1、Designation: G39 99 (Reapproved 2016)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,preparing, and using bent-beam stress-corrosion specimens.1.2 Differ
3、ent specimen configurations are given for use withdifferent product forms, such as sheet or plate. This practice isapplicable 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 (s
4、eeNote 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 interpre
5、tation 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 gaseo
6、us 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
7、four-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 refe
8、rred 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 st
9、rainvalues 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. Refer
11、enced 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 Prac
12、tice for Exposure of Metals andAlloys byAlternateImmersion in Neutral 3.5 % Sodium Chloride SolutionG50 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 Spe
13、cific 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.1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals
14、 and is the direct responsibility of Subcommittee G01.06 on Environmen-tally Assisted Cracking.Current edition approved May 1, 2016. Published May 2016. Originallyapproved in 1973. Last previous edition approved in 2011 as G39 99 (2011). DOI:10.1520/G0039-99R16.2The boldface numbers in parentheses r
15、efer 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 of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Avai
16、lable from NACE International (NACE), 1440 South Creek Dr., Houston,TX 77084-4906, http:/www.nace.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.1.1 DiscussionThe test begins when the stress isapplied and the stressed specime
17、n is exposed to the corrosiveenvironment, whichever occurs later.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, comparisonsshould
18、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 excludes in
19、tercrystalline 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 of the
20、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 utilized.5
21、. 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 range,
22、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 bent-beam
23、 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 2The do
24、uble-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 deadwei
25、ghtarrangement 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 holder
26、s 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 in an e
27、lectrolyte, 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(see No
28、te 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 specimen and
29、 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.6.2 Stre
30、ssing JigsThree-point and four-point loaded speci-men holders, Fig. 1(b and c), contain a stressing feature in theform of a loading screw. To stress two-point loaded specimens(Fig. 1(a), a separate stressing jig shall be used. A convenientstressing jig is shown in Fig. 2.NOTE 6The double-beam specim
31、en, described in 10.5, requires amechanical or hydraulic stressing frame (a universal tension testingmachine can also be used) as well as welding equipment.FIG. 1 Schematic Specimen and Holder ConfigurationsG39 99 (2016)26.3 Deflection GaugesDeflection of specimens is deter-mined by separate gages o
32、r by gages incorporated in a loadingapparatus as shown in Fig. 3. In designing a deflection gage tosuit individual circumstances care must be taken to referencethe deflection to the proper support distance as defined in 10.2 10.5.7. Hazards7.1 Bent-beam specimens made from high-strength materi-als m
33、ay 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 beprotected
34、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, for
35、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 The id
36、entification 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 materialbe
37、fore 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 spec
38、imens 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 of s
39、everal 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 desirabl
40、e 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 electroche
41、mical 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 occurre
42、dduring 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 equations
43、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 theFIG. 2 Stressing Jig and Two-Point Loaded Specimen withHolder (approximately1
44、4 actual size)FIG. 3 Specimen Loading Apparatus for Three-Point LoadedBeam Specimens with Integral Deflection GageG39 99 (2016)3equations (see Note 1). The equations must not be used abovethe yield strength of the material. The following paragraphsgive relationships used to calculate the maximum lon
45、gitudinalstress 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 minimize these stresses consistent with convenientuse of the specimens. The specimen dimensions gi
46、ven 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 temperatures,th
47、e 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 of b
48、ending 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 the
49、procedures given in 10.2.1 10.2.3,asreported by Haaijer and Loginow (3). 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
