1、Designation: G 49 85 (Reapproved 2005)Standard Practice forPreparation and Use of Direct Tension Stress-CorrosionTest Specimens1This standard is issued under the fixed designation G 49; the number immediately following the designation indicates the year of originaladoption or, in the case of revisio
2、n, the year of 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 covers procedures for designing, prepar-ing, and using ASTM standard tension test specim
3、ens forinvestigating susceptibility to stress-corrosion cracking. Axi-ally loaded specimens may be stressed quantitatively withequipment for application of either a constant load, constantstrain, or with a continuously increasing strain.1.2 Tension test specimens are adaptable for testing a widevari
4、ety of product forms as well as parts joined by welding,riveting, or various other methods.1.3 The exposure of specimens in a corrosive environmentis treated only briefly because other standards are beingprepared to deal with this aspect. Meanwhile, the investigatoris referred to Practices G 35, G 3
5、6, G 37, and G 44, and toASTM Special Technical Publication 425 (1).22. Referenced Documents2.1 ASTM Standards:3E8 Test Methods for Tension Testing of Metallic MaterialsG35 Practice for Determining the Susceptibility of Stain-less Steels and Related Nickel-Chromium-Iron Alloys toStress-Corrosion Cra
6、cking in Polythionic AcidsG36 Practice for Evaluating Stress-Corrosion CrackingResistance of Metals and Alloys in a Boiling MagnesiumChloride SolutionG37 Practice for Use of Mattssons Solution of pH 7.2 toEvaluate the Stress-Corrosion Cracking Susceptibility ofCopper-Zinc AlloysG44 Practice for Expo
7、sure of Metals and Alloys by Alter-nate Immersion in Neutral 3.5 % Sodium Chloride Solu-tion3. Summary of Practice3.1 This practice covers the use of axially loaded, quantita-tively stressed ASTM standard tension test specimens forinvestigating the resistance to stress-corrosion cracking ofmetallic
8、materials in all types of product forms. Considerationis given to important factors in the selection of appropriatespecimens, the design of loading equipment, and the effects ofthese factors on the state of stress in the specimen as corrosionoccurs.4. Significance and Use4.1 Axially loaded tension s
9、pecimens provide one of themost versatile methods of performing a stress-corrosion testbecause of the flexibility permitted in the choice of type andsize of test specimen, stressing procedures, and range of stresslevels.4.2 The uniaxial stress system is simple; hence, this testmethod is often used f
10、or studies of stress-corrosion mecha-nisms. This type of test is amenable to the simultaneousexposure of unstressed specimens (no applied load) withstressed specimens and subsequent tension testing to distin-guish between the effects of true stress corrosion and mechani-cal overload (2). Additional
11、considerations in regard to thesignificance of the test results and their interpretation are givenin Sections 6 and 10.4.3 Wide variations in test results may be obtained for agiven material and specimen orientation with different speci-men sizes and stressing procedures. This consideration issignif
12、icant especially in the standardization of a test procedurefor interlaboratory comparisons or quality control.5. Test Specimens5.1 Whenever possible, tension test specimens used inevaluating susceptibility to stress-corrosion cracking shouldconform to the dimensions of standard tension test specimen
13、sspecified in Test Methods E8, which contain details forspecimens machined from various product forms.5.2 A wide range of sizes for tension test specimens ispossible, depending primarily upon the dimensions of theproduct to be tested. Because the stress-corrosion test resultscan be markedly influenc
14、ed by the cross section of the testspecimen, this factor should be given careful consideration1This 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 Oc
15、t. 1, 2005. Published October 2005. Originallyapproved in 1976. Last previous edition approved in 2000 as G 49 85 (2000).2The boldface numbers in parentheses refer to the list of references at the end ofthis practice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact AST
16、M 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 Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.with regard to the object of
17、the investigation. Although largerspecimens may be more representative of most actual struc-tures, they often cannot be machined from product forms to beevaluated; and they present more difficulties in stressing andhandling in the laboratory. Also, larger specimens of somematerials may require longe
18、r exposure periods than smallerspecimens.5.3 Smaller cross-section specimens are widely used be-cause they (1) have a greater sensitivity to the initiation ofstress-corrosion cracking, (2) usually give test results morequickly, and (3) permit greater convenience in testing. On theother hand, the sma
19、ller specimens are more difficult to ma-chine, and their performance is more likely to be influenced byextraneous stress concentrations resulting from non-axial load-ing, corrosion pits, etc. Therefore, specimens less than about 10mm (0.4 in.) in gauge length or 3.0 mm (0.12 in.) in diameterare not
20、recommended for general use.5.4 Tension specimens containing machined notches havebeen used in studies of stress-corrosion cracking and hydrogenembrittlement (3). The presence of a notch induces a triaxialstress state at the root of the notch wherein the actual stress willbe greater by a concentrati
21、on factor dependent on the notchgeometry. Advantages of such specimens include the probablelocalization of cracking to the notch region and acceleration offailure. However, unless directly related to practical conditionsof usage, spurious results may ensue.5.5 Tension specimens containing a machined
22、 notch inwhich a mechanical precrack (for example, a fatigue or tensioncrack) has been started will be the subject of another ASTMstandard. Various types of precracked specimens are discussedin other publications (2, 4).6. Stress Considerations6.1 There are several factors that may introduce bending
23、moments on specimens, such as a longitudinal curvature,misalignment of threads on threaded-end round specimens, andthe corners of sheet-type specimens. The significance of thesefactors is greater for specimens with smaller cross sections.Even though eccentricity in loading can be minimized to equalt
24、he same standards accepted for tension testing machines,inevitably, there is some variation in the tensile stress aroundthe circumference of the test specimen which can be of suchmagnitude that it will introduce considerable error in thedesired stress. Tests should be made on specimens with strainga
25、ges affixed to the specimen surface (around the circumfer-ence in 90 or 120 intervals) to verify strain and stressuniformity and determine if machining practices and stressingjigs are of adequate tolerance and quality.6.2 Another consideration is the possible increase in netsection stress that will
26、occur when corrosion develops duringthe environmental exposure (1, 5). As shown schematically inFig. 1, there are two limiting curves: one for zero stiffness(dead weight) and the other for infinite stiffness (ideal constantstrain). In actual testing with various types of stressing frames,such as tho
27、se shown in Figs. 2-4, the increase in net sectionstress will be somewhere in between. When the net sectionstress becomes greater than the nominal gross section stressand increases to the point of fracture, either of two events canoccur: (1) fracture by mechanical overload of a material that isnot s
28、usceptible to stress-corrosion cracking, or (2) stress-corrosion cracking of a material at an unknown stress higherthan the intended nominal test stress. The occurrence of eitherof these phenomena would interfere with a valid evaluation ofmaterials with a relatively high resistance to stress corrosi
29、on.These considerations must be taken into account in experi-ments undertaken to determine “threshold” stresses. The sig-nificance of these factors is discussed further in Section 10.7. Stressing Methods7.1 General Considerations:7.1.1 Tension specimens may be subjected to a wide rangeof stress leve
30、ls associated with either elastic or elastic andplastic strain. Because the stress system is intended to beessentially uniaxial (except in the case of notched specimens),great care must be exercised in the construction of stressingframes so that bending stresses are avoided or minimized.7.1.2 Althou
31、gh a number of different stressing frames havebeen used with tension specimens, three basic types areconsidered herein: constant (sustained) load, constant strain(deformation), and continuously increasing strain. A constantload can be obtained with dead weight, but truly constant strainloading is se
32、ldom achieved because a stressing frame withinfinite stiffness would be required. Stress-corrosion test resultscan be influenced by the type of loading in combination withthe design of the test specimen; therefore, the investigatorshould select loading conditions most applicable to the purposeNOTETh
33、e behavior shown is generally representative, but the curveswill vary with specific alloys and tempers.FIG. 1 Effect of Loading Method and Extent of Cracking orCorrosion Pattern on Average Net Section StressG 49 85 (2005)2of the investigation. Further information in regard to the typeof loading most
34、 applicable to various types of structures isgiven in Ref (2).7.2 Stressing Frames:7.2.1 Constant Load:7.2.1.1 The simplest method is a dead weight hung on oneend of the specimen, and it is particularly useful for wirespecimens (6). For specimens of larger cross section, however,lever systems such a
35、s are used in creep testing machines aremore practical. The advantage of any dead-weight loadingdevice is the constancy of the applied load.7.2.1.2 An approximation of a constant-load system can beattained by the use of springs with a ring such as that shown inFig. 2 (7). The principle of the provin
36、g ring, as used in thecalibration of tension testing machines, has also been adaptedto stress-corrosion testing to provide a simple, compact, andeasily operated device to apply axial load (8); see Fig. 3(a).The load is applied by tightening a nut on one of the bolts andis determined by carefully mea
37、suring the change in ringdiameter.Another similar but less sophisticated ring device canalso be used, the difference being that the load is applied witha hydraulic jig (8) as shown in Fig. 3(b). In either ring device,the bolt contains a keyway to prevent a torsional stress frombeing applied to the s
38、pecimen while tightening the nut.7.2.2 Constant StrainStress-corrosion tests performed inlow-compliance tension testing machines are of the constant-strain type. The specimen is loaded to the required stress leveland the moving beam then locked in position. Other laboratorystressing frames have also
39、 been used, generally in testingspecimens of lower strength of smaller cross section (9). Fig.4(a) shows an exploded view of such a stressing frame, andFig. 4(b) shows a special loading device developed to ensureaxial loading with a minimum of torsion and bending of thespecimen.7.2.2.1 For stressing
40、 frames that do not contain any mecha-nism for the measurement of load, it is desirable to determinethe stress levels from measurement of the strain. It must benoted, however, that only when the intended stress is below theelastic limit of the test material is the average linear stress (s)proportion
41、al to the average linear strain (e), s/e = E, where theconstant E is the modulus of elasticity.7.2.2.2 When tests are conducted at elevated temperatureswith constant-strain loaded specimens, consideration should begiven to the possibility of stress relaxation.7.2.3 Continuously Increasing StrainA te
42、nsion testingmachine may be used to load the test specimen at a constantrate to failure (10). If the specimen is surrounded by a testenvironment and strain rate is slow enough, stress-corrosioncracking may occur during the test. This can result in shortertimes to fracture or in lower values of elong
43、ation or reductionof area, or both, than obtained for a specimen strained at thesame rate in air or in an inert environment at the sametemperature as the corrodent. Appropriate combinations ofspecimen cross section and corrosive environment must bedetermined, as well as the range of critical strain
44、rate forspecific alloy systems.FIG. 2 Spring-Loaded Stressing Frame (7)FIG. 3 Sustained Load Devices Using Ring Frames (8)G 49 85 (2005)38. Preparation of Specimens8.1 The pronounced effect of surface conditions on the timerequired to initiate stress-corrosion cracking in test specimensis well-known
45、. Unless it is desired to evaluate the as-fabricatedsurface, the final surface preparation generally preferred is amechanical process followed by simple degreasing. Suitablemechanical finishes include a machined or machine-groundsurface with a quality of about 32 in. rms or better.8.2 Care should be
46、 taken to avoid overheating or excessivepressure during the final preparation; otherwise, residualstresses or metallurgical changes may be induced in thesurface.8.3 When the final surface preparation involves a chemicaltreatment, care must be taken to ensure that the solution doesnot selectively att
47、ack alloy constituents in the metal or leaveundesirable residues on the surface.FIG. 4 Constant-Strain Type of Stressing Frame (9)G 49 85 (2005)48.4 Chemical or electrochemical treatments that producehydrogen on the specimen surface must not be used onmaterials that are subject to hydrogen embrittle
48、ment or thatreact with hydrogen to form a hydride.9. Exposure of Specimens9.1 The environmental testing conditions will depend uponthe intent of the test but, ideally, should be the same as thoseprevailing for the intended use of the alloy or relatable to theanticipated service conditions.9.2 The st
49、ressed specimens should be exposed to the testenvironment, either gaseous or liquid, as soon as possible afterstressing. When practicable, it is recommended that the speci-mens be stressed with the corrodent already present.9.3 In the experimental setup for exposure of the specimento the test environment (for example, total immersion, alternateimmersion, atmospheric exposure, etc.), appropriate precau-tions must be taken to avoid galvanic action or crevicecorrosion between the specimen, the stressing frame, andexposure racks. If necessary, protective coatings can be used t
