ASTM E2899-24

Standard Test Method for Measurement of Initiation Toughness in Surface Cracks Under Tension and Bending

SIGNIFICANCE AND USE
5.1 Surface cracks are among the most common defects found in structural components. An accurate characterization and understanding of crack-front behavior is necessary to ensure successful operation of a structure containing surface cracks. The testing of laboratory specimens with surface cracks provides a means to understand and quantify surface crack behavior, but the test results must be interpreted correctly to ensure transferability between the laboratory specimen and the structure.  
5.2 Transferability refers to the capacity of a fracture mechanics methodology to correlate the crack-tip stress and strain fields of different cracked bodies. Traditionally, the correlation has been based on the presence at fracture of a dominant, asymptotically singular, crack-tip field with amplitude set by the value of a single parameter, such as the stress intensity factor, KI, or the J-integral. For components and specimens with high crack-tip constraint, the singular crack-tip field dominates over microstructurally significant size scales for loads ranging from globally linear-elastic conditions to moderately large-scale plasticity. For specimens with low crack-tip constraint, a dominant single-parameter crack-tip field exists only at low levels of plasticity. At higher levels of plasticity, the opening mode stress of the low constraint specimen is lower than predicted by the single-parameter, asymptotically singular fields. Therefore, low constraint specimens often exhibit larger fracture toughness than do high constraint specimens. If feasible, users are strongly encouraged to generate high constraint fracture toughness data using methods such as Test Methods E399 or E1820 prior to testing the surface crack geometry.  
5.2.1 To address this phenomenon, two-parameter fracture criteria are used to include the influence of crack-tip constraint. Crack-tip constraint has been quantified using various scalar parameters including the T-stress (10, 11, 12), Q   (13, 14), stres...
SCOPE
1.1 This test method describes the method for testing fatigue-sharpened, semi-elliptically shaped surface cracks in rectangular flat panels subjected to monotonically increasing tension or bending. Tests quantify the crack-tip conditions at initiation of stable crack extension or immediate unstable crack extension.  
1.2 This test method applies to the testing of metallic materials not limited by strength, thickness, or toughness. Materials are assumed to be essentially homogeneous and free of residual stress. Tests may be conducted at any appropriate temperature. The effects of environmental factors and sustained or cyclic loads are not addressed in this test method.  
1.3 This test method describes all necessary details for the user to test for the initiation of crack extension in surface crack test specimens. Specific requirements and recommendations are provided for test equipment, instrumentation, test specimen design, and test procedures.  
1.4 Tests of surface cracked, laboratory-scale specimens as described in this test method may provide a more accurate understanding of full-scale structural performance in the presence of surface cracks. The provided recommendations help to assure test methods and data are applicable to the intended purpose.  
1.5 This test method prescribes a consistent methodology for test and analysis of surface cracks for research purposes and to assist in structural assessments. The methods described here utilize a constraint-based framework (1, 2)2 to evaluate the fracture behavior of surface cracks.
Note 1: Constraint-based framework. In the context of this test method, constraint is used as a descriptor of the three-dimensional stress and strain fields in the near vicinity of the crack tip, where material contractions due to the Poisson effect may be suppressed and therefore produce an elevated, tensile stress state (3, 4). (See further discussions in Terminology and Significance a...

General Information

• Status: Published
• Publication Date: 14-Feb-2024
• ICS: 19.040 - Environmental testing
• Technical Committee: E08 - Fatigue and Fracture
• Drafting Committee: E08.07 - Fracture Mechanics

Relations

• Replaces: ASTM E2899-19e1 - Standard Test Method for Measurement of Initiation Toughness in Surface Cracks Under Tension and Bending
• Effective Date: 15-Feb-2024
• Referred By: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Feb-2024

Overview

ASTM E2899-24: Standard Test Method for Measurement of Initiation Toughness in Surface Cracks Under Tension and Bending provides a comprehensive framework for evaluating the initiation toughness of semi-elliptical surface cracks in metallic materials. Developed by ASTM International, this test method is crucial for understanding the behavior of surface cracks in structural components subjected to tension and bending loads. Accurate characterization of surface crack initiation toughness supports safer and more efficient structural design, especially in applications where surface cracks are common.

This standard is intended for laboratory testing of metallic specimens with fatigue-sharpened surface cracks, allowing engineers and researchers to measure the crack-tip conditions at the onset of stable crack extension or immediate unstable crack growth. The methodology enables improved correlation between test specimen data and full-scale structural performance, promoting confidence in the integrity assessments of critical infrastructure.

Key Topics

• Surface Crack Testing: The method focuses on standardized testing of semi-elliptical surface cracks in rectangular, flat metallic panels under monotonically increasing tension or bending loads.

• Applicability: Suitable for metallic materials not limited by strength, thickness, or toughness, assuming the materials are homogenous and free of residual stress.

• Testing Regimes: The standard provides a framework to categorize specimens into three regimes:

  • Linear-elastic
  • Elastic-plastic
  • Field-collapse

• Constraint-Based Analysis: Utilizes both traditional single-parameter fracture mechanics (using K or J as crack-tip parameters) and more advanced two-parameter approaches accounting for crack-tip constraint (e.g., T-stress, Q factor).

• Specimen and Equipment Requirements: Details all requirements for specimen design, preparation, test equipment, and instrumentation to ensure high-quality, repeatable results.

• Data Interpretation: Guidance on interpreting results within a toughness-constraint locus, improving the utility of laboratory data for real-world structures.

Applications

• Structural Integrity Assessment: Enables engineers to accurately estimate the toughness and failure behavior of structural components with surface cracks, supporting safer design and inspection protocols.

• Material Characterization: Assists researchers in comparing materials’ resistance to crack initiation, facilitating development and qualification of advanced alloys and treatments.

• Failure Analysis and Life Prediction: Provides a quantitative basis for predicting crack extension and failure in service, informing repair and maintenance schedules.

• Benchmarking and Research: Useful for academic and industrial research to benchmark toughness data, study the effects of geometry and loading, and validate computational models related to fracture mechanics.

Related Standards

For comprehensive fracture mechanics testing, refer to the following ASTM standards:

• ASTM E399: Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
• ASTM E1820: Test Method for Measurement of Fracture Toughness
• ASTM E647: Test Method for Measurement of Fatigue Crack Growth Rates
• ASTM E8/E8M: Test Methods for Tension Testing of Metallic Materials
• ASTM E740: Practice for Fracture Testing with Surface-Crack Tension Specimens
• ASTM E1921: Test Method for Determination of Reference Temperature, T₀, for Ferritic Steels in the Transition Range

These related standards cover additional specimen types, loading conditions, and temperature effects, complementing ASTM E2899-24 for a complete evaluation of fracture behavior in metals.

---

By providing a standardized approach for surface crack initiation toughness testing, ASTM E2899-24 plays a key role in modern fracture mechanics and structural engineering, ensuring results are reliable, transferable, and directly applicable to real-world engineering challenges.

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: E2899 − 24
Standard Test Method for
Measurement of Initiation Toughness in Surface Cracks
Under Tension and Bending
This standard is issued under the fixed designation E2899; 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.
produce an elevated, tensile stress state (3, 4). (See further discussions in
1. Scope
Terminology and Significance and Use.) When a parameter describing this
1.1 This test method describes the method for testing
stress state, or constraint, is used with the standard measure of crack-tip
fatigue-sharpened, semi-elliptically shaped surface cracks in stress amplitude (K or J), the resulting two-parameter characterization
broadens the ability of fracture mechanics to accurately predict the
rectangular flat panels subjected to monotonically increasing
response of a crack under a wider range of loading. The two-parameter
tension or bending. Tests quantify the crack-tip conditions at
methodology produces a more complete description of the crack-tip
initiation of stable crack extension or immediate unstable crack
conditions at the initiation of crack extension. The effects of constraint on
extension.
measured fracture toughness are material dependent and are governed by
the effects of the crack-tip stress-strain state on the micromechanical
1.2 This test method applies to the testing of metallic
failure processes specific to the material. Surface crack tests conducted
materials not limited by strength, thickness, or toughness.
with this test method can help to quantify the material sensitivity to
Materials are assumed to be essentially homogeneous and free
constraint effects and to establish the degree to which the material
toughness correlates with a constraint-based fracture characterization.
of residual stress. Tests may be conducted at any appropriate
temperature. The effects of environmental factors and sustained
1.6 This test method provides a quantitative framework to
or cyclic loads are not addressed in this test method.
categorize test specimen conditions into one of three regimes:
(I) a linear-elastic regime, (II) an elastic-plastic regime, or (III)
1.3 This test method describes all necessary details for the
a field-collapse regime. Based on this categorization, analysis
user to test for the initiation of crack extension in surface crack
techniques and guidelines are provided to determine an appli-
test specimens. Specific requirements and recommendations
cable crack-tip parameter for the linear-elastic regime (K or J)
are provided for test equipment, instrumentation, test specimen
or the elastic-plastic regime (J), and an associated constraint
design, and test procedures.
parameter. Recommendations are provided to assess the test
1.4 Tests of surface cracked, laboratory-scale specimens as
data in the context of a toughness-constraint locus (2). For
described in this test method may provide a more accurate
tension loading, a computer program referred to as TASC
understanding of full-scale structural performance in the pres-
V1.0.2 (Tool for Analysis of Surface Cracks) may be used to
ence of surface cracks. The provided recommendations help to
perform the analytical assessments in Section 9, Analysis of
assure test methods and data are applicable to the intended
Results. The user is directed to other resources for evaluation
purpose.
of the test specimen in the field-collapse regime when exten-
1.5 This test method prescribes a consistent methodology
sive plastic deformation in the specimen eliminates the iden-
for test and analysis of surface cracks for research purposes and
tifiable crack-front fields of fracture mechanics.
to assist in structural assessments. The methods described here
NOTE 2—TASC. The computer program TASC is available at no charge
utilize a constraint-based framework (1, 2) to evaluate the
either at https://software.nasa.gov/software/MFS-33082-1 or at https://
fracture behavior of surface cracks.
sourceforge.net/projects/tascnasa/. The use of TASC relieves the user of
the burden of performing unique elastic-plastic finite element analyses for
NOTE 1—Constraint-based framework. In the context of this test
each test performed in the elastic-plastic regime. For the purposes of this
method, constraint is used as a descriptor of the three-dimensional stress
standard, TASC calculations are equivalent to finite element analysis
and strain fields in the near vicinity of the crack tip, where material
results. Users of TASC should follow the methodologies in Annex A6 for
contractions due to the Poisson effect may be suppressed and therefore
establishing analysis material property inputs. Documentation on the
development, verification and validation of TASC is provided in refer-
ences (5, 6, 7, 8).
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue
1.7 The specimen design and test procedures described in
and Fracture and is the direct responsibility of Subcommittee E08.07 on Fracture
this test method may be applied to evaluation of surface cracks
Mechanics.
Current edition approved Feb. 15, 2024. Published April 2024. Originally
in welds; however, the methods described in this test method to
ɛ1
approved in 2013. Last previous edition approved in 2019 as E2899 – 19 . DOI:
analyze test measurements may not be applicable. Weld frac-
10.1520/E2899-24.
ture tests generally have complicating features beyond the
The boldface numbers in parentheses refer to the list of references at the end of
this test method. scope of data analysis in this test method, including the effects
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2899 − 24
of residual stress, microstructural variability, and non-uniform 3. Terminology
strength. These effects will influence test results and must be
3.1 For definitions of terms used in this Test Method,
considered in the interpretation of measured quantities.
Terminologies E6 and E1823 apply.
1.8 This test method is not intended for testing surface
3.2 Symbols:
cracks in steel in the cleavage regime. Such tests are outside
3.2.1 crack depth, a [L]—see Terminology E1823 and Fig.
the scope of this test method. A methodology for evaluation of
1 in this test method.
cleavage fracture toughness in ferritic steels over the ductile-
3.2.1.1 Discussion—In this test method, the term a is the
o
to-brittle region using C(T) and SE(B) specimens can be found
original surface crack depth, as determined in subsection 8.4,
in Test Method E1921.
used in the evaluation of the test.
1.9 Units—The values stated in SI units are to be regarded
3.2.2 crack-mouth opening displacement, CMOD [L]—see
as the standard. The values given in parentheses are for
Terminology E1823 and Fig. 1 in this test method.
information only.
3.2.3 force, P [F]—see Terminology E1823.
1.10 This practice may involve hazardous materials,
-1 -2
3.2.4 J-integral, J [FL or FLL ]—see Terminology
operations, and equipment. This standard does not purport to
E1823.
address all of the safety concerns, if any, associated with its
-2
use. It is the responsibility of the user of this standard to
3.2.5 modulus of elasticity, E [FL ]—see Terminology
establish appropriate safety, health, and environmental prac-
E1823.
tices and determine the applicability of regulatory limitations
3.2.6 net section area, A [L ]—see Terminology E1823.
N
prior to use.
For surface cracks A = WB – πa c /2.
N 0 0
1.11 This international standard was developed in accor-
3.2.7 notch height, h [L]—the distance between the parallel
dance with internationally recognized principles on standard-
faces of the machined notch prior to specimen deformation
ization established in the Decision on Principles for the
(Fig. 6).
Development of International Standards, Guides and Recom-
-3/2
mendations issued by the World Trade Organization Technical
3.2.8 plane-strain fracture toughness, K [FL ]—see Ter-
Ic
Barriers to Trade (TBT) Committee.
minology E1823.
3.2.9 Poisson’s ratio, ν—see Terminology E6.
2. Referenced Documents
3.2.10 specimen width, W [L]—see Terminology E1823 and
2.1 ASTM Standards:
Fig. 1 from this test method.
C1421 Test Methods for Determination of Fracture Tough-
3.2.11 stable crack extension, [L]—see Terminology E1823.
ness of Advanced Ceramics at Ambient Temperature
E4 Practices for Force Calibration and Verification of Test- 3.2.12 stress ratio, R—see Terminology E1823.
ing Machines
3.2.13 surface crack length, 2c [L]—see Terminology
E6 Terminology Relating to Methods of Mechanical Testing
E1823 and Fig. 1 in this test method.
E8/E8M Test Methods for Tension Testing of Metallic Ma-
3.2.13.1 Discussion—In this test method, the term 2c is the
terials
original surface crack length, as determined in subsection 8.4,
E111 Test Method for Young’s Modulus, Tangent Modulus,
used in the evaluation of the test.
and Chord Modulus
-2
3.2.14 yield strength, σ [FL ]—see Terminology E1823,
E399 Test Method for Linear-Elastic Plane-Strain Fracture
YS
as determined by 0.2% offset strain method.
Toughness of Metallic Materials
E647 Test Method for Measurement of Fatigue Crack
3.3 Definitions of Terms Specific to This Standard:
Growth Rates
3.3.1 characteristic length, r , r [L]—a physical length
ϕa ϕb
E740 Practice for Fracture Testing with Surface-Crack Ten-
measured post-test on the specimen fracture surface and
sion Specimens
compared to the length scale provided by the deformation
E1012 Practice for Verification of Testing Frame and Speci-
limit. r is the distance measured on the crack plane normal to
ϕa
men Alignment Under Tensile and Compressive Axial
the crack front at the parametric angle ϕ to the front face
i
Force Application
(cracked face) of the specimen. r is the distance measured on
ϕb
E1820 Test Method for Measurement of Fracture Toughness
the crack plane normal to the crack front at the parametric
E1823 Terminology Relating to Fatigue and Fracture Testing
angle ϕ to the back face (uncracked face) or side of the
i
E1921 Test Method for Determination of Reference
specimen (Fig. A3.1).
Temperature, T , for Ferritic Steels in the Transition
3.3.2 constraint, Ω—in the context of this test method,
Range
constraint is a descriptor of the three dimensional stress and
strain fields in the near vicinity of the crack tip where material
contractions due to the Poisson effect may be suppressed and
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
therefore produce an elevated, three-dimensional tensile (hy-
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
drostatic) stress state. An elevated hydrostatic stress state
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. suppresses material yielding and permits larger stresses to
E2899 − 24
FIG. 1 Test Specimen and Crack Configurations
FIG. 2 Toughness-Constraint Locus with Example Trajectories
E2899 − 24
FIG. 3 Recommended Configuration of Tension Testing Clevis
NOTE 1—Flat bottomed holes are not required, but may be used in configurations found in Test Methods E399 or E1820.
FIG. 4 Specimen Design Principles
develop. The material, geometry, and externally applied loads fields. The non-dimensional parameters, C and C , define the
Ja Jb
influence the development of the elevated hydrostatic stress deformation limits for validity of the elastic-plastic regime in
state.
this test method.
3.3.3 elastic-plastic regime—conditions in a test specimen 3.3.3.1 Discussion—Non-dimensional deformation limits
where crack-tip deformations exceed limits of the linear-elastic
such as C , C and C are commonly designated by the letter
K Ja Jb
regime defined in this test method, but J alone or J and a
“M” in the literature (9).
constraint term still characterize the crack-tip stress and strain
E2899 − 24
FIG. 5 Recommended Configuration of Bend Testing Apparatus
FIG. 6 Fatigue Crack Starter Notch Configuration
3.3.4 elastic-plastic regime crack size deformation limit, elastic-plastic regime based on limiting the crack-tip opening
C —the non-dimensional, upper limit of deformation for the displacement relative to the crack size.
Ja
E2899 − 24
3.3.5 elastic-plastic regime ligament deformation limit, J-dominant fields permit the use of a single parameter charac-
C —the non-dimensional, upper limit of deformation for the terization of fracture toughness in terms of a critical J-value. In
Jb
elastic-plastic regime based on limiting plasticity in the re- this test method, J-dominant conditions prevail to higher levels
maining ligament. of crack-tip deformation than do K-dominant conditions.
-2
-1 -2
3.3.6 far field stress, σ [FL ]—stress far removed from the
3.3.14 J [FL or FLL ]—a value of the J-integral calcu-
K
crack plane resulting from applied forces or moments.
lated from K using the equation:
I
3.3.6.1 Discussion—For applied tensile forces, the far field
2 2
K 1 2 ν
~ !
I
stress is the average stress over the gross area, that is σ = P/WB. J 5 (1)
K
E
For applied bending moments, the far field stress is the
that is valid for linear-elastic, plane-strain conditions.
maximum tensile outer fiber stress across the gross area, that is
σ = 6M/(WB ). -1 -2
3.3.15 J [FL or FLL ]—the peak value of the J-integral
p
3.3.7 field-collapse regime—conditions in a test specimen around the perimeter of the surface crack during monotonic
where crack-tip deformations exceed the limit of the elastic-
loading.
plastic regime defined in this test method. Extensive plastic
-1 -2
3.3.16 J [FL or FLL ]—the J-integral value at the
ϕ
deformation in the specimen eliminates the identifiable crack-
initiation angle (ϕ ) when the specimen reaches the initiation
i
front fields of fracture mechanics, which precludes analysis of
crack mouth opening displacement (CMOD ).
i
test conditions in this test method.
3.3.17 K-dominance—crack-tip conditions where the stress
3.3.8 initiation angle, ϕ —the parametric angle determined
i
and strain fields immediately surrounding the crack-tip plastic
in accordance with Annex A5 that identifies the location along
zone are quantified by the stress intensity factor, K , without
I
the crack perimeter where the test result is evaluated.
constraint adjustment.
3.3.9 initiation of surface crack extension—in the context of
3.3.17.1 Discussion—Crack-tip fields defined as
this test method, the point during the test when, under
K-dominant exist when globally linear-elastic conditions pre-
monotonically increasing force or moment, the precrack ex-
vail in the specimen (see 3.3.22.1) together with high crack-tip
tends a small but consistently measurable amount by stable,
constraint conditions (for example, T-stress ≥ 0). K-dominant
ductile tearing, or when the precrack extends in an immediate,
fields permit the use of a single parameter fracture criterion
unstable ductile mode, failing the specimen.
expressed as a critical K-value, and are also J-dominant by
3.3.9.1 Discussion—Parameters associated with the initia-
definition.
tion of surface crack extension are designated herein with a
-3/2
3.3.18 K [FL ]—the peak value of the stress intensity
subscript i (for example, P ) and define the state at which the
p
i
factor around the perimeter of the surface crack during
crack front fields are characterized to render the toughness test
result. The initiation of surface crack extension will generally monotonic loading.
be a local occurrence along the perimeter of a surface crack. -3/2
3.3.19 K [FL ]—the stress intensity factor at the initia-
ϕ
Due to this localization, defining and experimentally quantify-
tion angle (ϕ ) with applied initiation force (P ), or moment
i i
ing a universal measure of relative or absolute crack extension
(M ).
i
for the surface crack geometry is not practical with commonly
-3/2
3.3.20 K [FL ]—the maximum value of stress inten-
available laboratory equipment. Therefore, if identifiable, the max-ϕ
sity occurring around the crack perimeter during fatigue
extent and location of stable crack extension is recorded as an
precracking.
integral part of the test result. See subsection 8.3.4. In this
context, the surface crack toughness result identifies a point on
3.3.21 length scale [L]—a calculated length that is com-
the material’s tearing resistance curve as influenced by the
pared to a characteristic length (r , r ) of the test specimen to
ϕa ϕb
local crack tip constraint conditions. See J-R curve and K-R
evaluate the test result or determine test validity.
curve definitions in Terminology E1823.
3.3.21.1 Discussion—The length scales are defined by a
3.3.10 initiation crack mouth opening displacement,
non-dimensional deformation limit, C, multiplied by the ratio
CMOD [L]—the CMOD at which initiation of surface crack
of J/σ in the form:
i
YS
extension occurs.
J
length scale 5 C (2)
3.3.11 initiation force, P [F]—the force at which initiation
i σ
YS
of surface crack extension occurs.
3.3.22 linear-elastic regime—conditions in a test specimen
3.3.12 initiation moment M [FL]—the applied moment at
i where the stress and strain fields enclosing the crack-tip plastic
which initiation of surface crack extension occurs.
zone are quantified by K alone, or by K and a constraint term.
I I
3.3.13 J-dominance—crack-tip conditions where the elastic-
3.3.22.1 Discussion—The linear-elastic regime applies
plastic stress and strain fields are quantified by the value of the
when the amount of deformation at the crack tip remains small
J-integral without constraint adjustment.
relative to the dimensions of the specimen. Conditions in the
3.3.13.1 Discussion—Crack-tip fields described as linear-elastic regime do not necessarily imply high constraint,
J-dominant in this test method exist when elastic-plastic for example, the T-stress may be positive or negative. The
conditions develop at the crack front and high crack-tip limit, C , sets the maximum deformation allowed at the crack
K
constraint conditions prevail (for example, T-stress ≥ 0). tip for the linear-elastic regime in this test method.
E2899 − 24
3.3.23 linear-elastic regime deformation limit, C —the non- plane (see Fig. 1), σ is the flow stress (average of the
K 0
dimensional, upper limit of deformation for the linear-elastic yield and ultimate strength). Alternatively σ can be sub-
YS
regime.
stituted for σ in the above equation.
3.3.24 moment, M [FL]—the value of the applied moment at
3.3.29 Q —value of Q at the initiation angle (ϕ ) at defor-
ϕ i
the crack plane of a specimen during a test.
mation level corresponding to CMOD .
i
M = (S – S ) P/4 for four-point bending.
outer inner
3.3.30 inner span, S , L[L]—distance between inner
inner
3.3.25 normalized T-stress, T/σ, T/σ —T-stress divided by
YS
specimen supports in the four-point bending configuration. See
far-field stress or yield strength.
Fig. 5.
3.3.25.1 Discussion—T/σ is used as a first order measure of
3.3.31 outer span, S , L[L]—distance between outer
constraint, providing a definition and relative comparison of
outer
specimen supports in the four-point bending configuration. See
constraint for different crack geometries and loading condi-
Fig. 5.
tions.
3.3.25.2 Discussion—T/σ is used as a first order, quanti-
YS
3.3.32 specimen uniform cross section length, L [L]—length
fiable measure of constraint to describe crack front stress and
of the center section of the specimen with uniform cross
strain fields.
section. See Fig. 1.
3.3.26 one-parameter fracture—the use of K or J alone to
I -3/2
3.3.33 stress intensity factor, K, K , K [FL ]—see Termi-
J I
describe fracture conditions when the crack-tip fields are K- or
nology E1823. All K-values in this test method refer to Mode
J-dominant as defined in this test method.
I fracture.
3.3.27 parametric angle, ϕ—the elliptic angle of position
3.3.34 surface crack extension, ℓ [L]—an increase in crack
along the crack front, whereby the physical angle is trans-
length measured normal to original crack front (Fig. 7). Differs
formed to a position on a semi-circle with radius a (Fig. 1).
o
from Terminology E1823 due to two-dimensional nature of the
3.3.28 Q—a non-dimensional parameter that describes the
crack extension.
difference between the crack front stress field of interest
relative to a common reference field.
3.3.35 two-parameter fracture—the use of K or J together
I
3.3.28.1 Discussion—Q can be inferred by subtracting the
with a constraint term (such as T-stress or Q) to describe
crack front stress field for the T = 0 reference state from the
fracture conditions when the crack-tip fields are not K- or
stress field of interest in the specimen at a chosen normalized
J-dominant.
radial location in front of the crack tip on the crack plane. A
-2
3.3.36 T-stress, T [FL ]—a linear-elastic parameter used to
commonly used definition of Q derives from a plane-strain, T
quantify the first-order effects of constraint on near crack-tip
= 0, reference field such that:
stress and strain fields, and on the measured values of fracture
σ 2 σ ! rσ
~
yy yy T50 0
toughness.
Q[ at θ 5 0 and 5 2 (3)
σ J
3.3.36.1 Discussion—T-stress is a scalar value appearing in
where σ is the stress normal to the crack plane, r is the second term of the Williams power series expansion of the
yy
the radial distance ahead of the crack tip on the crack crack-tip stress fields, where the first two terms are given as:
FIG. 7 Required Measurements of Precrack Dimensions and Crack Extension
E2899 − 24
T 0 0 ately describes the test conditions: linear-elastic regime,
K
I
0 0 0 elastic-plastic regime, or the field-collapse regime.
σ r , θ 5 f θ 1 (4)
~ ! ~ !
ij ij
F G
=2πr
0 0 vT
4.5 If the test conditions do not lead to the field-collapse
regime, the test result is classified into either the linear-elastic
The νT term in σ appears only for plane strain condi-
zz
or the elastic-plastic regime. For tests demonstrating stable
tions. The T-stress term does not vary with r and θ.
crack extension, the local length of surface crack extension is
3.3.36.2 Discussion—A specimen with geometry and load-
reported. If a one-parameter description of the crack tip fields
ing combinations that create compressive (negative) T-stress
is appropriate (T ≥ 0) the result includes only K or J ;
ϕ ϕ ϕ
has low crack front constraint (reduced hydrostatic stress) and,
otherwise, the result includes K or J along with the value of
ϕ ϕ
for most ductile fracture processes, may have a higher mea-
T /σ to complete a two-parameter description of the test.
ϕ YS
sured fracture toughness than specimens with a T ≥ 0 configu-
ration. A geometry and loading combination that creates tensile
5. Significance and Use
(positive) T-stress has high crack front constraint (increased
5.1 Surface cracks are among the most common defects
hydrostatic stress) and may have a slightly decreased measured
found in structural components. An accurate characterization
fracture toughness compared to the T ≤ 0 configuration. See
and understanding of crack-front behavior is necessary to
Appendix X4 for further discussion.
ensure successful operation of a structure containing surface
3.3.36.3 Discussion—Some common negative T-stress con-
cracks. The testing of laboratory specimens with surface cracks
figurations include SC(T), M(T), SE(B) with crack size to
provides a means to understand and quantify surface crack
width ratio (a/W) of a/W < 0.4, and SE(T) with a/W < 0.6.
behavior, but the test results must be interpreted correctly to
Some common positive T-stress configurations include SC(B)
ensure transferability between the laboratory specimen and the
with deep cracks, SE(B) with a/W > 0.4, SE(T) with a/W > 0.7,
structure.
C(T), and DC(B).
5.2 Transferability refers to the capacity of a fracture
-2
3.3.37 T [FL ]—T-stress at the initiation angle (ϕ ) at
ϕ i mechanics methodology to correlate the crack-tip stress and
deformation level corresponding to CMOD .
i strain fields of different cracked bodies. Traditionally, the
correlation has been based on the presence at fracture of a
3.3.38 unstable crack extension [L]—an abrupt crack exten-
dominant, asymptotically singular, crack-tip field with ampli-
sion occurring with or without prior, stable crack extension.
tude set by the value of a single parameter, such as the stress
intensity factor, K , or the J-integral. For components and
I
4. Summary of Test Method
specimens with high crack-tip constraint, the singular crack-tip
4.1 The objective of this test method is to obtain the fracture
field dominates over microstructurally significant size scales
toughness of fatigue sharpened surface cracks in a constraint-
for loads ranging from globally linear-elastic conditions to
based framework, where the toughness is measured either at
moderately large-scale plasticity. For specimens with low
the initiation of stable crack extension or immediate instability.
crack-tip constraint, a dominant single-parameter crack-tip
The fracture toughness is quantified by either a single tough-
field exists only at low levels of plasticity. At higher levels of
ness value, or by two quantities, a toughness and a measure of
plasticity, the opening mode stress of the low constraint
constraint.
specimen is lower than predicted by the single-parameter,
asymptotically singular fields. Therefore, low constraint speci-
4.2 The test method consists of notching and fatigue sharp-
mens often exhibit larger fracture toughness than do high
ening (see Section 7) surface cracks into flat rectangular test
constraint specimens. If feasible, users are strongly encouraged
specimens and then monotonically applying tension or bending
to generate high constraint fracture toughness data using
force until the initiation of stable tearing is detected or
methods such as Test Methods E399 or E1820 prior to testing
immediate instability fails the specimen. The method requires
the surface crack geometry.
at a minimum the continuous collection of force during the test.
5.2.1 To address this phenomenon, two-parameter fracture
The continuous collection of CMOD is recommended for all
criteria are used to include the influence of crack-tip constraint.
tests, and is required when the limit of the linear-elastic regime
Crack-tip constraint has been quantified using various scalar
is exceeded.
parameters including the T-stress (10, 11, 12), Q (13, 14), stress
4.3 The method of detecting the onset of stable crack
triaxiality (15, 16), and α (17, 18). Fracture toughness in a
h
extension is not mandated by this test method; however,
two-parameter methodology is not a single value, but rather is
suggested methods are provided including electric potential
a curve that defines a critical locus of fracture toughness and
drop, crack mouth opening displacement, acoustic emission,
constraint values (2). Fig. 2 illustrates a toughness-constraint
and replicate samples. Other methods are acceptable if vali-
locus for application of two-parameter fracture mechanics to
dated as part of the test procedure.
structures. A structural analysis provides the driving force
curve for the configuration of interest, and is plotted with the
4.4 The approach used to analyze the test results includes
toughness-constraint locus obtained from specimen test data.
determining the location around the surface crack front where
Crack extension is predicted when the driving force curve
the initiation of crack extension occurred (ϕ ). See Annex A5.
i
passes through the toughness-constraint locus.
Analysis of the test record then compares crack-front condi-
tions and material properties against specific geometric length 5.3 Tests conducted with this method provide data to assist
scales of the specimen to determine which regime appropri- in the prediction of structural capability in the presence of a
E2899 − 24
surface crack by including a measure of crack-tip constraint in within 1% of the full working range. Each gauge shall be
the interpretation of fracture toughness values. This improves verified for linearity using an extensometer calibrator or other
the correlation of test specimen and structural conditions. To suitable device. The resolution of the calibrator at each
achieve the most accurate comparison, the conditions tested in displacement interval shall be within 0.00051 mm (0.000020
accordance with this test method should match the structure as in.). Readings shall be taken at ten equally spaced intervals
closely as possible. For conservative structural assessment, the over the working range of the gauge. The verification proce-
user should ensure that conditions in the test specimen produce dure shall be performed three times, removing and reinstalling
higher levels of constraint relative to the structure in applica- the gauge in the calibration fixture after each run. The required
tion of the data. Factors that influence test specimen conditions linearity shall correspond to a maximum deviation of 0.003
include, but are not limited to, specimen geometry, a/c, a/B, mm (0.0001 in.) of the individual displacement readings from
loading conditions, as well as the amount and type of crack a least-squares-best-fit straight line through the data.
extension that occurred during the test.
6.4 Crack Extension Instrumentation—This test method
NOTE 3—The use of a constraint-based framework for the analysis of
does not dictate the method(s) used to detect surface crack
surface cracks permits a more realistic assessment of structural capability.
extension. Common methods include using the CMOD
This approach generally leads to a less conservative assessment than
would be achieved, for example, by using a measure of high-constraint measurement, electric potential drop, or acoustic emission.
fracture toughness obtained from testing standard C(T) and SE(B)
Instrumentation shall be sufficiently calibrated to produce a
specimens of the material following Test Method E1820. It is essential that
consistent indication of surface crack extension and shall be
constraint effects measured in surface crack tests with this method be
recorded as stated in subsection 6.1 for archival use in
applied to any structural assessment with the requisite understanding to
evaluating the test results.
maintain appropriate levels of conservatism.
5.4 This test method does not address environmental effects 6.5 System Verification—It is recommended that the perfor-
mance of the force and displacement measuring systems be
or loading rate effects that may be significant in assessing
service integrity. verified before beginning a series of continuous tests. Calibra-
tion accuracy of displacement transducers shall be verified with
6. Apparatus due consideration for the temperature and environment of the
test. Force calibrations shall be conducted periodically and
6.1 Proper apparatus is required to meet the following
documented in accordance with the latest revision of Practices
minimum requirements: suitable test machine with proper
E4.
measurement of applied force, instrumentation to record speci-
men displacements, and tension or bending clevises with 6.6 Fixtures:
associated fixturing. Additional apparatus may be useful to 6.6.1 Tension Fixtures—The design of tension fixtures shall
enhance the detection of surface crack extension. See subsec- produce a uniform tension stress across the width and thickness
tion 6.4. The force and displacement measurements along with of the specimen gauge section. Friction grips or pin and clevis
any supplemental instrumentation must be synchronized and arrangements are acceptable. Careful attention must be given
fully recorded throughout the test, either digitally for process- to specimen and test machine alignment in either case. It is
ing by computer or autographically with an x-y plotter. The recommended, particularly with new specimen or clevis
apparatus should be configured as mechanically stiff as pos- designs, that the uniformity of the tension stress be verified
sible to reduce stored elastic energy during the test. This using a specimen instrumented with opposing strain gauges on
an unnotched specimen. The uniformity of strain across all
significantly improves the ability to detect the initiation of
stable crack extension. gauges should be confirmed as described in subsection 8.2.5.1.
The clevis portion of a pinned specimen design is typical of
6.2 Force Measurement—Testing machines shall have a
those found in other fracture test standards. A common
force measurement capability conforming to the requirements
configuration is shown in Fig. 3. The flat bottomed holes
of Practices E4. Applied force may be measured by any force
required for clevises in other standards are not required for this
transducer capable of being recorded continuously. Accuracy
method because specimen rotation is not a concern; clevis
of force measurements shall be within 1% of the working
holes may be round. The clevis, pins and other fixturing must
range.
be fabricated from materials with sufficient strength to prevent
6.3 Displacement Measurement—A mechanical displace-
yielding, brinelling, or excessive elastic deflection up to the
ment gauge or other methods (for example digital image
maximum force encountered during test. Fixtures should be
correlation) is used to measure the CMOD during the test to
fabricated to high quality standards.
establish a force versus CMOD record. The CMOD measure-
NOTE 4—Forces may be very high when testing tension specimens.
ment will aid in identifying the onset of stable tearing and
Clevis designs must accommodate the stress and specimens using the pin
enable verification of test assessment. CMOD measurement is
and clevis design will often require reinforcement at the pin hole to
required for all tests except those satisfying subsection 9.2.1,
prevent bearing yield or failure. This reinforcement can come from
Linear-Elastic Regime Assessment, for which CMOD mea- reducing the width, thickness, or both of the test section relative to the grip
section or by adding supplemental doubler plates. See example specimen
surement and analytical confirmation are recommended, but
designs in Fig. 4.
not required.
6.3.1 All displacement gauges shall have a calibrated range 6.6.2 Bending Fixtures—Fig. 5 shows the general propor-
no more than twice the maximum expected displacement tions of acceptable four-point bend fixtures. The fixture design
during the test. The gauge accuracy shall be demonstrated to be minimizes frictional effects by allowing the support rollers to
E2899 − 24
rotate and move slightly apart as the force on the specimen length and depth with a regular semi-elliptical shape. The
increases, thus permitting rolling contact. The outer support method of producing the starter notch and precrack should not
rollers are allowed limited motion along plane surfaces parallel
influence the resulting fracture behavior of the test specimen.
to the specimen, but are initially held against the inner stops Fatigue loading may be applied through bending, tension, or a
with low tension springs (such as rubber bands).
combination of both. The method of applying precrack forces
may, and likely will, vary from that used for the actual
7. Specimen Size, Configuration, and Preparation
monotonic test for surface crack extension. Precise control of
7.1 Principles of Test Specimen Design—Basic features of
the stress distribution across the specimen thickness during
surface crack specimen design are shown in Fig. 4. As
fatigue cycling is necessary to ensure the surface crack
discussed in Section 5, the intent of surface crack testing is
develops in the desired shape.
commonly motivated by understanding the effects of surface
7.5.2 Fatigue Crack Starter Notch—Many different pre-
cracks in structurally relevant configurations. In these
crack starter notches are possible as shown in Fig. 6. The
situations, it is important that the test specimen represent the
semi-elliptical starter notch is recommended to maximize the
structure, primarily in thickness, crack size, and material
likelihood of producing a fatigue crack of proper shape with a
condition. If the surface crack tests are not relevant to a specific
minimum of fatigue crack growth, but other shapes may offer
structure, but are intended to characterize the general response
advantages or simplify to the notch machining. The starter
of the material to surface defects, the specimen dimensions
notch may be cut by any available means. The plunge electrical
should be established using the expected toughness and the
discharge machining (EDM) method is the most common, but
length scales provided in subsections 9.2.1 (Linear-Elastic
conventional machining techniques and laser cutting have been
Regime Assessment) and 9.2.2 (Elastic-Plastic Regime
used effectively. The height of the notch, h, should be mini-
Assessment), depending on which of these regimes is relevant
mized. In practice, it should not exceed 1.0 mm (0.04 in.). As
to the designed test conditions. For general characterization,
shown in Fig. 6, it is recommended that the notch end with a
the crack configurations are recommended to span the range of
sharp “V” shape, and as a minimum the notch should end with
0.2 ≤ a/B ≤ 0.8 and 0.1 ≤ a/c ≤ 1.0. For practical purposes, the
a radius ≤ h/2. Generally, the effort to develop a technique for
minimum crack dimensions permitted are: a ≥ 1.0 mm and c ≥
producing sharp notches is a good investment, because the time
1.0 mm (0.04 in.).
required to start the precrack is greatly reduced.
7.2 Specimen Quantities—The needed quantity of test speci-
7.5.3 Fatigue Precrack Shape and Length—The fatigue
mens depends on the required reliability of the data. If the test
precrack must be fully established around the full perimeter of
results are to be used for design and evaluation of critical
the semi-ellipse. At all locations around the perimeter, the
structures, sufficient tests to understand the variability of
fatigue precrack shall extend a minimum 2h from the notch.
surface crack performance are strongly recommended. For
The final shape shall be a semi-ellipse within the tolerance
general characterization, a minimum of three tests of a given
allowed in subsection 8.4. If additional features are machined
specimen configuration is recommended. If multiple crack
into the starter notch for purposes of mechanical CMOD
configurations are to be included in the test program, then
measurement, the precrack shall be sufficiently long to extend
replicates of each specimen are recommended.
to or beyond a 60-degree envelope enclosing the starter notch
7.3 Tension Specimen Configuration—Tensile test specimen
and any features machined at the surface. See Fig. X3.1 for an
proportions are shown in Fig. 4. The controlling proportions
illustration.
are W ≥ 5 × 2c and L ≥ 2W.
7.5.4 Fatigue Precrack Procedures—The following require-
7.4 Bending Specimen Configuration—Bend test specimen
ments shall be followed when producing the fatigue precrack.
proportions are shown in Fig. 4. The controlling proportions
7.5.4.1 Fixtures—The development of a regular semi-
are W ≥ 5 × 2c and S /W ≥ 4, where Fig. 5 defines the
outer
elliptical precrack is dependent on uniform stress distribution
dimension S .
outer
(tension or bending) over the specimen cross-section. Test
7.5 Specimen Precracking—All specimens shall be pre-
fixtures and specimen alignment should be carefully addressed.
cracked in fatigue. Experience has shown that it is impractical
The quality and precision of all precracking fixtures should be
to obtain a reproducibly sharp, narrow machined notch that
equivalent to those used for testing.
will simulate a natural crack well enough to provide a
7.5.4.2 Material Condition—Fatigue precracking shall be
satisfactory fracture toughness test result. The most effective
performed with the material in the final heat-treated, mechani-
artifice for this purpose is a narrow notch from which extends
cally worked, or environmentally conditioned state. Interme-
a comparatively short fatigue crack, called the precrack. (A
diate treatments between fatigue precracking and testing are
fatigue precrack is produced by cyclically loading the notched
4 acceptable only when such treatments are necessary to simulate
specimen for a number of cycles usually between about 10
6 the conditions of a specific structural application; such depar-
and 10 depending on specimen size, notch preparation, and
ture from recommended practice shall be explicitly reported.
stress intensity level.) The dimensions of the notch and
7.5.4.3 Fatigue Precrack Loading Requirements—The
precrack, and the sharpness of the precrack shall meet specified
maximum force applied to the specimen during precracking,
conditions that can be readily met with most engineering
including tension, bending, or combined tension/bending, shall
materials.
limit the stress intensity to the lesser of K < 0.6K or
7.5.1 Surface Crack Precracking Objectives—The precrack- maxϕ est
ing procedure must produce a fatigue crack of the intended 30MPa=m (27ksi=in) for the first 50% of the precrack and
E2899 − 24
the lesser of K < 0.5K or 25MPa=m (22.8ksi=in) for (30MPa=m and 25MPa=m) remain unchanged. Environ-
max-ϕ est
mental effects on material during precracking or test, other
the final 50% of the precrack, where K is a provisional
est
than temperature, are not within the scope of this test
estimated material toughness and K is the maximum value
max-ϕ
method.
of stress intensity occurring around the crack perimeter as
calculated by equations in Appendix X1. Precracking should
8. Procedure
be conducted at as low a K as practical. K is based on
max-ϕ max-ϕ
the instantaneous precrack size; therefore, forces required to
8.1 Overview and Objectives—The test procedure uses dis-
achieve K should be evaluated as the precrack grows.
placement control to apply monotonically increasing force or
max-ϕ
Small starting notches may result in high stresses to achieve the
moment to a properly precracked test specimen until the crack
initial K values allowed above. At no time during pre- begins to extend in a stable fashion, or until the specimen
max-ϕ
cracking shall the far field stress exceed 80% of the σ (0.2%
breaks due to unstable crack extension without any prior stable
YS
offset). crack extension. If the specimen breaks due to unstable fracture
(1) Precracking forces are evaluated following the test by without any prior stable crack extension, the initial crack size
is measured and the location of the initiation of surface crack
using K in place of the provisional estimated toughness, K .
ϕ est
To develop precracking parameters, K for the material may extension identified if possible. This information along with
est
the applied force or moment is then used to evaluate the test in
be estimated from the K of previous surface crack tests or
ϕ
accordance with Section 9. If unstable fracture does not occur,
from linear-elastic plane strain fracture toughness values de-
the force and crack extension instrumentation are monitored
termined by Test Method E399 or E1820. If no existing
continuously to detect the onset of stable crack extension.
material toughness information is available, an acceptable
When stable crack extension is detected, the force or moment
limiting value of K can often be estimated by ensuring
max-ϕ
on the specimen is removed (or reduced) and a method of
= =
K /E < 0.00016 m (0.001 in), though not to exceed the
max-ϕ
marking the current state of surface crack extension is applied
values in subsection 7.5.4.3. This relationship may not suffi-
to the test specimen. Finally, force or moment is re-applied in
ciently limit precracking conditions for high elastic modulus,
a monotonically increasing manner until specimen failure
low toughness materials such as very high strength steels.
occurs. The initial crack size and crack extension are measured
(2) The stress ratio, R, during precracking is not prescribed,
and are used along with the crack initiation force or moment to
but is most commonly s
...

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.
´1
Designation: E2899 − 19 E2899 − 24
Standard Test Method for
Measurement of Initiation Toughness in Surface Cracks
Under Tension and Bending
This standard is issued under the fixed designation E2899; 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.
ε NOTE—Editorial corrections were made throughout in May 2020.
1. Scope
1.1 This test method describes the method for testing fatigue-sharpened, semi-elliptically shaped surface cracks in rectangular flat
panels subjected to monotonically increasing tension or bending. Tests quantify the crack-tip conditions at initiation of stable crack
extension or immediate unstable crack extension.
1.2 This test method applies to the testing of metallic materials not limited by strength, thickness, or toughness. Materials are
assumed to be essentially homogeneous and free of residual stress. Tests may be conducted at any appropriate temperature. The
effects of environmental factors and sustained or cyclic loads are not addressed in this test method.
1.3 This test method describes all necessary details for the user to test for the initiation of crack extension in surface crack test
specimens. Specific requirements and recommendations are provided for test equipment, instrumentation, test specimen design,
and test procedures.
1.4 Tests of surface cracked, laboratory-scale specimens as described in this test method may provide a more accurate
understanding of full-scale structural performance in the presence of surface cracks. The provided recommendations help to assure
test methods and data are applicable to the intended purpose.
1.5 This test method prescribes a consistent methodology for test and analysis of surface cracks for research purposes and to assist
in structural assessments. The methods described here utilize a constraint-based framework (1, 2) to evaluate the fracture behavior
of surface cracks.
NOTE 1—Constraint-based framework. In the context of this test method, constraint is used as a descriptor of the three-dimensional stress and strain fields
in the near vicinity of the crack tip, where material contractions due to the Poisson effect may be suppressed and therefore produce an elevated, tensile
stress state (3, 4). (See further discussions in Terminology and Significance and Use.) When a parameter describing this stress state, or constraint, is used
with the standard measure of crack-tip stress amplitude (K or J), the resulting two-parameter characterization broadens the ability of fracture mechanics
to accurately predict the response of a crack under a wider range of loading. The two-parameter methodology produces a more complete description of
the crack-tip conditions at the initiation of crack extension. The effects of constraint on measured fracture toughness are material dependent and are
governed by the effects of the crack-tip stress-strain state on the micromechanical failure processes specific to the material. Surface crack tests conducted
with this test method can help to quantify the material sensitivity to constraint effects and to establish the degree to which the material toughness correlates
with a constraint-based fracture characterization.
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue and Fracture and is the direct responsibility of Subcommittee E08.07 on Fracture
Mechanics.
Current edition approved Nov. 15, 2019Feb. 15, 2024. Published January 2020April 2024. Originally approved in 2013. Last previous edition approved in 20152019 as
ɛ1
E2899 – 15.E2899 – 19 . DOI: 10.1520/E2899-19E01.10.1520/E2899-24.
The boldface numbers in parentheses refer to the list of references at the end of this test method.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2899 − 24
1.6 This test method provides a quantitative framework to categorize test specimen conditions into one of three regimes: (I) a
linear-elastic regime, (II) an elastic-plastic regime, or (III) a field-collapse regime. Based on this categorization, analysis techniques
and guidelines are provided to determine an applicable crack-tip parameter for the linear-elastic regime (K or J) or the
elastic-plastic regime (J), and an associated constraint parameter. Recommendations are provided to assess the test data in the
context of a toughness-constraint locus (2). For tension loading, a computer program referred to as TASC V1.0.2 (Tool for Analysis
of Surface Cracks) may be used to perform the analytical assessments in Section 9, Analysis of Results. The user is directed to
other resources for evaluation of the test specimen in the field-collapse regime when extensive plastic deformation in the specimen
eliminates the identifiable crack-front fields of fracture mechanics.
NOTE 2—TASC. The computer program TASC is available at no charge either at https://software.nasa.gov/software/MFS-33082-1 or at https://
sourceforge.net/projects/tascnasa/. The use of TASC relieves the user of the burden of performing unique elastic-plastic finite element analyses for each
test performed in the elastic-plastic regime. For the purposes of this standard, TASC calculations are equivalent to finite element analysis results. Users
of TASC should follow the methodologies in Annex A6 for establishing analysis material property inputs. Documentation on the development, verification
and validation of TASC is provided in references (5, 6, 7, 8).
1.7 The specimen design and test procedures described in this test method may be applied to evaluation of surface cracks in welds;
however, the methods described in this test method to analyze test measurements may not be applicable. Weld fracture tests
generally have complicating features beyond the scope of data analysis in this test method, including the effects of residual stress,
microstructural variability, and non-uniform strength. These effects will influence test results and must be considered in the
interpretation of measured quantities.
1.8 This test method is not intended for testing surface cracks in steel in the cleavage regime. Such tests are outside the scope of
this test method. A methodology for evaluation of cleavage fracture toughness in ferritic steels over the ductile-to-brittle region
using C(T) and SE(B) specimens can be found in Test Method E1921.
1.9 Units—The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information
only.
1.10 This practice may involve hazardous materials, operations, and equipment. 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.
1.11 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:
C1421 Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature
E4 Practices for Force Calibration and Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E8/E8M Test Methods for Tension Testing of Metallic Materials
E111 Test Method for Young’s Modulus, Tangent Modulus, and Chord Modulus
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
E647 Test Method for Measurement of Fatigue Crack Growth Rates
E740 Practice for Fracture Testing with Surface-Crack Tension Specimens
E1012 Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force
Application
E1820 Test Method for Measurement of Fracture Toughness
E1823 Terminology Relating to Fatigue and Fracture Testing
E1921 Test Method for Determination of Reference Temperature, T , for Ferritic Steels in the Transition Range
3. Terminology
3.1 For definitions of terms used in this Test Method, Terminologies E6 and E1823 apply.
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.
E2899 − 24
3.2 Symbols:
3.2.1 crack depth, a [L]—see Terminology E1823 and Fig. 1 in this test method.
3.2.1.1 Discussion—
In this test method, the term a is the original surface crack depth, as determined in subsection 8.4, used in the evaluation of the
o
test.
FIG. 1 Test Specimen and Crack Configurations
E2899 − 24
FIG. 2 Toughness-Constraint Locus with Example Trajectories
FIG. 3 Recommended Configuration of Tension Testing Clevis
NOTE 1—Flat bottomed holes are not required, but may be used in configurations found in Test Methods E399 or E1820.
3.2.2 crack-mouth opening displacement, CMOD [L]—see Terminology E1823 and Fig. 1 in this test method.
3.2.3 force, P [F]—see Terminology E1823.
E2899 − 24
FIG. 4 Specimen Design Principles
FIG. 5 Recommended Configuration of Bend Testing Apparatus
-1 -2
3.2.4 J-integral, J [FL or FLL ]—see Terminology E1823.
E2899 − 24
FIG. 6 Fatigue Crack Starter Notch Configuration
-2
3.2.5 modulus of elasticity, E [FL ]—see Terminology E1823.
3.2.6 net section area, A [L ]—see Terminology E1823. For surface cracks A = WB – πa c /2.
N N 0 0
3.2.7 notch height, h [L]—the distance between the parallel faces of the machined notch prior to specimen deformation (Fig. 6).
-3/2
3.2.8 plane-strain fracture toughness, K [FL ]—see Terminology E1823.
Ic
3.2.9 Poisson’s ratio, ν—see Terminology E6.
3.2.10 specimen thickness, B [L]—see Terminology E1823 and Fig. 1 from this test method.
3.2.10 specimen width, W [L]—see Terminology E1823 and Fig. 1 from this test method.
3.2.11 stable crack extension, [L]—see Terminology E1823.
3.2.12 stress ratio, R—see Terminology E1823.
3.2.13 surface crack length, 2c [L]—see Terminology E1823 and Fig. 1 in this test method.
3.2.13.1 Discussion—
In this test method, the term 2c is the original surface crack length, as determined in subsection 8.4, used in the evaluation of the
test.
-2
3.2.14 yield strength, σ [FL ]—see Terminology E1823, as determined by 0.2% offset strain method.
YS
3.3 Definitions of Terms Specific to This Standard:
E2899 − 24
3.3.1 characteristic length, r , r [L]—a physical length measured post-test on the specimen fracture surface and compared to
ϕa ϕb
the length scale provided by the deformation limit. r is the distance measured on the crack plane normal to the crack front at
ϕa
the parametric angle ϕ to the front face (cracked face) of the specimen. r is the distance measured on the crack plane normal
i ϕb
to the crack front at the parametric angle ϕ to the back face (uncracked face) or side of the specimen (Fig. A3.1).
i
3.3.2 constraint, Ω—in the context of this test method, constraint is a descriptor of the three dimensional stress and strain fields
in the near vicinity of the crack tip where material contractions due to the Poisson effect may be suppressed and therefore produce
an elevated, three-dimensional tensile (hydrostatic) stress state. An elevated hydrostatic stress state suppresses material yielding
and permits larger stresses to develop. The material, geometry, and externally applied loads influence the development of the
elevated hydrostatic stress state.
3.3.3 elastic-plastic regime—conditions in a test specimen where crack-tip deformations exceed limits of the linear-elastic regime
defined in this test method, but J alone or J and a constraint term still characterize the crack-tip stress and strain fields. The
non-dimensional parameters, C and C , define the deformation limits for validity of the elastic-plastic regime in this test method.
Ja Jb
3.3.3.1 Discussion—
Non-dimensional deformation limits such as C ,C and C are commonly designated by the letter “M” in the literature (9).
K Ja Jb
3.3.4 elastic-plastic regime crack size deformation limit, C —the non-dimensional, upper limit of deformation for the
Ja
elastic-plastic regime based on limiting the crack-tip opening displacement relative to the crack size.
3.3.5 elastic-plastic regime ligament deformation limit, C —the non-dimensional, upper limit of deformation for the elastic-
Jb
plastic regime based on limiting plasticity in the remaining ligament.
-2
3.3.6 far field stress, σ [FL ]—stress far removed from the crack plane resulting from applied forces or moments.
3.3.6.1 Discussion—
For applied tensile forces, the far field stress is the average stress over the gross area, that is σ = P/WB. For applied bending
moments, the far field stress is the maximum tensile outer fiber stress across the gross area, that is σ = 6M/(WB ).
3.3.7 field-collapse regime—conditions in a test specimen where crack-tip deformations exceed the limit of the elastic-plastic
regime defined in this test method. Extensive plastic deformation in the specimen eliminates the identifiable crack-front fields of
fracture mechanics, which precludes analysis of test conditions in this test method.
3.3.8 initiation angle, ϕ —the parametric angle determined in accordance with Annex A5 that identifies the location along the
i
crack perimeter where the test result is evaluated.
3.3.9 initiation of surface crack extension—in the context of this test method, the point during the test when, under monotonically
increasing force or moment, the precrack extends a small but consistently measurable amount by stable, ductile tearing, or when
the precrack extends in an immediate, unstable ductile mode, failing the specimen.
3.3.9.1 Discussion—
Parameters associated with the initiation of surface crack extension are designated herein with a subscript i (for example, P ) and
i
define the state at which the crack front fields are characterized to render the toughness test result. The initiation of surface crack
extension will generally be a local occurrence along the perimeter of a surface crack. Due to this localization, defining and
experimentally quantifying a universal measure of relative or absolute crack extension for the surface crack geometry is not
practical with commonly available laboratory equipment. Therefore, if identifiable, the extent and location of stable crack
extension is recorded as an integral part of the test result. See subsection 8.3.4. In this context, the surface crack toughness result
identifies a point on the material’s tearing resistance curve as influenced by the local crack tip constraint conditions. See J-R curve
and K-R curve definitions in Terminology E1823.
3.3.10 initiation crack mouth opening displacement, CMOD [L]—the CMOD at which initiation of surface crack extension
i
occurs.
3.3.11 initiation force, P [F]—the force at which initiation of surface crack extension occurs.
i
3.3.12 initiation moment M [FL]—the applied moment at which initiation of surface crack extension occurs.
i
E2899 − 24
3.3.13 J-dominance—crack-tip conditions where the elastic-plastic stress and strain fields are quantified by the value of the
J-integral without constraint adjustment.
3.3.13.1 Discussion—
Crack-tip fields described as J-dominant in this test method exist when elastic-plastic conditions develop at the crack front and high
crack-tip constraint conditions prevail (for example, T-stress ≥ 0). J-dominant fields permit the use of a single parameter
characterization of fracture toughness in terms of a critical J-value. In this test method, J-dominant conditions prevail to higher
levels of crack-tip deformation than do K-dominant conditions.
-1 -2
3.3.14 J [FL or FLL ]—a value of the J-integral calculated from K using the equation:
K I
2 2
K 1 2 ν
~ !
I
J 5 (1)
K
E
that is valid for linear-elastic, plane-strain conditions.
-1 -2
3.3.15 J [FL or FLL ]—the peak value of the J-integral around the perimeter of the surface crack during monotonic loading.
p
-1 -2
3.3.16 J [FL or FLL ]—the J-integral value at the initiation angle (ϕ ) when the specimen reaches the initiation crack mouth
ϕ i
opening displacement (CMOD ).
i
3.3.17 K-dominance—crack-tip conditions where the stress and strain fields immediately surrounding the crack-tip plastic zone are
quantified by the stress intensity factor, K , without constraint adjustment.
I
3.3.17.1 Discussion—
Crack-tip fields defined as K-dominant exist when globally linear-elastic conditions prevail in the specimen (see 3.3.22.1) together
with high crack-tip constraint conditions (for example, T-stress ≥ 0). K-dominant fields permit the use of a single parameter fracture
criterion expressed as a critical K-value, and are also J-dominant by definition.
-3/2
3.3.18 K [FL ]—the peak value of the stress intensity factor around the perimeter of the surface crack during monotonic
p
loading.
-3/2
3.3.19 K [FL ]—the stress intensity factor at the initiation angle (ϕ ) with applied initiation force (P ), or moment (M ).
ϕ i i i
-3/2
3.3.20 K [FL ]—the maximum value of stress intensity occurring around the crack perimeter during fatigue precracking.
max-ϕ
3.3.21 length scale [L]—a calculated length that is compared to a characteristic length (r , r ) of the test specimen to evaluate
ϕa ϕb
the test result or determine test validity.
3.3.21.1 Discussion—
The length scales are defined by a non-dimensional deformation limit, C, multiplied by the ratio of J/σ in the form:
YS
J
lengthscale5 C (2)
σ
YS
3.3.22 linear-elastic regime—conditions in a test specimen where the stress and strain fields enclosing the crack-tip plastic zone
are quantified by K alone, or by K and a constraint term.
I I
3.3.22.1 Discussion—
The linear-elastic regime applies when the amount of deformation at the crack tip remains small relative to the dimensions of the
specimen. Conditions in the linear-elastic regime do not necessarily imply high constraint, for example, the T-stress may be
positive or negative. The limit, C , sets the maximum deformation allowed at the crack tip for the linear-elastic regime in this test
K
method.
3.3.23 linear-elastic regime deformation limit, C —the non-dimensional, upper limit of deformation for the linear-elastic regime.
K
3.3.24 moment, M [FL]—the value of the applied moment at the crack plane of a specimen during a test.
M = (S – S ) P/4 for four-point bending.
outer inner
3.3.25 normalized T-stress, T/σ, T/σ —T-stress divided by far-field stress or yield strength.
YS
E2899 − 24
3.3.25.1 Discussion—
T/σ is used as a first order measure of constraint, providing a definition and relative comparison of constraint for different crack
geometries and loading conditions.
3.3.25.2 Discussion—
T/σ is used as a first order, quantifiable measure of constraint to describe crack front stress and strain fields.
YS
3.3.26 one-parameter fracture—the use of K or J alone to describe fracture conditions when the crack-tip fields are K- or
I
J-dominant as defined in this test method.
3.3.27 parametric angle, ϕ—the elliptic angle of position along the crack front, whereby the physical angle is transformed to a
position on a semi-circle with radius a (Fig. 1).
o
3.3.28 Q—a non-dimensional parameter that describes the difference between the crack front stress field of interest relative to a
common reference field.
3.3.28.1 Discussion—
Q can be inferred by subtracting the crack front stress field for the T = 0 reference state from the stress field of interest in the
specimen at a chosen normalized radial location in front of the crack tip on the crack plane. A commonly used definition of Q
derives from a plane-strain, T = 0, reference field such that:
σ 2 σ ! rσ
~
yy yy T50 0
Q[ atθ 5 0and 5 2 (3)
σ J
where σ is the stress normal to the crack plane, r is the radial distance ahead of the crack tip on the crack plane (see Fig.
yy
1), σ is the flow stress (average of the yield and ultimate strength). Alternatively σ can be substituted for σ in the above
0 YS 0
equation.
3.3.29 Q —value of Q at the initiation angle (ϕ ) at deformation level corresponding to CMOD .
ϕ i i
3.3.30 inner span, S , L[L]—distance between inner specimen supports in the four-point bending configuration. See Fig. 5.
inner
3.3.31 outer span, S , L[L]—distance between outer specimen supports in the four-point bending configuration. See Fig. 5.
outer
3.3.32 specimen uniform cross section length, L [L]—length of the center section of the specimen with uniform cross section. See
Fig. 1.
-3/2
3.3.33 stress intensity factor, K, K , K [FL ]—see Terminology E1823. All K-values in this test method refer to Mode I fracture.
J I
3.3.34 surface crack extension, ℓ [L]—an increase in crack length measured normal to original crack front (Fig. 7). Differs from
Terminology E1823 due to two-dimensional nature of the crack extension.
3.3.35 two-parameter fracture—the use of K or J together with a constraint term (such as T-stress or Q) to describe fracture
I
conditions when the crack-tip fields are not K- or J-dominant.
-2
3.3.36 T-stress, T [FL ]—a linear-elastic parameter used to quantify the first-order effects of constraint on near crack-tip stress
and strain fields, and on the measured values of fracture toughness.
3.3.36.1 Discussion—
T-stress is a scalar value appearing in the second term of the Williams power series expansion of the crack-tip stress fields, where
the first two terms are given as:
T 0 0
K
I
0 0 0
σ r , θ 5 f θ 1 (4)
~ ! ~ !
ij ij
F G
=2πr
0 0 vT
The νT term in σ appears only for plane strain conditions. The T-stress term does not vary with r and θ.
zz
3.3.36.2 Discussion—
A specimen with geometry and loading combinations that create compressive (negative) T-stress has low crack front constraint
E2899 − 24
FIG. 7 Required Measurements of Precrack Dimensions and Crack Extension
(reduced hydrostatic stress) and, for most ductile fracture processes, may have a higher measured fracture toughness than
specimens with a T ≥ 0 configuration. A geometry and loading combination that creates tensile (positive) T-stress has high crack
front constraint (increased hydrostatic stress) and may have a slightly decreased measured fracture toughness compared to the T
≤ 0 configuration. See Appendix X4 for further discussion.
3.3.36.3 Discussion—
Some common negative T-stress configurations include SC(T), M(T), SE(B) with crack size to width ratio (a/W) of a/W < 0.4, and
SE(T) with a/W < 0.6. Some common positive T-stress configurations include SC(B) with deep cracks, SE(B) with a/W > 0.4,
SE(T) with a/W > 0.7, C(T), and DC(B).
-2
3.3.37 T [FL ]—T-stress at the initiation angle (ϕ ) at deformation level corresponding to CMOD .
ϕ i i
3.3.38 unstable crack extension [L]—an abrupt crack extension occurring with or without prior, stable crack extension.
4. Summary of Test Method
4.1 The objective of this test method is to obtain the fracture toughness of fatigue sharpened surface cracks in a constraint-based
framework, where the toughness is measured either at the initiation of stable crack extension or immediate instability. The fracture
toughness is quantified by either a single toughness value, or by two quantities, a toughness and a measure of constraint.
4.2 The test method consists of notching and fatigue sharpening (see Section 7) surface cracks into flat rectangular test specimens
and then monotonically applying tension or bending force until the initiation of stable tearing is detected or immediate instability
fails the specimen. The method requires at a minimum the continuous collection of force during the test. The continuous collection
of CMOD is recommended for all tests, and is required when the limit of the linear-elastic regime is exceeded.
4.3 The method of detecting the onset of stable crack extension is not mandated by this test method; however, suggested methods
are provided including electric potential drop, crack mouth opening displacement, acoustic emission, and replicate samples. Other
methods are acceptable if validated as part of the test procedure.
4.4 The approach used to analyze the test results includes determining the location around the surface crack front where the
initiation of crack extension occurred (ϕ ). See Annex A5. Analysis of the test record then compares crack-front conditions and
i
material properties against specific geometric length scales of the specimen to determine which regime appropriately describes the
test conditions: linear-elastic regime, elastic-plastic regime, or the field-collapse regime.
4.5 If the test conditions do not lead to the field-collapse regime, the test result is classified into either the linear-elastic or the
E2899 − 24
elastic-plastic regime. For tests demonstrating stable crack extension, the local length of surface crack extension is reported. If a
one-parameter description of the crack tip fields is appropriate (T ≥ 0) the result includes only K or J ; otherwise, the result
ϕ ϕ ϕ
includes K or J along with the value of T /σ to complete a two-parameter description of the test.
ϕ ϕ ϕ YS
5. Significance and Use
5.1 Surface cracks are among the most common defects found in structural components. An accurate characterization and
understanding of crack-front behavior is necessary to ensure successful operation of a structure containing surface cracks. The
testing of laboratory specimens with surface cracks provides a means to understand and quantify surface crack behavior, but the
test results must be interpreted correctly to ensure transferability between the laboratory specimen and the structure.
5.2 Transferability refers to the capacity of a fracture mechanics methodology to correlate the crack-tip stress and strain fields of
different cracked bodies. Traditionally, the correlation has been based on the presence at fracture of a dominant, asymptotically
singular, crack-tip field with amplitude set by the value of a single parameter, such as the stress intensity factor, K , or the J-integral.
I
For components and specimens with high crack-tip constraint, the singular crack-tip field dominates over microstructurally
significant size scales for loads ranging from globally linear-elastic conditions to moderately large-scale plasticity. For specimens
with low crack-tip constraint, a dominant single-parameter crack-tip field exists only at low levels of plasticity. At higher levels
of plasticity, the opening mode stress of the low constraint specimen is lower than predicted by the single-parameter,
asymptotically singular fields. Therefore, low constraint specimens often exhibit larger fracture toughness than do high constraint
specimens. If feasible, users are strongly encouraged to generate high constraint fracture toughness data using methods such as Test
Methods E399 or E1820 prior to testing the surface crack geometry.
5.2.1 To address this phenomenon, two-parameter fracture criteria are used to include the influence of crack-tip constraint.
Crack-tip constraint has been quantified using various scalar parameters including the T-stress (10, 11, 12),Q (13, 14), stress
triaxiality (15, 16), and α (17, 18). Fracture toughness in a two-parameter methodology is not a single value, but rather is a curve
h
that defines a critical locus of fracture toughness and constraint values (2).Fig. 2 illustrates a toughness-constraint locus for
application of two-parameter fracture mechanics to structures. A structural analysis provides the driving force curve for the
configuration of interest, and is plotted with the toughness-constraint locus obtained from specimen test data. Crack extension is
predicted when the driving force curve passes through the toughness-constraint locus.
5.3 Tests conducted with this method provide data to assist in the prediction of structural capability in the presence of a surface
crack by including a measure of crack-tip constraint in the interpretation of fracture toughness values. This improves the correlation
of test specimen and structural conditions. To achieve the most accurate comparison, the conditions tested in accordance with this
test method should match the structure as closely as possible. For conservative structural assessment, the user should ensure that
conditions in the test specimen produce higher levels of constraint relative to the structure in application of the data. Factors that
influence test specimen conditions include, but are not limited to, specimen geometry, a/c,a/B, loading conditions, as well as the
amount and type of crack extension that occurred during the test.
NOTE 3—The use of a constraint-based framework for the analysis of surface cracks permits a more realistic assessment of structural capability. This
approach generally leads to a less conservative assessment than would be achieved, for example, by using a measure of high-constraint fracture toughness
obtained from testing standard C(T) and SE(B) specimens of the material following Test Method E1820. It is essential that constraint effects measured
in surface crack tests with this method be applied to any structural assessment with the requisite understanding to maintain appropriate levels of
conservatism.
5.4 This test method does not address environmental effects or loading rate effects that may be significant in assessing service
integrity.
6. Apparatus
6.1 Proper apparatus is required to meet the following minimum requirements: suitable test machine with proper measurement of
applied force, instrumentation to record specimen displacements, and tension or bending clevises with associated fixturing.
Additional apparatus may be useful to enhance the detection of surface crack extension. See subsection 6.4. The force and
displacement measurements along with any supplemental instrumentation must be synchronized and fully recorded throughout the
test, either digitally for processing by computer or autographically with an x-y plotter. The apparatus should be configured as
mechanically stiff as possible to reduce stored elastic energy during the test. This significantly improves the ability to detect the
initiation of stable crack extension.
6.2 Force Measurement—Testing machines shall have a force measurement capability conforming to the requirements of Practices
E2899 − 24
E4. Applied force may be measured by any force transducer capable of being recorded continuously. Accuracy of force
measurements shall be within 1% of the working range.
6.3 Displacement Measurement—A mechanical displacement gauge or other methods (for example digital image correlation) is
used to measure the CMOD during the test to establish a force versus CMOD record. The CMOD measurement will aid in
identifying the onset of stable tearing and enable verification of test assessment. CMOD measurement is required for all tests
except those satisfying subsection 9.2.1, Linear-Elastic Regime Assessment, for which CMOD measurement and analytical
confirmation are recommended, but not required.
6.3.1 All displacement gauges shall have a calibrated range no more than twice the maximum expected displacement during the
test. The gauge accuracy shall be demonstrated to be within 1% of the full working range. Each gauge shall be verified for linearity
using an extensometer calibrator or other suitable device. The resolution of the calibrator at each displacement interval shall be
within 0.00051 mm (0.000020 in.). Readings shall be taken at ten equally spaced intervals over the working range of the gauge.
The verification procedure shall be performed three times, removing and reinstalling the gauge in the calibration fixture after each
run. The required linearity shall correspond to a maximum deviation of 0.003 mm (0.0001 in.) of the individual displacement
readings from a least-squares-best-fit straight line through the data.
6.4 Crack Extension Instrumentation—This test method does not dictate the method(s) used to detect surface crack extension.
Common methods include using the CMOD measurement, electric potential drop, or acoustic emission. Instrumentation shall be
sufficiently calibrated to produce a consistent indication of surface crack extension and shall be recorded as stated in subsection
6.1 for archival use in evaluating the test results.
6.5 System Verification—It is recommended that the performance of the force and displacement measuring systems be verified
before beginning a series of continuous tests. Calibration accuracy of displacement transducers shall be verified with due
consideration for the temperature and environment of the test. Force calibrations shall be conducted periodically and documented
in accordance with the latest revision of Practices E4.
6.6 Fixtures:
6.6.1 Tension Fixtures—The design of tension fixtures shall produce a uniform tension stress across the width and thickness of the
specimen gauge section. Friction grips or pin and clevis arrangements are acceptable. Careful attention must be given to specimen
and test machine alignment in either case. It is recommended, particularly with new specimen or clevis designs, that the uniformity
of the tension stress be verified using a specimen instrumented with opposing strain gauges on an unnotched specimen. The
uniformity of strain across all gauges should be confirmed as described in subsection 8.2.5.1. The clevis portion of a pinned
specimen design is typical of those found in other fracture test standards. A common configuration is shown in Fig. 3. The flat
bottomed holes required for clevises in other standards are not required for this method because specimen rotation is not a concern;
clevis holes may be round. The clevis, pins and other fixturing must be fabricated from materials with sufficient strength to prevent
yielding, brinelling, or excessive elastic deflection up to the maximum force encountered during test. Fixtures should be fabricated
to high quality standards.
NOTE 4—Forces may be very high when testing tension specimens. Clevis designs must accommodate the stress and specimens using the pin and clevis
design will often require reinforcement at the pin hole to prevent bearing yield or failure. This reinforcement can come from reducing the width, thickness,
or both of the test section relative to the grip section or by adding supplemental doubler plates. See example specimen designs in Fig. 4.
6.6.2 Bending Fixtures—Fig. 5 shows the general proportions of acceptable four-point bend fixtures. The fixture design minimizes
frictional effects by allowing the support rollers to rotate and move slightly apart as the force on the specimen increases, thus
permitting rolling contact. The outer support rollers are allowed limited motion along plane surfaces parallel to the specimen, but
are initially held against the inner stops with low tension springs (such as rubber bands).
7. Specimen Size, Configuration, and Preparation
7.1 Principles of Test Specimen Design—Basic features of surface crack specimen design are shown in Fig. 4. As discussed in
Section 5, the intent of surface crack testing is commonly motivated by understanding the effects of surface cracks in structurally
relevant configurations. In these situations, it is important that the test specimen represent the structure, primarily in thickness,
crack size, and material condition. If the surface crack tests are not relevant to a specific structure, but are intended to characterize
the general response of the material to surface defects, the specimen dimensions should be established using the expected
toughness and the length scales provided in subsections 9.2.1 (Linear-Elastic Regime Assessment) and 9.2.2 (Elastic-Plastic
E2899 − 24
Regime Assessment), depending on which of these regimes is relevant to the designed test conditions. For general characterization,
the crack configurations are recommended to span the range of 0.2 ≤ a/B ≤ 0.8 and 0.1 ≤ a/c ≤ 1.0. For practical purposes, the
minimum crack dimensions permitted are: a ≥ 1.0 mm and c ≥ 1.0 mm (0.04 in.).
7.2 Specimen Quantities—The needed quantity of test specimens depends on the required reliability of the data. If the test results
are to be used for design and evaluation of critical structures, sufficient tests to understand the variability of surface crack
performance are strongly recommended. For general characterization, a minimum of three tests of a given specimen configuration
is recommended. If multiple crack configurations are to be included in the test program, then replicates of each specimen are
recommended.
7.3 Tension Specimen Configuration—Tensile test specimen proportions are shown in Fig. 4. The controlling proportions are W
≥ 5 × 2c and L ≥ 2W.
7.4 Bending Specimen Configuration—Bend test specimen proportions are shown in Fig. 4. The controlling proportions are W ≥
5 × 2c and S /W ≥ 4, where Fig. 5 defines the dimension S .
outer outer
7.5 Specimen Precracking—All specimens shall be precracked in fatigue. Experience has shown that it is impractical to obtain a
reproducibly sharp, narrow machined notch that will simulate a natural crack well enough to provide a satisfactory fracture
toughness test result. The most effective artifice for this purpose is a narrow notch from which extends a comparatively short
fatigue crack, called the precrack. (A fatigue precrack is produced by cyclically loading the notched specimen for a number of
4 6
cycles usually between about 10 and 10 depending on specimen size, notch preparation, and stress intensity level.) The
dimensions of the notch and precrack, and the sharpness of the precrack shall meet specified conditions that can be readily met
with most engineering materials.
7.5.1 Surface Crack Precracking Objectives—The precracking procedure must produce a fatigue crack of the intended length and
depth with a regular semi-elliptical shape. The method of producing the starter notch and precrack should not influence the
resulting fracture behavior of the test specimen. Fatigue loading may be applied through bending, tension, or a combination of
both. The method of applying precrack forces may, and likely will, vary from that used for the actual monotonic test for surface
crack extension. Precise control of the stress distribution across the specimen thickness during fatigue cycling is necessary to
ensure the surface crack develops in the desired shape.
7.5.2 Fatigue Crack Starter Notch—Many different precrack starter notches are possible as shown in Fig. 6. The semi-elliptical
starter notch is recommended to maximize the likelihood of producing a fatigue crack of proper shape with a minimum of fatigue
crack growth, but other shapes may offer advantages or simplify to the notch machining. The starter notch may be cut by any
available means. The plunge electrical discharge machining (EDM) method is the most common, but conventional machining
techniques and laser cutting have been used effectively. The height of the notch, h, should be minimized. In practice, it should not
exceed 1.0 mm (0.04 in.). As shown in Fig. 6, it is recommended that the notch end with a sharp “V” shape, and as a minimum
the notch should end with a radius ≤ h/2. Generally, the effort to develop a technique for producing sharp notches is a good
investment, because the time required to start the precrack is greatly reduced.
7.5.3 Fatigue Precrack Shape and Length—The fatigue precrack must be fully established around the full perimeter of the
semi-ellipse. At all locations around the perimeter, the fatigue precrack shall extend a minimum 2h from the notch. The final shape
shall be a semi-ellipse within the tolerance allowed in subsection 8.4. If additional features are machined into the starter notch for
purposes of mechanical CMOD measurement, the precrack shall be sufficiently long to extend to or beyond a 60-degree envelope
enclosing the starter notch and any features machined at the surface. See Fig. X3.1 for an illustration.
7.5.4 Fatigue Precrack Procedures—The following requirements shall be followed when producing the fatigue precrack.
7.5.4.1 Fixtures—The development of a regular semi-elliptical precrack is dependent on uniform stress distribution (tension or
bending) over the specimen cross-section. Test fixtures and specimen alignment should be carefully addressed. The quality and
precision of all precracking fixtures should be equivalent to those used for testing.
7.5.4.2 Material Condition—Fatigue precracking shall be performed with the material in the final heat-treated, mechanically
worked, or environmentally conditioned state. Intermediate treatments between fatigue precracking and testing are acceptable only
when such treatments are necessary to simulate the conditions of a specific structural application; such departure from
recommended practice shall be explicitly reported.
E2899 − 24
7.5.4.3 Fatigue Precrack Loading Requirements—The maximum force applied to the specimen during precracking, including
tension, bending, or combined tension/bending, shall limit the stress intensity to the lesser of K < 0.6K or 30MPa=m (27ksi
maxϕ est
=in) for the first 50% of the precrack and the lesser of K < 0.5K or 25MPa=m (22.8ksi=in) for the final 50% of the
max-ϕ est
precrack, where K is a provisional estimated material toughness and K is the maximum value of stress intensity occurring
est max-ϕ
around the crack perimeter as calculated by equations in Appendix X1. Precracking should be conducted at as low a K as
max-ϕ
practical. K is based on the instantaneous precrack size; therefore, forces required to achieve K should be evaluated as
max-ϕ max-ϕ
the precrack grows. Small starting notches may result in high stresses to achieve the initial K values allowed above. At no time
max-ϕ
during precracking shall the far field stress exceed 80% of the σ (0.2% offset).
YS
(1) Precracking forces are evaluated following the test by using K in place of the provisional estimated toughness, K . To
ϕ est
develop precracking parameters, K for the material may be estimated from the K of previous surface crack tests or from
est ϕ
linear-elastic plane strain fracture toughness values determined by Test Method E399 or E1820. If no existing material toughness
=
information is available, an acceptable limiting value of K can often be estimated by ensuring K /E < 0.00016 m (0.001
max-ϕ max-ϕ
=
in), though not to exceed the values in subsection 7.5.4.3. This relationship may not sufficiently limit precracking conditions for
high elastic modulus, low toughness materials such as very high strength steels.
(2) The stress ratio, R, during precracking is not prescribed, but is most commonly set at R = 0.1. Precracking may proceed
as a single-step, multiple step, or continuous shedding process. If using the higher initial values of K to hasten the initial 50%
max-ϕ
or less of precrack growth, then at least one additional step is required to complete the remaining 50% of the precrack with K
max-ϕ
equal to or less than the values shown. Additional steps or automated load shedding may also be used to achieve this effect. An
acceptable method for promoting fatigue crack initiation from the notch is to first apply compressive force cycles not exceeding
the planned magnitude of the tensile fatigue precrack loads. If compressive forces are applied to tensile specimen designs (as
opposed to bending), then buckling of the specimen must be avoided.
7.5.4.4 Precracking and Test Temperature—If the precrack and testing temperature are not the same, in addition to considering
the potential for differing material toughness at the test temperature, the change in material strength must
...