1、Designation: G30 97 (Reapproved 2015)Standard Practice forMaking and Using U-Bend Stress-Corrosion TestSpecimens1This standard is issued under the fixed designation G30; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of l
2、ast revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers procedures for making and usingU-bend specimens for the evaluation of stress-corrosion crack-ing in me
3、tals. The U-bend specimen is generally a rectangularstrip which is bent 180 around a predetermined radius andmaintained in this constant strain condition during the stress-corrosion test. Bends slightly less than or greater than 180 aresometimes used. Typical U-bend configurations showing sev-eral d
4、ifferent methods of maintaining the applied stress areshown in Fig. 1.1.2 U-bend specimens usually contain both elastic andplastic strain. In some cases (for example, very thin sheet orsmall diameter wire) it is possible to form a U-bend andproduce only elastic strain. However, bent-beam (Practice G
5、39or direct tension (Practice G49) specimens are normally usedto study stress-corrosion cracking of strip or sheet under elasticstrain only.1.3 This practice is concerned only with the test specimenand not the environmental aspects of stress-corrosion testingwhich are discussed elsewhere (1)2and in
6、Practices G35, G36,G37, G41, G44, G103 and Test Method G123.1.4 The values stated in SI units are to be regarded asstandard. The inch-pound units in parentheses are provided forinformation.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is
7、 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:3E3 Guide for Preparation of Metallographic SpecimensG1 Practice for Preparing, Clea
8、ning, and Evaluating Corro-sion Test SpecimensG15 Terminology Relating to Corrosion and Corrosion Test-ing (Withdrawn 2010)4G35 Practice for Determining the Susceptibility of StainlessSteels and Related Nickel-Chromium-Iron Alloys toStress-Corrosion Cracking in Polythionic AcidsG36 Practice for Eval
9、uating 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 ofCopper-Zinc AlloysG39 Practice for Preparation and Use of Bent-Beam Stress-Corrosion
10、 Test SpecimensG41 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-Corrosio
11、n Test SpecimensG103 Practice for Evaluating Stress-Corrosion Cracking Re-sistance of Low Copper 7XXX Series Al-Zn-Mg-CuAlloys in Boiling 6 % Sodium Chloride SolutionG123 Test Method for Evaluating Stress-Corrosion Crackingof Stainless Alloys with Different Nickel Content inBoiling Acidified Sodium
12、Chloride Solution1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-tally Assisted Cracking.Current edition approved Nov. 1, 2015. Published November 2015. Originallyapproved in 1972. Last previou
13、s edition approved in 2009 as G3097(2009). DOI:10.1520/G0030-97R15.2The boldface numbers in parentheses refer to a list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book o
14、f ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.
15、 Terminology3.1 For definitions of corrosion-related terms used in thispractice see Terminology G15.4. Summary of Practice4.1 This practice involves the stressing of a specimen bentto a U shape. The applied strain is estimated from the bendconditions. The stressed specimens are then exposed to the t
16、estenvironment and the time required for cracks to develop isdetermined. This cracking time is used as an estimate of thestress corrosion resistance of the material in the test environ-ment.5. Significance and Use5.1 The U-bend specimen may be used for any metal alloysufficiently ductile to be forme
17、d into the U-shape withoutmechanically cracking.The specimen is most easily made fromstrip or sheet but can be machined from plate, bar, castings, orweldments; wire specimens may be used also.5.2 Since the U-bend usually contains large amounts ofelastic and plastic strain, it provides one of the mos
18、t severetests available for smooth (as opposed to notched or pre-cracked) stress-corrosion test specimens. The stress conditionsare not usually known and a wide range of stresses exist in asingle stressed specimen. The specimen is therefore unsuitablefor studying the effects of different applied str
19、esses on stress-corrosion cracking or for studying variables which have only aminor effect on cracking. The advantage of the U-bendspecimen is that it is simple and economical to make and use.It is most useful for detecting large differences between thestress-corrosion cracking resistance of (a) dif
20、ferent metals inthe same environment, (b) one metal in different metallurgicalconditions in the same environment, or (c) one metal in severalenvironments.6. Hazards6.1 U-bends made from high strength material may besusceptible to high rates of crack propagation and a specimencontaining more than one
21、 crack may splinter into two or morepieces. Due to the highly stressed condition in a U-bendspecimen, these pieces may leave the specimen at high velocityand can be dangerous.7. Sampling7.1 Specimens shall be taken from a location in the bulksample so that they are representative of the material to
22、betested; however, the bulk sampling of mill products is outsidethe scope of this standard.7.2 In performing tests to simulate a service condition it isessential that the thickness of the test specimen, its orientationwith respect to the direction of metal working and the surfacefinish, etc., be rel
23、evant to the anticipated application.8. Test Specimen8.1 Specimen OrientationWhen specimens are cut fromsheet or plate and in some cases strip or bar, it is possible to cutthem transverse or longitudinal to the direction of rolling. Inmany cases the stress-corrosion cracking resistance in theseFIG.
24、1 Typical Stressed U-bendsG30 97 (2015)2two directions is quite different so it is important to define theorientation of the test specimen.8.2 Specimen DimensionsFig. 2 shows a typical testspecimen and lists, by way of example, several dimensioncombinations that have been used successfully to test a
25、 widerange of materials. Other dimensional characteristics may beused as necessary. For example, some special types of U-bendconfiguration have been used for simulating exposure condi-tions encountered in high temperature water environmentsrelative to the nuclear power industry. These include double
26、U-bend (2) and split tube U-bend (or reverse U-bend) (3)specimens.8.2.1 Whether or not the specimen contains holes is depen-dent upon the method of maintaining the applied stress (seeFig. 1).8.2.2 The length (L) and width (W) of the specimen aredetermined by the amount and form of the material avail
27、able,the stressing method used, and the size of the test environmentcontainer.8.2.3 The thickness (T) is usually dependent upon the formof the material, its strength and ductility, and the meansavailable to perform the bending. For example, it is difficult tomanually form U-bends of thickness greate
28、r than approxi-mately 3 mm (0.125 in.) if the yield strength exceeds about1400 MPa (200 ksi).8.2.4 For comparison purposes, it is desirable to keep thespecimen dimensions, especially the ratio of thickness to bendradius, constant. This produces approximately the same maxi-mum strain in the materials
29、 being compared (see 9.3).However, it does not necessarily provide tests of equal severityif the mechanical properties of the materials being comparedare widely different.8.2.5 When wire is to be evaluated, the specimen is simplya wire of a length suitable for the restraining jig. It may bedesirable
30、 to loop the wire rather than use just a simple U-shape(4).8.3 Surface Finish:8.3.1 Any necessary heat treatment should be performedbefore the final surface preparation.8.3.2 Surface preparation is generally a mechanical processbut in some cases it may be more convenient and acceptable tochemically
31、finish (see 8.3.4).8.3.3 Grinding or machining should be done in stages sothat the final cut leaves the surface with a finish of 0.76 m(30 in.) or better. Care must be taken to avoid excessiveheating during preparation because this may induce undesir-able residual stresses and in some cases cause me
32、tallurgical orchemical changes, or both, at the surface. The edges of thespecimen should receive the same finish as the faces.8.3.4 When the final surface preparation involves chemicaldissolution, care must be taken to ensure that the solution useddoes not induce hydrogen embrittlement, selectively
33、attackconstituents in the metal, or leave undesirable residues on thesurface.8.3.5 It may be desirable to test a surface (for example, coldrolled or cold rolled, annealed, and pickled) without surfacemetal removal. In such cases the edges of the specimen shouldbe milled. Sheared edges should be avoi
34、ded in all cases.Examples of Typical Dimensions (SI Units)Example L, mm M, mm W, mm T, mm D, mm X, mm Y, mm R, mm ,rada 80 50 20 2.5 10 32 14 5 1.57b 100 90 9 3.0 7 25 38 16 1.57c 120 90 20 1.5 8 35 35 16 1.57d 130 100 15 3.0 6 45 32 13 1.57e 150 140 15 0.8 3 61 20 9 1.57f 310 250 25 13.0 13 105 90
35、32 1.57g 510 460 25 6.5 13 136 165 76 1.57h 102 83 19 3.2 9.6 40 16 4.8 1.57FIG. 2 Typical U-Bend Specimen Dimensions (Examples only, not for specification)G30 97 (2015)38.3.6 The final stage of surface preparation is degreasing.Depending upon the method of stressing, this may be donebefore or after
36、 stressing.8.4 Identification of the specimen is best achieved bystamping or scribing near one of the ends of the test specimen,well away from the area to be stressed. Alternatively, nonme-tallic tags may be attached to the bolt or fixture used tomaintain the specimen in a stressed condition during
37、the test.9. Stress Considerations9.1 The stress of principal interest in the U-bend specimenis circumferential. It is nonuniform because (a) there is a stressgradient through the thickness varying from a maximumtension on the outer surface to a maximum compression on theinner surface, (b) the stress
38、 varies from zero at the ends of thespecimen to a maximum at the center of the bend, and (c) thestress may vary across the width of the bend. The stressdistribution has been studied (5).9.2 When a U-bend specimen is stressed, the material in theouter fibers of the bend is strained into the plastic p
39、ortion of thetrue stress-true strain curve; for example, into Section AB inFig. 3(a). Fig. 3(be) show several stress-strain relationshipsthat can exist in the outer fibers of the U-bend test specimen;the actual relationship obtained will depend upon the methodof stressing (see Section 10). For the c
40、onditions shown in Fig.3(d), a quantitative measure of the maximum test stress can bemade (6).9.3 The total strain () on the outside of the bend can beclosely approximated to the equation: 5 T/2R when T,Rwhere:T = specimen thickness, andR = radius of bend curvature.10. Stressing the Specimen10.1 Str
41、essing is usually achieved by either a one- or atwo-stage operation.10.2 Single-stage stressing is accomplished by bending thespecimen into shape and maintaining it in that shape withoutallowing relaxation of the tensile elastic strain. Typical stress-ing sequences are shown in Fig. 4. The method sh
42、own in Fig.4(a) may be performed in a tension testing machine and isoften the most suitable method for stressing U-bends that aredifficult to form manually due to large thickness or high-strength material or both. The techniques shown in Fig. 4(b andc) may be suitable for thin or low-strength materi
43、al, or both,but are generally inferior to the method shown in Fig. 4(a). Themethod shown in Fig. 4(b) results in a more complex strainsystem in the outer surface and may cause scratching. Thetechnique shown in Fig. 4(c) suffers from greater lack ofcontrol of the bend radius. The two types of stress
44、conditionsthat can be obtained by the single-stage stressing method aredefined by point X in Fig. 3(b and c). In the latter case, someFIG. 3 True Stress-True Strain Relationships for Stressed U-BendsG30 97 (2015)4elastic strain relaxation has occurred as a result of allowing theU-bend legs to spring
45、 back slightly at the end of the stressingsequence.10.3 Two-stage stressing involves first forming the approxi-mate U-shape, then allowing the elastic strain to relax com-pletely before the second stage of applying the test stress. Atypical sequence of operations is shown in Fig. 5. The type ofequip
46、ment shown in Fig. 4(a and b) can also be used topreform the U-shape. The test strain applied may be apercentage of the tensile elastic strain that occurred duringpreforming (Fig. 3(d) or may involve additional plastic strain(Fig. 3(e).10.4 The slope, MN, of the curve shown in Fig. 3(d) is steep(equ
47、al to Youngs modulus). Therefore, it is often difficult toreproducibly apply a constant percentage of the total elasticprestrain and there is a danger of leaving the specimen surfaceunder compressive stress. For this reason and also because itresults in a more severe test (that is, higher applied st
48、ress), it isrecommended that the stress conditions shown in Fig. 3(bore)be achieved. Hence, the final applied strain prior to testingconsists of plastic and elastic strain. To achieve the conditionsshown in Fig. 3(b and e), it is necessary (a) to avoidprestraining a greater amount than the final tes
49、t strain and (b)to avoid “springback” of the U-bend legs after achieving thefinal plastic strain.10.5 The bolt or restraining jig used to maintain the stressshould be insulated from the test specimen to avoid galvaniccorrosion effects. The insulators should have mechanicalstrength adequate to stand the stressing pressure, should notcreep significantly during the test, and should be inert to thetest environment. Insulators (Fig. 4 and Fig. 5) made ofzirconia or other non-compressible non-conducting materialshave proven satisfactory for this purpose. It is advisible
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