1、Designation: G38 01 (Reapproved 2013)Standard Practice forMaking and Using C-Ring Stress-Corrosion Test Specimens1This standard is issued under the fixed designation G38; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of
2、last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () 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 featur
3、es 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 appropriat
4、e test environments areoutside the scope of this practice.1.3 The values stated in SI units are to be regarded asstandard. 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 therespon
5、sibility 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 Resis-tance to Sulfide Stress Cracking and Stres
6、s 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 discussed.
7、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 for mak
8、ing 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 sectio
9、ns 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 thestressing bolt) is in the direction of minimum resistance tostress-corrosion cracking. For example, in the case of alumi-num alloys (1),3this
10、 is 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
11、mm (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
12、. Stress 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 th
13、e opposite 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 stress
14、ed by measuring the strain on the tensionsurface of the C-ring, the strain gage should be positioned at1This 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 ap
15、proved May 1, 2013. Published July 2013. Originally approvedin 1973. Last previous edition approved in 2007 as G38-01 (2007). DOI:10.1520/G0038-01R13.2Available from NationalAssociation of Corrosion Engineers (NACE), P.O. Box218340, Houston, TX 772188340.3The boldface numbers in parentheses refer to
16、 the list of references at the end ofthis practice.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1the middle of the arc in order to indicate the maximum strain.Thirdly, the circumferential stress may vary across the width ofthe ring,
17、 the extent of the variation depending on the width-to-thickness and 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), t
18、he tensilestress on the inner surface will be less at the edge than 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 stre
19、ss will vary from a maximum at themid-width to zero at the edges, 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
20、transverse tensile stress at the mid-width of a19.00 mm (0.748 in.) 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
21、most of the width of the C-ring; measurementswere not made at the 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 stre
22、ss and generally may beexpected to be in the plastic range.7.4 The 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 beingmach
23、ined. It is generally not advisable to heat treat finish-machined 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 inve
24、stigated. Relaxation can be estimated from knowncreep data for both 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 us
25、e of measuring instruments (micrometers, straingages, etc.) as discussed 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.8. Stres
26、sing Methods8.1 The C-ring, as generally used, is a constant-strainspecimen 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.
27、C-rings also canbe stressed in the reverse direction by spreading the ring 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 st
28、rain or constant load stressing.8.2 The most accurate stressing procedure is 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 (C) and
29、transverse (T), stresses are calculatedas follows:C5 E/1 2 2!C1T!, andT5 E/1 2 2!T1C!where:E = Youngs modulus of elasticity, = Poissons ratio,C= circumferential strain, andT= transverse strain.NOTE 2When using electrical strain gages with thin-walled C-rings, acorrection should be allowed for the di
30、splacement of the gage from thesurface of the ring. All traces 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
31、.8.3 When several rings of the same alloy and dimensionsare to be loaded, it is convenient to determine a calibrationFIG. 1 Sampling Procedure for Testing Various ProductsG38 01 (2013)2curve of circumferential stress versus ring deflection as in Fig.4 to avoid the inconvenience of strain gaging each
32、 ring.8.4 The amount of compression required on the C-ring toproduce elastic straining only, and the degree of elastic strainscan be predicted theoretically (2, 3). Therefore, C-rings may bestressed by calculating the deflection required to develop adesired elastic stress by using the individual rin
33、g dimensions ina modified curved beam equation as shown in Table A1.1. Theaccuracy of calculated stresses is shown in Fig. 4 by theagreement 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
34、stress isassumed using the ring outside diameter measured at the rootof the notch. Consideration then should be given to the stressconcentration factor (KT) for the specific notch when calculat-ing the required to develop the intended stress.NOTE 4The National Association of Corrosion Engineers (NAC
35、E)Standard TM017796 provides procedures for stressing C-Rings to the0.2% offset yield strength of the material to be tested. Experimentationunder the review and scrutiny of the ASTM subcommittee holdingjurisdiction of this standard was conducted to assess the accuracy andvalidity of such procedures.
36、 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 whichNOTE 1If stock is undersize or tube stock is used dimensions can be varied to suit size of section from which th
37、e specimen must be cut.FIG. 2 C-Ring Type of Stress-Corrosion SpecimenNOTE 1For Fig 3 (d) a similar notch could be used on the tension 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)G38 01 (2013)3would be of signifi
38、cance. However, 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 mach
39、ined from 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 ma
40、chining sequences, 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 tes
41、t purposes 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
42、 used. The 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
43、undergo hydrogen 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
44、preparation; 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
45、 bolt holes. 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
46、 that nothing 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, an
47、d exposure 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 t
48、he selection 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
49、 intended corrosiveenvironment as soon as possible after being stressed, as some alloys maycrack in 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 p
