ASTM_G_168-17.pdf

This international standard was developed indastablished in the Decisionon Principles for theDevelopment of International Standards,Guides andI by theWorld Trade Organization Technical Barriers to Trade(TBT)Committee.Designation:G168-17NTERNATIONALStandard Practice forMaking and Using Precracked Double Beam StressCorrosion Specimens1This standard is issued under the fixed designation G168:the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon(e)indicates an editorial change since the last revision or reapproval.1.Scope2.Referenced Documents1.1 This practice covers procedures for fabricating,2.1 ASTM Standards:2preparing,and using precracked double beam stress corrosionD1193 Specification for Reagent Watertest specimens.This specimen configuration was formerlyE8/E8M Test Methods for Tension Testing of Metallic Ma-designated the double cantilever beam(DCB)specimen.terialsGuidelines are given for methods of exposure and inspection.E399 Test Method for Linear-Elastic Plane-Strain Fracture1.2 The precracked double beam specimen,as described inToughness Ki of Metallic Materialsthis practice,is applicable for evaluation of a wide variety ofE1823 Terminology Relating to Fatigue and Fracture Testingmetals exposed to corrosive environments.It is particularlyG15 Terminology Relating to Corrosion and Corrosion Test-suited to evaluation of products having a highly directionaling(Withdrawn 2010)grain structure,such as rolled plate,forgings,and extrusions,G35 Practice for Determining the Susceptibility of Stainlesswhen stressed in the short transverse direction.Steels and Related Nickel-Chromium-Iron Alloys toStress-Corrosion Cracking in Polythionic Acids1.3 The precracked double beam specimen may be stressedG36 Practice for Evaluating Stress-Corrosion-Cracking Re-in constant displacement by bolt or wedge loading or insistance of Metals and Alloys in a Boiling Magnesiumconstant load by use of proof rings or dead weight loading.TheChloride Solutionprecracked double beam specimen is amenable to exposure toG37 Practice for Use of Mattssons Solution of pH 7.2 toaqueous or other liquid solutions by specimen immersion or byEvaluate the Stress-Corrosion Cracking Susceptibility ofperiodic dropwise addition of solution to the crack tip,orCopper-Zinc Alloysexposure to the atmosphere.G41 Practice for Determining Cracking Susceptibility of1.4 This practice is concerned only with precracked doubleMetals Exposed Under Stress to a Hot Salt Environmentbeam specimen and not with the detailed environmentalG44 Practice for Exposure of Metals and Alloys by Alternateaspects of stress corrosion testing,which are covered inImmersion in Neutral 3.5 Sodium Chloride SolutionPractices G35,G36.G37.G41,G44,and G50.G49 Practice for Preparation and Use of Direct Tension1.5 This standard does not purport to address all of theStress-Corrosion Test Specimenssafety concerns,if any,associated with its use.It is theG50 Practice for Conducting Atmospheric Corrosion Testsresponsibility of the user of this standard to establish appro-on Metalspriate safety,health,and environmental practices and deter-mine the applicability of regulatory limitations prior to use.3.Terminology1.6 This international standard was developed in accor-3.1 Definitions of Terms Specific to This Standard:dance with internationally recognized principles on standard-3.1.1 stress corrosion cracking(SCC)threshold stressization established in the Decision on Principles for theintensiry,Kis-the stress intensity level below which stressDevelopment of International Standards,Guides and Recom-corrosion cracking does not occur for a specific combination ofmendations issued by the World Trade Organization Technicalmaterial and environment when plane strain conditions areBarriers to Trade(TBT)Committee.satisfied.This practice is under the jurisdiction of ASTM Committee GOl on Corrosion2 For referenced ASTM standards,visit the ASTM website,www.astm.ong.orof Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-contact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMtally Assisted Cracking.Standards volume information,refer to the standards Document Summary page onCurrent edition approved Nov.1.2017.Published December 2017.Originallythe ASTM website.approved in 000.Last previous edition approved in 2013 asG168-00(2013).The last approved version of this historical standard is referenced onD0L:10.1520/G0168-17.www.astm.org.*A Summary of Changes section appears at the end of this standardCopyright ASTM Intemational,100 Barr Harbor Drive,PO Box C700.West Conshohocken,PA 19428-2959.United States4G168-173.1.1.1 Discussion-Terms relative to this subject matteron crack growth versus time curve is shown schematically incan be found in Terminologies G15 and E1823.Fig.I(9).When wedging forces occur,they can invalidatefurther results and the test should be ended.4.Summary of Practice6.1.2 Crack-tip blunting or branching out,or both,of the4.1 This practice covers the preparation and testing ofplane of the precrack can invalidate the test.For valid tests,theprecracked double beam specimens for investigating the resis-crack must remain within+10 of the centerline of thetance to SCC(see Terminology G15)of metallic materials inspecimen.various product forms.Precracking by fatigue loading and by6.1.3 Drying or contamination of the corrodent in the crackmechanical overload are described.Procedures for stressingduring interim measurements of the crack length may affect thespecimens in constant displacement with loading bolts arecracking behavior during subsequent exposure.described,and expressions are given for specimen stressNorE 1-Do not allow corrodent in the crack to dry during periodicintensity and crack mouth opening displacement.Guidance ismeasurements to avoid repassivation at the crack tip and the resultinggiven for methods of exposure and inspection of precrackedchange in corrosion conditions.Remove one specimen at a time fromdouble beam specimens.corrodent.For tests conducted in deaerated test environments or inenvironments that contain readily oxidizable species or corrosion5.Significance and Useproducts,interim crack length examinations may produce changes in theconditions at the crack tip that can,in turn,affect cracking behavior during5.1 Precracked specimens offer the opportunity to use thethe subsequent exposure period.principles of linear elastic fracture mechanics(1)to evaluate6.2 Interferences in Visual Crack Length Measurements:resistance to stress corrosion cracking in the presence of a6.2.1 Corrosion products on the side surfaces of the speci-pre-existing crack.This type of evaluation is not included inmen can interfere with accurate crack length measurementsconventional bent beam,C-ring,U-bend,and tension speci-Corrosion products on these surfaces may be removed bymens.The precracked double beam specimen is particularlycareful scrubbing with a nonmetallic abrasive pad.However.useful for evaluation of materials that display a strong depen-for interim measurements,a minimum area of surface shoulddence on grain orientation.Since the specimen dimension inbe cleaned to allow for visual crack length measurements ifthe direction of applied stress is small for the precrackedreexposure is planned.double beam specimen,it can be successfully used to evaluate6.2.2 Measurement on side grooved specimens may beshort transverse stress corrosion cracking of wrought products,difficult if the advancing crack travels up the side of the groove.such as rolled plate or extrusions.The research applicationsThis is especially difficult with V-shaped grooves.Adjustmentand analysis of precracked specimens in general,and theof the direction and intensity of the lighting may highlight theprecracked double beam specimen in particular,are discussedlocation of the crack tip.in Appendix XI.6.2.3 Often the crack length measured at the specimen5.2 The precracked double beam specimen may be stressedsurface is less than in the interior,due to decreased stressin either constant displacement or constant load.Constantdisplacement specimens stressed by loading bolts or wedgesare compact and self-contained.By comparison,constant loadspecimens stressed with springs(for example,proof rings,discussed in Test Method G49,7.2.1.2)or by deadweightloading require additional fixtures that remain with the speci-Alloy Dmen during exposure5.3 The recommendations of this practice are based on the毛results of interlaboratory programs to evaluate precrackedspecimen test procedures(2,3)as well as considerableindustrial experience with the precracked double beam speci-men and other precracked specimen geometries(4-8).Alloy C6.Interferences6.1 Interferences in Testing:Alloy B6.1.1 The accumulation of solid corrosion products or oxidefilms on the faces of an advancing stress corrosion crack canAlloy Agenerate wedge forces that add to the applied load,therebyincreasing the effective stress intensity at the crack tip(6-9).Exposure timeThis self-loading condition caused by corrosion product wedg-Nore 1-Schematic of the influence of corrosion product wedging oning can accelerate crack growth and can prevent crack arrestSCC growth versus time curves in a decreasing K(constant displacement)test.Solid lines:actually measured curve for case of corrosion productfrom being achieved.The effect of corrosion product wedgingwedging that results in increase in crack growth with time:asterisksindicate temporary crack arrest.Dashed lines:true crack growth curveexcluding the effect of corrosion product wedging(9).4The boldface numbers in parentheses refer to the list of references at the end ofFIG.1 Effect of Corrosion Product Wedging on Growth Crackthis standard.Versus Time Curve4G168-17triaxiality at the specimen surface.Alternatively,some condi-specimen should be parallel;the machined notch should betions produce an increase in crack length at the surface due tocentered:and the bolt holes should be aligned and centered.Aavailability of the corrodent.Ultrasonic methods can be used totypical bolt loaded specimen is shown in Fig.4.obtain interim crack length measurements at the interior of the7.1.3 Recommendations for determining the minimumspecimen but not near the specimen surface.specimen thickness,B.which will ensure that plane strain6.2.4 Transport of species in solution in the through-conditions are maintained at the tip of an SCC crack,arethickness direction can be important for precracked doublediscussed in Brown(1)and Dorward and Helfrich(8).Basedbeam specimens.This may affect measurement of crack lengthon a conservative estimate for plane strain conditions,thesince it can produce curvature of the crack front(that is,minimum specimen thickness shall be calculated as B 2.5variation in crack length from the edge to the center of the(K/oys)2,where Ki is determined per Test Method E399 andspecimen).Gys is the 0.2%offset yield strength in tension per TestMethod E8/E8M.For bolt loaded precracked double beam7.Specimen Size,Configuration,and Preparationspecimens,the thickness,B.may also be influenced by the size7.1 Specimen Dimensions and Fabrication:of the loading bolts and the minimum thickness needed to7.1.1 Dimensions for the recommended specimen are givensupport the bolt loading.in Figs.2 and 3.As a general guideline,specimen dimensions7.1.4 The specimen half-height.H,may be reduced forshould ensure that plane strain conditions are maintained at thematerial under 25 mm(1 in.thick.The minimum A that cancrack tip(1,10).While there are no established criteria forbe used is constrained by the onset of plastic deformation uponensuring adequate constraint for a plane strain SCC test,someprecracking or stresses in the leg of the specimen since thisguidelines are given herein regarding specimen dimensionsinfluences the calculation of K.Outer fiber stresses shall notsee7.1.3).exceed the yield strength of the test material during precrack-7.1.2 Specimen machining shall be in accordance with theing or stressing.standards outlined in Test Method E399.The principal consid-NorE 2-The effect of notch geometry on specimen compliance anderations in machining are that the sides,top,and bottom of thestress intensity solutions,noted in 7.3.4.4,Note 4,8.1.3,and Note 5,is2.5(0.10Fatque or mechancal precrackM101.25-5G6G(3/8-24UNF-2BAm5.10.225.4(1.00)Co=9.5(0.375)Smooth FaceU-Shape Side GrooveV-Shape Side GrooveL=136.5(5.375)B=25.4(1.00)m8=22.9(0.90)maxB=22.9(0.90)maxNorE 1-All dimensions in mm(in.).Top and front views are shown for smooth specimen only;side view is shown for both smooth and side groovedconfiguration.NorE 2-For Chevron notch crack starter.cutter tip angle 90 max.NorE 3-Radius at notch bottom to be 0.25 mm(0.01 in.)or less.NorE 4-Crack starter to be perpendicular to specimen length and thickness to within=2.Nor 5-Initial COD(A)may be increased to 12.7 mm(0.5 in.)to accommodate COD gageNorE 6-All surfaces 32 uin.or better,tolerances not specified+0.127(0.005).NorE 7-For V-shape side groove,continue with Chevron cutter on surface to machine grooves.For U-shape side groove,machine groove with radiuscutting tool such as a ball end mill,size equal to notch height.NorE 8Loading bolt holes shall be perpendicular to specimen center lines within5.Nor 9Center line of holes shall be parallel and perpendicular to specimen surfaces within2NoTE 10-Center line of holes shall be coincident within 0.127 mm(0.005 in.).NorE 11-The crack length at the start of the exposure test(a)is achieved by fatigue or mechanical precracking.Precracking length shall extend 2.5to 3.8 mm(0.10 to 0.15 in.)from the tip of the machine notch at the specimen surface,see 7.3.4.3.FIG.2 Detailed Machine Drawing for Smooth Face and Side Grooved DCB Specimen34G168-17susceptible material that are loaded in constant deflection tohigh starting stress intensities may require additional crackgrowth to achieve crack arrest as defined in 10.1.7.2 Specimen Configuration:8.267.2.1 The recommended specimen configuration includes a(0.325)sharp starter notch,which may be either a straight through orchevron configuration.The chevron configuration is recom-mended for both the fatigue and the mechanical overloadprecracking operations(see Fig.2).7.2.2 The use of side grooves is optional.They may behelpful if any difficulty is experienced in keeping the crack inthe center of the specimen.The side groove configuration mayM101.25-5G6Gbe machined with the chevron V-shaped cutter or with a(.375-24-UNF-2A)U-shaped radius cutting tool.The depth of each side grooveshould not exceed 5%of B.such that the net thickness,B,will,25.40be at least 90 of B.(1.00)dia.7.2.3 Specimens machined from rectangular product canhave six possible orientations(see Test Method E399)relativeto the direction of loading and the direction of crackpropagation,namely,S-L,S-T,T-L,T-S,LT,and L-S.Inwrought products,the S-L orientation is usually the mostcritical and is the most frequently used to avoid crack branch-.1.90ing(0.75)7.2.4 More detailed discussions of the factors described inNorE 1-All dimensions in mm(in).Tolerances not specified+0.127this section are given in Brown(1).Sprowls et al(6),and(0.005).Sprowls(9).NorE 2-Suggested material:Strong enough not to fail in tension7.3 Specimen Preparation:during loading or mechanical precracking.Nor 3-Bolt head design optional.Commercial stainless steel socket7.3.1 Specimen surfaces along the path of expected crackhead cap screws or hex head bolts are satisfactory.propagation may be polished to assist in crack measurement.NorE 4-Use one rounded end and one flat end bolt for loading each7.3.2 Specimens shall be cleaned and degreased prior tospecimen.Commercial bolts or screws should be modified accordingly.precracking and testing.Successive ultrasonic cleaning inNorE 5-To avoid galvanic corrosion between dissimilar bolt andacetone and methyl alcohol is suggested.Specimens shall notspecimen metals,see 8.2.be recleaned after precracking to prevent contamination of theFIG.3 Machine Drawing for DCB Loading Boltscrack with cleaning or degreasing chemicals.If cleaning of theside surfaces of the specimen following precracking isnecessary,then this should be performed by lightly wipingthese surfaces and not by immersion of the specimen into thecleaning or degreasing media.NorE 3-Only chemicals appropriate for the metal or alloy of interestshall be used.All chemicals shall be of reagent grade purity.7.3.3 Specimens shall be fully machined,including surfacegrooves,prior to precracking.Precracked specimens shall bestored in a dry atmosphere prior to environmental exposure.7.3.4 Fatigue Precracking:7.3.4.1 Fatigue precracking shall be performed under sinu-soidal cyclic loading with a stress ratio 0.05 R25 MPa-m12.Regardless of theaffect the subsequent SCC growth.material toughness,mechanical precracking is also difficult forspecimens that are machined with the crack propagation8.General Proceduredirection normal to predominant grain orientation;forexample,L-T or S-T(see Test Method E399)orientations in8.1 Stressing Procedure:rolled plate.8.1.1 Precracked double beam specimens may be stressed7.3.5.2 Crack mouth opening displacement,V.shall beeither in constant displacement or constant load.The constantmonitored with a clip-on crack mouth opening displacementdisplacement condition may be achieved by a wedge inserted(COD)gage during precracking.A typical COD gage isin the machined notch or by loading bolts.The constant loaddescribed in Test Method E399.Annex Al.condition may be achieved through the use of dead weight7.3.5.3 The mechanical precrack shall be extended 2.5 toloading or approximated with the use of proof rings with3.8 mm(0.10 to 0.15 in.)from the tip of the machined notch atadequate compliance to minimize load reduction that willthe specimen surface.The resulting crack length,a shall beoccur during the test due to crack growth in the specimen(3).measured on both specimen surfaces,and the two values8.1.2 Suggested loading bolts are shown in Fig.3.Aaveraged.The measuring instrument shall have an accuracy ofprecracked double beam specimen stressed in constant dis-0.025mm(0.001in.).placement with two bolts is shown in Fig.4.The loading bolts7.3.5.4 The resulting stress intensity after mechanical pre-shall be tightened until the crack mouth opening displacementcracking will be Ki the stress intensity for mechanical crack(V)reaches a value corresponding to the desired target startingarrest.If K is greater than the target starting stress intensity,stress intensity value for the measured precrack length.Thethen Ki shall be used as the starting stress intensity for thebolts shall be tightened in small increments,alternating be-stress corrosion test(that is,K).If a mechanically precrackedtween the two,such that the specimen is deflected symmetri-specimen is inadvertently overloaded,no attempt shall be madecally about the centerline.Another approach is to mount theto reduce the initial stress by partially unloading the specimen.nonstressed end of the specimen in a vice and use twoThis will produce compressive stresses at the crack tip,whichwrenches,turning both wrenches simultaneously and attempt-will retard or prevent crack initiation.If Ki is less than theing similar movement of both wrenches.target starting stress intensity,then adjustment of crack mouth8.1.3 The required crack mouth opening displacement toopening,V should be made following procedures provided inachieve the target starting stress intensity level is calculated8.1(Eq3).with the following relationship(11).Crack mouth opening7.3.5.5 The resulting stress intensity factor,K should bedisplacement during loading shall be measured with a clip-oncomputed from the following equation(11):crack opening displacement(COD)gage.5