ASTM E740/E740M-03(2010)e1
(Practice)Standard Practice for Fracture Testing with Surface-Crack Tension Specimens
Standard Practice for Fracture Testing with Surface-Crack Tension Specimens
SIGNIFICANCE AND USE
The surface-crack tension (SCT) test is used to estimate the load-carrying capacity of simple sheet- or plate-like structural components having a type of flaw likely to occur in service. The test is also used for research purposes to investigate failure mechanisms of cracks under service conditions.
The residual strength of an SCT specimen is a function of the crack depth and length and the specimen thickness as well as the characteristics of the material. This relationship is extremely complex and cannot be completely described or characterized at present.
The results of the SCT test are suitable for direct application to design only when the service conditions exactly parallel the test conditions. Some methods for further analysis are suggested in Appendix X1.
In order that SCT test data can be comparable and reproducible and can be correlated among laboratories, it is essential that uniform SCT testing practices be established.
The specimen configuration, preparation, and instrumentation described in this practice are generally suitable for cyclic- or sustained-force testing as well. However, certain constraints are peculiar to each of these tests. These are beyond the scope of this practice but are discussed in Ref. (1).
SCOPE
1.1 This practice covers the design, preparation, and testing of surface-crack tension (SCT) specimens. It relates specifically to testing under continuously increasing force and excludes cyclic and sustained loadings. The quantity determined is the residual strength of a specimen having a semielliptical or circular-segment fatigue crack in one surface. This value depends on the crack dimensions and the specimen thickness as well as the characteristics of the material.
1.2 Metallic materials that can be tested are not limited by strength, thickness, or toughness. However, tests of thick specimens of tough materials may require a tension test machine of extremely high capacity. The applicability of this practice to nonmetallic materials has not been determined.
1.3 This practice is limited to specimens having a uniform rectangular cross section in the test section. The test section width and length must be large with respect to the crack length. Crack depth and length should be chosen to suit the ultimate purpose of the test.
1.4 Residual strength may depend strongly upon temperature within a certain range depending upon the characteristics of the material. This practice is suitable for tests at any appropriate temperature.
1.5 Residual strength is believed to be relatively insensitive to loading rate within the range normally used in conventional tension tests. When very low or very high rates of loading are expected in service, the effect of loading rate should be investigated using special procedures that are beyond the scope of this practice.
Note 1—Further information on background and need for this type of test is given in the report of ASTM Task Group E24.01.05 on Part-Through-Crack Testing (1).
1.6 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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´1
Designation: E740/E740M − 03(Reapproved 2010)
Standard Practice for
Fracture Testing with Surface-Crack Tension Specimens
This standard is issued under the fixed designation E740/E740M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—The units statement (1.6) and the designation were editorially revised in January 2011.
1. Scope 1.6 The values stated in either SI units or inch-pound units
are to be regarded separately as standard. The values stated in
1.1 This practice covers the design, preparation, and testing
each system may not be exact equivalents; therefore, each
of surface-crack tension (SCT) specimens. It relates specifi-
system shall be used independently of the other. Combining
cally to testing under continuously increasing force and ex-
values from the two systems may result in non-conformance
cludes cyclic and sustained loadings. The quantity determined
with the standard.
istheresidualstrengthofaspecimenhavingasemiellipticalor
circular-segment fatigue crack in one surface. This value 1.7 This standard does not purport to address all of the
dependsonthecrackdimensionsandthespecimenthicknessas safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
well as the characteristics of the material.
priate safety and health practices and determine the applica-
1.2 Metallic materials that can be tested are not limited by
bility of regulatory limitations prior to use.
strength, thickness, or toughness. However, tests of thick
specimens of tough materials may require a tension test
2. Referenced Documents
machine of extremely high capacity. The applicability of this
practice to nonmetallic materials has not been determined.
2.1 ASTM Standards:
E4Practices for Force Verification of Testing Machines
1.3 This practice is limited to specimens having a uniform
E8Test Methods for Tension Testing of Metallic Materials
rectangular cross section in the test section. The test section
E338Test Method of Sharp-Notch Tension Testing of High-
widthandlengthmustbelargewithrespecttothecracklength.
Strength Sheet Materials (Withdrawn 2010)
Crack depth and length should be chosen to suit the ultimate
E399Test Method for Linear-Elastic Plane-Strain Fracture
purpose of the test.
Toughness K of Metallic Materials
Ic
1.4 Residual strength may depend strongly upon tempera-
E466Practice for Conducting Force Controlled Constant
ture within a certain range depending upon the characteristics
Amplitude Axial Fatigue Tests of Metallic Materials
of the material. This practice is suitable for tests at any
E561Test Method forK-R Curve Determination
appropriate temperature.
E1823TerminologyRelatingtoFatigueandFractureTesting
1.5 Residual strength is believed to be relatively insensitive
to loading rate within the range normally used in conventional
3. Terminology
tension tests. When very low or very high rates of loading are
3.1 Definitions:
expected in service, the effect of loading rate should be
3.1.1 Definitions given in Terminology E1823 are appli-
investigatedusingspecialproceduresthatarebeyondthescope
cable to this practice.
of this practice.
3.1.2 crack mouth opening displacement (CMOD), 2v
m
NOTE 1—Further information on background and need for this type of
(L)—the Mode 1 (also called opening mode) component of
test is given in the report of ASTM Task Group E24.01.05 on Part-
crack displacement due to elastic and plastic deformation,
Through-Crack Testing (1).
measured at the location on the crack surface that has the
greatest elastic displacement per unit load.
ThispracticeisunderthejurisdictionofASTMCommitteeE08onFatigueand
Fracture and is the direct responsibility of Subcommittee E08.07 on Fracture
Mechanics. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Oct. 1, 2010. Published January 2011. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1988. Last previous edition approved in 2003 as E740–03. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0740-03R10E01. the ASTM website.
2 4
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof The last approved version of this historical standard is referenced on
this standard. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
E740/E740M − 03 (2010)
NOTE 2—In surface-crack tension (SCT) specimens, CMOD is mea-
5.2 FatiguePrecrackingApparatus—Axialtensionorthree-
sured on the specimen surface along the normal bisector of the crack
point, four-point, or cantilever bending are all acceptable
length.
modes for fatigue precracking. Fixture design is not critical as
3.1.3 fracture toughness—a generic term for measures of
long as the crack growth is symmetrical and the plane of the
resistance to extension of a crack. E616
crack remains perpendicular to the specimen face and the
3.1.4 original crack size, a [L]—the physical crack size at tensileforcevector.Theeffectofcyclicfrequencyisthoughtto
o
be negligible below 100 Hz in a nonaggressive environment.
the start of testing. (E616)
3.2 Definitions of Terms Specific to This Standard:
NOTE 4—Certain crack shapes are more readily produced in axial
tension, others in bending (see Annex A1).
3.2.1 crack depth, a [L]—in surface-crack tension (SCT)
specimens, the normal distance from the cracked plate surface
5.2.1 Devices and fixtures for cantilever bending of sheet
tothepointofmaximumpenetrationofthecrackfrontintothe
and plate specimens are described in Refs. (2) and (3),
material. Crack depth is a fraction of the specimen thickness.
respectively. Others may be equally suitable. The axial fatigue
3.2.1.1 Discussion—In this practice, crack depth is the
machines described in Practice E466 are suitable for precrack-
original depth a and the subscript o is everywhere implied.
ing in tension; however, since the precracking operation is
o
terminated prior to specimen failure, one should ensure that
3.2.2 crack length, 2c [L]—in surface-crack tension speci-
load variations during slowdown or shutdown do not exceed
mens, a distance measured on the specimen surface between
those desired.
the two points at which the crack front intersects the specimen
5.2.2 A magnifier of about 20 power should be used to
surface. Crack length is a fraction of specimen width.
monitor the fatigue precracking process. Ease of observation
3.2.2.1 Discussion—In this practice, crack length is the
willbeenhancedifthecyclicratecanbereducedtoabout1Hz
original length 2c and the subscript o is everywhere implied.
o
when desired.Alternatively, a stroboscopic light synchronized
−2
3.2.3 residual strength, σ (FL )—the maximum value of
r
with the maximum application of tensile force may serve as
the nominal stress, neglecting the area of the crack, that a
well.
cracked specimen is capable of sustaining.
5.3 Testing Machine—The test should be conducted with a
NOTE3—Insurface-cracktension(SCT)specimens,residualstrengthis
tension testing machine that conforms to the requirements of
the ratio of the maximum load (P ) to the product of test section width
max
Practices E4.
(W) times thickness (B), P /(BW). It represents the stress at fracture
max
5.3.1 The devices for transmitting force to the specimen
normal to and remote from the plane of the crack.
shallbesuchthatthemajoraxisofthespecimencoincideswith
4. Significance and Use theloadaxis.Thepin-and-clevisarrangementdescribedinTest
Method E338 should be suitable for specimens whose width is
4.1 The surface-crack tension (SCT) test is used to estimate
less than about 4 in. [100 mm]. An arrangement such as that
the load-carrying capacity of simple sheet- or plate-like struc-
shown in Fig. 2 of Practice E561 should be suitable for wider
tural components having a type of flaw likely to occur in
specimens.
service. The test is also used for research purposes to investi-
5.3.2 For tests at other than room temperature, the tempera-
gate failure mechanisms of cracks under service conditions.
turecontrolandtemperaturemeasurementrequirementsofTest
4.2 The residual strength of an SCT specimen is a function
Method E338 are appropriate.
of the crack depth and length and the specimen thickness as
5.4 Displacement Gage (Optional)—If used to measure
well as the characteristics of the material. This relationship is
CMOD, the displacement gage output should accurately indi-
extremely complex and cannot be completely described or
cate the relative displacement of two gage points on the
characterized at present.
cracked surface, spanning the crack at the midpoint of its
4.2.1 The results of the SCT test are suitable for direct
length. Further information on displacement gages appears in
application to design only when the service conditions exactly
Appendix X2.
parallel the test conditions. Some methods for further analysis
5.5 For some combinations of material and crack geometry,
are suggested in Appendix X1.
thecrackmaypropagateentirelythroughthethicknesspriorto
4.3 In order that SCT test data can be comparable and
totalfailure.Methodsofdetectingthisoccurrence,shoulditbe
reproducible and can be correlated among laboratories, it is
of interest, are discussed briefly in Ref. (1).
essential that uniform SCT testing practices be established.
6. Test Specimen
4.4 The specimen configuration, preparation, and instru-
mentation described in this practice are generally suitable for 6.1 Configuration and Notation—The SCT test specimen
cyclic- or sustained-force testing as well. However, certain
and the notation used herein are shown in Fig. 1. Grip details
constraintsarepeculiartoeachofthesetests.Thesearebeyond
have been omitted, since grip design may depend on specimen
the scope of this practice but are discussed in Ref. (1).
size (5.3.1) and material toughness. In general, the only
gripping requirements are that the arrangement be strong
5. Apparatus
enough to carry the maximum expected force and that it allow
uniform distribution of force over the specimen cross section.
5.1 The procedure involves testing of specimens that have
been precracked in fatigue. force versus CMOD, if CMOD is 6.2 Dimensions—The crack depth and length and specimen
measured, is recorded autographically or digitally. thickness should be chosen according to the ultimate purpose
´1
E740/E740M − 03 (2010)
whose subsequent fracture behavior will not be influenced by
any detail of the preparation process. A small slit or crack
starter is machined into the specimen surface at the center of
the test section (Fig. 2) to locate and help initiate the fatigue
crack.Regularityofcrackconfigurationisinfluencedprimarily
by fatigue force uniformity, which can be maximized by
careful alignment of force train and fixtures. Material inhomo-
geneity,residualstresses,andstarternotchrootradiusvariation
can produce irregularities which may be beyond control.
Fatigue crack size and shape control are discussed in Annex
A1.
6.3.1 Crack starters have been produced by a variety of
methods. The following procedures are known to produce
acceptable results.
6.3.1.1 The crack starter should be machined, either by
slittingwithathinjeweler’scircularsaworsimilarcutterorby
electrical discharge machining (EDM) with a thin, shaped
electrode.
6.3.1.2 The crack starter plane should be perpendicular to
the specimen face and the tensile force vector within 10°.
6.3.1.3 The starter notch root radius should be less than
0.010 in. [0.25 mm].
6.3.1.4 The crack starter length and depth should be chosen
FIG. 1 Typical Surface-Crack Specimen (Grip Details Omitted)
with the desired crack dimensions and the requirements of
and Nomenclature
6.3.2.2 in mind.
6.3.2 The following procedures should ensure the produc-
of the test. Further discussion of this subject may be found in
tion of an effective sharp fatigue crack.
AppendixX3.ThespecimenwidthWshouldbeatleast5times
6.3.2.1 Fatigue crack with the specimen in the heat treat-
the crack length 2c and the specimen test section length L
ment condition in which it is to be tested, if at all possible.
should be at least twice the width W. Should these width and
6.3.2.2 Whenever it is physically possible, the crack should
length dimensions exceed actual service dimensions, the ser-
be extended at least 0.05 in. [1.3 mm]; in any event the fatigue
vice dimensions should be used but one should not then
crack extension must not be less than 5% of the final crack
attempt to generalize data from such tests.
depth, and the crack and its starter must lie entirely within an
6.3 Fatigue Precracking—The object is to produce at a imaginary 30° wedge whose apex is at the crack tip. These
prescribed location a fatigue crack whose configuration is two-dimensional descriptions shall apply around the entire
regular (that is, a half-ellipse or a segment of a circle), whose crackfront,thatis,inallplanesnormaltotangentstoallpoints
depth and length are close to predetermined target values, and on the crack periphery (Fig. 2).
NOTE 1—Section A-A refers to the plane normal to any tangent to the crack periphery and containing the point of tangency.
FIG. 2 Fatigue Crack and Starter Details
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E740/E740M − 03 (2010)
6.3.2.3 The ratio of minimum to maximum cyclic stress, R, shape;itshouldcloselyapproximateasemiellipseorasegment
should not be greater than 0.1. of a circle. If the crack shape is irregular or unsymmetric the
6.3.2.4 For at least the final 2.5% of the total crack depth, test should be discarded. Using the actual crack dimensions,
1/2 1/2
theratioK /Eshouldnotexceed0.002in. [0.00032m ], verify that the requirement 6.3.2.4 was indeed met.
max
where K is the maximum stress intensity factor during
max
7.6 Residual Strength—Calculate the residual strength asσ
r
fatigue cracking and E is the material’s elastic modulus. An
=P /(BW).
max
estimate of K can be computed based on the cyclic stress
max
8. Report
and the target crack dimensions using the appropriate equation
from AnnexA2. Compute K at the surface or at the deepest
max 8.1 The report should include the following for each speci-
point, whichever is greater.
men tested:
8.1.1 Test section width, W, and thickness, B.
7. Procedure
8.1.2 Maximum stress intensity factor during fatigue pre-
cracking, K , based on actual crack dimensions.
7.1 Number of Tests—If only one crack geometry (that
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation:E740–03 Designation:E740/E740M–03 (Reapproved 2010)
Standard Practice for
Fracture Testing with Surface-Crack Tension Specimens
This standard is issued under the fixed designation E740/E740M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—The units statement (1.6) and the designation were editorially revised in January 2011.
1. Scope
1.1 This practice covers the design, preparation, and testing of surface-crack tension (SCT) specimens. It relates specifically to
testing under continuously increasing force and excludes cyclic and sustained loadings. The quantity determined is the residual
strength of a specimen having a semielliptical or circular-segment fatigue crack in one surface. This value depends on the crack
dimensions and the specimen thickness as well as the characteristics of the material.
1.2 Metallic materials that can be tested are not limited by strength, thickness, or toughness. However, tests of thick specimens
of tough materials may require a tension test machine of extremely high capacity.The applicability of this practice to nonmetallic
materials has not been determined.
1.3 This practice is limited to specimens having a uniform rectangular cross section in the test section. The test section width
and length must be large with respect to the crack length. Crack depth and length should be chosen to suit the ultimate purpose
of the test.
1.4 Residual strength may depend strongly upon temperature within a certain range depending upon the characteristics of the
material. This practice is suitable for tests at any appropriate temperature.
1.5 Residual strength is believed to be relatively insensitive to loading rate within the range normally used in conventional
tensiontests.Whenveryloworveryhighratesofloadingareexpectedinservice,theeffectofloadingrateshouldbeinvestigated
using special procedures that are beyond the scope of this practice.
NOTE 1—Further information on background and need for this type of test is given in the report of ASTM Task Group E24.01.05 on
Part-Through-Crack Testing (1).
1.6
1.6 The values stated in either SI units or inch-pound units are to be regarded separately as standard.The values stated in each
system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the
two systems may result in non-conformance with the standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E4 Practices for Force Verification of Testing Machines
E8 Test Methods for Tension Testing of Metallic Materials
E338 Test Method of Sharp-Notch Tension Testing of High-Strength Sheet Materials
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness K of Metallic Materials
Ic
E466 Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials
E561 Test Method for K-R Curve Determination
E1823 Terminology Relating to Fatigue and Fracture Testing
This practice is under the jurisdiction of ASTM Committee E08 on Fatigue and Fracture and is the direct responsibility of Subcommittee E08.07 on Linear–Elastic
Fracture.
´1
Current edition approved Nov. 1, 2003. Published December 2003. Originally approved in 1988. Last previous edition approved in 1995 as E740–88 (1995) . DOI:
10.1520/E0740-03.on Fracture Mechanics.
Current edition approved Oct. 1, 2010. Published January 2011. Originally approved in 1988. Last previous edition approved in 2003 as E740–03. DOI:
10.1520/E0740-03R10E01.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
´1
E740/E740M–03 (2010)
3. Terminology
3.1 Definitions—Definitions given in Terminology E1823 are applicable to this practice.
3.1.1 crack mouth opening displacement (CMOD), 2v (L)—the Mode 1 (also called opening mode) component of crack
m
displacement due to elastic and plastic deformation, measured at the location on the crack surface that has the greatest elastic
displacement per unit load.
NOTE 2—In surface-crack tension (SCT) specimens, CMOD is measured on the specimen surface along the normal bisector of the crack length.
3.1.2 fracture toughness—a generic term for measures of resistance to extension of a crack. E616
3.1.3 original crack size, a [L] —the physical crack size at the start of testing. (E616)
o
3.2 Definitions of Terms Specific to This Standard:
3.2.1 crack depth, a [L]—in surface-crack tension (SCT) specimens, the normal distance from the cracked plate surface to the
point of maximum penetration of the crack front into the material. Crack depth is a fraction of the specimen thickness.
3.2.1.1 Discussion—In this practice, crack depth is the original depth a and the subscript o is everywhere implied.
o
3.2.2 crack length, 2c [L]—in surface-crack tension specimens, a distance measured on the specimen surface between the two
points at which the crack front intersects the specimen surface. Crack length is a fraction of specimen width.
3.2.2.1 Discussion—In this practice, crack length is the original length 2c and the subscript o is everywhere implied.
o
−2
3.2.3 residual strength, s (FL )—the maximum value of the nominal stress, neglecting the area of the crack, that a cracked
r
specimen is capable of sustaining.
NOTE 3—In surface-crack tension (SCT) specimens, residual strength is the ratio of the maximum load (P ) to the product of test section width (W)
max
times thickness (B), P /(BW). It represents the stress at fracture normal to and remote from the plane of the crack.
max
4. Significance and Use
4.1 The surface-crack tension (SCT) test is used to estimate the load-carrying capacity of simple sheet- or plate-like structural
components having a type of flaw likely to occur in service. The test is also used for research purposes to investigate failure
mechanisms of cracks under service conditions.
4.2 The residual strength of an SCT specimen is a function of the crack depth and length and the specimen thickness as well
as the characteristics of the material. This relationship is extremely complex and cannot be completely described or characterized
at present.
4.2.1 The results of the SCT test are suitable for direct application to design only when the service conditions exactly parallel
the test conditions. Some methods for further analysis are suggested in Appendix X1.
4.3 InorderthatSCTtestdatacanbecomparableandreproducibleandcanbecorrelatedamonglaboratories,itisessentialthat
uniform SCT testing practices be established.
4.4 The specimen configuration, preparation, and instrumentation described in this practice are generally suitable for cyclic- or
sustained-force testing as well. However, certain constraints are peculiar to each of these tests.These are beyond the scope of this
practice but are discussed in Ref. (1).
5. Apparatus
5.1 The procedure involves testing of specimens that have been precracked in fatigue. force versus CMOD, if CMOD is
measured, is recorded autographically or digitally.
5.2 Fatigue PrecrackingApparatus—Axialtensionorthree-point,four-point,orcantileverbendingareallacceptablemodesfor
fatigue precracking. Fixture design is not critical as long as the crack growth is symmetrical and the plane of the crack remains
perpendicular to the specimen face and the tensile force vector. The effect of cyclic frequency is thought to be negligible below
100 Hz in a nonaggressive environment.
NOTE 4—Certain crack shapes are more readily produced in axial tension, others in bending (see Annex A1).
5.2.1 Devices and fixtures for cantilever bending of sheet and plate specimens are described in Refs. (2) and (3), respectively.
Others may be equally suitable. The axial fatigue machines described in Practice E466 are suitable for precracking in tension;
however, since the precracking operation is terminated prior to specimen failure, one should ensure that load variations during
slowdown or shutdown do not exceed those desired.
5.2.2 A magnifier of about 20 power should be used to monitor the fatigue precracking process. Ease of observation will be
enhanced if the cyclic rate can be reduced to about 1 Hz when desired.Alternatively, a stroboscopic light synchronized with the
maximum application of tensile force may serve as well.
5.3 Testing Machine—The test should be conducted with a tension testing machine that conforms to the requirements of
Practices E4.
5.3.1 The devices for transmitting force to the specimen shall be such that the major axis of the specimen coincides with the
load axis. The pin-and-clevis arrangement described in Test Method E338 should be suitable for specimens whose width is less
than about 4 in. (100 mm).[100 mm].An arrangement such as that shown in Fig. 2 of Practice E561 should be suitable for wider
specimens.
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E740/E740M–03 (2010)
5.3.2 For tests at other than room temperature, the temperature control and temperature measurement requirements of Test
Method E338 are appropriate.
5.4 Displacement Gage (Optional)—If used to measure CMOD, the displacement gage output should accurately indicate the
relative displacement of two gage points on the cracked surface, spanning the crack at the midpoint of its length. Further
information on displacement gages appears in Appendix X2.
5.5 Forsomecombinationsofmaterialandcrackgeometry,thecrackmaypropagateentirelythroughthethicknesspriortototal
failure. Methods of detecting this occurrence, should it be of interest, are discussed briefly in Ref. (1).
6. Test Specimen
6.1 Configuration and Notation—The SCT test specimen and the notation used herein are shown in Fig. 1. Grip details have
been omitted, since grip design may depend on specimen size (5.3.1) and material toughness. In general, the only gripping
requirementsarethatthearrangementbestrongenoughtocarrythemaximumexpectedforceandthatitallowuniformdistribution
of force over the specimen cross section.
6.2 Dimensions—Thecrackdepthandlengthandspecimenthicknessshouldbechosenaccordingtotheultimatepurposeofthe
test. Further discussion of this subject may be found inAppendix X3. The specimen width W should be at least 5 times the crack
length 2c and the specimen test section length L should be at least twice the width W. Should these width and length dimensions
exceed actual service dimensions, the service dimensions should be used but one should not then attempt to generalize data from
such tests.
6.3 Fatigue Precracking—The object is to produce at a prescribed location a fatigue crack whose configuration is regular (that
is, a half-ellipse or a segment of a circle), whose depth and length are close to predetermined target values, and whose subsequent
fracture behavior will not be influenced by any detail of the preparation process.Asmall slit or crack starter is machined into the
specimen surface at the center of the test section (Fig. 2) to locate and help initiate the fatigue crack. Regularity of crack
configuration is influenced primarily by fatigue force uniformity, which can be maximized by careful alignment of force train and
fixtures. Material inhomogeneity, residual stresses, and starter notch root radius variation can produce irregularities which may be
beyond control. Fatigue crack size and shape control are discussed in Annex A1.
6.3.1 Crack starters have been produced by a variety of methods. The following procedures are known to produce acceptable
results.
6.3.1.1 Thecrackstartershouldbemachined,eitherbyslittingwithathinjeweler’scircularsaworsimilarcutterorbyelectrical
discharge machining (EDM) with a thin, shaped electrode.
6.3.1.2 The crack starter plane should be perpendicular to the specimen face and the tensile force vector within 10°.
6.3.1.3 The starter notch root radius should be less than 0.010 in. (0.25 mm). [0.25 mm].
6.3.1.4 The crack starter length and depth should be chosen with the desired crack dimensions and the requirements of 6.3.2.2
in mind.
FIG. 1 Typical Surface-Crack Specimen (Grip Details Omitted)
and Nomenclature
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E740/E740M–03 (2010)
NOTE 1—Section A-A refers to the plane normal to any tangent to the crack periphery and containing the point of tangency.
FIG. 2 Fatigue Crack and Starter Details
6.3.2 The following procedures should ensure the production of an effective sharp fatigue crack.
6.3.2.1 Fatigue crack with the specimen in the heat treatment condition in which it is to be tested, if at all possible.
6.3.2.2 Whenever it is physically possible, the crack should be extended at least 0.05 in. (1.3 mm);[1.3 mm]; in any event the
fatigue crack extension must not be less than 5% of the final crack depth, and the crack and its starter must lie entirely within an
imaginary 30° wedge whose apex is at the crack tip.These two-dimensional descriptions shall apply around the entire crack front,
that is, in all planes normal to tangents to all points on 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.
1/2 [0.00032
6.3.2.4 For at least the final 2.5% of the total crack depth, the ratio K /E should not exceed 0.002 in. (0.00032 m
max
1/2
m ), where ], where K is the maximum stress intensity factor during fatigue cracking and E is the material’s elastic modulus.
max
An estimate of K can be computed based on the cyclic stress and the target crack dimensions using the appropriate equation
max
from Annex A2. Compute K at the surface or at the deepest point, whichever is greater.
max
7. Procedure
7.1 Number of Tests—Ifonlyonecrackgeometry(thatis,fixedcrackdepthandlength)istobestudied,atleastthreespecimens
sho
...
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