1、Designation: G 38 01 (Reapproved 2007)Standard Practice forMaking and Using C-Ring Stress-Corrosion Test Specimens1This standard is issued under the fixed designation G 38; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year o
2、f last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This practice covers the essential fea
3、tures of the designand machining, and procedures for stressing, exposing, andinspecting C-ring type of stress-corrosion test specimens. Ananalysis is given of the state and distribution of stress in theC-ring.1.2 Specific considerations relating to the sampling processand to the selection of appropr
4、iate test environments areoutside the scope of this practice.1.3 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is th
5、eresponsibility 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.2. Referenced Documents2.1 NACE Document:NACE TM017796 Laboratory Testing of Metals for Re-sistance to Sulfide Stress Cracking an
6、d Stress CorrosionCracking in H2S Environments23. Summary of Practice3.1 This practice involves the preparation of and the quan-titative stressing of a C-ring stress-corrosion test specimen byapplication of a bending load. Characteristics of the stresssystem and the distribution of stresses are disc
7、ussed. Guidanceis given for methods of exposure and inspection.4. Significance and Use4.1 The C-ring is a versatile, economical specimen forquantitatively determining the susceptibility to stress-corrosioncracking of all types of alloys in a wide variety of productforms. It is particularly suitable
8、for making transverse tests oftubing and rod and for making short-transverse tests of variousproducts as illustrated for plate in Fig. 1.5. Sampling5.1 Test specimens shall be taken from a location and withan orientation so that they adequately represent the material tobe tested.5.2 In testing thick
9、 sections that have a directional grainstructure, it is essential that the C-ring be oriented in thesection so that the direction of principal stress (parallel to the1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee
10、G01.06 on Environmen-tally Assisted Cracking.Current edition approved May 1, 2007. Published May 2007. Originallyapproved in 1973. Last previous edition approved in 2001 as G 38-01.2Available from NationalAssociation of Corrosion Engineers (NACE), P.O. Box218340, Houston, TX 772188340.FIG. 1 Samplin
11、g Procedure for Testing Various Products1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.stressing bolt) is in the direction of minimum resistance tostress-corrosion cracking. For example, in the case of alumi-num alloys (1),3this is
12、the short-transverse direction relative tothe grain structure. If the ring is not so oriented it will tend tocrack off-center at a location where the stress is unknown.6. Specimen Design6.1 Sizes for C-rings may be varied over a wide range, butC-rings with an outside diameter less than about 16 mm (
13、58 in.)are not recommended because of increased difficulties inmachining and decreased precision in stressing. The dimen-sions of the ring can affect the stress state, and these consid-erations are discussed in Section 7. A typical shop drawing forthe manufacture of a C-ring is shown in Fig. 2.7. St
14、ress Considerations7.1 The stress of principal interest in the C-ring specimen isthe circumferential stress. It should be recognized that thisstress is not uniform (2, 3). First, there is a gradient through thethickness, varying from a maximum tension on one surface toa maximum compression on the op
15、posite surface. Secondly, thestress varies around the circumference of the C-ring from zeroat each bolt hole to a maximum at the middle of the arcopposite the stressing bolt; the nominal stress is present onlyalong a line across the ring at the middle of the arc. Thus, whenthe specimen is stressed b
16、y measuring the strain on the tensionsurface of the C-ring, the strain gage should be positioned atthe middle of the arc in order to indicate the maximum strain.Thirdly, the circumferential stress may vary across the width ofthe ring, the extent of the variation depending on the width-to-thickness a
17、nd diameter-to-thickness ratios of the C-ring. Ingeneral, when loaded as shown in Fig. 3 (a, b), the tensile stresson the outer surface will be greater at the extreme edge than atthe center, while when loaded as shown in Fig. 3 (c), the tensilestress on the inner surface will be less at the edge tha
18、n at thecenter (4).7.2 Another characteristic of the stress system in the C-ringis the presence of biaxial stresses; that is, transverse as well ascircumferential stresses are developed on the critical testsection. The transverse stress will vary from a maximum at themid-width to zero at the edges,
19、and will be the same sign as thecircumferential stress. In general, the transverse stress may beexpected to decrease with decreasing width to thickness andincreasing diameter to thickness ratios.An example is shown inFig. 4 where the transverse tensile stress at the mid-width of a19.00 mm (0.748 in.
20、) outside diameter by 1.537 mm (0.0605in.) thick by 19.0 mm (0.75 in.) wide C-ring of aluminum alloy7075-T6 was equal to about one third of the circumferentialtensile stress. In this example the circumferential stress wasuniform over most of the width of the C-ring; measurementswere not made at the
21、extreme edge.7.3 In the case of the notched C-ring (Fig. 3(d) a triaxialstress state is present adjacent to the root of the notch (5).Inaddition, the circumferential stress at the root of the notch willbe greater than the nominal stress and generally may beexpected to be in the plastic range.7.4 The
22、 possibility of residual stress should always beconsidered, especially when C-rings are machined from prod-ucts that contain appreciable residual stress or when C-ringsover about 6.35 mm (14 in.) thick are heat treated after beingmachined. It is generally not advisable to heat treat finish-machined
23、C-rings because of the likelihood of developingresidual stresses in the ring.NOTE 1When specimens are exposed to corrosive media at elevatedtemperatures, the possibility of relaxation of stress during the exposureperiod should be investigated. Relaxation can be estimated from knowncreep data for bot
24、h the ring and the stressing bolt.7.5 An advantage of the C-ring is that it can be stressed withhigh precision and bias by application of a measured deflec-tion. The sources of error in stressing are those that are inherentwith the use of measuring instruments (micrometers, straingages, etc.) as dis
25、cussed in 7.2-7.4 and Annex A1.7.6 The calculated stress applies only to the state of stressbefore initiation of cracks. Once cracking has initiated thestress at the tip of the crack, as well as in uncracked areas, haschanged.3The boldface numbers in parentheses refer to the list of references at th
26、e end ofthis practice.NOTEIf stock is undersize or tube stock is used dimensions can be varied to suit size of section from which the specimen must be cut.FIG. 2 C-Ring Type of Stress-Corrosion SpecimenG 38 01 (2007)28. Stressing Methods8.1 The C-ring, as generally used, is a constant-strainspecimen
27、 with tensile stress produced on the exterior of thering by tightening a bolt centered on the diameter of the ring.However, a nearly constant load can be developed by the use ofa calibrated spring placed on the loading bolt. C-rings also canbe stressed in the reverse direction by spreading the ring
28、andcreating a tensile stress on the inside surface. These methods ofstressing are illustrated in Fig. 3. Proper choice of a minimumbolt diameter or a spring constant is, of course, required toassure achieving true constant strain or constant load stressing.8.2 The most accurate stressing procedure i
29、s to attachcircumferential and transverse electrical strain gages to thesurface stressed in tension and to tighten the bolt until the strainmeasurements indicate the desired circumferential stress. Thecircumferential (sC) and transverse (sT), stresses are calcu-lated as follows:sC5 E/1 2 2!eC1 eT!,
30、andsT5 E/1 2 2!eT1 eC!where:E = Youngs modulus of elasticity, = Poissons ratio,eC= circumferential strain, andeT= transverse strain.NOTE 2When using electrical strain gages with thin-walled C-rings, acorrection should be allowed for the displacement of the gage from thesurface of the ring. All trace
31、s of the gage and the adhesive must beremoved from the C-ring before it is exposed.NOTE 3Stresses may be calculated from measured strains using themodulus of elasticity, provided the stresses and strains do not exceed theproportional limit.8.3 When several rings of the same alloy and dimensionsare t
32、o be loaded, it is convenient to determine a calibrationcurve of circumferential stress versus ring deflection as in Fig.4 to avoid the inconvenience of strain gaging each ring.8.4 The amount of compression required on the C-ring toproduce elastic straining only, and the degree of elastic strainscan
33、 be predicted theoretically (2, 3). Therefore, C-rings may bestressed by calculating the deflection required to develop adesired elastic stress by using the individual ring dimensions ina modified curved beam equation as shown in Table A1.1. Theaccuracy of calculated stresses is shown in Fig. 4 by t
34、heagreement of the calculated curve and the actual data points.See Annex A1 for the equation for stressing C-ring specimens.8.5 In the case of notched specimens a nominal stress isassumed using the ring outside diameter measured at the rootof the notch. Consideration then should be given to the stre
35、ssconcentration factor (KT) for the specific notch when calculat-ing the D required to develop the intended stress.NOTE 4The National Association of Corrosion Engineers (NACE)Standard TM017796 provides procedures for stressing C-Rings to theNOTEFor Fig 3 (d) a similar notch could be used on the tens
36、ion side of (b) or (c).FIG. 3 Methods of Stressing C-RingsFIG. 4 Stresses in 7075-T6 Aluminum Alloy C-Ring Stress-Corrosion Specimen (4)G 38 01 (2007)30.2% offset yield strength of the material to be tested. Experimentationunder the review and scrutiny of the ASTM subcommittee holdingjurisdiction of
37、 this standard was conducted to assess the accuracy andvalidity of such procedures. It was found that for a wide range of alloysystems, heat treatments, and test specimen dimensions, errors in thetarget strain associated with the 0.2% offset yield strength occurred whichwould be of significance. How
38、ever, it was also determined that in all casesthe actual strain realized following the procedures exceeded that associ-ated with the 0.2% offset yield stress, rendering results following suchprocedures conservative from an engineering analysis standpoint.9. Machining9.1 When rings are machined from
39、solid stock, precautionsshould be taken to avoid practices that overheat, plasticallydeform, or develop residual stress in the metal surface.Machining should be done in stages so that the final cut leavesthe principal surface with a clean finish of 0.7 m (30 in.) rmsor better. Necessary machining se
40、quences, type of tool, feedrate, etc., depend upon the alloy and temper of the test piece.Lapping, mechanical polishing, and similar operations thatproduce flow of the metal should be avoided.10. Surface Preparation10.1 A high-quality machined surface is the most desirablefor corrosion test purposes
41、 unless one wants to test theas-fabricated surface of a tube or bar; it should, of course, bedegreased before exposing the specimen. In order to removeheat treat films or thin layers of surface metal that may havebecome distorted during machining, chemical or electrochemi-cal etches may be used. The
42、 choice of such a treatment willdepend upon the alloy of the test piece. Care should beexercised to choose an etchant that will not selectively attackconstituents in the metal or will not deposit undesirableresidues on the surface. Etching or pickling should not be usedfor alloys that may undergo hy
43、drogen embrittlement.10.2 It is generally the best procedure to complete thesurface preparation before the C-ring is stressed except for apossible final degreasing of the critically stressed area.10.3 Every precaution should be taken to maintain theintegrity of the surface after the final preparatio
44、n; that is, avoidfinger printing and any rough handling that could mar thefinish.11. Specimen Identification11.1 Specimen numbers may be scribed on one of the tipsadjacent to the cut-away segment of the C-ring. No markingsof any kind should be made on the critically stressed arcbetween the bolt hole
45、s. Nonmetallic tags may be attached tothe stressing bolt by means of a second nut.12. Exposure Methods12.1 The C-ring, because of its small size and the simplemethods of stressing, can be exposed to almost any kind ofcorrosive environment (6). The specimens should be supportedin such a way that noth
46、ing except the corrosive medium comesin contact with the critically stressed area. No part of anexposure rack should be allowed to touch the surface or theedges of the critically stressed region.12.2 Care must be exercised to avoid galvanic effectsbetween the C-ring, the stressing bolt, and exposure
47、 racks. It isessential also to prevent crevice corrosion that could developcorrosion products between ring and bolt and alter the stress inthe C-ring. Protection can readily be applied by means ofsuitable coatings or by insulating bushing as shown in Fig. 5.Consideration must be given to the selecti
48、on of coatings orinsulators that will neither contaminate the corroding mediumnor be deteriorated by it. An insulating bushing, for example,that would deteriorate or creep, and thus allow the stress in thespecimen to decrease, would be unsatisfactory.NOTE 5Specimens should be placed in the intended
49、corrosive envi-ronment as soon as possible after being stressed, as some alloys may crackin moderately humid air.NOTE 6Hemispheric glazed ceramic insulators (S-151 Steatite) areexcellent for use outdoors and in neutral aqueous solutions.4Beeswax, andother adherent wax-type coatings, are suitable for room temperature testsin aqueous solutions. For tests in acidic or alkaline solutions, fast dryingvinyl-type lacquers have been used successfully; an example is anelectroplaters stop-off.512.3 Determination of cracking time is a subjective proce-dure involving v
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