covers procedures for fabricating, preparing, and using precracked double beam
3、 stress corrosion testspecimens. This specimen configuration was formerly designated the double cantilever beam (DCB) specimen. Guidelines aregiven for methods of exposure and inspection.1.2 The precracked double beam specimen, as described in this practice, is applicable for evaluation of a wide va
4、riety of metalsexposed to corrosive environments. It is particularly suited to evaluation of products having a highly directional grain structure,such as rolled plate, forgings, and extrusions, when stressed in the short transverse direction.1.3 The precracked double beam specimen may be stressed in
5、 constant displacement by bolt or wedge loading or in constantload by use of proof rings or dead weight loading. The precracked double beam specimen is amenable to exposure to aqueous orother liquid solutions by specimen immersion or by periodic dropwise addition of solution to the crack tip, or exp
6、osure to theatmosphere.1.4 This practice is concerned only with precracked double beam specimen and not with the detailed environmental aspects ofstress corrosion testing, which are covered in Practices G35, G36, G37, G41, G44, and G50.1.5 This standard does not purport to address all of the safety
7、concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulatory limitations prior to use.1.6 This international standard was developed in acc
8、ordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM
9、Standards:2D1193 Specification for Reagent WaterE8/E8M Test Methods for Tension Testing of Metallic MaterialsE399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIc of Metallic MaterialsE1823 Terminology Relating to Fatigue and Fracture TestingG15 Terminology Relating to Corrosion an
10、d Corrosion Testing (Withdrawn 2010)3G35 Practice for Determining the Susceptibility of Stainless Steels and Related Nickel-Chromium-Iron Alloys to Stress-Corrosion Cracking in Polythionic AcidsG36 Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesi
11、um ChlorideSolutionG37 Practice for Use of Mattssons Solution of pH 7.2 to Evaluate the Stress-Corrosion Cracking Susceptibility of Copper-ZincAlloysG41 Practice for Determining Cracking Susceptibility of Metals Exposed Under Stress to a Hot Salt EnvironmentG44 Practice for Exposure of Metals and Al
12、loys by Alternate Immersion in Neutral 3.5 % Sodium Chloride Solution1 This practice is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.06 on EnvironmentallyAssisted Cracking.Current edition approved May 1, 2013Nov. 1, 2017. Pu
13、blished July 2013December 2017. Originally approved in 2000. Last previous edition approved in 20062013 asG168 00 (2006).(2013). DOI: 10.1520/G0168-00R13.10.1520/G0168-17.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annu
14、al Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 The last approved version of this historical standard is referenced on www.astm.org.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an in
15、dication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be
16、 considered the official document.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1G49 Practice for Preparation and Use of Direct Tension Stress-Corrosion Test SpecimensG
17、50 Practice for Conducting Atmospheric Corrosion Tests on Metals3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 stress corrosion cracking (SCC) threshold stress intensity, KIsccthe stress intensity level below which stress corrosioncracking does not occur for a specific combin
18、ation of material and environment when plane strain conditions are satisfied.3.1.1.1 DiscussionTerms relative to this subject matter can be found in Terminologies G15 and E1823.4. Summary of Practice4.1 This practice covers the preparation and testing of precracked double beam specimens for investig
19、ating the resistance toSCC (see Terminology G15) of metallic materials in various product forms. Precracking by fatigue loading and by mechanicaloverload are described. Procedures for stressing specimens in constant displacement with loading bolts are described, andexpressions are given for specimen
20、 stress intensity and crack mouth opening displacement. Guidance is given for methods ofexposure and inspection of precracked double beam specimens.5. Significance and Use5.1 Precracked specimens offer the opportunity to use the principles of linear elastic fracture mechanics (1)4 to evaluateresista
21、nce to stress corrosion cracking in the presence of a pre-existing crack. This type of evaluation is not included inconventional bent beam, C-ring, U-bend, and tension specimens. The precracked double beam specimen is particularly useful forevaluation of materials that display a strong dependence on
22、 grain orientation. Since the specimen dimension in the direction ofapplied stress is small for the precracked double beam specimen, it can be successfully used to evaluate short transverse stresscorrosion cracking of wrought products, such as rolled plate or extrusions. The research applications an
23、d analysis of precrackedspecimens in general, and the precracked double beam specimen in particular, are discussed in Appendix X1.5.2 The precracked double beam specimen may be stressed in either constant displacement or constant load. Constantdisplacement specimens stressed by loading bolts or wedg
24、es are compact and self-contained. By comparison, constant loadspecimens stressed with springs (for example, proof rings, discussed inTest Method G49, 7.2.1.2) or by deadweight loading requireadditional fixtures that remain with the specimen during exposure.5.3 The recommendations of this practice a
25、re based on the results of interlaboratory programs to evaluate precracked specimentest procedures (2, 3) as well as considerable industrial experience with the precracked double beam specimen and other precrackedspecimen geometries (4-8).6. Interferences6.1 Interferences in Testing:6.1.1 The accumu
26、lation of solid corrosion products or oxide films on the faces of an advancing stress corrosion crack cangenerate wedge forces that add to the applied load, thereby increasing the effective stress intensity at the crack tip (6-9). Thisself-loading condition caused by corrosion product wedging can ac
27、celerate crack growth and can prevent crack arrest from beingachieved. The effect of corrosion product wedging on crack growth versus time curve is shown schematically in Fig. 1 (9). Whenwedging forces occur, they can invalidate further results and the test should be ended.6.1.2 Crack-tip blunting o
28、r branching out, or both, of the plane of the precrack can invalidate the test. For valid tests, the crackmust remain within 610 of the centerline of the specimen.6.1.3 Drying or contamination of the corrodent in the crack during interim measurements of the crack length may affect thecracking behavi
29、or during subsequent exposure.NOTE 1Do not allow corrodent in the crack to dry during periodic measurements to avoid repassivation at the crack tip and the resulting change incorrosion conditions. Remove one specimen at a time from corrodent. For tests conducted in deaerated test environments or in
30、environments that containreadily oxidizable species or corrosion products, interim crack length examinations may produce changes in the conditions at the crack tip that can, inturn, affect cracking behavior during the subsequent exposure period.6.2 Interferences in Visual Crack Length Measurements:6
31、.2.1 Corrosion products on the side surfaces of the specimen can interfere with accurate crack length measurements. Corrosionproducts on these surfaces may be removed by careful scrubbing with a nonmetallic abrasive pad. However, for interimmeasurements, a minimum area of surface should be cleaned t
32、o allow for visual crack length measurements if reexposure isplanned.4 The boldface numbers in parentheses refer to the list of references at the end of this standard.G168 1726.2.2 Measurement on side grooved specimens may be difficult if the advancing crack travels up the side of the groove. Thisis
33、 especially difficult with V-shaped grooves. Adjustment of the direction and intensity of the lighting may highlight the locationof the crack tip.6.2.3 Often the crack length measured at the specimen surface is less than in the interior, due to decreased stress triaxiality atthe specimen surface. Al
34、ternatively, some conditions produce an increase in crack length at the surface due to availability of thecorrodent. Ultrasonic methods can be used to obtain interim crack length measurements at the interior of the specimen but not nearthe specimen surface.6.2.4 Transport of species in solution in t
35、he through-thickness direction can be important for precracked double beamspecimens. This may affect measurement of crack length since it can produce curvature of the crack front (that is, variation in cracklength from the edge to the center of the specimen).7. Specimen Size, Configuration, and Prep
36、aration7.1 Specimen Dimensions and Fabrication:7.1.1 Dimensions for the recommended specimen are given in Figs. 2 and 3. As a general guideline, specimen dimensionsshould ensure that plane strain conditions are maintained at the crack tip (1, 10). While there are no established criteria for ensuring
37、adequate constraint for a plane strain SCC test, some guidelines are given herein regarding specimen dimensions (see 7.1.3).7.1.2 Specimen machining shall be in accordance with the standards outlined in Test Method E399. The principal considerationsin machining are that the sides, top, and bottom of
38、 the specimen should be parallel; the machined notch should be centered; andthe bolt holes should be aligned and centered. A typical bolt loaded specimen is shown in Fig. 4.7.1.3 Recommendations for determining the minimum specimen thickness, B, which will ensure that plane strain conditions aremain
39、tained at the tip of an SCC crack, are discussed in Brown (1) and Dorward and Helfrich (8). A Based on a conservativeestimate for the plane strain conditions, the minimum specimen thickness shall be made by adopting the thickness criteriacalculatedas B 2.5 (KIc/YS)2 for plane, where KIc strain fract
40、ure toughness testing, as describedis determined per Test Method E399 inTestand YS Method is the 0.2 % offset yield strength in tension per Test Method E399E8/E8M. For bolt loaded precracked doublebeam specimens, the thickness, B, may also be influenced by the size of the loading bolts and the minim
41、um thickness needed tosupport the bolt loading.7.1.4 The specimen half-height, H, may be reduced for material under 25 mm (1 in.) thick. The minimum H that can be usedis constrained by the onset of plastic deformation upon precracking or stresses in the leg of the specimen since this influences thec
42、alculation of K. Outer fiber stresses shall not exceed the yield strength of the test material during precracking or stressing.NOTE 2The effect of notch geometry on specimen compliance and stress intensity solutions, noted in 7.3.4.4, Note 4, 8.1.3, and Note 5, is magnifiedas H is reduced.7.1.5 The
43、overall length of the specimen, L, can be increased to allow for more crack growth. Specimens of SCC susceptiblematerial that are loaded in constant deflection to high starting stress intensities may require additional crack growth to achievecrack arrest as defined in 10.1.7.2 Specimen Configuration
44、:NOTE 1Schematic of the influence of corrosion product wedging on SCC growth versus time curves in a decreasing K (constant displacement) test.Solid lines: actually measured curve for case of corrosion product wedging that results in increase in crack growth with time; asterisks indicate temporarycr
45、ack arrest. Dashed lines: true crack growth curve excluding the effect of corrosion product wedging (9).FIG. 1 Effect of Corrosion Product Wedging on Growth Crack Versus Time CurveG168 1737.2.1 The recommended specimen configuration includes a sharp starter notch, which may be either a straight thro
46、ugh orchevron configuration. The chevron configuration is recommended for both the fatigue and the mechanical overload precrackingoperations (see Fig. 2).7.2.2 The use of side grooves is optional. They may be helpful if any difficulty is experienced in keeping the crack in the centerof the specimen.
47、 The side groove configuration may be machined with the chevron V-shaped cutter or with a U-shaped cutter.radius cutting tool. The depth of each side groove should not exceed 5 % of B, such that the net thickness, Bn, will be at least 90 %of B.7.2.3 Specimens machined from rectangular product can ha
48、ve six possible orientations (see Test Method E399) relative to thedirection of loading and the direction of crack propagation, namely, S-L, S-T, T-L, T-S, L-T, and L-S. In wrought products, the S-Lorientation is usually the most critical and is the most frequently used to avoid crack branching7.2.4
49、 More detailed discussions of the factors described in this section are given in Brown (1), Sprowls et al (6), and Sprowls(9).7.3 Specimen Preparation:7.3.1 Specimen surfaces along the path of expected crack propagation may be polished to assist in crack measurement.7.3.2 Specimens shall be cleaned and degreased prior to precracking and testing. Successive ultrasonic cleaning in acetone andmethyl alcohol is suggested. Specimens shall not be recleaned after precracking to prevent contamination of the crack withcleaning or degreasing chemicals. If cleaning of the side s
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