1、Designation: G 30 97 (Reapproved 2009)Standard Practice forMaking and Using U-Bend Stress-Corrosion TestSpecimens1This standard is issued under the fixed designation G 30; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of
2、 last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon () 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
3、 in metals. 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-
4、eral different 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 (Prac
5、tice G39or 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)2a
6、nd in Practices G35,G36, G37, G41, G44, G 103 and Test Method G 123.1.4 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 us
7、e. 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:3E3 Guide for Preparation of Metallographic SpecimensG1 Practice for Prepari
8、ng, Cleaning, and Evaluating Cor-rosion Test SpecimensG15 Terminology Relating to Corrosion and CorrosionTestingG35 Practice for Determining the Susceptibility of Stain-less Steels and Related Nickel-Chromium-Iron Alloys toStress-Corrosion Cracking in Polythionic AcidsG36 Practice for Evaluating Str
9、ess-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 AlloysG39 Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test SpecimensG
10、41 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 Spec
11、imensG 103 Practice for Evaluating Stress-Corrosion CrackingResistance of Low Copper 7XXX Series Al-Zn-Mg-CuAlloys in Boiling 6 % Sodium Chloride SolutionG 123 Test Method for Evaluating Stress-Corrosion Crack-ing of Stainless Alloys with Different Nickel Content inBoiling Acidified Sodium Chloride
12、Solution3. 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 expose
13、d to the testenvironment 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.1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the dire
14、ct responsibility of Subcommittee G01.06 on Environmen-tally Assisted Cracking.Current edition approved May 1, 2009. Published May 2009. Originallyapproved in 1972. Last previous edition approved in 2003 as G 3097(2003).2The boldface numbers in parentheses refer to a list of references at the end of
15、this standard.3For referenced ASTM standards, 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
16、Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Significance and Use5.1 The U-bend specimen may be used for any metal alloysufficiently ductile to be formed into the U-shape withoutmechanically cracking. The specimen is most easily made fromstrip or sheet but can be machined f
17、rom 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 most severetests available for smooth (as opposed to notched or pre-cracked) stress-corrosion test specimens. The stress con
18、ditionsare 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 stresses on stress-corrosion cracking or for studying variables which have only aminor effect on cracking. The advantage of
19、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) different metals inthe same environment, (b) one metal in different metallurgicalconditions in the same environment, or (c)
20、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 crack may splinter into two or morepieces. Due to the highly stressed condition in a U-bendspecimen, these pieces may le
21、ave 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 betested; however, the bulk sampling of mill products is outsidethe scope of this standard.7.2 In performing tests to sim
22、ulate 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 relevant to the anticipated application.8. Test Specimen8.1 Specimen OrientationWhen specimens are cut fromsheet or plate an
23、d 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 thesetwo directions is quite different so it is important to define theorientation of the test specimen.8.2 Specimen DimensionsFig.
24、 2 shows a typical testspecimen and lists, by way of example, several dimensioncombinations that have been used successfully to test a widerange of materials. Other dimensional characteristics may beused as necessary. For example, some special types of U-bendconfiguration have been used for simulati
25、ng exposure condi-tions encountered in high temperature water environmentsrelative to the nuclear power industry. These include doubleU-bend (2) and split tube U-bend (or reverse U-bend) (3)specimens.FIG. 1 Typical Stressed U-bendsG 30 97 (2009)28.2.1 Whether or not the specimen contains holes is de
26、pen-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 available,the stressing method used, and the size of the test environmentcontainer.8.2.3 The thickness (T) is usually depende
27、nt 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 greater than approxi-mately 3 mm (0.125 in.) if the yield strength exceeds about1400 MPa (200 ksi).8.2.4 For comparison purpos
28、es, 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 being compared (see 9.3). How-ever, it does not necessarily provide tests of equal severity ifthe mechanical properties
29、 of the materials being compared arewidely 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 to loop the wire rather than use just a simple U-shape(4).8.3 Surface Finish:8.3.1 Any necessary heat treatment shoul
30、d 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 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
31、 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 metallurgical orchemical changes, or both, at the surface. The edges of thespecimen should receive the same finish as th
32、e 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 attackconstituents in the metal, or leave undesirable residues on thesurface.8.3.5 It may be desirable to test a surfa
33、ce (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 avoided in all cases.8.3.6 The final stage of surface preparation is degreasing.Depending upon the method of stressing, th
34、is may be donebefore or after 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
35、 a stressed condition during 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 the
36、Examples of Typical Dimensions (SI Units)Example L, mm M, mm W, mm T, mm D, mm X, mm Y, mm R, mm a,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 32 1.57g 510 460
37、 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)G 30 97 (2009)3inner surface, (b) the stress varies from zero at the ends of thespecimen to a maximum at the center of the bend, and (c) thestress may vary acro
38、ss 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 portion of thetrue stress-true strain curve; for example, into Section AB inFig. 3(a). Fig. 3(be) show several stress
39、-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 conditions shown in Fig.3(d), a quantitative measure of the maximum test stress can bemade (6).9.3 The total strain (
40、) on the outside of the bend can beclosely approximated to the equation:5T/2R when T , Rwhere:T = specimen thickness, andR = radius of bend curvature.10. Stressing the Specimen10.1 Stressing is usually achieved by either a one- or atwo-stage operation.10.2 Single-stage stressing is accomplished by b
41、ending 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 shown in Fig.4(a) may be performed in a tension testing machine and isoften the most suitable method for stressing U-b
42、ends 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 material, or both,but are generally inferior to the method shown in Fig. 4(a). Themethod shown in Fig. 4(b) results in a m
43、ore 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 conditionsthat can be obtained by the single-stage stressing method aredefined by point X in Fig. 3(b and c). In the
44、 latter case, someelastic strain relaxation has occurred as a result of allowing theU-bend legs to spring 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 secon
45、d stage of applying the test stress. Atypical sequence of operations is shown in Fig. 5. The type ofequipment 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 inv
46、olve additional plastic strain(Fig. 3(e).10.4 The slope, MN, of the curve shown in Fig. 3(d) is steep(equal to Youngs modulus). Therefore, it is often difficult toreproducibly apply a constant percentage of the total elasticFIG. 3 True Stress-True Strain Relationships for Stressed U-BendsG 30 97 (20
47、09)4prestrain 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 stress), it isrecommended that the stress conditions shown in Fig. 3(bore)be achieved. Hence, the final applied strai
48、n prior to testingconsists of plastic and elastic strain. To achieve the conditionsFIG. 4 Methods of Stressing U-Bend SpecimensSingle-Stage StressingFIG. 5 Method of Stressing U-BendTwo-Stage MethodG 30 97 (2009)5shown in Fig. 3(b and e), it is necessary (a) to avoidprestraining a greater amount tha
49、n the final test 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 purpos
