ASTM E740-88(1995)e1
(Practice)Standard Practice for Fracture Testing with Surface-Crack Tension Specimens
Standard Practice for Fracture Testing with Surface-Crack Tension Specimens
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 load 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 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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e1
Designation: E 740 – 88 (Reapproved 1995)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Practice for
Fracture Testing with Surface-Crack Tension Specimens
This standard is issued under the fixed designation E 740; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorial corrections were made throughout in January 1996.
1. Scope 2. Referenced Documents
1.1 This practice covers the design, preparation, and testing 2.1 ASTM Standards:
of surface-crack tension (SCT) specimens. It relates specifi- E 4 Practices for Force Verification of Testing Machines
cally to testing under continuously increasing load and ex- E 8 Test Methods for Tension Testing of Metallic Materials
cludes cyclic and sustained loadings. The quantity determined E 338 Test Method for Sharp-Notch Tension Testing of
is the residual strength of a specimen having a semielliptical or High-Strength Sheet Materials
circular-segment fatigue crack in one surface. This value E 399 Test Method for Plane-Strain Fracture Toughness of
depends on the crack dimensions and the specimen thickness as Metallic Materials
well as the characteristics of the material. E 466 Practice for Conducting Force Controlled Constant
1.2 Metallic materials that can be tested are not limited by Amplitude Axial Fatigue Tests of Metallic Materials
strength, thickness, or toughness. However, tests of thick E 561 Practice for R-Curve Determination
specimens of tough materials may require a tension test E 616 Terminology Relating to Fracture Testing
machine of extremely high capacity. The applicability of this
3. Terminology
practice to nonmetallic materials has not been determined.
3.1 Definitions—Definitions given in Terminology E 616
1.3 This practice is limited to specimens having a uniform
rectangular cross section in the test section. The test section are applicable to this practice.
3.1.1 crack mouth opening displacement (CMOD), 2v
width and length must be large with respect to the crack length.
m
Crack depth and length should be chosen to suit the ultimate (L)—the Mode 1 (also called opening mode) component of
crack displacement due to elastic and plastic deformation,
purpose of the test.
1.4 Residual strength may depend strongly upon tempera- measured at the location on the crack surface that has the
greatest elastic displacement per unit load.
ture within a certain range depending upon the characteristics
of the material. This practice is suitable for tests at any
NOTE 2—In surface-crack tension (SCT) specimens, CMOD is mea-
appropriate temperature.
sured on the specimen surface along the normal bisector of the crack
1.5 Residual strength is believed to be relatively insensitive
length.
to loading rate within the range normally used in conventional
3.1.2 fracture toughness—a generic term for measures of
tension tests. When very low or very high rates of loading are
resistance to extension of a crack. E 616
expected in service, the effect of loading rate should be
3.1.3 original crack size, a [L]—the physical crack size at
o
investigated using special procedures that are beyond the scope
the start of testing. (E 616)
of this practice.
3.2 Definitions of Terms Specific to This Standard:
NOTE 1—Further information on background and need for this type of 3.2.1 crack depth, a [L]—in surface-crack tension (SCT)
test is given in the report of ASTM Task Group E24.01.05 on Part-
specimens, the normal distance from the cracked plate surface
Through-Crack Testing (1).
to the point of maximum penetration of the crack front into the
material. Crack depth is a fraction of the specimen thickness.
1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 3.2.1.1 Discussion—In this practice, crack depth is the
original depth a and the subscript o is everywhere implied.
responsibility of the user of this standard to establish appro-
o
priate safety and health practices and determine the applica- 3.2.2 crack length, 2c [L]—in surface-crack tension speci-
mens, a distance measured on the specimen surface between
bility of regulatory limitations prior to use.
the two points at which the crack front intersects the specimen
surface. Crack length is a fraction of specimen width.
This practice is under the jurisdiction of ASTM Committee E-8 on Fatigue and
3.2.2.1 Discussion—In this practice, crack length is the
Fracture and is the direct responsibility of Subcommittee E08.07on Linear–Elastic
Fracture. original length 2c and the subscript o is everywhere implied.
o
Current edition approved March 28, 1988. Published May 1988.
The boldface numbers in parentheses refer to the list of references at the end of
this standard. Annual Book of ASTM Standards, Vol 03.01.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 740
−2
3.2.3 residual strength, s (FL )—the maximum value of 5.3 Testing Machine—The test should be conducted with a
r
the nominal stress, neglecting the area of the crack, that a tension testing machine that conforms to the requirements of
cracked specimen is capable of sustaining. Practices E 4.
5.3.1 The devices for transmitting load to the specimen shall
NOTE 3—In surface-crack tension (SCT) specimens, residual strength is
be such that the major axis of the specimen coincides with the
the ratio of the maximum load (P ) to the product of test section width
max
load axis. The pin-and-clevis arrangement described in Test
(W) times thickness (B), P /(BW). It represents the stress at fracture
max
Method E 338 should be suitable for specimens whose width is
normal to and remote from the plane of the crack.
less than about 4 in. (100 mm). An arrangement such as that
4. Significance and Use
shown in Fig. 2 of Practice E 561 should be suitable for wider
4.1 The surface-crack tension (SCT) test is used to estimate
specimens.
the load-carrying capacity of simple sheet- or plate-like struc-
5.3.2 For tests at other than room temperature, the tempera-
tural components having a type of flaw likely to occur in
ture control and temperature measurement requirements of Test
service. The test is also used for research purposes to investi-
Method E 338 are appropriate.
gate failure mechanisms of cracks under service conditions.
5.4 Displacement Gage (Optional)—If used to measure
4.2 The residual strength of an SCT specimen is a function
CMOD, the displacement gage output should accurately indi-
of the crack depth and length and the specimen thickness as
cate the relative displacement of two gage points on the
well as the characteristics of the material. This relationship is
cracked surface, spanning the crack at the midpoint of its
extremely complex and cannot be completely described or
length. Further information on displacement gages appears in
characterized at present.
Appendix X2.
4.2.1 The results of the SCT test are suitable for direct
5.5 For some combinations of material and crack geometry,
application to design only when the service conditions exactly
the crack may propagate entirely through the thickness prior to
parallel the test conditions. Some methods for further analysis
total failure. Methods of detecting this occurrence, should it be
are suggested in Appendix X1.
of interest, are discussed briefly in Ref. (1).
4.3 In order that SCT test data can be comparable and
reproducible and can be correlated among laboratories, it is
6. Test Specimen
essential that uniform SCT testing practices be established.
6.1 Configuration and Notation—The SCT test specimen
4.4 The specimen configuration, preparation, and instru-
and the notation used herein are shown in Fig. 1. Grip details
mentation described in this practice are generally suitable for
have been omitted, since grip design may depend on specimen
cyclic- or sustained-load testing as well. However, certain
size (5.3.1) and material toughness. In general, the only
constraints are peculiar to each of these tests. These are beyond
gripping requirements are that the arrangement be strong
the scope of this practice but are discussed in Ref. (1).
enough to carry the maximum expected load and that it allow
5. Apparatus uniform distribution of load over the specimen cross section.
5.1 The procedure involves testing of specimens that have
been precracked in fatigue. Load versus CMOD, if CMOD is
measured, is recorded autographically or digitally.
5.2 Fatigue Precracking Apparatus—Axial tension or
three-point, four-point, or cantilever bending are all acceptable
modes for 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 load 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 E 466 are suitable for precrack-
ing 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 load may serve as
FIG. 1 Typical Surface-Crack Specimen (Grip Details Omitted)
well. and Nomenclature
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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E 740
6.2 Dimensions—The crack depth and length and specimen tion of an effective sharp fatigue crack.
thickness should be chosen according to the ultimate purpose 6.3.2.1 Fatigue crack with the specimen in the heat treat-
of the test. Further discussion of this subject may be found in ment condition in which it is to be tested, if at all possible.
Appendix X3. The specimen width W should be at least 5 times 6.3.2.2 Whenever it is physically possible, the crack should
the crack length 2c and the specimen test section length L be extended at least 0.05 in. (1.3 mm); in any event the fatigue
should be at least twice the width W. Should these width and crack extension must not be less than 5 % of the final crack
length dimensions exceed actual service dimensions, the ser- depth, and the crack and its starter must lie entirely within an
vice dimensions should be used but one should not then imaginary 30° wedge whose apex is at the crack tip. These
attempt to generalize data from such tests. two-dimensional descriptions shall apply around the entire
6.3 Fatigue Precracking—The object is to produce at a crack front, that is, in all planes normal to tangents to all points
prescribed location a fatigue crack whose configuration is on the crack periphery (Fig. 2).
regular (that is, a half-ellipse or a segment of a circle), whose 6.3.2.3 The ratio of minimum to maximum cyclic stress, R,
depth and length are close to predetermined target values, and should not be greater than 0.1.
whose subsequent fracture behavior will not be influenced by 6.3.2.4 For at least the final 2.5 % of the total crack depth,
1/2 1/2
any detail of the preparation process. A small slit or crack the ratio K /E should not exceed 0.002 in. (0.00032 m ),
max
starter is machined into the specimen surface at the center of where K is the maximum stress intensity factor during
max
the test section (Fig. 2) to locate and help initiate the fatigue fatigue cracking and E is the material’s elastic modulus. An
crack. Regularity of crack configuration is influenced primarily estimate of K can be computed based on the cyclic stress
max
by fatigue load uniformity, which can be maximized by careful and the target crack dimensions using the appropriate equation
alignment of load train and fixtures. Material inhomogeneity, from Annex A2. Compute K at the surface or at the deepest
max
residual stresses, and starter notch root radius variation can point, whichever is greater.
produce irregularities which may be beyond control. Fatigue
7. Procedure
crack size and shape control are discussed in Annex A1.
6.3.1 Crack starters have been produced by a variety of 7.1 Number of Tests—If only one crack geometry (that is,
methods. The following procedures are known to produce fixed crack depth and length) is to be studied, at least three
acceptable results. specimens should be tested. If geometry is to be varied, at least
6.3.1.1 The crack starter should be machined, either by two specimens should be tested for each combination of
slitting with a thin jeweler’s circular saw or similar cutter or by depth-to-length (a/2c) and depth-to-thickness (a/B) ratios.
electrical discharge machining (EDM) with a thin, shaped 7.2 Specimen Measurements—Measure the specimen thick-
electrode. ness B at the points midway between each crack tip and the
6.3.1.2 The crack starter plane should be perpendicular to nearest specimen edge, to the nearest 0.001 in. (0.025 mm) or
the specimen face and the tensile load vector within 10°. 0.1 %, whichever is larger. If these measurements are not
6.3.1.3 The starter notch root radius should be less than within 3 % of their average, the specimen should be discarded
0.010 in. (0.25 mm). or remachined as appropriate. Measure the specimen width W
6.3.1.4 The crack starter length and depth should be chosen at the crack plane to within 1 % of W.
with the desired crack dimensions and the requirements of 7.3 Testing—Conduct the test in a manner similar to that for
an ordinary tension specimen. The test loading rate shall be
6.3.2.2 in mind.
6.3.2 The following procedures should ensure the produc- such that the rate of increase of the nominal stress P/BW is less
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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