1、Designation: E 740 03Standard Practice forFracture Testing with Surface-Crack Tension Specimens1This standard is issued under the fixed designation E 740; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A
2、 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 practice covers the design, preparation, and testingof surface-crack tension (SCT) specimens. It relates specifi-cally to testin
3、g under continuously increasing force and ex-cludes cyclic and sustained loadings. The quantity determinedis the residual strength of a specimen having a semielliptical orcircular-segment fatigue crack in one surface. This valuedepends on the crack dimensions and the specimen thickness aswell as the
4、 characteristics of the material.1.2 Metallic materials that can be tested are not limited bystrength, thickness, or toughness. However, tests of thickspecimens of tough materials may require a tension testmachine of extremely high capacity. The applicability of thispractice to nonmetallic materials
5、 has not been determined.1.3 This practice is limited to specimens having a uniformrectangular cross section in the test section. The test sectionwidth and length must be large with respect to the crack length.Crack depth and length should be chosen to suit the ultimatepurpose of the test.1.4 Residu
6、al strength may depend strongly upon tempera-ture within a certain range depending upon the characteristicsof the material. This practice is suitable for tests at anyappropriate temperature.1.5 Residual strength is believed to be relatively insensitiveto loading rate within the range normally used i
7、n conventionaltension tests. When very low or very high rates of loading areexpected in service, the effect of loading rate should beinvestigated using special procedures that are beyond the scopeof this practice.NOTE 1Further information on background and need for this type oftest is given in the r
8、eport of ASTM Task Group E24.01.05 on Part-Through-Crack Testing (1).21.6 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
9、 applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E 4 Practices for Force Verification of Testing MachinesE 8 Test Methods for Tension Testing of Metallic MaterialsE 338 Test Method for Sharp-Notch Tension Testing ofHigh-Strength Sheet MaterialsE 399 T
10、est Method for Plane-Strain Fracture Toughness ofMetallic MaterialsE 466 Practice for Conducting Force Controlled ConstantAmplitude Axial Fatigue Tests of Metallic MaterialsE 561 Practice for R-Curve DeterminationE 1823 Terminology Relating to Fatigue and Fracture3. Terminology3.1 DefinitionsDefinit
11、ions given in Terminology E 1823are applicable to this practice.3.1.1 crack mouth opening displacement (CMOD), 2vm(L)the Mode 1 (also called opening mode) component ofcrack displacement due to elastic and plastic deformation,measured at the location on the crack surface that has thegreatest elastic
12、displacement per unit load.NOTE 2In surface-crack tension (SCT) specimens, CMOD is mea-sured on the specimen surface along the normal bisector of the cracklength.3.1.2 fracture toughnessa generic term for measures ofresistance to extension of a crack. E 6163.1.3 original crack size, aoLthe physical
13、crack size atthe start of testing. (E 616)3.2 Definitions of Terms Specific to This Standard:3.2.1 crack depth, a Lin surface-crack tension (SCT)specimens, the normal distance from the cracked plate surfaceto the point of maximum penetration of the crack front into thematerial. Crack depth is a frac
14、tion of the specimen thickness.3.2.1.1 DiscussionIn this practice, crack depth is theoriginal depth aoand the subscript o is everywhere implied.3.2.2 crack length, 2c Lin surface-crack tension speci-mens, a distance measured on the specimen surface betweenthe two points at which the crack front inte
15、rsects the specimensurface. Crack length is a fraction of specimen width.1This practice is under the jurisdiction of ASTM Committee E08 on Fatigue andFracture and is the direct responsibility of Subcommittee E08.07 on LinearElasticFracture.Current edition approved Nov. 1, 2003. Published December 20
16、03. Originallyapproved in 1988. Last previous edition approved in 1995 as E 740 88 (1995)e1.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service
17、astm.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.3.2.2.1 DiscussionIn this practice, crack length is theori
18、ginal length 2coand the subscript o is everywhere implied.3.2.3 residual strength, sr(FL2)the maximum value ofthe nominal stress, neglecting the area of the crack, that acracked specimen is capable of sustaining.NOTE 3In surface-crack tension (SCT) specimens, residual strength isthe ratio of the max
19、imum load (Pmax) to the product of test section width(W) times thickness (B), Pmax/(BW). It represents the stress at fracturenormal to and remote from the plane of the crack.4. Significance and Use4.1 The surface-crack tension (SCT) test is used to estimatethe load-carrying capacity of simple sheet-
20、 or plate-like struc-tural components having a type of flaw likely to occur inservice. The test is also used for research purposes to investi-gate failure mechanisms of cracks under service conditions.4.2 The residual strength of an SCT specimen is a functionof the crack depth and length and the spe
21、cimen thickness aswell as the characteristics of the material. This relationship isextremely complex and cannot be completely described orcharacterized at present.4.2.1 The results of the SCT test are suitable for directapplication to design only when the service conditions exactlyparallel the test
22、conditions. Some methods for further analysisare suggested in Appendix X1.4.3 In order that SCT test data can be comparable andreproducible and can be correlated among laboratories, it isessential that uniform SCT testing practices be established.4.4 The specimen configuration, preparation, and inst
23、ru-mentation described in this practice are generally suitable forcyclic- or sustained-force testing as well. However, certainconstraints are peculiar to each of these tests. These are beyondthe scope of this practice but are discussed in Ref. (1).5. Apparatus5.1 The procedure involves testing of sp
24、ecimens that havebeen precracked in fatigue. force versus CMOD, if CMOD ismeasured, is recorded autographically or digitally.5.2 Fatigue Precracking ApparatusAxial tension orthree-point, four-point, or cantilever bending are all acceptablemodes for fatigue precracking. Fixture design is not critical
25、 aslong as the crack growth is symmetrical and the plane of thecrack remains perpendicular to the specimen face and thetensile force vector. The effect of cyclic frequency is thought tobe negligible below 100 Hz in a nonaggressive environment.NOTE 4Certain crack shapes are more readily produced in a
26、xialtension, others in bending (see Annex A1).5.2.1 Devices and fixtures for cantilever bending of sheetand plate specimens are described in Refs. (2) and (3),respectively. Others may be equally suitable. The axial fatiguemachines described in Practice E 466 are suitable for precrack-ing in tension;
27、 however, since the precracking operation isterminated prior to specimen failure, one should ensure thatload variations during slowdown or shutdown do not exceedthose desired.5.2.2 A magnifier of about 20 power should be used tomonitor the fatigue precracking process. Ease of observationwill be enha
28、nced if the cyclic rate can be reduced to about 1 Hzwhen desired. Alternatively, a stroboscopic light synchronizedwith the maximum application of tensile force may serve aswell.5.3 Testing MachineThe test should be conducted with atension testing machine that conforms to the requirements ofPractices
29、 E 4.5.3.1 The devices for transmitting force to the specimenshall be such that the major axis of the specimen coincides withthe load axis. The pin-and-clevis arrangement described in TestMethod E 338 should be suitable for specimens whose width isless than about 4 in. (100 mm). An arrangement such
30、as thatshown in Fig. 2 of Practice E 561 should be suitable for widerspecimens.5.3.2 For tests at other than room temperature, the tempera-ture control and temperature measurement requirements of TestMethod E 338 are appropriate.5.4 Displacement Gage (Optional)If used to measureCMOD, the displacemen
31、t gage output should accurately indi-cate the relative displacement of two gage points on thecracked surface, spanning the crack at the midpoint of itslength. Further information on displacement gages appears inAppendix X2.5.5 For some combinations of material and crack geometry,the crack may propag
32、ate entirely through the thickness prior tototal failure. Methods of detecting this occurrence, should it beof interest, are discussed briefly in Ref. (1).6. Test Specimen6.1 Configuration and NotationThe SCT test specimenand the notation used herein are shown in Fig. 1. Grip detailshave been omitte
33、d, since grip design may depend on specimensize (5.3.1) and material toughness. In general, the onlyFIG. 1 Typical Surface-Crack Specimen (Grip Details Omitted)and NomenclatureE740032gripping requirements are that the arrangement be strongenough to carry the maximum expected force and that it allowu
34、niform distribution of force over the specimen cross section.6.2 DimensionsThe crack depth and length and specimenthickness should be chosen according to the ultimate purposeof the test. Further discussion of this subject may be found inAppendix X3. The specimen width W should be at least 5 timesthe
35、 crack length 2c and the specimen test section length Lshould be at least twice the width W. Should these width andlength dimensions exceed actual service dimensions, the ser-vice dimensions should be used but one should not thenattempt to generalize data from such tests.6.3 Fatigue PrecrackingThe o
36、bject is to produce at aprescribed location a fatigue crack whose configuration isregular (that is, a half-ellipse or a segment of a circle), whosedepth and length are close to predetermined target values, andwhose subsequent fracture behavior will not be influenced byany detail of the preparation p
37、rocess. A small slit or crackstarter is machined into the specimen surface at the center ofthe test section (Fig. 2) to locate and help initiate the fatiguecrack. Regularity of crack configuration is influenced primarilyby fatigue force uniformity, which can be maximized bycareful alignment of force
38、 train and fixtures. Material inhomo-geneity, residual stresses, and starter notch root radius variationcan produce irregularities which may be beyond control.Fatigue crack size and shape control are discussed in AnnexA1.6.3.1 Crack starters have been produced by a variety ofmethods. The following p
39、rocedures are known to produceacceptable results.6.3.1.1 The crack starter should be machined, either byslitting with a thin jewelers circular saw or similar cutter or byelectrical discharge machining (EDM) with a thin, shapedelectrode.6.3.1.2 The crack starter plane should be perpendicular tothe sp
40、ecimen face and the tensile force vector within 10.6.3.1.3 The starter notch root radius should be less than0.010 in. (0.25 mm).6.3.1.4 The crack starter length and depth should be chosenwith the desired crack dimensions and the requirements of6.3.2.2 in mind.6.3.2 The following procedures should en
41、sure the produc-tion of an effective sharp fatigue crack.6.3.2.1 Fatigue crack with the specimen in the heat treat-ment condition in which it is to be tested, if at all possible.6.3.2.2 Whenever it is physically possible, the crack shouldbe extended at least 0.05 in. (1.3 mm); in any event the fatig
42、uecrack extension must not be less than 5 % of the final crackdepth, and the crack and its starter must lie entirely within animaginary 30 wedge whose apex is at the crack tip. Thesetwo-dimensional descriptions shall apply around the entirecrack front, that is, in all planes normal to tangents to al
43、l pointson the crack periphery (Fig. 2).6.3.2.3 The ratio of minimum to maximum cyclic stress, R,should not be greater than 0.1.6.3.2.4 For at least the final 2.5 % of the total crack depth,the ratio Kmax/E should not exceed 0.002 in.1/2(0.00032 m1/2),where Kmaxis the maximum stress intensity factor
44、 duringfatigue cracking and E is the materials elastic modulus. Anestimate of Kmaxcan be computed based on the cyclic stressand the target crack dimensions using the appropriate equationfrom Annex A2. Compute Kmaxat the surface or at the deepestpoint, whichever is greater.7. Procedure7.1 Number of T
45、estsIf only one crack geometry (that is,fixed crack depth and length) is to be studied, at least threespecimens should be tested. If geometry is to be varied, at leasttwo specimens should be tested for each combination ofdepth-to-length (a/2c) and depth-to-thickness (a/B) ratios.7.2 Specimen Measure
46、mentsMeasure the specimen thick-ness B at the points midway between each crack tip and thenearest specimen edge, to the nearest 0.001 in. (0.025 mm) or0.1 %, whichever is larger. If these measurements are notwithin 3 % of their average, the specimen should be discardedNOTE 1Section A-A refers to the
47、 plane normal to any tangent to the crack periphery and containing the point of tangency.FIG. 2 Fatigue Crack and Starter DetailsE740033or remachined as appropriate. Measure the specimen width Wat the crack plane to within 1 % of W.7.3 TestingConduct the test in a manner similar to that foran ordina
48、ry tension specimen. The test loading rate shall besuch that the rate of increase of the nominal stress P/BW is lessthan 100 000 psi (690 MPa)/min. Record the maximum force,Pmax, reached during the test.7.4 Test RecordIf CMOD is measured, a test recordshould be made consisting of an autographic plot
49、 or digitalrecord of the output of a force-sensing transducer versus thedisplacement gage output.7.5 Crack MeasurementsAfter fracture, measure thecrack depth a and the crack length 2c to the nearest 0.001 in.(0.025 mm) or 0.1 %, whichever is larger. A low-power (20 to50 3 ) traveling microscope is usually satisfactory. Observethe crack shape; it should closely approximate a semiellipse ora segment of a circle. If the crack shape is irregular orunsymmetric the test should be discarded. Using the actualcrack dimensions, verify that the requirement 6
