1、Designation: G 30 97 (Reapproved 2003)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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes procedures for making and usingU-bend specimens for the evaluation of stress-corrosion crack
3、-ing 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
4、sev-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 (
5、Practice G 39or direct tension (Practice G 49) 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
6、 (1),2in Practices G 35, G 36,G 37, G 41, G 44, 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 wit
7、h 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. (For more specificsafety hazard information see Section 10.)2. Referenced Documents2.1 ASTM Standards:E 3 Met
8、hods of Preparation of Metallographic Specimens3G 1 Practice for Preparing, Cleaning, and Evaluating Cor-rosion Test Specimens3G 15 Terminology Relating to Corrosion and CorrosionTesting4G 35 Practice for Determining the Susceptibility of Stain-less Steels and Related Nickel-Chromium-Iron Alloys toS
9、tress Corrosion Cracking in Polythionic Acids3G 36 Practice for Evaluating Stress-Corrosion CrackingResistance of Metals and Alloys in a Boiling MagnesiumChloride Solution3G 37 Practice for Use of Mattssons Solution of pH 7.2 toEvaluate the Stress-Corrosion Cracking Susceptibility ofCopper-Zinc Allo
10、ys3G 39 Practice for Preparation and Use of Bent-Beam Stress-Corrosion Specimens3G 41 Practice for Determining Cracking Susceptibility ofMetals Exposed Under Stress to a Hot Salt Environment3G 44 Practice for Evaluating Stress Corrosion CrackingResistance of Metals and Alloys by Alternate Immersion
11、in3.5 % Sodium Chloride Solution3G 49 Practice for Preparation and Use of Direct TensionStress-Corrosion Test Specimens3G 103 Practice for Performing a Stress-Corrosion CrackingTest of Low Copper Containing Al-Zn-Mg Alloys inBoiling 6 % Sodium Chloride Solution4G 123 Test Method for Evaluating Stres
12、s-Corrosion Crack-ing of Stainless Alloys with Different Nickel Content in aBoiling Acidified Sodium Chloride Solution3. Terminology3.1 For definitions of corrosion-related terms used in thispractice see Terminology G 15.4. Summary of Practice4.1 This practice involves the stressing of a specimen be
13、ntto a U shape. The applied strain is estimated from the bendconditions. The stressed specimens are then exposed 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 en
14、viron-ment.1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G01.06 on Stress-Corrosion Cracking and Corrosion Fatigue.Current edition approved April 10, 1997. Published February 1998. Originallyapproved in 1972. Las
15、t previous edition approved in 1994 as G 30 94.2The boldface numbers in parentheses refer to the list of references at the end ofthis practice.3Annual Book of ASTM Standards, Vol 03.01.4Annual Book of ASTM Standards, Vol 03.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West C
16、onshohocken, 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 from plate, bar, castings,
17、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 conditionsare not usually kno
18、wn 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 the U-bendspecimen is that
19、 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) one metal in severalenviro
20、nments.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 leave the specimen at high v
21、elocityand 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 simulate a service condition
22、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 and in some cases strip or b
23、ar, 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. 2 shows a typical testspe
24、cimen 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 simulating exposure condi-tions en
25、countered 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 (2003)28.2.1 Whether or not the specimen contains holes is depen-dent upon the method o
26、f 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 dependent upon the formof the mat
27、erial, 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 purposes, it is desirable to kee
28、p 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 of the materials being co
29、mpared 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 should be performedbefore the f
30、inal 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 finish of 0.76 m (30in.)
31、or better. Care must be taken to avoid excessive heatingduring preparation because this may induce undesirable re-sidual stresses and in some cases cause metallurgical orchemical changes, or both, at the surface. The edges of thespecimen should receive the same finish as the faces.8.3.4 When the fin
32、al 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 surface (for example, coldrolle
33、d 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, this may be donebefore or af
34、ter 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 duri
35、ng 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 theExamples of Typical Dimens
36、ions (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 25 6.5 13 136 165 76 1.5
37、7h 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 (2003)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 across the width of the bend
38、. 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-strain relationshipstha
39、t 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 (e) on the outside of the
40、 bend can beclosely approximated to the equation:e5T/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 bending thespecimen int
41、o 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-bends that aredifficult
42、 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 more complex strainsyst
43、em 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 latter case, someelas
44、tic 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 second stage of applying th
45、e 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 involve additional plasti
46、c 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 (2003)4prestrain and ther
47、e 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 strain prior to testingcons
48、ists 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 (2003)5shown in Fig. 3(b and e), it is necessary (a) to avoidprestraining a greater amount than the final test strai
49、n 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 to usef
