ASTM E1290-2008 Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement.pdf

1、Designation: E 1290 08Standard Test Method forCrack-Tip Opening Displacement (CTOD) FractureToughness Measurement1This standard is issued under the fixed designation E 1290; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year

2、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. Scope1.1 This test method covers the determination of criticalcrack-tip opening displacement (CTOD) values at one or moreof

3、 several crack extension events, and may be used to measurecleavage crack initiation toughness for materials that exhibit achange from ductile to brittle behavior with decreasing tem-perature, such as ferritic steels. This test method appliesspecifically to notched specimens sharpened by fatigue cra

4、ck-ing. The recommended specimens are three-point bendSE(B), compact C(T), or arc-shaped bend A(B) speci-mens. The loading rate is slow and influences of environment(other than temperature) are not covered. The specimens aretested under crosshead or clip gage displacement controlledloading.1.1.1 The

5、 recommended specimen thickness, B, for theSE(B) and C(T) specimens is that of the material in thicknessesintended for an application. For the A(B) specimen, therecommended depth, W, is the wall thickness of the tube orpipe from which the specimen is obtained. Superficial surfacemachining may be use

6、d when desired.1.1.2 For the recommended three-point bend specimensSE(B), width, W, is either equal to, or twice, the specimenthickness, B, depending upon the application of the test. (See4.3 for applications of the recommended specimens.) ForSE(B) specimens the recommended initial normalized cracks

7、ize is 0.45 # ao/W # 0.70. The span-to-width ratio (S/W)isspecified as 4.1.1.3 For the recommended compact specimen C(T) theinitial normalized crack size is 0.45 # ao/W # 0.70. Thehalf-height-to-width ratio (H/W) equals 0.6 and the width tothickness ratio W/B is specified to be 2.1.1.4 For the recom

8、mended arc-shaped bend A(B) speci-men, B is one-half the specimen depth, W. The initial normal-ized crack size is 0.45 0.6 to 1.0, a span to width ratio, S/W, of 4 may beused. For r1/r2ratios from 0.4 to 0.6, an S/W of 3 may be used.1.2 The values stated in SI units are to be regarded as thestandard

9、. The values given in parentheses are for informationonly.1.3 This standard does not purport to address all 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-bil

10、ity of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E4 Practices for Force Verification of Testing MachinesE 8/E 8M Test Methods for Tension Testing of MetallicMaterialsE 399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE 182

11、0 Test Method for Measurement of Fracture Tough-nessE 1823 Terminology Relating to Fatigue and Fracture Test-ingE 1921 Test Method for Determination of Reference Tem-perature, To, for Ferritic Steels in the Transition Range3. Terminology3.1 Terminology E 1823 is applicable to this test method.3.2 De

12、finitions:3.2.1 crack-tip opening displacement, (CTOD), dLthecrack displacement resulting from the total deformation (elasticplus plastic) at variously defined locations near the original(prior to an application of force) crack tip.3.2.1.1 DiscussionIn common practice, d is estimated forMode I by in

13、ference from observations of crack displacementnearby or away, or both for the crack tip. In this test method,CTOD is the displacement of the crack surfaces normal to theoriginal (unloaded) crack plane at the tip of the fatigueprecrack, ao.In CTOD testing, dcL is the value of CTOD at the onset of un

14、stablebrittle crack extension (see 3.2.12) or pop-in (see 3.2.6) when Dap0.2 mm (0.008 in.). The force Puand the clip gagedisplacement vu, for duare indicated in Fig. 1.In CTOD testing deotL is the value of CTOD at the end-of-test forstable ductile crack extension. The corresponding force Peotand cl

15、ipgage displacement veotfor deotare indicated in Fig. 1.3.2.2 effective yield strength, sYFL2an assumed valueof uniaxial yield strength that represents the influence of plasticyielding upon fracture test parameters.3.2.2.1 DiscussionThe calculation of sYis the average ofthe 0.2 % offset yield streng

16、th (sYS), and the tensile strength(sTS), that is (sYS+ sTS)/2. Both sYSand sTSare determined inaccordance with Test Methods E 8/E 8M. In estimating sY,influences of the testing conditions, such as loading rate andtemperature, should be considered.3.2.3 original ligament, boLthe distance from the ori

17、gi-nal crack front to the back surface of the specimen at the startof testing, bo= W ao.3.2.4 physical crack extension, DapLan increase inphysical crack size, Dap= ap ao.3.2.5 physical crack size, apLsee Terminology E 1823.3.2.5.1 DiscussionIn CTOD testing, ap= ao+ Dap. Thistest method uses a 9point

18、 method (see 8.9.5) to measure ap.3.2.6 pop-ina discontinuity in the force versus clip gagedisplacement record. This discontinuity is characterized by asudden increase in displacement and, generally, a decrease inforce. Subsequently, the displacement and force increase toabove their respective value

19、s at pop-in.3.2.7 slow stable crack extension La displacement con-trolled crack extension beyond the stretch zone width (see3.2.11). The extension stops when the applied displacement isheld constant.3.2.8 specimen span, S Lthe distance between specimensupports.3.2.9 specimen thickness, BLsee Termino

20、logy E 1823.3.2.10 specimen width, W Lsee Terminology E 1823.3.2.11 stretch zone width, (SZW)Lthe length of crackextension that occurs during crack-tip blunting, for example,prior to the onset of unstable brittle crack extension, pop-in, orslow stable crack extension. The SZW is co-planar with theor

21、iginal (unloaded) fatigue precrack and refers to an extensionof the original crack.3.2.12 unstable brittle crack extension Lan abrupt crackextension occurring with or without prior stable crack exten-sion in a standard fracture test specimen under crosshead orclip gage displacement control.NOTE 1Con

22、struction lines drawn parallel to the elastic loading slope to give vp, the plastic component of total displacement, vg.NOTE 2In curves b and d, the behavior after pop-in is a function of machine/specimen compliance, instrument response, etc.FIG. 1 Types of Force Versus Clip Gage Displacement Record

23、sE12900824. Summary of Test Method4.1 The objective of the test is to determine the value ofCTOD at one of the following crack extension events. Thevalues of CTOD may correspond to: dc, the onset of unstablebrittle crack extension with no significant prior slow stablecrack extension (see 3.2.1), du,

24、 the onset of unstable brittlecrack extension following prior slow stable crack extension, ordeot, the CTOD value at the end-of-test test with only slowstable crack extension.4.2 The test method involves crosshead or clip gage dis-placement controlled three-point bend loading or pin loading offatigu

25、e precracked specimens. Force versus clip gage crackopening displacement is recorded, for example, Fig. 1. Theforces and displacements corresponding to the specific eventsin the crack initiation and extension process are used todetermine the corresponding CTOD values. For values of dc, duand deot, t

26、he corresponding force and clip gage displacementsare obtained directly from the test records.4.3 The rectangular section bend specimen and the compactspecimen are intended to maximize constraint and these aregenerally recommended for those through-thickness cracktypes and orientations for which suc

27、h geometries are feasible.For the evaluation of surface cracks in structural applicationsfor example, orientationsT-S or L-S (Terminology E 1823), thesquare section bend specimen is recommended. Also forcertain situations in curved geometry source material or weldedjoints, the square section bend sp

28、ecimen may be preferred.Square section bend specimens may be necessary in order tosample an acceptable volume of a discrete microstructure.4.4 The arc-shaped bend specimen permits toughness test-ing in the C-R orientation (Terminology E 1823), for pipe ortube. This orientation is of interest since p

29、ipes and tubes underpressure often fail with longitudinal cracks. The specimengeometry is convenient for obtaining samples with minimal useof material.5. Significance and Use5.1 This test method characterizes the fracture toughness ofmaterials through the determination of crack-tip opening dis-place

30、ment (CTOD) at one of three events: (a) onset of unstablecrack extension without significant prior stable crack exten-sion, or (b) onset of unstable crack extension with significantprior stable crack extension, or (c) the end-of-test after signifi-cant slow stable crack extension. This test method m

31、ay also beused to characterize the toughness of materials for which theproperties and thickness of interest preclude the determinationof Klcfracture toughness in accordance with Test MethodE 399.5.2 The different values of CTOD determined by this testmethod can be used to characterize the resistance

32、 of a materialto crack initiation and early crack extension at a giventemperature.5.3 The values of CTOD may be affected by specimendimensions. It has been shown that values of CTOD deter-mined on SE(B) specimens using the square section geometrymay not be the same as those using the rectangular sec

33、tiongeometry, and may differ from those obtained with either theC(T) or A(B) specimens.5.4 The values of CTOD determined by this test methodmay serve the following purposes:5.4.1 In research and development, CTOD testing can showthe effects of certain parameters on the fracture toughness ofmetallic

34、materials significant to service performance. Theseparameters include material composition, thermo-mechanicalprocessing, welding, and thermal stress relief.5.4.2 CTOD testing may be used in specifications of accep-tance and manufacturing quality control of base materials,weld metals, and weld heat a

35、ffected zones. Previous versions ofTest Method E 1290 made effective use of the value of CTODat the first attainment of a maximum force plateau for suchpurposes. Qualitative comparisons of this type can only bemade if a consistent specimen geometry is used and thematerials compared have similar cons

36、titutive properties. Thevalue of CTOD at the first attainment of a maximum forceplateau was removed from this test method because is was notassociated with a measurement of crack extension and there-fore cannot be considered a measurement of fracture tough-ness. The deotvalue may be used in place of

37、 the value of CTODat the first attainment of a maximum force plateau for qualitycontrol and specifications.5.4.3 The dcand deotvalues from CTOD testing can be usedfor inspection and flaw assessment criteria, when used inconjunction with other standards such as Test Methods E 1921and E 1820 and infor

38、med fracture mechanics analyses. Aware-ness of differences that may exist between laboratory test andfield conditions is required to make proper flaw assessment(see 4.3 and 4.4).6. Apparatus6.1 This procedure involves measurement of applied force,P, and clip gage crack opening displacement, v. Force

39、 versusdisplacement is autographically recorded on an x-y plotter forvisual display, or converted to and recorded in digital form forsubsequent processing. Testing is performed under crossheador clip gage displacement control in a compression or tensiontesting machine, or both, that conforms to the

40、requirements ofPractices E4.6.2 Fixturing for Three-Point Bend SpecimensA recom-mended SE(B) or A(B) specimen fixture is shown in Fig. 2.Friction effects between the support rollers and specimen arereduced by allowing the rollers to rotate during the test. Theuse of high hardness steel of the order

41、of 40 HRC or more isrecommended for the fixture and rollers to prevent indentationof the platen surfaces.6.3 Tension Testing ClevisA loading clevis suitable fortesting C(T) specimens is shown in Fig. 3. Each leg of thespecimen is held by such a clevis and loaded through pins, inorder to allow rotati

42、on of the specimen during testing. Toprovide rolling contact between the loading pins and the clevisholes, these holes are produced with small flats on the loadingsurfaces. Other clevis designs may be used if it can bedemonstrated that they will accomplish the same result as thedesign shown. Clevise

43、s and pins should be fabricated fromsteels of sufficient strength and hardness (greater than 40 HRC)to elastically resist indentation forces. The critical tolerancesand suggested proportions of the clevis and pins are given inFig. 3. These proportions are based on specimens having W/BE1290083= 2 for

44、 B 12.7 mm (0.5 in.) and W/B = 4 for B #12.7 mm(0.5 in.). If a 1930-MPa (280 000-psi) yield strength maragingsteel is used for the clevis and pins, adequate strength will beobtained. If lower strength grip material is used, or if substan-tially larger specimens are required at a given sYS/E ratio, t

45、henheavier grips will be required.As indicated in Fig. 3, the cleviscorners may be cut off sufficiently to accommodate seating ofthe clip gage in specimens less than 9.5 mm (0.375 in.) thick.Attention should be given to achieving good alignment throughcareful machining of all auxiliary gripping fixt

46、ures.NOTE 1Roller pins and specimen contact surface of loading ram must be parallel to each other within 0.002W.NOTE 2 0.10 in. = 2.54 mm; 0.15 in. = 3.81 mm.FIG. 2 SE(B) Test Fixture DesignNOTE 1Corners of the clevis may be removed as necessary to accommodate the clip gage.FIG. 3 Clevis for C(T) Sp

47、ecimen TestingE12900846.4 Displacement Measuring Devices:6.4.1 Displacement measuring gages are used to measureopening displacements on SE(B) specimens at either knifeedges a distance z beyond the crack mouth, Fig. 4a, or at thecrack mouth (z = 0) in the case of integral knife edges, Fig. 4b.For C(T

48、) specimens, where the opening displacement is notmeasured on the load line, the difference between the load lineand the displacement measuring point shall constitute thedimension z (see 9.2). Alternatively, when the opening dis-placements on C(T) specimens are made on or within 60.002W of the load

49、line, it may be assumed that z = 0. For A(B)specimens, special measurements must be taken to establishlocation of the clip gage knife edges with respect to the crackplane. Paragaphs 8.9.4 and 8.9.5 provide more detail onrequired measurements.6.4.2 The clip gage recommended in Test Method E 399may be used in cases where the total expected displacement is2.5 mm (0.1 in.) or less. Sensitivity and linearity requirementsspecified in Test Method E 399, shall be met over the fullworking range of the gage. In addition, the gage is t