1、Designation: G39 99 (Reapproved 2011)Standard Practice forPreparation and Use of Bent-Beam Stress-Corrosion TestSpecimens1This standard is issued under the fixed designation G39; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the
2、year of last 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 designing, prepar-ing, and using bent-beam stress-corrosion specimens.1.2 Diff
3、erent specimen configurations are given for use withdifferent product forms, such as sheet or plate. This practiceapplicable to specimens of any metal that are stressed to levelsless than the elastic limit of the material, and therefore, theapplied stress can be accurately calculated or measured (se
4、eNote 1). Stress calculations by this practice are not applicableto plastically stressed specimens.NOTE 1It is the nature of these practices that only the applied stresscan be calculated. Since stress-corrosion cracking is a function of the totalstress, for critical applications and proper interpret
5、ation of results, theresidual stress (before applying external stress) or the total elastic stress(after applying external stress) should be determined by appropriatenondestructive methods, such as X-ray diffraction (1).21.3 Test procedures are given for stress-corrosion testing byexposure to gaseou
6、s and liquid environments.1.4 The bent-beam test is best suited for flat product forms,such as sheet, strip, and plate. For plate material the bent-beamspecimen is more difficult to use because more rugged speci-men holders must be built to accommodate the specimens. Adouble-beam modification of a f
7、our-point loaded specimen toutilize heavier materials is described in 10.5.1.5 The exposure of specimens in a corrosive environmentis treated only briefly since other practices deal with thisaspect, for example, Specification D1141, and Practices G30,G36, G44, G50, and G85. The experimenter is refer
8、red toASTM Special Technical Publication 425 (2).1.6 The bent-beam practice generally constitutes a constantstrain (deflection) test. Once cracking has initiated, the state ofstress at the tip of the crack as well as in uncracked areas haschanged, and therefore, the known or calculated stress or str
9、ainvalues discussed in this practice apply only to the state of stressexisting before initiation of cracks.1.7 The values stated in SI units are to be regarded asstandard. The inch-pound values in parentheses are providedfor information.1.8 This standard does not purport to address all of thesafety
10、concerns, if any, associated 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. (For more specificsafety hazard information see Section 7 and 12.1.)2. Refere
11、nced Documents2.1 ASTM Standards:3D1141 Practice for the Preparation of Substitute OceanWaterG30 Practice for Making and Using U-Bend Stress-Corrosion Test SpecimensG36 Practice for Evaluating Stress-Corrosion-Cracking Re-sistance of Metals and Alloys in a Boiling MagnesiumChloride SolutionG44 Pract
12、ice for Exposure of Metals and Alloys by Alter-nate Immersion in Neutral 3.5 % Sodium Chloride Solu-tionG50 Practice for Conducting Atmospheric Corrosion Testson MetalsG85 Practice for Modified Salt Spray (Fog) Testing2.2 NACE Documents:4NACE TM0177-96 Laboratory Testing of Metals for Resis-tance to
13、 Specific Forms of Environmental Cracking in H2SEnvironments3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 cracking timethe time elapsed from the inception oftest until the appearance of cracking.3.1.1.1 DiscussionThe test begins when the stress isapplied and the stressed spe
14、cimen is exposed to the corrosiveenvironment, whichever occurs later.1This 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 March 1, 2011. Published Ap
15、ril 2011. Originallyapproved in 1973. Last previous edition approved in 2005 as G3999(2005). DOI:10.1520/G0039-99R11.2The 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 Cust
16、omer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from NACE International (NACE), 1440 South Creek Dr., Houston,TX 77084-4906, http:/www.nace.org.1Copyright ASTM International, 100 Barr Har
17、bor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.1.2 DiscussionThe specimen is considered to havefailed when cracks are detected. Presence of cracks can bedetermined with or without optical, mechanical, or electronicaids. However, for meaningful interpretation, comparisons
18、should be made only among tests employing crack detectionmethods of equivalent sensitivity.3.1.2 stress-corrosion crackinga cracking process requir-ing the simultaneous action of a corrodent and sustained tensilestress. This excludes corrosion-reduced sections that fail byfast fracture. It also excl
19、udes intercrystalline or transcrystallinecorrosion which can disintegrate an alloy without eitherapplied or residual stress.4. Summary of Practice4.1 This practice involves the quantitative stressing of abeam specimen by application of a bending stress. The appliedstress is determined from the size
20、of the specimen and thebending deflection. The stressed specimens then are exposed tothe test environment and the time required for cracks todevelop is determined. This cracking time is used as a measureof the stress-corrosion resistance of the material in the testenvironment at the stress level uti
21、lized.5. Significance and Use5.1 The bent-beam specimen is designed for determiningthe stress-corrosion behavior of alloy sheets and plates in avariety of environments. The bent-beam specimens are de-signed for testing at stress levels below the elastic limit of thealloy. For testing in the plastic
22、range, U-bend specimens shouldbe employed (see Practice G30). Although it is possible tostress bent-beam specimens into the plastic range, the stresslevel cannot be calculated for plastically-stressed three- andfour-point loaded specimens as well as the double-beamspecimens. Therefore, the use of be
23、nt-beam specimens in theplastic range is not recommended for general use.6. Apparatus6.1 Specimen HoldersBent-beam specimens require aspecimen holder for each specimen, designed to retain theapplied stress on the specimen. Typical specimen holderconfigurations are shown schematically in Fig. 1.NOTE
24、2The double-beam specimen, more fully described in 10.5,isself-contained and does not require a holder.NOTE 3Specimen holders can be modified from the constant defor-mation type shown in Fig. 1 to give a constant-load type of stressing. Forinstance, the loading bolt can be supplanted by a spring or
25、deadweightarrangement to change the mode of loading.6.1.1 The holder shall be made of a material that wouldwithstand the influence of the environment without deteriora-tion or change in shape.NOTE 4It should be recognized that many plastics tend to creep whensubjected to sustained loads. If specimen
26、 holders or insulators are made ofsuch materials, the applied stress on the specimen may change appreciablywith time. By proper choice of holder and insulator materials, however,many plastics can be used, especially in short-time tests.6.1.2 When the stress-corrosion test is conducted by immer-sion
27、in an electrolyte, galvanic action between specimen andholder (or spacer) shall be prevented (see Note 5). This isaccomplished by (1) making the holder of the same material asthe individual specimens, (2) inserting electrically insulatingmaterials between specimen and holder at all points of contact
28、(see Note 4), (3) making the entire holder out of a nonmetallicmaterial (see Note 4), or (4) coating the holder with anelectrically nonconducting coating that effectively preventscontact between holder and electrolyte.6.1.3 Crevice corrosion may occur in an electrolyte atcontact points between speci
29、men and holder (or spacer). Inthese instances the critical areas should be packed with ahydrophobic filler (such as grease or wax).NOTE 5In atmospheres (gas) galvanic action between specimen andholder either does not exist or is confined to a very small area asexperienced in outdoor exposure tests.6
30、.2 Stressing JigsThree-point and four-point loadedspecimen holders, Fig. 1(b and c), contain a stressing feature inthe form of a loading screw. To stress two-point loadedspecimens (Fig. 1(a), a separate stressing jig shall be used. Aconvenient stressing jig is shown in Fig. 2.NOTE 6The double-beam s
31、pecimen, described in 10.5, requires amechanical or hydraulic stressing frame (a universal tension testingmachine can also be used) as well as welding equipment.6.3 Deflection GaugesDeflection of specimens is deter-mined by separate gages or by gages incorporated in a loadingapparatus as shown in Fi
32、g. 3. In designing a deflection gage toFIG. 1 Schematic Specimen and Holder ConfigurationsG39 99 (2011)2suit individual circumstances care must be taken to referencethe deflection to the proper support distance as defined in10.2-10.5.7. Hazards7.1 Bent-beam specimens made from high-strength materi-a
33、ls may exhibit high rates of crack propagation and a specimenmay splinter into several pieces. Due to high stresses in aspecimen, these pieces may leave the specimen at high velocityand can be dangerous. Personnel installing and examiningspecimens should be cognizant of this possibility and beprotec
34、ted against injury.8. Sampling8.1 Test specimens shall be selected so that they representthe material to be tested. In simulating a service condition, thedirection of load application in the specimen shall represent theanticipated loading direction in service with respect to process-ing conditions,
35、for example, rolling direction.8.2 Paragraphs 9.4 and 9.5 deal specifically with specimenselection as related to the original material surface.9. Test Specimen9.1 The bent-beam, stress-corrosion specimens shall be flatstrips of metal of uniform, rectangular cross section, anduniform thickness.9.2 Th
36、e identification of individual specimens should bepermanently inscribed at each end of the specimen because thisis the area of lowest stress and cracking is not expected to beinitiated by the identification markings. If stenciling is used foridentification, this shall be done only on softened materi
37、albefore any hardening heat treatments to prevent cracking in thestenciled area. Care must be taken to prevent the identificationfrom being obliterated by corrosion.9.3 Mechanical properties should be determined on thesame heat-treatment lot from which stress-corrosion specimensare obtained.9.4 The
38、specimens can be cut from sheet or plate in such afashion that the original material surface is retained. Thisprocedure is recommended when it is desired to include theeffect of surface condition in the test.9.5 If, however, it is desired that surface conditions shouldnot influence the test results
39、of several materials with differentsurface conditions, the surfaces of all specimens must beprepared in the same way. It is recommended that grinding ormachining to a surface finish of at least 0.7 m (30 in.) and toa depth of at least 0.25 mm (0.01 in.) be utilized for surfacepreparation. It is desi
40、rable to remove the required amount ofmetal in several steps by alternately grinding opposite surfaces.This practice minimizes warpage due to residual stressescaused by machining. All edges should be similarly ground ormachined to remove cold-worked material from previousshearing. Chemical or electr
41、ochemical treatments that producehydrogen on the specimen surface must not be used onmaterials that may be subject to embrittlement by hydrogen orthat react with hydrogen to form a hydride.9.6 Immediately before stressing, the specimens should bedegreased and cleaned to remove contamination that occ
42、urredduring specimen preparation. Only chemicals appropriate forthe given metal or alloy should be used. Care must be exercisednot to contaminate cleaned specimens.Also, it is suggested thatspecimens be examined for cracks before exposure to the testenvironment.10. Stress Calculations10.1 The equati
43、ons given in this section are valid only forstresses below the elastic limit of the material. At stressesabove the elastic limit, but below the engineering yield strength(0.2 % offset) only a small error results from use of theequations (see Note 1). The equations must not be used abovethe yield str
44、ength of the material. The following paragraphsgive relationships used to calculate the maximum longitudinalstress in the outer fibers of the specimen convex surface.Calculations for transverse stress or edge-to-edge variation oflongitudinal stress are not given; the specimen dimensions arechosen to
45、 minimize these stresses consistent with convenientFIG. 2 Stressing Jig and Two-Point Loaded Specimen with Holder(approximately14 actual size)FIG. 3 Specimen Loading Apparatus for Three-Point LoadedBeam Specimens with Integral Deflection GageG39 99 (2011)3use of the specimens. The specimen dimension
46、s given here canbe modified to suit specific needs. However, if this is done, theapproximate specimen proportions should be preserved to givea similar stress distribution (for instance, if the length isdoubled the width should be doubled also).10.1.1 When specimens are tested at elevated temperature
47、s,the possibility of stress relaxation should be investigated.Relaxation can be estimated from known creep data for thespecimen, holder, and insulating materials. Differences inthermal expansion also should be considered.10.1.2 The applied stress is determined by specimen dimen-sions and the amount
48、of bending deflection. Thus, the errors inthe applied stress are related to those inherent in the use ofmeasuring instruments (micrometers, deflection gages, straingages, and so forth). For the two-point loaded specimens, mostmeasured values lie within 5 % of the values calculated inaccordance with
49、the procedures given in 10.2.1-10.2.3,asreported by Haaijer and Loginow (4). The calculated stressapplies only to the state of stress before initiation of cracks.Once cracking is initiated, the stress at the tip of the crack, aswell as in uncracked areas, has changed.10.2 Two-Point Loaded SpecimensThis specimen can beused for materials that do not deform plastically when bent to(L H)/H = 0.01 (see section 10.2.5). The specimens shall beapproximately 25 by 254-mm (1- by 10-in.) flat strips cut toappropriate lengths to produce the desired stress after bendingas s
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