ASTM E1921-23b

Standard Test Method for Determination of Reference Temperature, <emph type="bdit">T<inf >0</inf></emph>, for Ferritic Steels in the Transition Range

SIGNIFICANCE AND USE
5.1 Fracture toughness is expressed in terms of an elastic-plastic stress-intensity factor, KJc, that is derived from the J-integral calculated at fracture.  
5.2 Ferritic steels are microscopically inhomogeneous with respect to the orientation of individual grains. Also, grain boundaries have properties distinct from those of the grains. Both contain carbides or nonmetallic inclusions that can act as nucleation sites for cleavage microcracks. The random location of such nucleation sites with respect to the position of the crack front manifests itself as variability of the associated fracture toughness (13). This results in a distribution of fracture toughness values that is amenable to characterization using the statistical methods in this test method.  
5.3 The statistical methods in this test method assume that the data set represents a macroscopically homogeneous material, such that the test material has both the uniform tensile and toughness properties. The fracture toughness evaluation of nonuniform materials is not amenable to the statistical analysis procedures employed in this test method. For example, multi-pass weldments can create heat-affected and brittle zones with localized properties that are quite different from either the bulk or weld materials. Thick-section steels also often exhibit some variation in properties near the surfaces. Metallographic analysis can be used to identify possible nonuniform regions in a material. These regions can then be evaluated through mechanical testing such as hardness, microhardness, and tensile testing for comparison with the bulk material. It is also advisable to measure the toughness properties of these nonuniform regions distinctly from the bulk material. Section 10.6 provides a screening criterion to assess whether the data set may not be representative of a macroscopically homogeneous material, and therefore, may not be amenable to the statistical analysis procedures employed in this test method. If the data ...
SCOPE
1.1 This test method covers the determination of a reference temperature, T0, which characterizes the fracture toughness of ferritic steels that experience onset of cleavage cracking at elastic, or elastic-plastic KJc instabilities, or both. The specific types of ferritic steels (3.2.2) covered are those with yield strengths ranging from 275 MPa to 825 MPa (40 ksi to 120 ksi) and weld metals, after stress-relief annealing, that have 10 % or less strength mismatch relative to that of the base metal.  
1.2 The specimens covered are fatigue precracked single-edge notched bend bars, SE(B), and standard or disk-shaped compact tension specimens, C(T) or DC(T). A range of specimen sizes with proportional dimensions is recommended. The dimension on which the proportionality is based is specimen thickness.  
1.3 Median KJc values tend to vary with the specimen type at a given test temperature, presumably due to constraint differences among the allowable test specimens in 1.2. The degree of KJc variability among specimen types is analytically predicted to be a function of the material flow properties (1)2 and decreases with increasing strain hardening capacity for a given yield strength material. This KJc dependency ultimately leads to discrepancies in calculated T0 values as a function of specimen type for the same material. T0 values obtained from C(T) specimens are expected to be higher than T0 values obtained from SE(B) specimens. Best estimate comparisons of several materials indicate that the average difference between C(T) and SE(B)-derived T0 values is approximately 10°C (2). C(T) and SE(B) T0 differences up to 15 °C have also been recorded (3). However, comparisons of individual, small datasets may not necessarily reveal this average trend. Datasets which contain both C(T) and SE(B) specimens may generate T0 results which fall between the T0 values calculated using solely C(T) or SE(B) specimens. It is therefore strongl...

General Information

• Status: Published
• Publication Date: 14-Dec-2023
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: E08 - Fatigue and Fracture
• Drafting Committee: E08.07 - Fracture Mechanics

Relations

• Replaces: ASTM E1921-23a - Standard Test Method for Determination of Reference Temperature, <emph type="bdit">T<inf >0</inf></emph>, for Ferritic Steels in the Transition Range
• Effective Date: 15-Dec-2023
• Refers: ASTM E23-24 - Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
• Effective Date: 01-Apr-2024
• Refers: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Feb-2024
• Refers: ASTM E1823-24 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Feb-2024
• Refers: ASTM E8/E8M-24 - Standard Test Methods for Tension Testing of Metallic Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM E23-23a - Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
• Effective Date: 01-Mar-2023
• Refers: ASTM E8/E8M-22 - Standard Test Methods for Tension Testing of Metallic Materials
• Effective Date: 01-May-2022
• Referred By: ASTM E1820-23b - Standard Test Method for Measurement of Fracture Toughness
• Effective Date: 15-Dec-2023
• Referred By: ASTM E1823-23 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Dec-2023
• Referred By: ASTM E509/E509M-21 - Standard Guide for In-Service Annealing of Light-Water Moderated Nuclear Reactor Vessels
• Effective Date: 15-Dec-2023
• Referred By: ASTM E1253-21 - Standard Guide for Reconstitution of Charpy-Sized Specimens
• Effective Date: 15-Dec-2023
• Referred By: ASTM E2899-19e1 - Standard Test Method for Measurement of Initiation Toughness in Surface Cracks Under Tension and Bending
• Effective Date: 15-Dec-2023
• Referred By: ASTM E399-23 - Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
• Effective Date: 15-Dec-2023
• Referred By: ASTM E185-21 - Standard Practice for Design of Surveillance Programs for Light-Water Moderated Nuclear Power Reactor Vessels
• Effective Date: 15-Dec-2023
• Referred By: ASTM E636-20 - Standard Guide for Conducting Supplemental Surveillance Tests for Nuclear Power Reactor Vessels
• Effective Date: 15-Dec-2023

Overview

ASTM E1921-23b is the internationally recognized standard test method for the determination of reference temperature, T₀, for ferritic steels in the ductile-to-brittle transition range. Issued by ASTM International, this standard provides a statistical approach for characterizing the fracture toughness of ferritic steels, which include many carbon, low-alloy, and higher-alloy grades commonly used in the construction of pressure vessels, bridges, and other structures where reliable fracture resistance is vital. The test method specifies the use of fatigue-precracked specimens and emphasizes the significance of material homogeneity for accurate measurement and analysis.

Key Topics

• Fracture Toughness Characterization: The standard expresses fracture toughness using the elastic-plastic stress-intensity factor, K_Jc, derived from the J-integral at the point of fracture. This method helps quantify cleavage cracking behavior in ferritic steels.

• Reference Temperature, T₀: T₀ is defined as the temperature at which the median K_Jc for 1T-size specimens equals 100 MPa√m. This value is an essential material parameter for design and safety assessments.

• Specimen Types and Constraints: The standard covers testing using fatigue precracked single-edge notched bend bars (SE(B)) and compact tension specimens (C(T) and DC(T)), with recommendations for specimen proportions based on thickness. Variation in K_Jc and T₀ values may occur depending on specimen type due to differences in crack-tip constraint.

• Statistical Treatment: ASTM E1921 employs a three-parameter Weibull distribution and weakest-link theory to analyze the variability of fracture toughness values. Homogeneity of the test material is critical for the validity of the statistical analysis.

• Material Homogeneity and Testing Limitations: This standard is applicable primarily to macroscopically homogeneous ferritic steels. Material nonuniformities-such as those caused by multi-pass welding, heat-affected zones, or surface property variations in thick sections-require separate evaluation and may not be suitable for the statistical procedures in this method.

Applications

ASTM E1921-23b is widely used in quality assurance, material certification, engineering design, and failure analysis where reliable fracture toughness data is needed for ferritic steels in critical applications. Typical use cases include:

• Pressure Vessels and Nuclear Components: Ensuring that reactor pressure vessel steels and similar components meet the required fracture resistance at service temperatures, especially during potential transition from ductile to brittle failure.
• Structural Engineering: Supporting safe design and material selection for bridges, pipelines, and heavy machinery.
• Weld Quality Assessment: Determining the toughness of weld metals and heat-affected zones post stress-relief annealing, provided their strength mismatch with base materials is within 10%.
• Material Research and Development: Establishing master curves for transition temperature analysis and for benchmarking new ferritic steel grades.
• Life Assessment and Integrity Management: Used in predictive models for shift in fracture toughness due to service-induced degradation, such as irradiation embrittlement.

Related Standards

ASTM E1921-23b builds upon and cross-references several related ASTM and ASME standards that are crucial for comprehensive material test and analysis processes, including:

• ASTM E1820 - Measurement of Fracture Toughness
• ASTM E399 - Linear-Elastic Plane-Strain Fracture Toughness
• ASTM E23 / E208 / E436 - Impact and drop-weight testing of metallic materials
• ASTM E4 and E8/E8M - Force calibration and tension testing of metals
• ASTM E74 - Calibration of force-measuring instruments
• ASTM E111 - Modulus determination
• ASME Boiler and Pressure Vessel Code, Section II, Part D

These referenced standards support the calibration, testing, and terminology required for accurate fracture mechanics assessments in accordance with ASTM E1921.

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Keywords: ASTM E1921-23b, fracture toughness, T₀ reference temperature, ferritic steels, transition range, K_Jc, statistical analysis, Weibull distribution, mechanical testing, material homogeneity, pressure vessel steels, structural integrity, engineering standards, master curve.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1921 − 23b
Standard Test Method for
Determination of Reference Temperature, T , for Ferritic
Steels in the Transition Range
This standard is issued under the fixed designation E1921; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope recommended that the specimen type be reported along with
the derived T value in all reporting, analysis, and discussion of
1.1 This test method covers the determination of a reference
results. This recommended reporting is in addition to the
temperature, T , which characterizes the fracture toughness of
requirements in 11.1.1.
ferritic steels that experience onset of cleavage cracking at
elastic, or elastic-plastic K instabilities, or both. The specific 1.4 Requirements are set on specimen size and the number
Jc
types of ferritic steels (3.2.2) covered are those with yield
of replicate tests that are needed to establish acceptable
strengths ranging from 275 MPa to 825 MPa (40 ksi to 120 ksi) characterization of K data populations.
Jc
and weld metals, after stress-relief annealing, that have 10 % or
1.5 T is dependent on the K-rate. T is evaluated for a
0 0
less strength mismatch relative to that of the base metal.
¯
˙
quasi-static K-rate range with 0.5 < K < 2 MPa√m/s. T values
I 0
1.2 The specimens covered are fatigue precracked single-
¯
˙
for slowly loaded specimens (K < 0.5 MPa√m) can be
I
edge notched bend bars, SE(B), and standard or disk-shaped
considered valid if environmental effects are known to be
compact tension specimens, C(T) or DC(T). A range of
¯
˙
specimen sizes with proportional dimensions is recommended. negligible. Provision is also made for higher K-rates (K > 2
I
The dimension on which the proportionality is based is
MPa√m/s) in Annex A1. Note that this threshold K-rate for
specimen thickness. application of Annex A1 is a much lower threshold than is
required in other fracture toughness test methods such as E399
1.3 Median K values tend to vary with the specimen type
Jc
and E1820.
at a given test temperature, presumably due to constraint
differences among the allowable test specimens in 1.2. The
1.6 The statistical effects of specimen size on K in the
Jc
degree of K variability among specimen types is analytically
transition range are treated using the weakest-link theory (4)
Jc
predicted to be a function of the material flow properties (1)
applied to a three-parameter Weibull distribution of fracture
and decreases with increasing strain hardening capacity for a
toughness values. A limit on K values, relative to the
Jc
given yield strength material. This K dependency ultimately
specimen size, is specified to ensure high constraint conditions
Jc
leads to discrepancies in calculated T values as a function of
along the crack front at fracture. For some materials, particu-
specimen type for the same material. T values obtained from
larly those with low strain hardening, this limit may not be
C(T) specimens are expected to be higher than T values
sufficient to ensure that a single-parameter (K ) adequately
Jc
obtained from SE(B) specimens. Best estimate comparisons of
describes the crack-front deformation state (5).
several materials indicate that the average difference between
1.7 Statistical methods are employed to predict the transi-
C(T) and SE(B)-derived T values is approximately 10°C (2).
tion toughness curve and specified tolerance bounds for 1T
C(T) and SE(B) T differences up to 15 °C have also been
specimens of the material tested. The standard deviation of the
recorded (3). However, comparisons of individual, small data-
data distribution is a function of Weibull slope and median K .
Jc
sets may not necessarily reveal this average trend. Datasets
The procedure for applying this information to the establish-
which contain both C(T) and SE(B) specimens may generate
ment of transition temperature shift determinations and the
T results which fall between the T values calculated using
0 0
establishment of tolerance limits is prescribed.
solely C(T) or SE(B) specimens. It is therefore strongly
1.8 The procedures described in this test method assume
that the data set represents a macroscopically homogeneous
material, such that the test material has uniform tensile and
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue
toughness properties. Application of this test method to an
and Fracture and is the direct responsibility of E08.07 on Fracture Mechanics.
Current edition approved Dec. 15, 2023. Published March 2024. Originally
inhomogeneous material will result in an inaccurate estimate of
approved in 1997. Last previous edition approved in 2023 as E1921 – 23a. DOI:
the transition reference value T and nonconservative confi-
10.1520/E1921-23B.
dence bounds. For example, multi-pass weldments can create
The boldface numbers in parentheses refer to the list of references at the end of
this standard. heat-affected and brittle zones with localized properties that are
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1921 − 23b
quite different from either the bulk or weld materials. Thick- 3. Terminology
section steels also often exhibit some variation in properties
3.1 Terminology given in Terminology E1823 is applicable
near the surfaces. Metallography and initial screening may be
to this test method.
necessary to verify the applicability of these and similarly
3.2 Definitions:
graded materials. Section 10.6 provides a screening criterion to
-2
3.2.1 effective yield strength, σ [FL ]— an assumed value
assess whether the data set may not be representative of a Y
of uniaxial yield strength that represents the influence of plastic
macroscopically homogeneous material, and therefore, may
yielding upon fracture test parameters.
not be amenable to the statistical analysis procedures employed
3.2.1.1 Discussion—It is calculated as the average of the
in this test method. If the data set fails the screening criterion
0.2 % offset yield strength σ , and the ultimate tensile
in 10.6, the homogeneity of the material and its fracture
YS
strength, σ as follows:
toughness can be more accurately assessed using the analysis
TS
methods described in Appendix X5.
σ 1σ
YS TS
σ 5
Y
1.9 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.2.2 ferritic steels—typically carbon, low-alloy, and higher
responsibility of the user of this standard to establish appro-
alloy grades. Typical microstructures are bainite, tempered
priate safety, health, and environmental practices and deter-
bainite, tempered martensite, and ferrite and pearlite. All
mine the applicability of regulatory limitations prior to use.
ferritic steels have body centered cubic crystal structures that
1.10 This international standard was developed in accor-
display ductile-to-cleavage transition temperature fracture
dance with internationally recognized principles on standard-
toughness characteristics. See also Test Methods E23, E208
ization established in the Decision on Principles for the
and E436.
Development of International Standards, Guides and Recom-
3.2.2.1 Discussion—This definition is not intended to imply
mendations issued by the World Trade Organization Technical
that all of the many possible types of ferritic steels have been
Barriers to Trade (TBT) Committee.
verified as being amenable to analysis by this test method.
2. Referenced Documents
– 3/2
3.2.3 stress-intensity factor, K [FL ]—the magnitude of
2.1 ASTM Standards:
the mathematically ideal crack-tip stress field coefficient (stress
E4 Practices for Force Calibration and Verification of Test-
field singularity) for a particular mode of crack-tip region
ing Machines
deformation in a homogeneous body.
E8/E8M Test Methods for Tension Testing of Metallic Ma-
3.2.3.1 Discussion—In this test method, Mode I is assumed.
terials
See Terminology E1823 for further discussion.
E23 Test Methods for Notched Bar Impact Testing of Me-
–1
3.2.4 J-integral, J [FL ]—a mathematical expression; a
tallic Materials
line or surface integral that encloses the crack front from one
E74 Practices for Calibration and Verification for Force-
crack surface to the other; used to characterize the local
Measuring Instruments
stress-strain field around the crack front (6). See Terminology
E111 Test Method for Young’s Modulus, Tangent Modulus,
E1823 for further discussion.
and Chord Modulus
E177 Practice for Use of the Terms Precision and Bias in
3.3 Definitions of Terms Specific to This Standard:
¯
ASTM Test Methods -3/2 -1
˙
3.3.1 average stress-intensity-factor rate K [FL T ]—
I
E208 Test Method for Conducting Drop-Weight Test to
average rate of increase of applied stress-intensity factor up to
Determine Nil-Ductility Transition Temperature of Fer-
K .
Jc
ritic Steels
E399 Test Method for Linear-Elastic Plan-Strain Fracture
3.3.1.1 Discussion—It is evaluated as the ratio between K
Jc
Toughness K of Metallic Materials and the corresponding time to cleavage. For tests where partial
Ic
E436 Test Method for Drop-Weight Tear Tests of Ferritic unloading/reloading sequences are used to measure
Steels compliance, an equivalent time to cleavage (t ) shall be used to
c
E561 Test Method for K Curve Determination calculate the stress-intensity-factor rate (that is, K-rate). The
R
E691 Practice for Conducting an Interlaboratory Study to value of t is calculated as the ratio between the value of
c
Determine the Precision of a Test Method load-line displacement at cleavage and the load-line displace-
E1820 Test Method for Measurement of Fracture Toughness ment rate applied during the monotonic loading portions of the
E1823 Terminology Relating to Fatigue and Fracture Testing test (that is, the periods between partial unloading/reloading
2.2 ASME Standards: sequences used for compliance measurement). Additionally, in
ASME Boiler and Pressure Vessel Code, Section II, Part D such tests, the total time of unloading/loading sequences
should be shorter than the total time of monotonic loading.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.3.2 control force, P [F]—a calculated value of maximum
m
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
force, used in 7.8.1 to stipulate allowable precracking limits.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3.3.3 crack initiation—describes the onset of crack propa-
Available from American Society of Mechanical Engineers (ASME), ASME
gation from a preexisting macroscopic crack created in the
International Headquarters, Two Park Ave., New York, NY 10016-5990, http://
www.asme.org. specimen by a stipulated procedure.
E1921 − 23b
–2
3.3.4 effective modulus, E [FL ]—an elastic modulus that specimens, the net thickness, B , is the distance between the
eff N
allows a theoretical (modulus normalized) compliance to roots of the side-groove notches.
match an experimentally measured compliance for an actual
3.3.20 specimen size, nT—a code used to define specimen
initial crack size, a .
o
dimensions, where n is expressed in multiples of 1 in.
–2
3.3.5 elastic modulus, E' [FL ]—a linear-elastic factor
3.3.20.1 Discussion—In this method, specimen proportion-
relating stress to strain, the value of which is dependent on the
ality is required. For compact specimens and bend bars,
degree of constraint. For plane stress, E' = E is used, and for
specimen thickness B = n inches.
plane strain, E/(1 – v ) is used, with E being Young’s modulus
3.3.21 temperature, T [°C]—For K values that are devel-
Q Jc
and v being Poisson’s ratio.
oped using specimens or test practices, or both, that do not
–1
3.3.6 elastic plastic J [FL ]—J-integral at the onset of
c conform to the requirements of this test method, a temperature
cleavage fracture.
at which K = 100 MPa√m is defined as T . T is not a
Jc (med) Q Q
–3/2
provisional value of T .
3.3.7 elastic-plastic K [FL ]—An elastic-plastic equiva-
J
lent stress-intensity factor derived from the J-integral.
3.3.22 time to control force, t [T],—time to P .
m m
3.3.7.1 Discussion—In this test method, K also implies a
J
3.3.23 Weibull fitting parameter, K —a scale parameter
stress-intensity factor determined at the test termination point
located at the 63.2 % cumulative failure probability level (9).
under conditions that require censoring the data by 8.9.2.
K = K when p = 0.632.
0 f
Jc
–3/2
3.3.8 elastic-plastic K [FL ]—an elastic-plastic equiva-
Jc
3.3.24 Weibull slope, b—with p and K data pairs plotted in
f Jc
lent stress-intensity factor derived from the J-integral at the
linearized Weibull coordinates obtainable by rearranging Eq
point of onset of cleavage fracture, J .
c
22, b is the slope of a line that defines the characteristics of the
eq
3.3.9 equivalent value of median toughness, K
Jc~med! typical scatter of K data.
Jc
-3/2
[FL ]—an equivalent value of the median toughness for a
3.3.24.1 Discussion—A Weibull slope of 4 is used exclu-
multi-temperature data set.
sively in this method.
3.3.10 Eta (η)—a dimensionless parameter that relates plas-
−2
3.3.25 yield strength, σ [FL ]—the stress at which a
YS
tic work done on a specimen to crack growth resistance defined
material exhibits a specific limiting deviation from the propor-
in terms of deformation theory J-integral (7).
tionality of stress to strain at the test temperature. This
3.3.11 failure probability, p —the probability that a single
deviation is expressed in terms of strain.
f
selected specimen chosen at random from a population of
3.3.25.1 Discussion—It is customary to determine yield
specimens will fail at or before reaching the K value of
strength by either (1) Offset Method (usually a strain of 0.2 %
Jc
interest.
is specified) or (2) Total-Extension-Under-Force Method (usu-
ally a strain of 0.5 % is specified although other values of strain
3.3.12 initial ligament length, b [L]— the distance from the
o
may be used).
initial crack tip, a , to the back face of a specimen.
o
-1 3.3.25.2 Discussion—Whenever yield strength is specified,
˙
3.3.13 load-line displacement rate,∆ [LT ]—rate of in-
LL
the method of test must be stated along with the percent offset
crease of specimen load-line displacement.
or total strain under force. The values obtained by the two
3.3.14 pop-in—a discontinuity in a force versus displace-
methods may differ.
ment test record (8).
3.3.14.1 Discussion—A pop-in event is usually audible, and
4. Summary of Test Method
is a sudden cleavage crack initiation event followed by crack
4.1 This test method involves the testing of notched and
arrest. The test record will show increased displacement and
fatigue precracked bend or compact specimens in a tempera-
drop in applied force if the test frame is stiff. Subsequently, the
ture range where either cleavage cracking or crack pop-in
test record may continue on to higher forces and increased
develop during the loading of specimens. Crack aspect ratio,
displacements.
a/W, is nominally 0.5. Specimen width in compact specimens
3.3.15 precracked Charpy, PCC, specimen—SE(B) speci-
is two times the thickness. In bend bars, specimen width can be
men with W = B = 10 mm (0.394 in.).
either one or two times the thickness.
3.3.16 provisional reference temperature, (T ) [°C]—
0Q
4.2 Force versus displacement across the notch at a speci-
Interim T value calculated using the standard test method
fied location is recorded by autographic recorder or computer
described herein. T is validated as T in 10.5.
0Q 0
data acquisition, or both. Fracture toughness is calculated at a
3.3.17 reference temperature, T [°C]—The test tempera-
defined condition of crack instability. The J-integral value at
ture at which the median of the K distribution from 1T size
Jc
instability, J , is calculated and converted into its equivalent in
c
specimens will equal 100 MPa√m (91.0 ksi√in.).
units of stress-intensity factor, K . Censoring limits are based
Jc
on K to determine the suitability of data for statistical
3.3.18 SE(B) specimen span, S [L]—the distance between
Jc
specimen supports (See Test Method E1820 Fig. 4). analyses.
3.3.19 specimen thickness, B [L]—the distance between the
4.3 A minimum of six tests are required to estimate the
parallel sides of a test specimen as depicted in Fig. 1–3.
median K of the Weibull distribution for the data population
Jc
3.3.19.1 Discussion—In the case of side-grooved (10). Extensive data scatter among replicate tests is expected.
E1921 − 23b
Statistical methods are used to characterize these data popula- analysis procedures employed in this test method. If the data
tions and to predict changes in data distributions with changed set fails the screening criterion in 10.6, the homogeneity of the
specimen size. material and its fracture toughness can be more accurately
assessed using the analysis methods described in Appendix X5.
4.4 The statistical relationship between specimen size and
K fracture toughness is assessed using weakest-link theory, 5.4 Distributions of K data from replicate tests can be used
Jc
Jc
thereby providing a relationship between the specimen size and to predict distributions of K for different specimen sizes.
Jc
K (4). Limits are placed on the fracture toughness range over Theoretical reasoning (9), confirmed by experimental data,
Jc
which this model can be used. suggests that a fixed Weibull slope of 4 applies to all data
distributions and, as a consequence, standard deviation on data
4.5 For the definition of the toughness transition curve, a
scatter can be calculated. Data distribution and specimen size
master curve concept is used (11, 12). The position of the curve
effects are characterized using a Weibull function that is
on the temperature coordinate is established from the experi-
coupled with weakest-link statistics (14). An upper limit on
mental determination of the temperature, designated T , at
constraint loss and a lower limit on test temperature are defined
which the median K for 1T size specimens is 100 MPa√m
Jc
between which weakest-link statistics can be used.
(91.0 ksi√in.). Selection of a test temperature close to that at
which the median K value will be 100 MPa√m is encouraged 5.5 The experimental results can be used to define a master
Jc
and a means of estimating this temperature is suggested. Small curve that describes the shape and location of median K
Jc
specimens such as precracked Charpy’s may have to be tested transition temperature fracture toughness for 1T specimens
at temperatures below T where K is well below 100 (15). The curve is positioned on the abscissa (temperature
0 Jc(med)
MPa√m. In such cases, additional specimens may be required coordinate) by an experimentally determined reference
as stipulated in 8.5. temperature, T . Shifts in reference temperature are a measure
of transition temperature change caused, for example, by
4.6 Tolerance bounds can be determined that define the
metallurgical damage mechanisms.
range of scatter in fracture toughness throughout the transition
range. 5.6 Tolerance bounds on K can be calculated based on
Jc
theory and generic data. For added conservatism, an offset can
5. Significance and Use
be added to tolerance bounds to cover the uncertainty associ-
ated with estimating the reference temperature, T , from a
5.1 Fracture toughness is expressed in terms of an elastic-
relatively small data set. From this it is possible to apply a
plastic stress-intensity factor, K , that is derived from the
Jc
margin adjustment to T in the form of a reference temperature
J-integral calculated at fracture.
shift.
5.2 Ferritic steels are microscopically inhomogeneous with
5.7 For some materials, particularly those with low strain
respect to the orientation of individual grains. Also, grain
hardening, the value of T may be influenced by specimen size
boundaries have properties distinct from those of the grains.
due to a partial loss of crack-tip constraint (5). When this
Both contain carbides or nonmetallic inclusions that can act as
occurs, the value of T may be lower than the value that would
nucleation sites for cleavage microcracks. The random location
be obtained from a data set of K values derived using larger
of such nucleation sites with respect to the position of the crack
Jc
specimens.
front manifests itself as variability of the associated fracture
toughness (13). This results in a distribution of fracture
5.8 As discussed in 1.3, there is an expected bias among T
toughness values that is amenable to characterization using the
values as a function of the standard specimen type. The
statistical methods in this test method.
magnitude of the bias may increase inversely to the strain
hardening ability of the test material at a given yield strength,
5.3 The statistical methods in this test method assume that
as the average crack-tip constraint of the data set decreases
the data set represents a macroscopically homogeneous
(16). On average, T values obtained from C(T) specimens are
material, such that the test material has both the uniform tensile
higher than T values obtained from SE(B) specimens. Best
and toughness properties. The fracture toughness evaluation of
estimate comparison indicates that the average difference
nonuniform materials is not amenable to the statistical analysis
between C(T) and SE(B)-derived T values is approximately
procedures employed in this test method. For example, multi-
10 °C (2). However, individual C(T) and SE(B) datasets may
pass weldments can create heat-affected and brittle zones with
show much larger T differences (3, 17, 18), or the SE(B) T
localized properties that are quite different from either the bulk
0 0
values may be higher than the C(T) values (2). On the other
or weld materials. Thick-section steels also often exhibit some
hand, comparisons of individual, small datasets may not
variation in properties near the surfaces. Metallographic analy-
necessarily reveal this average trend. Datasets which contain
sis can be used to identify possible nonuniform regions in a
both C(T) and SE(B) specimens may generate T results which
material. These regions can then be evaluated through me-
fall between the T values calculated using solely C(T) or
chanical testing such as hardness, microhardness, and tensile
SE(B) specimens.
testing for comparison with the bulk material. It is also
advisable to measure the toughness properties of these nonuni-
6. Apparatus
form regions distinctly from the bulk material. Section 10.6
provides a screening criterion to assess whether the data set 6.1 Precision of Instrumentation—Measurements of applied
may not be representative of a macroscopically homogeneous forces and load-line displacements are needed to obtain work
material, and therefore, may not be amenable to the statistical done on the specimen. Force versus load-line displacement
E1921 − 23b
shall be recorded digitally on computers or autographically on described in 9.1.4 is used to calculate the plastic part of J.
x-y plotters. For computers, digital signal resolution shall be at However, it is recommended that the plastic part of J be
least 1/32,000 of the displacement transducer signal range and estimated from the direct CMOD or load-line displacement
shall be at least 1/4,000 of the force transducer signal range. measurement rather than inferring load-line displacement from
CMOD. Additionally, CMOD measurement is more accurate
6.2 Grips for C(T) Specimens—A clevis with flat-bottom
than load-line displacement for estimating crack size from
holes is recommended. See Test Method E399, Fig. A6.2, for a
compliance.
recommended design. Clevises and pins should be fabricated
6.5.3 Crack growth can be measured by alternative methods
from steels of sufficient strength to elastically resist indentation
such as electric potential, but care must be taken to minimize
loads (greater than 40 Rockwell hardness C scale (HRC)).
specimen heating effects in low-temperature tests (see also
6.3 Bend Test Fixture—A suitable bend test fixture scheme
6.4.2) (21).
is shown in Fig. A3.2 of Test Method E399. It allows for roller
6.6 Force Measurement:
pin rotation and minimizes friction effects during the test.
6.6.1 Testing shall be performed in a machine conforming to
Fixturing and rolls should be made of high-hardness steel
Practices of E4 and Test Methods E8/E8M. Applied force may
(HRC greater than 40).
be measured by any transducer with a noise-to-signal ratio less
6.4 Displacement Gage for Compact Specimens:
than 1/2,000 of the transducer signal range.
6.4.1 Displacement measurements are made so that J values
6.6.2 Calibrate force measurement instruments by way of
are determined from area under force versus displacement test
Practice E74, 10.2. Annual calibration using calibration equip-
records (a measure of work done). If the test temperature
ment traceable to the National Institute of Standards and
selection recommendations of this practice are followed, crack
Technology is a mandatory requirement.
growth measurement will probably prove to be unimportant.
6.7 Temperature Control—Specimen temperature shall be
Results that fall within the limits of uncertainty of the
measured with thermocouple wires and potentiometers. It is
recommended test temperature estimation scheme will prob-
recommended that the two thermocouple wires be attached to
ably not have significant slow-stable crack growth prior to
the specimen surface separately, either by welding, spot
instability. Nevertheless, crack growth measurements are rec-
welding, or by being affixed mechanically. Mechanical attach-
ommended to provide supplementary information, and these
ment schemes must be verified to provide equivalent tempera-
results may be reported.
ture measurement accuracy. The purpose is to use the test
6.4.2 Unloading compliance is the primary recommendation
material as a part of the thermocouple circuit (see also 8.6.1).
for measuring slow-stable crack growth. See Test Method
Accuracy of temperature measurement shall be within 3 °C of
E1820. When multiple tests are performed sequentially at low
true temperature and repeatability among specimens shall be
test temperatures, there will be condensation and ice buildup
within 2 °C. Precision of measurement shall be 61 °C or
on the grips between the loading pins and flats of the clevis
better. The temperature measuring apparatus shall be checked
holes. Ice will interfere with the accuracy of the unloading
every six months using instruments traceable to the National
compliance method. Alternatively, crack growth can be mea-
Institute of Standards and Technology in order to ensure the
sured by other methods such as electric potential, but care must
required accuracy.
be taken to avoid specimen heating when low test temperatures
are used.
7. Specimen Configuration, Dimensions, and Preparation
6.4.3 In compact C(T) specimens, displacement measure-
ments on the load-line are recommended for J determinations.
7.1 Compact Specimens—Four recommended C(T) speci-
However, other positions up to 0.35W in front of the load-line
men designs are shown in Fig. 1. One C(T) specimen configu-
can be used, as suggested in 7.1.
ration is taken from Test Method E399; the other two C(T)
6.4.4 Clip gage (or other similar displacement gage) accu-
specimen configurations with cutout sections to permit load-
racy shall be verified according to 6.2.2 of Test Method E1820.
line displacement measurement are taken from Test Method
E1820. A fourth C(T) specimen configuration with cutout
6.5 Displacement Gages for Bend Bars, SE(B):
sections to permit both outboard load-line displacement mea-
6.5.1 The SE(B) specimen has two displacement gage
surement and front face razor blade attachment is provided.
locations. A load-line displacement transducer is primarily
Room is provided for attachment of razor blade tips. Care
intended for J computation, but may also be used for calcula-
should be taken to maintain parallel alignment of the blade
tions of crack size based on elastic compliance, if provision is
edges. Other C(T) specimen configurations that provide provi-
made to subtract the extra displacement due to the elastic
sions for measuring displacement at locations up to 0.35W in
compliance of the fixturing. The load-line gage shall display
front of the load-line are allowed. When other than load-line
accuracy of 1 % over the working range of the gage. The gages
displacements are measured, the elastic load-line compliance,
used shall not be temperature sensitive.
C , can be inferred from the measured compliance, C , as
o x
6.5.2 Alternatively, a crack-mouth opening displacement
follows:
(CMOD) gage can also be used to determine the plastic part of
J. However, it is necessary to employ a plastic eta (η) value
A
C 5 × C (1)
o x
developed specifically for the CMOD location (19) or infer
x
A1
load-point displacement from CMOD using an expression that
W
relates the two displacements (20). In either case, the procedure where:
E1921 − 23b
FIG. 1 Four Compact Specimen Designs That Have Been Used Successfully for Fracture Toughness Testing

E1921 − 23b
x a x a Fig. 5 summarizes the maximum starter notch dimensions.
0 0
A 5 1.07 1 0.976 · 1 0.35 2 4.056 · 2 2.874
S D S D S D S
W W W W The notch cutout for measurement gages shall be no greater
3 4
than 0.2W wide by 0.1W deep. The allowable starter notch
x a x a
0 0
2 8.981 · 1 4.99 2 10.23 · 2 2.547
D S D S D S D S
height shall be no greater than 0.063W. The centerline of the
W W W W
crack-starter notch shall not deviate from the specimen center-
x a
2 4.318 · (2)
D S D
line by more than 0.005W. Fig. 5 also defines the notch tip
W W
length for both a V-notch type and a narrow-notch type. The
In Eq 1, x is the distance in front of the load-line. It is valid
for 0≤x≤0.35W. When other than load-line displacements are
narrow notch is often machined using a wire or plunge
measured, the load-line displacement for area A determina-
p electrical discharge machining technique with no additional
tion shall be inferred according to 9.1.4(22).The ratio of
machining to further sharpen the notch root radius.
specimen height to width, 2H/W is 1.2, and this ratio is to be
7.5 Specimen Dimension Requirements—The crack front
the same for all types and sizes of C(T) specimens. The ini-
straightness criterion defined in 8.9.1 must be satisfied. The
tial crack size, a , shall be 0.5W 6 0.05W. Specimen width,
o
specimen remaining ligament, b , must have sufficient size to
W, shall be 2B.
o
maintain a condition of high crack-front constraint at fracture.
7.2 Disk-shaped Compact Specimens—A recommended
The maximum K capacity of a specimen is given by:
Jc
DC(T) specimen design is shown in Fig. 2. Initial crack size,
a , shall be 0.5W6 0.05W. Specimen width shall be 2B.
o Eb σ
o YS
K 5Π(3)
Jclimit 2
7.3 Single-edge Notched Bend—The recommended SE(B) 30~1 2 v !
specimen designs, shown in Fig. 3, are made for use with a
where:
span-to-width ratio, S/W = 4. The width, W, can be either 1B or
b = W-a
o o
2B. The initial crack size, a , shall be 0.5W 6 0.05W.
o
Measurement of σ at the test temperature (T) using Test
7.4 Machined Notch Design—Three designs of fatigue ys
Methods E8/E8M is preferred for use in Eq 3. When σ has not
crack starter notches are shown in Fig. 4. These notches can be ys
been measured at T, any of the following three methods are
straight through the specimen thickness or incorporate the
acceptable for estimating σ at T for use in Eq 3:
chevron form (Fig. 4). To facilitate fatigue cracking at low
ys
(1) Using a value of σ measured at a higher temperature
stress-intensity levels, it is recommended that the root radius
ys
than T.
for either a straight-through slot terminating in a V-notch, or a
(2) Interpolating between measurements of σ at tempera-
narrow notch, be 0.08 mm (0.003 in.) or less. If a chevron form
ys
of notch is used, the root radius may be 0.25 mm (0.010 in.) or tures above and below T, as long as the σ measurement
ys
temperatures are within 75 °C of each other. Extrapolation of
less. In the case of a notch ending in a drilled hole, a sharp
stress raiser at the end of the hole will facilitate fatigue the fit to infer yield strength outside of the measurement points
precracking and help ensure that the precrack centering re- is not allowed with this method. This method is applicable over
quirement in 7.8.2 is met. a test temperature range of - 200 °C and 300 °C
NOTE 1—A surfaces shall be perpendicular and parallel as applicable to within 0.002W TIR.
NOTE 2—The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top and bottom extremes of
the disk within 0.005W TIR.
NOTE 3—Integral or attached knife edges for clip gage attachment may be used. See also Fig. 6, Test Method E399.
FIG. 2 Disk-shaped Compact Specimen DC(T) Standard Proportions
E1921 − 23b
NOTE 1—All surfaces shall be perpendicular and parallel within 0.001W TIR; surface finish 64v.
NOTE 2—Crack starter notch shall be perpendicular to specimen surfaces to within6 2°.
FIG. 3 Recommended Bend Bar Specimen Design
FIG. 4 Envelope Crack Starter Notches
E1921 − 23b
FIG. 5 Envelope of Fatigue Crack and Crack Starter Notches
(3) Determining σ from the following equation which can The fatigue precracking shall be conducted with the specimen
ys
be used for temperatures between -200 °C and 300 °C. (23) See
fully heat-treated to the condition in which it is to be tested. No
Note 1.
intermediate heat treatments between precracking and testing
are allowed. There are several ways of promoting early crack
σ 5 σ 110 ⁄~491 1 1.8 T! 2 189~MPa! (4)
ys ysRT
initiation: (1) by providing a very sharp notch tip, (2) by using
where:
a chevron notch (Fig. 4), (3) by statically preloading the
T = test temperature (°C), and
specimen in such a way that the notch tip is compressed in a
σ = the material yield strength at room temperature
ysRT
direction normal to the intended crack plane (to a force not to
(MPa)
exceed P ), and (4) by using a negative fatigue force ratio; for
m
NOTE 1—Eq 4 should not be used to determine σ from σ values
ysRT ys
a given maximum fatigue force, the more negative the force
obtained at other temperatures.
ratio, the earlier crack initiation is likely to occur. The peak
K data that exceed this requirement (that is, Eq 3) are used
Jc
compressive force shall not exceed P as defined in the
m
in a data censoring procedure. Details of this procedure are
equations below:
described in 10.2.2.
0.5Bb σ
7.6 Small Specimens—At high values of fracture toughness o Y
For SE~B! specimens, P 5 (5)
m
S
relative to specimen size and material flow properties, the
values of K that meet the requirements of Eq 3 may not
0.4Bb σ
Jc
o Y
For C~T! and DC~T! specimens, P 5 (6)
m
always provide a unique description of the crack-front stress-
2W1a
o
strain fields due to some loss of constraint caused by excessive
7.8.2 Fatigue Precracking Procedure—Fatigue precracking
plastic flow (5). This condition may develop in materials with
can be conducted under either force control, displacement
low strain hardening. When this occurs, the highest K values
Jc
of the data set could possibly cause the value of T to be lower control, or K control. If the force cycle is maintained constant,
than the value that would be obtained from testing specimens
the maximum K and ∆K will increase with crack size; if the
with higher constraint. displacement cycle is maintained constant, the reverse will
happen. If K is maintained constant, force has to be reduced as
7.7 Side Grooves— Side grooves are optional. Precracking
a function of increasing crack size. Fatigue cycling is con-
prior to side-grooving is recommended, despite the fact that
ducted using a sinusoidal waveform and a frequency close to
crack growth on the surfaces might be slightly behind. Speci-
the highest practical value. There is no known marked fre-
mens may be side-grooved after precracking to decrease the
quency effect on fatigue precrack formation up to at least 100
curvature of the initial crack front. In fact, side-grooving may
Hz in the absence of adverse environments. The specimen shall
be indispensable as a means for controlling crack front
straightness in bend bars of square cross section. The total be accurately located in the loading fixture to achieve uniform,
side-grooved depth shall not exceed 0.25B. Side grooves with symmetric loading. The specimen should be carefully moni-
an included angle of 45° and a root radius of 0.5 mm 6 0.2 mm
tored until crack initiation is observed on one side. If crack
(0.02 in. 6 0.01 in.) usually produce the desired results.
initiation is not observed on the other side before appreciable
growth is observed on the first side, then fatigue cycling should
7.8 Precracking:
be stopped to try to determine the cause and find a remedy for
7.8.1 Fatigue Loading Requirements—Allowable fatigue
the unsymmetrical behavior. Sometimes, simply turning the
force values are limited to keep the maximum stress-intensity
specimen around in relation to the fixture will solve the
factor applied during precracking, K , well below the mate-
max
rial fracture toughness measured during the subsequent test. problem.
E1921 − 23b
NOTE 2—If the yield strength (σ ) is not known, a low estimate should
Precracking can be performed either by some method of
ys
be used to obtain a conservatively high estimate of ∆a .
sh
smoothly and continually decreasing the maximum stress-
intensity factor (K ) or by using discrete steps. It is suggested Also, as summarized in Fig. 5, the length of the fatigue
max
that the reduction in K between any discrete step be no precrack extension from the machined notch, ∆a (determined
max pc
greater than 20 % because reducing K too rapidly can result using the measured initial crack size defined in 8.8.1), shall
max
in precrack growth rate retardation. It is also suggested that equal or exceed the larger of 0.5h, (∆a + ∆a ), or 0.25 mm at
sh f
measurable crack extension occur before proceeding to the each of the nine measurement locations defined in 8.8.1.
next step. Precracking is generally most effectively conducted Additionally, the sum of Δa and the notch tip length shall
pc
using R = P /P = 0.1. Maximum force values shall be exceed 2.0h at each of the nine measurement locations defined
min max
accurate to within 6 5 % of their target values. in 8.8.1. The precrack must also meet the curvature require-
Fig. 6 shows the allowable envelope for K during ment in 8.9.1. Ensuring that the average ∆a is long enough
max
pc
precracking. The precracking K and crack extension re- such that the minimum fatigue precrack extension occurs at
max
quirements are summarized in Table 1, and Table 2. Precrack- each measurement point in 8.8.1 for a crack having the
ing can be conducted in any manner such that K remains maximum curvature allowed in 8.9.1 will provide some con-
max
within the envelope and the maximum fatigue force is less than fidence that these requirements are met before testing the
P . The K applied to the specimen shall not exceed 25
specimen.
m max
MPa√m (22.8 ksi√in) at any crack size, and may be limited by
P for small specimens or low yield strength materials, or both.
8. Procedure
m
As the testing temperature decreases compared to the precrack-
8.1 Testing Procedure—The objective of the procedure de-
ing temperature, the warm prestressing effect increases, which
scribed here is to determine the J-integral at the point of crack
can elevate the measured fracture toughness. To minimize the
instability, J . Crack growth can be measured by partial
c
warm prestressing effect, the maximum K that may be applied
unloading compliance, or by any other method that has
to the specimen during Δa (K in Fig. 6) shall not exceed 15
f f
precision and accuracy, as defined below. However, the
MPa√m (13.7 ksi√in). Alternatively, when the testing tempera-
J-integral is not corrected for slow-stable crack growth in this
ture is equal to or above the precracking temperature, K shall
f
test method.
not exceed 20 MPa√m (18.3 ksi√in). The minimum length of
8.2 Test Preparation—Prior to each test, certain specimen
∆a (Fig. 6) is 0.2 mm (0.008 in.). ∆a is greater than or equal
f sh
to the change in plastic zone size in going from a maximum K dimensions should be measured, and the average starting crack
size estimated. The average starting crack size can be estimated
of 25 MPa√m (22.8 ksi√in) to K . The minimum value for ∆a
f sh
defines the condition where the leading edge of the plastic zone using a variety of techniques including precrack compliance,
back-face strain, and using the average of the optical side face
remains stationary as K is decreased.
max
measurements.
Δa $ r 2 r (7)
sh p1 p2
NOTE 3—When side-grooving is to be used, first precrack without side
where:
grooves and then visually estimate the precrack size.
If estimates are available from multiple techniques, the user
1 K
max
r 5 with K
S D
p1 max
shall select the value that is believed to be most representative
3π σ
ys
of the average crack size.
5 25 MPa=m ~22.8 ksi=in.!
8.2.1 The dimensions B, B , and W shall be measured to
N
1 K within 0.05 mm (0.002 in.) accuracy or 0.5 %, whichever is
f
r 5
S D
p2
3π σ larger.
ys
FIG. 6 Envelope of Allowable K During Precracking
max
E1921 − 23b
TABLE 1 K Requirements
max
Intial: K cannot exceed 25 MPa=m (22.8 ksi=in.) and the maximum fatigue force cannot exceed P .
max m
Final: K depends on the test temperature:
f
Test Temperature K throughout Δa
f f
< precracking temperature < 15MPa=m (13.7 ksi=in.)
$ precrackeing temperature < 20MPa=m (18.3 ksi=in.)
TABLE 2 Crack Extension Requirements
Δa $ MAX {0.5h, (Δa + Δa ),
pc sh f
0.25 mm}
Δa $ r – r
sh p1 p2
Where:
1 K
max
r 5 with
S D
p1
3π σ
ys
r =
pl
K ≤ 25 MPa√m (22.8
max
ksi√in.)
1 K
f
r 5
r = S D
p2 p2
3π σ
ys
Δa $ 0.2 mm (0.008 in.)
f
8.2.2 Follow Test Method E1820, 8.5 for crack size 8.4.2 The procedure outlined in 8.4.1 is only appropriate for
measurement, 8.3.2 for testing compact specimens and 8.3.1 determining an initial test temperature. The iterative scheme
for testing bend specimens. described in 10.3.1 may be necessary to refine this test
temperature in order to increase T accuracy. Testing below the
8.3 The required minimum number of K results that are
Jc
temperature specified in Eq 8 may be appropriate for low
uncensored is specified according to the value of K . See
Jc(med)
upper-shelf toughness materials to avoid ductile crack growth
also 8.5.
before cleavage onset, and for low yield strength materials to
8.4 Test Temperature Selection—It is recommended that the
avoid obtaining data that must be censored because it exceeds
selected temperatures be close to that at which the K 6
Jc(med)
K in accordance with Eq 3.
Jclimit
value will be about 100 MPa√m for the specimen size selected.
8.5 Testing Below Temperature, T —When the equivalent
8.4.1 Quasi-static loading rates—If K-rate complies with 0
value of K for 1T specimens is greater than 83 MPa√m,
the limits stated in 8.7.1, Charpy V-notch data can be used as Jc(med)
the required number of uncensored K values to perform the
an aid for predicting a viable test temperature. If a Charpy Jc
analyses covered in Section 10 is six. However, small speci-
transition temperature, T , is known corresponding to a 28 J
CVN
mens such as precracked Charpy specimens can develop
Charpy V-notch energy or a 41 J Charpy V-notch energy, the
excessive numbers of K values that exceed the K (Eq 3)
constant C can be chosen from Table 3 corresponding to the
Jc Jclimit
when testing close to the T temperature. In such cases it is
test specimen size (defined in 3.3.20), and used to estimate the 0
advisable to test at temperatures below T , where most, if not
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM 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.
Designation: E1921 − 23a E1921 − 23b
Standard Test Method for
Determination of Reference Temperature, T , for Ferritic
Steels in the Transition Range
This standard is issued under the fixed designation E1921; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of a reference temperature, T , which characterizes the fracture toughness of ferritic
steels that experience onset of cleavage cracking at elastic, or elastic-plastic K instabilities, or both. The specific types of ferritic
Jc
steels (3.2.2) covered are those with yield strengths ranging from 275 MPa to 825 MPa (40 ksi to 120 ksi) and weld metals, after
stress-relief annealing, that have 10 % or less strength mismatch relative to that of the base metal.
1.2 The specimens covered are fatigue precracked single-edge notched bend bars, SE(B), and standard or disk-shaped compact
tension specimens, C(T) or DC(T). A range of specimen sizes with proportional dimensions is recommended. The dimension on
which the proportionality is based is specimen thickness.
1.3 Median K values tend to vary with the specimen type at a given test temperature, presumably due to constraint differences
Jc
among the allowable test specimens in 1.2. The degree of K variability among specimen types is analytically predicted to be a
Jc
function of the material flow properties (1) and decreases with increasing strain hardening capacity for a given yield strength
material. This K dependency ultimately leads to discrepancies in calculated T values as a function of specimen type for the same
Jc 0
material. T values obtained from C(T) specimens are expected to be higher than T values obtained from SE(B) specimens. Best
0 0
estimate comparisons of several materials indicate that the average difference between C(T) and SE(B)-derived T values is
approximately 10°C (2). C(T) and SE(B) T differences up to 15 °C have also been recorded (3). However, comparisons of
individual, small datasets may not necessarily reveal this average trend. Datasets which contain both C(T) and SE(B) specimens
may generate T results which fall between the T values calculated using solely C(T) or SE(B) specimens. It is therefore strongly
0 0
recommended that the specimen type be reported along with the derived T value in all reporting, analysis, and discussion of
results. This recommended reporting is in addition to the requirements in 11.1.1.
1.4 Requirements are set on specimen size and the number of replicate tests that are needed to establish acceptable characterization
of K data populations.
Jc
¯
˙
1.5 T is dependent on the K-rate. T is evaluated for a quasi-static K-rate range with 0.5 < K < 2 MPa√m/s. T values for slowly
0 0 I 0
¯
˙
loaded specimens (K < 0.5 MPa√m) can be considered valid if environmental effects are known to be negligible. Provision is also
I
¯
˙
made for higher K-rates (K > 2 MPa√m/s) in Annex A1. Note that this threshold K-rate for application of Annex A1 is a much
I
lower threshold than is required in other fracture toughness test methods such as E399 and E1820.
1.6 The statistical effects of specimen size on K in the transition range are treated using the weakest-link theory (4) applied to
Jc
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue and Fracture and is the direct responsibility of E08.07 on Fracture Mechanics.
Current edition approved Nov. 1, 2023Dec. 15, 2023. Published March 2024. Originally approved in 1997. Last previous edition approved in 2023 as
E1921 – 23.E1921 – 23a. DOI: 10.1520/E1921-23A.10.1520/E1921-23B.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1921 − 23b
a three-parameter Weibull distribution of fracture toughness values. A limit on K values, relative to the specimen size, is specified
Jc
to ensure high constraint conditions along the crack front at fracture. For some materials, particularly those with low strain
hardening, this limit may not be sufficient to ensure that a single-parameter (K ) adequately describes the crack-front deformation
Jc
state (5).
1.7 Statistical methods are employed to predict the transition toughness curve and specified tolerance bounds for 1T specimens
of the material tested. The standard deviation of the data distribution is a function of Weibull slope and median K . The procedure
Jc
for applying this information to the establishment of transition temperature shift determinations and the establishment of tolerance
limits is prescribed.
1.8 The procedures described in this test method assume that the data set represents a macroscopically homogeneous material,
such that the test material has uniform tensile and toughness properties. Application of this test method to an inhomogeneous
material will result in an inaccurate estimate of the transition reference value T and nonconservative confidence bounds. For
example, multi-pass weldments can create heat-affected and brittle zones with localized properties that are quite different from
either the bulk or weld materials. Thick-section steels also often exhibit some variation in properties near the surfaces.
Metallography and initial screening may be necessary to verify the applicability of these and similarly graded materials. Section
10.6 provides a screening criterion to assess whether the data set may not be representative of a macroscopically homogeneous
material, and therefore, may not be amenable to the statistical analysis procedures employed in this test method. If the data set fails
the screening criterion in 10.6, the homogeneity of the material and its fracture toughness can be more accurately assessed using
the analysis methods described in Appendix X5.
1.9 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.10 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E4 Practices for Force Calibration and Verification of Testing Machines
E8/E8M Test Methods for Tension Testing of Metallic Materials
E23 Test Methods for Notched Bar Impact Testing of Metallic Materials
E74 Practices for Calibration and Verification for Force-Measuring Instruments
E111 Test Method for Young’s Modulus, Tangent Modulus, and Chord Modulus
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E208 Test Method for Conducting Drop-Weight Test to Determine Nil-Ductility Transition Temperature of Ferritic Steels
E399 Test Method for Linear-Elastic Plan-Strain Fracture Toughness K of Metallic Materials
Ic
E436 Test Method for Drop-Weight Tear Tests of Ferritic Steels
E561 Test Method for K Curve Determination
R
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E1820 Test Method for Measurement of Fracture Toughness
E1823 Terminology Relating to Fatigue and Fracture Testing
2.2 ASME Standards:
ASME Boiler and Pressure Vessel Code, Section II, Part D
3. Terminology
3.1 Terminology given in Terminology E1823 is applicable to this test method.
3.2 Definitions:
-2
3.2.1 effective yield strength, σ [FL ]— an assumed value of uniaxial yield strength that represents the influence of plastic
Y
yielding upon fracture test parameters.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American Society of Mechanical Engineers (ASME), ASME International Headquarters, Two Park Ave., New York, NY 10016-5990, http://
www.asme.org.
E1921 − 23b
3.2.1.1 Discussion—
It is calculated as the average of the 0.2 % offset yield strength σ , and the ultimate tensile strength, σ as follows:
YS TS
σ 1σ
YS TS
σ 5
Y
3.2.2 ferritic steels—typically carbon, low-alloy, and higher alloy grades. Typical microstructures are bainite, tempered bainite,
tempered martensite, and ferrite and pearlite. All ferritic steels have body centered cubic crystal structures that display
ductile-to-cleavage transition temperature fracture toughness characteristics. See also Test Methods E23, E208 and E436.
3.2.2.1 Discussion—
This definition is not intended to imply that all of the many possible types of ferritic steels have been verified as being amenable
to analysis by this test method.
– 3/2
3.2.3 stress-intensity factor, K [FL ]—the magnitude of the mathematically ideal crack-tip stress field coefficient (stress field
singularity) for a particular mode of crack-tip region deformation in a homogeneous body.
3.2.3.1 Discussion—
In this test method, Mode I is assumed. See Terminology E1823 for further discussion.
–1
3.2.4 J-integral, J [FL ]—a mathematical expression; a line or surface integral that encloses the crack front from one crack
surface to the other; used to characterize the local stress-strain field around the crack front (6). See Terminology E1823 for further
discussion.
3.3 Definitions of Terms Specific to This Standard:
¯
-3/2 -1
˙
3.3.1 average stress-intensity-factor rate K [FL T ]—average rate of increase of applied stress-intensity factor up to K .
I Jc
3.3.1.1 Discussion—
It is evaluated as the ratio between K and the corresponding time to cleavage. For tests where partial unloading/reloading
Jc
sequences are used to measure compliance, an equivalent time to cleavage (t ) shall be used to calculate the stress-intensity-factor
c
rate (that is, K-rate). The value of t is calculated as the ratio between the value of load-line displacement at cleavage and the
c
load-line displacement rate applied during the monotonic loading portions of the test (that is, the periods between partial
unloading/reloading sequences used for compliance measurement). Additionally, in such tests, the total time of unloading/loading
sequences should be shorter than the total time of monotonic loading.
3.3.2 control force, P [F]—a calculated value of maximum force, used in 7.8.1 to stipulate allowable precracking limits.
m
3.3.3 crack initiation—describes the onset of crack propagation from a preexisting macroscopic crack created in the specimen by
a stipulated procedure.
–2
3.3.4 effective modulus, E [FL ]—an elastic modulus that allows a theoretical (modulus normalized) compliance to match an
eff
experimentally measured compliance for an actual initial crack size, a .
o
–2
3.3.5 elastic modulus, E' [FL ]—a linear-elastic factor relating stress to strain, the value of which is dependent on the degree of
constraint. For plane stress, E' = E is used, and for plane strain, E/(1 – v ) is used, with E being Young’s modulus and v being
Poisson’s ratio.
–1
3.3.6 elastic plastic J [FL ]—J-integral at the onset of cleavage fracture.
c
–3/2
3.3.7 elastic-plastic K [FL ]—An elastic-plastic equivalent stress-intensity factor derived from the J-integral.
J
3.3.7.1 Discussion—
In this test method, K also implies a stress-intensity factor determined at the test termination point under conditions that require
J
censoring the data by 8.9.2.
–3/2
3.3.8 elastic-plastic K [FL ]—an elastic-plastic equivalent stress-intensity factor derived from the J-integral at the point of
Jc
onset of cleavage fracture, J .
c
E1921 − 23b
eq
3.3.9 equivalent value of median toughness, K
Jc~med!
-3/2
[FL ]—an equivalent value of the median toughness for a multi-temperature data set.
3.3.10 Eta (η)—a dimensionless parameter that relates plastic work done on a specimen to crack growth resistance defined in terms
of deformation theory J-integral (7).
3.3.11 failure probability, p —the probability that a single selected specimen chosen at random from a population of specimens
f
will fail at or before reaching the K value of interest.
Jc
3.3.12 initial ligament length, b [L]— the distance from the initial crack tip, a , to the back face of a specimen.
o o
-1
˙
3.3.13 load-line displacement rate,∆ [LT ]—rate of increase of specimen load-line displacement.
LL
3.3.14 pop-in—a discontinuity in a force versus displacement test record (8).
3.3.14.1 Discussion—
A pop-in event is usually audible, and is a sudden cleavage crack initiation event followed by crack arrest. The test record will
show increased displacement and drop in applied force if the test frame is stiff. Subsequently, the test record may continue on to
higher forces and increased displacements.
3.3.15 precracked Charpy, PCC, specimen—SE(B) specimen with W = B = 10 mm (0.394 in.).
3.3.16 provisional reference temperature, (T ) [°C]—Interim T value calculated using the standard test method described herein.
0Q 0
T is validated as T in 10.5.
0Q 0
3.3.17 reference temperature, T [°C]—The test temperature at which the median of the K distribution from 1T size specimens
0 Jc
will equal 100 MPa√m (91.0 ksi√in.).
3.3.18 SE(B) specimen span, S [L]—the distance between specimen supports (See Test Method E1820 Fig. 4).
3.3.19 specimen thickness, B [L]—the distance between the parallel sides of a test specimen as depicted in Fig. 1–3.
3.3.19.1 Discussion—
In the case of side-grooved specimens, the net thickness, B , is the distance between the roots of the side-groove notches.
N
3.3.20 specimen size, nT—a code used to define specimen dimensions, where n is expressed in multiples of 1 in.
3.3.20.1 Discussion—
In this method, specimen proportionality is required. For compact specimens and bend bars, specimen thickness B = n inches.
3.3.21 temperature, T [°C]—For K values that are developed using specimens or test practices, or both, that do not conform
Q Jc
to the requirements of this test method, a temperature at which K = 100 MPa√m is defined as T . T is not a provisional
Jc (med) Q Q
value of T .
3.3.22 time to control force, t [T],—time to P .
m m
3.3.23 Weibull fitting parameter, K —a scale parameter located at the 63.2 % cumulative failure probability level (9).K = K
0 Jc 0
when p = 0.632.
f
3.3.24 Weibull slope, b—with p and K data pairs plotted in linearized Weibull coordinates obtainable by rearranging Eq 22, b
f Jc
is the slope of a line that defines the characteristics of the typical scatter of K data.
Jc
3.3.24.1 Discussion—
A Weibull slope of 4 is used exclusively in this method.
−2
3.3.25 yield strength, σ [FL ]—the stress at which a material exhibits a specific limiting deviation from the proportionality of
YS
stress to strain at the test temperature. This deviation is expressed in terms of strain.
3.3.25.1 Discussion—
E1921 − 23b
It is customary to determine yield strength by either (1) Offset Method (usually a strain of 0.2 % is specified) or (2)
Total-Extension-Under-Force Method (usually a strain of 0.5 % is specified although other values of strain may be used).
3.3.25.2 Discussion—
Whenever yield strength is specified, the method of test must be stated along with the percent offset or total strain under force.
The values obtained by the two methods may differ.
4. Summary of Test Method
4.1 This test method involves the testing of notched and fatigue precracked bend or compact specimens in a temperature range
where either cleavage cracking or crack pop-in develop during the loading of specimens. Crack aspect ratio, a/W, is nominally 0.5.
Specimen width in compact specimens is two times the thickness. In bend bars, specimen width can be either one or two times
the thickness.
4.2 Force versus displacement across the notch at a specified location is recorded by autographic recorder or computer data
acquisition, or both. Fracture toughness is calculated at a defined condition of crack instability. The J-integral value at instability,
J , is calculated and converted into its equivalent in units of stress-intensity factor, K . Censoring limits are based on K to
c Jc Jc
determine the suitability of data for statistical analyses.
4.3 A minimum of six tests are required to estimate the median K of the Weibull distribution for the data population (10).
Jc
Extensive data scatter among replicate tests is expected. Statistical methods are used to characterize these data populations and to
predict changes in data distributions with changed specimen size.
4.4 The statistical relationship between specimen size and K fracture toughness is assessed using weakest-link theory, thereby
Jc
providing a relationship between the specimen size and K (4). Limits are placed on the fracture toughness range over which this
Jc
model can be used.
4.5 For the definition of the toughness transition curve, a master curve concept is used (11, 12). The position of the curve on the
temperature coordinate is established from the experimental determination of the temperature, designated T , at which the median
K for 1T size specimens is 100 MPa√m (91.0 ksi√in.). Selection of a test temperature close to that at which the median K value
Jc Jc
will be 100 MPa√m is encouraged and a means of estimating this temperature is suggested. Small specimens such as precracked
Charpy’s may have to be tested at temperatures below T where K is well below 100 MPa√m. In such cases, additional
0 Jc(med)
specimens may be required as stipulated in 8.5.
4.6 Tolerance bounds can be determined that define the range of scatter in fracture toughness throughout the transition range.
5. Significance and Use
5.1 Fracture toughness is expressed in terms of an elastic-plastic stress-intensity factor, K , that is derived from the J-integral
Jc
calculated at fracture.
5.2 Ferritic steels are microscopically inhomogeneous with respect to the orientation of individual grains. Also, grain boundaries
have properties distinct from those of the grains. Both contain carbides or nonmetallic inclusions that can act as nucleation sites
for cleavage microcracks. The random location of such nucleation sites with respect to the position of the crack front manifests
itself as variability of the associated fracture toughness (13). This results in a distribution of fracture toughness values that is
amenable to characterization using the statistical methods in this test method.
5.3 The statistical methods in this test method assume that the data set represents a macroscopically homogeneous material, such
that the test material has both the uniform tensile and toughness properties. The fracture toughness evaluation of nonuniform
materials is not amenable to the statistical analysis procedures employed in this test method. For example, multi-pass weldments
can create heat-affected and brittle zones with localized properties that are quite different from either the bulk or weld materials.
Thick-section steels also often exhibit some variation in properties near the surfaces. Metallographic analysis can be used to
identify possible nonuniform regions in a material. These regions can then be evaluated through mechanical testing such as
hardness, microhardness, and tensile testing for comparison with the bulk material. It is also advisable to measure the toughness
properties of these nonuniform regions distinctly from the bulk material. Section 10.6 provides a screening criterion to assess
whether the data set may not be representative of a macroscopically homogeneous material, and therefore, may not be amenable
E1921 − 23b
to the statistical analysis procedures employed in this test method. If the data set fails the screening criterion in 10.6, the
homogeneity of the material and its fracture toughness can be more accurately assessed using the analysis methods described in
Appendix X5.
5.4 Distributions of K data from replicate tests can be used to predict distributions of K for different specimen sizes. Theoretical
Jc Jc
reasoning (9), confirmed by experimental data, suggests that a fixed Weibull slope of 4 applies to all data distributions and, as a
consequence, standard deviation on data scatter can be calculated. Data distribution and specimen size effects are characterized
using a Weibull function that is coupled with weakest-link statistics (14). An upper limit on constraint loss and a lower limit on
test temperature are defined between which weakest-link statistics can be used.
5.5 The experimental results can be used to define a master curve that describes the shape and location of median K transition
Jc
temperature fracture toughness for 1T specimens (15). The curve is positioned on the abscissa (temperature coordinate) by an
experimentally determined reference temperature, T . Shifts in reference temperature are a measure of transition temperature
change caused, for example, by metallurgical damage mechanisms.
5.6 Tolerance bounds on K can be calculated based on theory and generic data. For added conservatism, an offset can be added
Jc
to tolerance bounds to cover the uncertainty associated with estimating the reference temperature, T , from a relatively small data
set. From this it is possible to apply a margin adjustment to T in the form of a reference temperature shift.
5.7 For some materials, particularly those with low strain hardening, the value of T may be influenced by specimen size due to
a partial loss of crack-tip constraint (5). When this occurs, the value of T may be lower than the value that would be obtained from
a data set of K values derived using larger specimens.
Jc
5.8 As discussed in 1.3, there is an expected bias among T values as a function of the standard specimen type. The magnitude
of the bias may increase inversely to the strain hardening ability of the test material at a given yield strength, as the average
crack-tip constraint of the data set decreases (16). On average, T values obtained from C(T) specimens are higher than T values
0 0
obtained from SE(B) specimens. Best estimate comparison indicates that the average difference between C(T) and SE(B)-derived
T values is approximately 10 °C (2). However, individual C(T) and SE(B) datasets may show much larger T differences (3, 17,
0 0
18), or the SE(B) T values may be higher than the C(T) values (2). On the other hand, comparisons of individual, small datasets
may not necessarily reveal this average trend. Datasets which contain both C(T) and SE(B) specimens may generate T results
which fall between the T values calculated using solely C(T) or SE(B) specimens.
6. Apparatus
6.1 Precision of Instrumentation—Measurements of applied forces and load-line displacements are needed to obtain work done
on the specimen. Force versus load-line displacement shall be recorded digitally on computers or autographically on x-y plotters.
For computers, digital signal resolution shall be at least 1/32,000 of the displacement transducer signal range and shall be at least
1/4,000 of the force transducer signal range.
6.2 Grips for C(T) Specimens—A clevis with flat-bottom holes is recommended. See Test Method E399, Fig. A6.2, for a
recommended design. Clevises and pins should be fabricated from steels of sufficient strength to elastically resist indentation loads
(greater than 40 Rockwell hardness C scale (HRC)).
6.3 Bend Test Fixture—A suitable bend test fixture scheme is shown in Fig. A3.2 of Test Method E399. It allows for roller pin
rotation and minimizes friction effects during the test. Fixturing and rolls should be made of high-hardness steel (HRC greater than
40).
6.4 Displacement Gage for Compact Specimens:
6.4.1 Displacement measurements are made so that J values are determined from area under force versus displacement test records
(a measure of work done). If the test temperature selection recommendations of this practice are followed, crack growth
measurement will probably prove to be unimportant. Results that fall within the limits of uncertainty of the recommended test
temperature estimation scheme will probably not have significant slow-stable crack growth prior to instability. Nevertheless, crack
growth measurements are recommended to provide supplementary information, and these results may be reported.
6.4.2 Unloading compliance is the primary recommendation for measuring slow-stable crack growth. See Test Method E1820.
E1921 − 23b
When multiple tests are performed sequentially at low test temperatures, there will be condensation and ice buildup on the grips
between the loading pins and flats of the clevis holes. Ice will interfere with the accuracy of the unloading compliance method.
Alternatively, crack growth can be measured by other methods such as electric potential, but care must be taken to avoid specimen
heating when low test temperatures are used.
6.4.3 In compact C(T) specimens, displacement measurements on the load-line are recommended for J determinations. However,
other positions up to 0.35W in front of the load-line can be used, as suggested in 7.1.
6.4.4 Clip gage (or other similar displacement gage) accuracy shall be verified according to 6.2.2 of Test Method E1820.
6.5 Displacement Gages for Bend Bars, SE(B):
6.5.1 The SE(B) specimen has two displacement gage locations. A load-line displacement transducer is primarily intended for J
computation, but may also be used for calculations of crack size based on elastic compliance, if provision is made to subtract the
extra displacement due to the elastic compliance of the fixturing. The load-line gage shall display accuracy of 1 % over the working
range of the gage. The gages used shall not be temperature sensitive.
6.5.2 Alternatively, a crack-mouth opening displacement (CMOD) gage can also be used to determine the plastic part of J.
However, it is necessary to employ a plastic eta (η) value developed specifically for the CMOD location (19) or infer load-point
displacement from CMOD using an expression that relates the two displacements (20). In either case, the procedure described in
9.1.4 is used to calculate the plastic part of J. However, it is recommended that the plastic part of J be estimated from the direct
CMOD or load-line displacement measurement rather than inferring load-line displacement from CMOD. Additionally, CMOD
measurement is more accurate than load-line displacement for estimating crack size from compliance.
6.5.3 Crack growth can be measured by alternative methods such as electric potential, but care must be taken to minimize
specimen heating effects in low-temperature tests (see also 6.4.2) (21).
6.6 Force Measurement:
6.6.1 Testing shall be performed in a machine conforming to Practices of E4 and Test Methods E8/E8M. Applied force may be
measured by any transducer with a noise-to-signal ratio less than 1/2,000 of the transducer signal range.
6.6.2 Calibrate force measurement instruments by way of Practice E74, 10.2. Annual calibration using calibration equipment
traceable to the National Institute of Standards and Technology is a mandatory requirement.
6.7 Temperature Control—Specimen temperature shall be measured with thermocouple wires and potentiometers. It is
recommended that the two thermocouple wires be attached to the specimen surface separately, either by welding, spot welding,
or by being affixed mechanically. Mechanical attachment schemes must be verified to provide equivalent temperature measurement
accuracy. The purpose is to use the test material as a part of the thermocouple circuit (see also 8.6.1). Accuracy of temperature
measurement shall be within 3 °C of true temperature and repeatability among specimens shall be within 2 °C. Precision of
measurement shall be 61 °C or better. The temperature measuring apparatus shall be checked every six months using instruments
traceable to the National Institute of Standards and Technology in order to ensure the required accuracy.
7. Specimen Configuration, Dimensions, and Preparation
7.1 Compact Specimens—Four recommended C(T) specimen designs are shown in Fig. 1. One C(T) specimen configuration is
taken from Test Method E399; the other two C(T) specimen configurations with cutout sections to permit load-line displacement
measurement are taken from Test Method E1820. A fourth C(T) specimen configuration with cutout sections to permit both
outboard load-line displacement measurement and front face razor blade attachment is provided. Room is provided for attachment
of razor blade tips. Care should be taken to maintain parallel alignment of the blade edges. Other C(T) specimen configurations
that provide provisions for measuring displacement at locations up to 0.35W in front of the load-line are allowed. When other than
load-line displacements are measured, the elastic load-line compliance, C , can be inferred from the measured compliance, C , as
o x
follows:
A
C 5 ×C (1)
o x
x
A1
W
where:
E1921 − 23b
FIG. 1 Four Compact Specimen Designs That Have Been Used Successfully for Fracture Toughness Testing

E1921 − 23b
2 3 4 5
x a x a x a x a x a
0 0 0 0 0
A 5 1.07 1 0.976 · 1 0.35 2 4.056 · 2 2.874 2 8.981 · 1 4.99 2 10.23 · 2 2.547 2 4.318 · (2)
S D S D S D S D S D S D S D S D S D
W W W W W W W W W W
In Eq 1, x is the distance in front of the load-line. It is valid for 0≤x≤0.35W. When other than load-line displacements are
measured, the load-line displacement for area A determination shall be inferred according to 9.1.4(22).The ratio of specimen
p
height to width, 2H/W is 1.2, and this ratio is to be the same for all types and sizes of C(T) specimens. The initial crack size,
a , shall be 0.5W 6 0.05W. Specimen width, W, shall be 2B.
o
7.2 Disk-shaped Compact Specimens—A recommended DC(T) specimen design is shown in Fig. 2. Initial crack size, a , shall be
o
0.5W6 0.05W. Specimen width shall be 2B.
7.3 Single-edge Notched Bend—The recommended SE(B) specimen designs, shown in Fig. 3, are made for use with a
span-to-width ratio, S/W = 4. The width, W, can be either 1B or 2B. The initial crack size, a , shall be 0.5W 6 0.05W.
o
7.4 Machined Notch Design—Three designs of fatigue crack starter notches are shown in Fig. 4. These notches can be straight
through the specimen thickness or incorporate the chevron form (Fig. 4). To facilitate fatigue cracking at low stress-intensity levels,
it is recommended that the root radius for either a straight-through slot terminating in a V-notch, or a narrow notch, be 0.08 mm
(0.003 in.) or less. If a chevron form of notch is used, the root radius may be 0.25 mm (0.010 in.) or less. In the case of a notch
ending in a drilled hole, a sharp stress raiser at the end of the hole will facilitate fatigue precracking and help ensure that the
precrack centering requirement in 7.8.2 is met.
Fig. 5 summarizes the maximum starter notch dimensions. The notch cutout for measurement gages shall be no greater than
0.2W wide by 0.1W deep. The allowable starter notch height shall be no greater than 0.063W. The centerline of the crack-starter
notch shall not deviate from the specimen centerline by more than 0.005W. Fig. 5 also defines the notch tip length for both a
V-notch type and a narrow-notch type. The narrow notch is often machined using a wire or plunge electrical discharge machining
technique with no additional machining to further sharpen the notch root radius.
7.5 Specimen Dimension Requirements—The crack front straightness criterion defined in 8.9.1 must be satisfied. The specimen
remaining ligament, b , must have sufficient size to maintain a condition of high crack-front constraint at fracture. The maximum
o
K capacity of a specimen is given by:
Jc
Eb σ
o YS
K 5Π(3)
Jclimit
30 12 v
~ !
NOTE 1—A surfaces shall be perpendicular and parallel as applicable to within 0.002W TIR.
NOTE 2—The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top and bottom extremes of
the disk within 0.005W TIR.
NOTE 3—Integral or attached knife edges for clip gage attachment may be used. See also Fig. 6, Test Method E399.
FIG. 2 Disk-shaped Compact Specimen DC(T) Standard Proportions
E1921 − 23b
NOTE 1—All surfaces shall be perpendicular and parallel within 0.001W TIR; surface finish 64v.
NOTE 2—Crack starter notch shall be perpendicular to specimen surfaces to within6 2°.
FIG. 3 Recommended Bend Bar Specimen Design
FIG. 4 Envelope Crack Starter Notches
where:
b = W-a
o o
E1921 − 23b
FIG. 5 Envelope of Fatigue Crack and Crack Starter Notches
Measurement of σ at the test temperature (T) using Test Methods E8/E8M is preferred for use in Eq 3. When σ has not been
ys ys
measured at T, any of the following three methods are acceptable for estimating σ at T for use in Eq 3:
ys
(1) Using a value of σ measured at a higher temperature than T.
ys
(2) Interpolating between measurements of σ at temperatures above and below T, as long as the σ measurement temperatures
ys ys
are within 75 °C of each other. Extrapolation of the fit to infer yield strength outside of the measurement points is not allowed with
this method. This method is applicable over a test temperature range of - 200 °C and 300 °C
(3) Determining σ from the following equation which can be used for temperatures between -200 °C and 300 °C. (23) See
ys
Note 1.
σ 5 σ 110 ⁄~491 1 1.8 T! 2 189~MPa! (4)
ys ysRT
where:
T = test temperature (°C), and
σ = the material yield strength at room temperature (MPa)
ysRT
NOTE 1—Eq 4 should not be used to determine σ from σ values obtained at other temperatures.
ysRT ys
K data that exceed this requirement (that is, Eq 3) are used in a data censoring procedure. Details of this procedure are
Jc
described in 10.2.2.
7.6 Small Specimens—At high values of fracture toughness relative to specimen size and material flow properties, the values of
K that meet the requirements of Eq 3 may not always provide a unique description of the crack-front stress-strain fields due to
Jc
some loss of constraint caused by excessive plastic flow (5). This condition may develop in materials with low strain hardening.
When this occurs, the highest K values of the data set could possibly cause the value of T to be lower than the value that would
Jc 0
be obtained from testing specimens with higher constraint.
7.7 Side Grooves— Side grooves are optional. Precracking prior to side-grooving is recommended, despite the fact that crack
growth on the surfaces might be slightly behind. Specimens may be side-grooved after precracking to decrease the curvature of
the initial crack front. In fact, side-grooving may be indispensable as a means for controlling crack front straightness in bend bars
of square cross section. The total side-grooved depth shall not exceed 0.25B. Side grooves with an included angle of 45° and a
root radius of 0.5 mm 6 0.2 mm (0.02 in. 6 0.01 in.) usually produce the desired results.
7.8 Precracking:
7.8.1 Fatigue Loading Requirements—Allowable fatigue force values are limited to keep the maximum stress-intensity factor
applied during precracking, K , well below the material fracture toughness measured during the subsequent test. The fatigue
max
precracking shall be conducted with the specimen fully heat-treated to the condition in which it is to be tested. No intermediate
heat treatments between precracking and testing are allowed. There are several ways of promoting early crack initiation: (1) by
providing a very sharp notch tip, (2) by using a chevron notch (Fig. 4), (3) by statically preloading the specimen in such a way
that the notch tip is compressed in a direction normal to the intended crack plane (to a force not to exceed P ), and (4) by using
m
E1921 − 23b
a negative fatigue force ratio; for a given maximum fatigue force, the more negative the force ratio, the earlier crack initiation is
likely to occur. The peak compressive force shall not exceed P as defined in the equations below:
m
0.5Bb σ
o Y
For SE~B! specimens,P 5 (5)
m
S
0.4Bb σ
o Y
For C~T! and DC~T! specimens,P 5 (6)
m
2W1a
o
7.8.2 Fatigue Precracking Procedure—Fatigue precracking can be conducted under either force control, displacement control, or
K control. If the force cycle is maintained constant, the maximum K and ∆K will increase with crack size; if the displacement cycle
is maintained constant, the reverse will happen. If K is maintained constant, force has to be reduced as a function of increasing
crack size. Fatigue cycling is conducted using a sinusoidal waveform and a frequency close to the highest practical value. There
is no known marked frequency effect on fatigue precrack formation up to at least 100 Hz in the absence of adverse environments.
The specimen shall be accurately located in the loading fixture to achieve uniform, symmetric loading. The specimen should be
carefully monitored until crack initiation is observed on one side. If crack initiation is not observed on the other side before
appreciable growth is observed on the first side, then fatigue cycling should be stopped to try to determine the cause and find a
remedy for the unsymmetrical behavior. Sometimes, simply turning the specimen around in relation to the fixture will solve the
problem.
Precracking can be performed either by some method of smoothly and continually decreasing the maximum stress-intensity
factor (K ) or by using discrete steps. It is suggested that the reduction in K between any discrete step be no greater than 20 %
max max
because reducing K too rapidly can result in precrack growth rate retardation. It is also suggested that measurable crack
max
extension occur before proceeding to the next step. Precracking is generally most effectively conducted using R = P /P = 0.1.
min max
Maximum force values shall be accurate to within 6 5 % of their target values.
Fig. 6 shows the allowable envelope for K during precracking. The precracking K and crack extension requirements are
max max
summarized in Table 1, and Table 2. Precracking can be conducted in any manner such that K remains within the envelope and
max
the maximum fatigue force is less than P . The K applied to the specimen shall not exceed 25 MPa√m (22.8 ksi√in) at any crack
m max
size, and may be limited by P for small specimens or low yield strength materials, or both. As the testing temperature decreases
m
compared to the precracking temperature, the warm prestressing effect increases, which can elevate the measured fracture
toughness. To minimize the warm prestressing effect, the maximum K that may be applied to the specimen during Δa (K in Fig.
f f
6) shall not exceed 15 MPa√m (13.7 ksi√in). Alternatively, when the testing temperature is equal to or above the precracking
temperature, K shall not exceed 20 MPa√m (18.3 ksi√in). The minimum length of ∆a (Fig. 6) is 0.2 mm (0.008 in.). ∆a is greater
f f sh
than or equal to the change in plastic zone size in going from a maximum K of 25 MPa√m (22.8 ksi√in) to K . The minimum value
f
for ∆a defines the condition where the leading edge of the plastic zone remains stationary as K is decreased.
sh max
FIG. 6 Envelope of Allowable K During Precracking
max
E1921 − 23b
TABLE 1 K Requirements
max
Intial: K cannot exceed 25 MPa=m (22.8 ksi=in.) and the maximum fatigue force cannot exceed P .
max m
Final: K depends on the test temperature:
f
Test Temperature K throughoutΔa
f f
< precracking temperature < 15MPa=m (13.7 ksi=in.)
$ precrackeing temperature < 20MPa=m (18.3 ksi=in.)
TABLE 2 Crack Extension Requirements
Δa $ MAX {0.5h, (Δa + Δa ),
pc sh f
0.25 mm}
Δa $ r – r
sh p1 p2
Where:
1 K
max
r 5 with K
S D
p1 max
3π σ
ys
r =
pl
≤ 25 MPa√m (22.8
ksi√in.)
1 K
f
r 5
r = S D
p2 p2
3π σ
ys
Δa $ 0.2 mm (0.008 in.)
f
Δa $ r 2 r (7)
sh p1 p2
where:
1 K
max
r 5 with K 5 25 MPa=m ~22.8 ksi=in.!
S D
p1 max
3π σ
ys
1 K
f
r 5
S D
p2
3π σ
ys
NOTE 2—If the yield strength (σ ) is not known, a low estimate should be used to obtain a conservatively high estimate of ∆a .
ys sh
Also, as summarized in Fig. 5, the length of the fatigue precrack extension from the machined notch, ∆a (determined using
pc
the measured initial crack size defined in 8.8.1), shall equal or exceed the larger of 0.5h, (∆a + ∆a ), or 0.25 mm at each of the
sh f
nine measurement locations defined in 8.8.1. Additionally, the sum of Δa and the notch tip length shall exceed 2.0h at each of
pc
the nine measurement locations defined in 8.8.1. The precrack must also meet the curvature requirement in 8.9.1. Ensuring that
the average ∆a is long enough such that the minimum fatigue precrack extension occurs at each measurement point in 8.8.1 for
pc
a crack having the maximum curvature allowed in 8.9.1 will provide some confidence that these requirements are met before
testing the specimen.
8. Procedure
8.1 Testing Procedure—The objective of the procedure described here is to determine the J-integral at the point of crack instability,
J . Crack growth can be measured by partial unloading compliance, or by any other method that has precision and accuracy, as
c
defined below. However, the J-integral is not corrected for slow-stable crack growth in this test method.
8.2 Test Preparation—Prior to each test, certain specimen dimensions should be measured, and the average starting crack size
estimated. The average starting crack size can be estimated using a variety of techniques including precrack compliance, back-face
strain, and using the average of the optical side face measurements.
NOTE 3—When side-grooving is to be used, first precrack without side grooves and then visually estimate the precrack size.
If estimates are available from multiple techniques, the user shall select the value that is believed to be most representative of
the average crack size.
E1921 − 23b
8.2.1 The dimensions B,B , and W shall be measured to within 0.05 mm (0.002 in.) accuracy or 0.5 %, whichever is larger.
N
8.2.2 Follow Test Method E1820, 8.5 for crack size measurement, 8.3.2 for testing compact specimens and 8.3.1 for testing bend
specimens.
8.3 The required minimum number of K results that are uncensored is specified according to the value of K . See also 8.5.
Jc Jc(med)
8.4 Test Temperature Selection—It is recommended that the selected temperatures be close to that at which the K value will
Jc(med)
be about 100 MPa√m for the specimen size selected.
8.4.1 Quasi-static loading rates—If K-rate complies with the limits stated in 8.7.1, Charpy V-notch data can be used as an aid for
predicting a viable test temperature. If a Charpy transition temperature, T , is known corresponding to a 28 J Charpy V-notch
CVN
energy or a 41 J Charpy V-notch energy, the constant C can be chosen from Table 3 corresponding to the test specimen size (defined
in 3.3.20), and used to estimate the test temperature from (12, 24).
T 5 T 1C (8)
CVN
8.4.2 The procedure outlined in 8.4.1 is only appropriate for determining an initial test temperature. The iterative scheme described
in 10.3.1 may be necessary to refine this test temperature in order to increase T accuracy. Testing below the temperature specified
in Eq 8 may be appropriate for low upper-shelf toughness materials to avoid ductile crack growth before cleavage onset, and for
low yield strength materials to avoid obtaining data that must be censored because it exceeds K in accordance with Eq 3.
Jclimit
8.5 Testing Below Temperature, T —When the equivalent value of K for 1T specimens is greater than 83 MPa√m, the
0 Jc(med)
required number of uncensored K values to perform th
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