1、Designation: G 39 99 (Reapproved 2005)Standard Practice forPreparation and Use of Bent-Beam Stress-Corrosion TestSpecimens1This standard is issued under the fixed designation G 39; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, th
2、e year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon (e) 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 D
3、ifferent 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
4、(seeNote 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 interp
5、retation 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 gas
6、eous 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
7、a 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 D 1141, and Practices G 30,G 36, G 44, G 50, and G85. The experimenter
8、is referred 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 stres
9、s or strainvalues 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 equivalents in parentheses are pro-vided for information.1.8 This standard does not purport to address al
10、l of thesafety 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
11、12.1.)2. Referenced Documents2.1 ASTM Standards:3D 1141 Practice for the Preparation of Substitute OceanWaterG30 Practice for Making and Using U-Bend Stress-Corrosion Test SpecimensG36 Practice for Performing Stress-Corrosion-CrackingResistance of MEtals and Alloys in a Boiling MagnesiumChloride Sol
12、utionG44 Practice for Exposure of Metals and Alloys by Alter-nate Immersion in 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-tan
13、ce to 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 stresse
14、d specimen 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 May 1, 2005. Publishe
15、d May 2005. Originallyapproved in 1973. Last previous edition approved in 1999 as G 39 99.2The boldface numbers in parentheses refer to the list of references appended tothis practice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm
16、.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from National Association of Corrosion Engineers (NACE), 1440South Creek Dr., Houston, TX 77084-4906.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700
17、, 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, comparisonsshould be made only am
18、ong 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 intercrystalline
19、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 specimen and th
20、ebending 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. Significance
21、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, U-bend specimen
22、s 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 specimens in t
23、heplastic 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 double-beam speci
24、men, 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 dead-weightarrangement
25、 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 holders or insulator
26、s 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 electrolyte, ga
27、lvanic 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 Note 4), (3) mak
28、ing 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 holder (or sp
29、acer). 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 Stressing JigsThre
30、e-point and four-point loadedspecimen holders, Fig. 1 ( b and c), contain a stressing featurein the 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 specimen, described
31、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 GagesDeflection of specimens is deter-mined by separate gages or by gages incorporated in a loadingapparatus as shown in Fig. 3. In designing a
32、 deflection gage toFIG. 1 Schematic Specimen and Holder ConfigurationsG 39 99 (2005)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-als may exhibit high
33、 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 against injury.
34、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 example, rollin
35、g direction.8.2 Paragraphs 7.4 and 7.5 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 identification of
36、 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 materialbefore any harden
37、ing 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 specimens can be cu
38、t 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 several material
39、s 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 desirable to remove the
40、 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 electrochemical treatment
41、s 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 occurredduring specime
42、n 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 given in this s
43、ection 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 strength of the materi
44、al. 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 minimize these str
45、esses 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 GageG 39 99 (2005)3use of the specimens. The specimen dimensions given here canbe
46、 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,the possibility
47、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 bending deflect
48、ion. 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 procedures giv
49、en 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 shown in Fig. 1(a).1
