ASTM E1681-23e1

Standard Test Method for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials

SIGNIFICANCE AND USE
5.1 The parameters KEAC  or KIEAC  determined by this test method characterize the resistance to crack growth of a material with a sharp crack in specific environments under loading conditions in which the crack-tip plastic region is small compared with the crack depth and the uncracked ligament. The less restrictive thickness requirements of KEAC  are intended for those conditions in which the results are a strong function of the thickness of the specimen and the application requires the testing of specimens with thickness representative of the application. Since the chemical and mechanical influences cannot be separated, in some material/environment combinations, the thickness must be treated as a variable. A KEAC  or KIEAC  value is believed to represent a characteristic measurement of environment-assisted cracking resistance in a precracked specimen exposed to an environment under sustained tensile loading. A KEAC  or KIEAC  value may be used to estimate the relationship between failure stress and defect size for a material under any service condition, where the combination of crack-like defects, sustained tensile loading and the same specific environment would be expected to occur. (Background information concerning the development of this test method can be found in Refs (3-18).  
5.1.1 The apparent KEAC  or KIEAC  of a material under a given set of chemical and electrochemical environmental conditions is a function of the test duration. It is difficult to furnish a rigorous and scientific proof for the existence of a threshold (4, 5). Therefore, application of KEAC  or KIEAC  data in the design of service components should be made with awareness of the uncertainty inherent in the concept of a true threshold for environment-assisted cracking in metallic materials (6, 18). A measured KEAC  or KIEAC  value for a particular combination of material and environment may, in fact, represent an acceptably low rate of crack growth rather than an absolute upper limit f...
SCOPE
1.1 This test method covers the determination of the environment-assisted cracking threshold stress intensity factor parameters, KIEAC  and KEAC, for metallic materials from constant-force testing of fatigue precracked beam or compact fracture specimens and from constant-displacement testing of fatigue precracked bolt-load compact fracture specimens.  
1.2 This test method is applicable to environment-assisted cracking in aqueous or other aggressive environments.  
1.3 Materials that can be tested by this test method are not limited by thickness or by strength as long as specimens are of sufficient thickness and planar size to meet the size requirements of this test method.  
1.4 A range of specimen sizes with proportional planar dimensions is provided, but size may be variable and adjusted for yield strength and applied force. Specimen thickness is a variable independent of planar size.  
1.5 Specimen configurations other than those contained in this test method may be used, provided that well-established stress intensity calibrations are available and that specimen dimensions are of sufficient size to meet the size requirements of this test method during testing.  
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.7 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.

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 91.080.10 - Metal structures
• Technical Committee: E08 - Fatigue and Fracture
• Drafting Committee: E08.06 - Crack Growth Behavior

Relations

• Replaces: ASTM E1681-23 - Standard Test Method for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials
• Effective Date: 01-May-2023
• Referred By: ASTM F1624-12(2018) - Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique
• Effective Date: 01-May-2023
• Referred By: ASTM E2818-11(2019) - Standard Practice for Determination of Quasistatic Fracture Toughness of Welds
• Effective Date: 01-May-2023
• Referred By: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-May-2023
• Referred By: ASTM G129-21 - Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking
• Effective Date: 01-May-2023

Overview

ASTM E1681-23e1 is an international standard developed by ASTM International, establishing a reliable test method for determining the threshold stress intensity factors (K_IEAC and K_EAC) for environment-assisted cracking (EAC) in metallic materials. This standard is crucial for characterizing a material’s resistance to crack growth when exposed to aggressive environments-such as aqueous or corrosive solutions-under sustained tensile loads. The method accommodates variations in specimen thickness, size, and strength, ensuring broad applicability for different materials and service conditions. The intent is to provide essential data in evaluating the susceptibility of metallic components to environmentally induced cracking, supporting material selection, component design, and integrity assessment in various industries.

Key Topics

• Threshold Stress Intensity Factors: Defines K_IEAC (for plane strain conditions) and K_EAC (where specimen thickness affects results), establishing the highest stress intensity below which crack growth is not observed for a given material-environment combination.
• Specimen Preparation and Testing: Describes procedures for fatigue precracking, loading, and appropriate specimen configurations (including beam, compact, and bolt-load specimens).
• Environmental Influence: Emphasizes the role of chemical, electrochemical, and mechanical variables in the onset and progression of environment-assisted cracking.
• Validity and Uncertainty: Stresses the importance of specimen size validity, acknowledges uncertainties in defining a true threshold, and provides best practices for data interpretation.
• Test Duration and Force Bracketing: Provides guidance on suitable test durations and stresses the need to bracket force levels to accurately determine threshold values, especially in long-term exposures.

Applications

ASTM E1681-23e1 is applicable across multiple sectors where metallic structures are at risk of environmental cracking, including:

• Aerospace: Evaluating alloys for airframes and engine components exposed to stressful, corrosive conditions.
• Oil & Gas: Assessing pipeline materials and offshore structures for crack resistance in saline or sour service environments.
• Power Generation: Guiding material selection and maintenance planning for components exposed to water, steam, or other corrosive fluids.
• Manufacturing and Transportation: Ensuring the durability of metallic parts subject to environmental and mechanical stress.
• Quality Assurance: Defining acceptance criteria during production and for risk-based inspection intervals.

The data derived from this standard-K_IEAC and K_EAC values-enables engineers to estimate relationships between applied stress, defect size, and the likelihood of environment-assisted failure. This supports safe component design and life prediction in critical applications, helping mitigate hazardous or costly failures due to EAC.

Related Standards

ASTM E1681-23e1 is closely integrated with other ASTM standards that cover related testing and material evaluation methods, including:

• ASTM E8/E8M: Test methods for tension testing of metallic materials.
• ASTM E399: Methods for linear-elastic plane-strain fracture toughness in metals.
• ASTM E647: Measurement of fatigue crack growth rates.
• ASTM G1: Procedures for preparing, cleaning, and evaluating corrosion test specimens.
• ASTM D1141: Practice for preparation of substitute ocean water.

These standards collectively strengthen the robustness and reproducibility of data relevant to EAC susceptibility and collectively underpin reliable material performance assessment.

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By following ASTM E1681-23e1, industries can systematically evaluate metallic materials for susceptibility to environment-assisted cracking, ensuring safe operation and cost-effective maintenance of critical infrastructure under challenging environmental and loading conditions. This standard supports informed engineering decisions, regulatory compliance, and best practices in corrosion engineering and fracture mechanics.

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.
´1
Designation: E1681 − 23
Standard Test Method for
Determining Threshold Stress Intensity Factor for
Environment-Assisted Cracking of Metallic Materials
This standard is issued under the fixed designation E1681; 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—Section 3.1.9 was editorially corrected in April 2024.
1. Scope 2. Referenced Documents
1.1 This test method covers the determination of the 2.1 ASTM Standards:
environment-assisted cracking threshold stress intensity factor D1141 Practice for Preparation of Substitute Ocean Water
parameters, K and K , for metallic materials from E8/E8M Test Methods for Tension Testing of Metallic Ma-
IEAC EAC
constant-force testing of fatigue precracked beam or compact terials
fracture specimens and from constant-displacement testing of E399 Test Method for Linear-Elastic Plane-Strain Fracture
fatigue precracked bolt-load compact fracture specimens. Toughness of Metallic Materials
E647 Test Method for Measurement of Fatigue Crack
1.2 This test method is applicable to environment-assisted
Growth Rates
cracking in aqueous or other aggressive environments.
E1823 Terminology Relating to Fatigue and Fracture Testing
1.3 Materials that can be tested by this test method are not
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
limited by thickness or by strength as long as specimens are of
sion Test Specimens
sufficient thickness and planar size to meet the size require-
G5 Reference Test Method for Making Potentiodynamic
ments of this test method.
Anodic Polarization Measurements
1.4 A range of specimen sizes with proportional planar G15 Terminology Relating to Corrosion and Corrosion Test-
ing (Withdrawn 2010)
dimensions is provided, but size may be variable and adjusted
for yield strength and applied force. Specimen thickness is a
3. Terminology
variable independent of planar size.
1.5 Specimen configurations other than those contained in 3.1 Definitions:
this test method may be used, provided that well-established 3.1.1 For definitions of terms relating to fracture testing
stress intensity calibrations are available and that specimen used in this test method, refer to Terminology E1823.
dimensions are of sufficient size to meet the size requirements 3.1.2 For definitions of terms relating to corrosion testing
of this test method during testing. used in this test method, refer to Terminology G15.
3.1.3 stress-corrosion cracking (SCC)—a cracking process
1.6 This standard does not purport to address all of the
that requires the simultaneous action of a corrodent and
safety concerns, if any, associated with its use. It is the
sustained tensile stress.
responsibility of the user of this standard to establish appro-
3.1.4 stress intensity factor threshold for plane strain
priate safety, health, and environmental practices and deter-
–3/2
mine the applicability of regulatory limitations prior to use. environment-assisted cracking (K [FL ])—the highest
IEAC
value of the stress intensity factor (K) at which crack growth is
1.7 This international standard was developed in accor-
dance with internationally recognized principles on standard- not observed for a specified combination of material and
environment and where the specimen size is sufficient to meet
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom- requirements for plane strain as described in Test Method
E399.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1 2
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue For referenced ASTM standards, visit the ASTM website, www.astm.org, or
and Fracture and is the direct responsibility of Subcommittee E08.06 on Crack contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Growth Behavior. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved May 1, 2023. Published June 2023. Originally the ASTM website.
approved in 1995. Last previous edition approved in 2020 as E1681 - 03(2020). The last approved version of this historical standard is referenced on
DOI: 10.1520/E1681-23E01. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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E1681 − 23
3.1.5 stress intensity factor threshold for environment- 4. Summary of Test Method
–3/2
assisted cracking (K [FL ])—the highest value of the
EAC
4.1 This test method involves testing of single-edge notched
stress intensity factor (K) at which crack growth is not
[SE(B)] specimens, compact [C(T)] specimens, or bolt-load
observed for a specified combination of material and environ-
compact [MC(W)] specimens, precracked in fatigue. The
ment and where the measured value may depend on specimen
single-edge notched beam specimen is tested by dead weight
thickness.
loading. An environmental chamber is either attached to the
specimen, or the specimen is contained within the chamber.
3.1.6 physical crack size (a [L])—the distance from a ref-
p
erence plane to the observed crack front. This distance may The chamber must enclose the portion of the specimen where
the crack tip is located. Prescribed environmental conditions
represent an average of several measurements along the crack
front. The reference plane depends on the specimen form, and must be established and maintained within the chamber at all
times during the test.
it is normally taken to be either the boundary or a plane
containing either the loadline or the centerline of a specimen or 4.1.1 Specimens shall be deadweight loaded or otherwise
held under constant force or held under constant displacement
plate. The reference plane is defined prior to specimen defor-
mation. (defined in 6.2) for a prescribed length of time, during which
failure by crack growth leading to fracture may or may not
3.1.7 original crack size (a [L])—the physical crack size at
o
occur. K and K are defined as the highest value of stress
IEAC EAC
the start of testing.
intensity factor at which neither failure nor crack growth
3.1.8 original uncracked ligament (b [L])—distance from
occurs. The stress intensity factor (K ) is calculated from an
o
the original crack front to the back edge of the specimen (b =
expression based on linear elastic stress analysis. To establish
o
W – a ).
a suitable crack-tip condition for constant force tests, the
o
stress-intensity level at which the fatigue precracking of the
3.1.9 specimen thickness (B)[L]—the distance between the
specimen is conducted is limited to a value substantially less
parallel sides of a test specimen.
than the measured K or K values. For constant dis-
IEAC EAC
–2
3.1.10 tensile strength (σ [FL ])—the maximum tensile
TS
placement tests, the stress-intensity level at which the fatigue
stress that a material is capable of sustaining. Tensile strength
precracking of the specimen is conducted is limited to the
is calculated from the maximum force during a tension test
requirements of Test Method E399. The validity of the K
IEAC
carried to rupture and the original cross-section area of the
value determined by this test method depends on meeting the
specimen.
size requirements to ensure plane strain conditions, as stated in
3.2 Definitions of Terms Specific to This Standard: Test Method E399. The validity of the K value depends on
EAC
meeting the size requirements for linear elastic behavior, as
3.2.1 environment-assisted cracking (EAC)—a cracking
stated in the Test Method E647.
process in which the environment promotes crack growth or
4.1.2 This test method can produce information on the onset
higher crack growth rates than would occur without the
of environment-assisted crack growth. Crack growth rate
presence of the environment.
information can be obtained after crack nucleation, but the
3.2.2 normalized crack size (a/W)—the ratio of crack size, a,
method for obtaining this information is not part of this test
to specimen width, W. Specimen width is measured from a 4
method (1).
reference position such as the front edge in a bend specimen or
4.2 The mechanisms of environment-assisted cracking are
the loadline in the compact specimen to the back edge of the
varied and complex. Measurement of a K or K value
EAC IEAC
specimen.
for a given combination of material and environmental pro-
–2
3.2.3 yield strength (σ [FL ])—the stress at which a
YS
vides no insight into the particular cracking mechanism that
material exhibits a specific limiting deviation from the propor-
was either operative or dominant. Two prominent theories of
tionality of stress to strain. This deviation is expressed in terms
environment-assisted cracking are anodic reaction and hydro-
of strain.
gen embrittlement (2). The data obtained from this test method
may be interpreted by either theory of environment-assisted
NOTE 1—In this test method, the yield strength determined by the 0.2 %
cracking.
offset method is used.
–2
4.3 Specimen thickness governs the proportions of plane
3.2.4 effective yield strength (σ [FL ])—an assumed value
Y
strain and plane stress deformation local to the crack tip, along
of uniaxial yield strength that represents the influences of
with the environmental contribution to cracking. Since these
plastic yielding upon fracture test parameters. For use in this
chemical and mechanical influences cannot be separated in
method, it is calculated as the average of the 0.2 % offset yield
some material/environment combinations, thickness must be
strength σ , and the ultimate tensile strength, σ , or
YS TS
treated as a variable. In this test method, however, the stress in
σ 5 ~σ 1σ !/2 (1)
Y YS TS
the specimen must remain elastic. For these reasons, two
3.2.5 notch length (a (L))—the distance from a reference
n threshold values of EAC are defined by this test method. The
plane to the front of the machined notch. The reference plane
measurement of K requires that the thickness requirements
IEAC
depends on the specimen form and normally is taken to be
either the boundary or a plane containing either the loadline or
the centerline of a specimen or plate. The reference plane is
The boldface numbers in parentheses refer to the list of references at the end of
defined prior to specimen deformation. this standard.
´1
E1681 − 23
of plane strain constraint are met. The less restrictive require- 5.1.3 In some material/environment combinations, the
ments of K are intended for those conditions in which the smaller the specimen, the lower the measured K value,
EAC EAC
results are a strong function of the thickness of the specimen while in other material/environment combinations the mea-
and the application requires the testing of specimens with sured K value will be the lowest value (5, 9, 10, 11, 12). If,
IEAC
thickness representative of the application. for the material/environment combination of interest, it is not
known which specimen size will result in the lower measured
4.4 A variety of environmental (temperature, environment
value, then it is suggested that the use of both specimen sizes
composition, and electrode potential, for example) and metal-
should be considered; that is, specimens with thicknesses
lurgical (yield strength, alloy composition, and specimen
representative of the application and specimens in which the
orientation) variables affect K and K .
EAC IEAC
thickness meets the requirements (see 7.2.1) of a K value.
IEAC
5.1.3.1 The user may optionally determine and report a
5. Significance and Use
K value or a K value. The specimen size validity
IEAC
EAC
5.1 The parameters K or K determined by this test
EAC IEAC requirements for a K value meet the size requirements
EAC
method characterize the resistance to crack growth of a
developed for Test Method E647 to achieve predominately
material with a sharp crack in specific environments under
elastic behavior in the specimen. Test Method E647 size
loading conditions in which the crack-tip plastic region is small
requirements for compact specimens should be applied to both
compared with the crack depth and the uncracked ligament.
the compact specimen and the beam specimen. The specimen
The less restrictive thickness requirements of K are in-
EAC size validity requirements for a K value meet the size
IEAC
tended for those conditions in which the results are a strong
requirements developed for plane strain conditions for Test
function of the thickness of the specimen and the application
Method E399.
requires the testing of specimens with thickness representative
5.1.4 Evidence of environment-assisted crack growth under
of the application. Since the chemical and mechanical influ-
conditions that do not meet the validity requirements of 7.2
ences cannot be separated, in some material/environment
may provide an important indication of susceptibility to
combinations, the thickness must be treated as a variable. A
environmental cracking but cannot be used to determine a valid
K or K value is believed to represent a characteristic
IEAC K value (14).
EAC
EAC
measurement of environment-assisted cracking resistance in a
5.1.5 Environment-assisted cracking is influenced by both
precracked specimen exposed to an environment under sus-
mechanical and electrochemical driving forces. The latter can
tained tensile loading. A K or K value may be used to
EAC IEAC vary with crack depth, opening, or shape and may not be
estimate the relationship between failure stress and defect size
uniquely described by the fracture mechanics stress intensity
for a material under any service condition, where the combi-
factor. As an illustrative example, note the strong decrease
nation of crack-like defects, sustained tensile loading and the
reported in K with decreasing crack size below 5 mm for
ISCC
same specific environment would be expected to occur. (Back-
steels in 3 % NaCl in water solution (15). Geometry effects on
ground information concerning the development of this test
K similitude should be experimentally assessed for specific
method can be found in Refs (3-18).
material/environment systems. Application modeling based on
5.1.1 The apparent K or K of a material under a
K similitude should be conducted with caution when
EAC IEAC
EAC
given set of chemical and electrochemical environmental
substantial differences in crack and specimen geometry exist
conditions is a function of the test duration. It is difficult to
between the specimen and the component.
furnish a rigorous and scientific proof for the existence of a
5.1.6 Not all combinations of material and environment will
threshold (4, 5). Therefore, application of K or K data
EAC IEAC result in environment-assisted cracking. In general, suscepti-
in the design of service components should be made with
bility to aqueous stress-corrosion cracking decreases with
awareness of the uncertainty inherent in the concept of a true
decreasing material strength level. When a material in a certain
threshold for environment-assisted cracking in metallic mate-
environment is not susceptible to environment-assisted
rials (6, 18). A measured K or K value for a particular
EAC IEAC cracking, it will not be possible to measure K or K .
EAC IEAC
combination of material and environment may, in fact, repre-
This method can serve the following purposes:
sent an acceptably low rate of crack growth rather than an
5.1.6.1 In research and development, valid K or K
EAC IEAC
absolute upper limit for crack stability. Care should be exer-
data can quantitatively establish the effects of metallurgical and
cised when service times are substantially longer than test
environmental variables on the environment-assisted cracking
times.
resistance of materials.
5.1.2 The degree to which force deviations from static
5.1.6.2 In service evaluation, valid K or K data can
EAC IEAC
tensile stress will influence the apparent K or K of a
EAC IEAC be utilized to establish the suitability of a material for an
material is largely unknown. Small-amplitude cyclic loading,
application with specific stress, flaw size, and environmental
well below that needed to produce fatigue crack growth,
conditions.
superimposed on sustained tensile loading was observed to
5.1.6.3 In acceptance and quality control specifications,
significantly lower the apparent threshold for stress corrosion
valid K or K data can be used to establish criteria for
EAC IEAC
cracking in certain instances (7, 8). Therefore, caution should
material processing and component inspection.
be used in applying K or K data to service situations
EAC IEAC
involving cyclic loading. In addition, since this standard is for
K has been used in the literature as a special case of K in which the
ISCC IEAC
static loading, small-amplitude cyclic loading should be
crack growth is known to be due to the simultaneous action of a stress and a
avoided during testing. corrodent.
´1
E1681 − 23
5.1.7 Test results will be affected by force relaxation in
constant displacement bolt-loaded compact specimens for
some material/environment conditions. For relatively low
strength material, non-agressive environments, or high test
temperatures, force relaxation can occur independently from
environment-assisted cracking. Significant force relaxation
would make cracking results difficult to interpret. If force
relaxation is suspected of influencing the data, the following
trial specimen test is recommended. Test a trial specimen with
all the test conditions of interest, except with no environment
applied. Monitor the force on the sample using a bolt with an
electronic load cell attached. Instrumented bolts of this type are
commercially available. A force relaxation of more than 5 %
after 24 h indicates that the constant displacement test method
may not be suitable for these test conditions, and a constant
force test should be considered.
NOTE 1—The length of the moment arm (L) should be equal to or
5.1.8 Residual stresses can have an influence on
greater than 8W
environment-assisted cracking. The effect can be significant
FIG. 1 Typical Configuration of a Dead-Weight Beam Loading
when test specimens are removed from material in which
Fixture
complete stress relief is impractical, such as weldments,
as-heat-treated materials, complex wrought parts, and parts
with intentionally produced residual stresses. Residual stresses
superimposed on the applied stress can cause the local crack-
tip stress-intensity factor to be different from that calculated
from externally applied forces or displacements. Irregular
crack growth during precracking, such as excessive crack front
curvature or out-of-plane crack growth, often indicates that
residual stresses will affect the subsequent environment-
assisted crack growth behavior. Changes in the zero-force
value of crack-mouth-opening displacement as a result of
precrack growth is another indication that residual stresses will
affect the subsequent environment-assisted crack growth.
5.1.9 For bolt loaded specimens, the user should realize that
material being tested at an non-ambient temperature may have
a different displacement-to-force ratio from that at ambient
temperature, and also the bolt material may have a different
coefficient of thermal expansion from that of the material being
tested. Care should be taken to minimize these effects.
NOTE 1—Pin diameter = 0.24 W (+0.000W/–0.005W). For Specimens
a with σ > 1379 MPa the holes in the specimen and in the clevis may be
vs
6. Apparatus 0.3W (+0.005W/–.0000W) and the pin diameter = 0.288W (+0.000W/
–0.005W)
6.1 Fixtures:
NOTE 2—Corners of the clevis may be removed if necessary to
6.1.1 Beam Specimens—Specimens should be loaded with accommodate a clip gage
FIG. 2 Tension Test Clevis Design
one end clamped in a stable rigid fixture and the other end
clamped to a horizontal moment arm to which a force is
applied. In a fixture of this type, the long axis of the specimen
Both ends of the specimen are held in a clevis and loaded
is placed horizontally with the notch opening upward. A
through pins to allow rotation of the specimen during testing.
schematic representation of a suitable loading fixture is given
To provide rolling contact between the loading pins and the
in Fig. 1. Note that limits are placed on the proximity of fixture
clevis holes, the holes are machined with small flats on the
contact points to the specimen notch and on the length of the
loading surface. Other clevis designs may be used if it can be
moment arm. The fixture should have enough stiffness to
demonstrated that they will accomplish the same result.
ensure that moment arm deflection under force application is
6.1.3 Bolt-Load Compact Specimens—A test arrangement
primarily caused by test specimen compliance. In situations in
suitable for constant-displacement testing of bolt-load compact
which a single loading fixture simultaneously accommodates
specimens is shown in Fig. 3. The displacement is applied to
multiple specimens, it is important that the loading fixture be
the specimen containing a machined notch and fatigue pre-
rigid enough to minimize transmission of transient deflections
crack. The displacement is applied with a bolt tightened against
from specimen to specimen through the fixture.
a flattened pin and measured with an electronic crack-mouth-
6.1.2 Compact Specimens—A loading clevis suitable for opening-displacement (CMOD) gage (see Test Method E399).
constant force testing of compact specimens is shown in Fig. 2. Reference marks on the face of the specimen on both sides of
´1
E1681 − 23
and repetitive displacement fluctuations of more than 1 % must
be avoided during the experiment.
6.3 Displacement Gauge—It may be desirable to attach a
displacement gage to a constant force specimen to detect crack
growth during testing. It is required that a displacement gage
be used with the constant displacement specimen to measure
the amount of applied displacement (see 6.1.3). An electronic
CMOD gage can provide a highly sensitive indicator of crack
growth for this purpose (see Test Method E399). However,
when placed directly above an environmental chamber con-
taining an aqueous solution for prolonged periods, corrosion
may degrade CMOD gages. Also, the CMOD gage should not
be allowed to come into direct contact with the solution to
FIG. 3 Typical Test Arrangement for Constant Displacement K
IEAC
avoid possible galvanic action between the gage and the test
Tests with Modified Bolt-Load Compact Specimen; H/W = 0.486
specimen. A mechanical dial gage placed near the extremity of
the moment arm also may be used to detect crack growth.
6.4 Environmental Chamber—It is important that the envi-
ronmental chamber does not influence the test results either by
modifying the environment or the electrochemical potential of
the specimen. Influence of the environment chamber or the
pressure of the environment should be accounted for in the
calibration of the applied K value. The environmental chamber
shall enclose the portion of the specimen that contains the
NOTE 1—A Surface Perpendicular and parallel within 0.001 W TIR
crack tip. It shall be configured so that either the test specimen
FIG. 4 Beam Specimen
is the only metallic component in contact with the solution or
the specimen is electrically isolated from any other metals in
contact with the solution. Nonmetallic or corrosion resistant
the notch may also be used to verify the CMOD measurement
materials are recommended for the environmental chamber. A
of the applied displacement. The gage is attached to the
sealant might be required between the specimen and the
specimen using integral knife edges machined into the speci-
environmental chamber. Sealants selected must not alter the
men or using knife edges affixed to the specimen. Other types
bulk solution chemistry of the test environment. It is recom-
of gages and attachments may be used if it can be demonstrated
mended that the volume of the environmental chamber be large
that they will accomplish the same result. It is recommended
enough to contain at least 40 mL/cm of specimen surface area
that, if possible, the bolt pin be isolated from the environment
exposed to the solution.
and that an electric insulator be used between the bolt and pin.
For some test conditions, environmental isolation and electrical
6.5 Potentiostatic Control—Where potentiostatic control of
insulation may not be possible.
the specimen is desired, an electrochemical cell is required
(including an auxiliary electrode, such as platinum or graphite,
6.2 Displacement Application:
and a reference electrode with specimen potential controlled by
6.2.1 Constant-Force Specimens—Specimens must be dead-
a potentiostat). Care must be taken to avoid ground loops and
weight loaded or loaded so that the force remains constant
galvanic interference from the clamping and loading fixtures.
throughout the test. Weights or a servo-controlled actuator are
Oxides on the specimen surface may hamper the achievement
suitable for this purpose. A means must be provided to
of the desired specimen potential. Under some conditions, it
accurately measure the force, including the weight of the
may be necessary to mask off a portion of the specimen surface
moment arm and associated load train fixtures. This may be
so that proper potentiostatic control can be achieved. It is
done by including an electronic load cell in the load train or by
desirable to include apparatus for measuring and recording
using calibrated weights. The force applied to the specimen
electrode potential and applied current (Reference Method
must be known, with an accuracy of 61 % of the indicated
G5).
reading. Overloads of more than 3 % and repetitive force
fluctuations of more than 1 % must be avoided during the
7. Specimen Configuration, Size, and Preparation
experiment. In addition, extraneous bending and torsional
7.1 Specimen Configuration:
forces must be minimized (see 8.3).
6.2.2 Constant Displacement Specimens—The crack-
The ratio of the specimen free surface area, exposed to the test solution in the
mouth-opening-displacement applied to the bolt-load specimen
chamber, to the crack size affects the anode/cathode area and can affect the corrosion
must be known, with an accuracy of +1 % of the indicated
potential in the crack. The area external to the crack should be significantly greater
reading. Overapplications of displacement of more than 5 %
than the crack area.
´1
E1681 − 23
7.1.1 The recommended beam specimen configuration is
shown in Fig. 4. It is recommended that 1≤W/B≤2, provided
that B, a , and W-a meet the validity criteria of 7.2. The
o o
specimen configuration shown in Fig. 4 does not include side
grooves.
NOTE 2—Caution should be exercised to avoid preferential crack
growth near the side grooves when testing in more aggressive environ-
ments.
7.1.2 The recommended compact specimen configuration is
shown in Fig. 5. The configuration does not include side
grooves. For the determination of K , it is recommended
IEAC
that 1≤W/B≤2, provided that B, a , and W-a meet the validity
o o
criteria of 7.2.
NOTE 1—A surfaces perpendicular and parallel as applicable to within
7.1.3 The recommended bolt-load compact specimen con-
0.002W TIR.
figuration is shown in Fig. 6. The configuration does not
NOTE 2—The intersection of the crack starter notch tips with the two
include side grooves. While for the determination of K , it
IEAC
specimen surfaces shall be equally distant from the top and bottom edges
is recommended that W/B is 2:1, a 1:1 ratio can also be used,
of the specimen within 0.005W.
provided that B, a, and W-a meet the validity criteria of 7.2.
NOTE 3—Integral or attachable knife edges for clip gage attachment to
7.1.4 Other specimen and loading configurations, for which
the crack mouth may be used.
well-established stress intensity calibrations are available, are
NOTE 4—For starter notch and fatigue crack configuration see Fig. 7.
FIG. 6 Standard Configuration for the Modified Bolt-Load Com-
acceptable as long as the specimen size requirements of 7.2 are
pact Specimen; H/W = 0.486
met.
7.2 Specimen Size—For the results to be valid in accordance
7.2.1 For the measurement of K , it is required that B, a ,
with this test method, it is required that the specimen be IEAC o
and W-a equal or exceed the quantity 2.5 (K /σ ) , where
predominantly elastic in its behavior and that one or more of o IEAC YS
σ is the yield strength of the material determined at the
the following criteria be satisfied. YS
temperature of the K experiment.
IEAC
7.2.2 For the measurement of K , it is required that W-a
EAC o
equal or exceed the quantity (4/π)(K /σ ) . In this
EAC YS
calculation, σ may be replaced by σ for high work harden-
YS Y
If crack growth rate information is to be obtained in addition to K , side
EAC
ing materials with an ultimate to yield strength ratio greater
grooves may be desirable. Side grooves may promote straight fronted crack growth
than 1.3. These requirements are consistent with those used in
with some materials in some environments. Side groove depths with a total
Test Method E647.
thickness reduction of 20 % are suggested. Side groove root radii of less than 0.4
mm (0.016 in.) are suggested. Alternative methods to obtain crack growth rate
7.2.3 For the beam and compact specimens, it is recom-
information are available (see Test Method E647) (1).
mended that the crack length (total length of the machined
notch plus the fatigue precrack) be between 0.45 and 0.55 W
whenever possible. However, normalized crack length values,
a/W, may range from 0.25 to 0.75 in extreme instances,
provided the requirements of 9.3 are met.
7.2.4 For the bolt-load compact specimen, applied K values
continuously decrease with increasing crack length so that
large crack lengths can be used. It is recommended that the
total crack length (total length of the machined notch plus the
fatigue precrack and the crack growth) be between 0.30 and .95
W, provided the requirements of 8.8.2.5 are met.
7.3 Specimen Preparation:
7.3.1 The dimensional tolerances and surface finishes
shown in Figs. 4-7 shall be followed in the specimen prepara-
tion.
7.3.2 Care should be taken in machining to prevent con-
NOTE 1—A surface shall be perpendicular and parallel as applicable to
within 0.002W TIR tamination of specimen and notch surfaces that are difficult or
NOTE 2—The intersection of the crack starter notch tips with the two impossible to clean. An example of this is the copper deposit
specimen surfaces shall be equally distant from the top and bottom edges
left by electric discharge machining (EDM) with a copper
of the specimen within 0.005W
electrode.
NOTE 3—Integral or attachable knife edges for clip gage attachment to
7.3.3 Prior to fatigue precracking and testing, specimens
the crack mouth may be used
should be cleaned in accordance with Practice G1.
NOTE 4—For starter notch and fatigue crack configuration see Fig. 7
7.3.4 It is required that the specimen be fatigue precracked
FIG. 5 Standard Proportions and Tolerances for the Compact
Specimen before testing. Fatigue precracking may be conducted in an
´1
E1681 − 23
8. General Procedure
8.1 Number of Tests—It is difficult to prescribe in advance
the number of tests required to establish a valid K or K
EAC IEAC
value by this test method. The K or K value is
EAC IEAC
determined from several experiments at K levels in which
specimens failed after a relatively long time under load or did
not fail within a prescribed period (discussed in 8.4). For the
beam and compact specimens to meet the force-bracketing
requirements of 8.5, it is suggested that at least four K levels,
and perhaps up to six, be investigated to ensure a measurement
of K or K . For the bolt-load compact specimen it is
EAC IEAC
suggested that at least two and perhaps up to four specimens be
tested to ensure a measurement of K or K . As a general
EAC IEAC
practice, it is recommended that test data be displayed graphi-
cally in terms of initial applied K (K based upon the applied
force or displacement and a ) versus logarithmic time to
o
failure. Guidance for the estimation of K or K can be
EAC IEAC
obtained for steels, aluminum alloys and titanium alloys
(15-18). If neither past experience nor these references are
helpful in making this estimate, a screening program with a
limited number of specimens may be needed as a first phase in
NOTE 1—Fatigue crack extension on each surface of the specimen
the testing program.
NOTE 2—Fatigue crack extension on each surface of the specimen from
the stress riser tipping the hole shall be at least 0.5 D or 1.3 mm whichever
8.2 Exposure to the Environment—With some environment-
is larger
material combinations, preconditioning of the specimen in the
NOTE 3—Crack starter notch shall be perpendicular to the specimen
environment prior to force or displacement application will
surface and to the intended direction of crack propagation within 62°
greatly influence the resulting K or K values. When this
NOTE 4—Notch height h need not be less than 1.6 mm EAC IEAC
is the case, the specimen shall be exposed to the environment
FIG. 7 Crack Starter Notch and Fatigue Crack Configurations
immediately preceding the test for at least 10 % of the total test
time, or 8 h, whichever is less. The specimen shall then be
loaded after this pre-exposure, either incrementally or continu-
ambient-air environment. The single-edge notched specimen
ously; however, the rate of force application should not exceed
may be fatigue precracked either in cantilever bending or in
100 MPa√m per min.
three-point bending. Fatigue precracking should be performed
8.3 Displacement Changes—Any significant change or in-
with the specimen fully heat treated to the condition in which
terruption in loading, displacement, temperature, environmen-
it is to be tested.
tal exposure, or applied potential (if appropriate) needs to be
7.3.4.1 The fatigue precrack shall extend to a depth of not
evaluated and may invalidate the measurement of K or
EAC
less than 0.10B, or 1.0 mm (0.04 in.), whichever is greater,
K . Such interruptions need to be reported with the results.
IEAC
beyond the tip of the machined notch as measured on each face
Occasional interruption of the force usually does not influence
of the specimen. It is required that the final 1 mm (0.04 in.)
the results, but overloads of more than 5 % and repetitive force
increment of fatigue precracking be conducted at a maximum
fluctuations of more than 1 % must be avoided and would
stress intensity factor (K ) of not more than 60 % of the
max
invalidate the results.
expected K value. The plane of the crack shall be parallel to
EAC
both the specimen width and thickness directions within 610°. 8.4 Test Duration—A test will continue until one of the
7.3.4.2 Note that in some materials highly sensitive to stress following occurs: (1) fracture, (2) evidence of subcritical crack
corrosion cracking (such as ultrahigh-strength alloys), K growth is observed in the specimen, (3) a pre-established
EAC
values can be very low (less than 20 MPa√m). Thus, permis- period of time has elapsed. Determining an adequate, but not
sible K levels for precracking highly sensitive materials excessive, test duration for threshold measurement is one of the
max
might be restricted to small values. This restriction could most difficult aspects of K testing (5). The test duration that
EAC
dictate lengthy periods of fatigue precracking. Under these is adequate for a valid threshold measurement depends strongly
circumstances, it may be necessary to initiate fatigue precrack- on the material and the environment. For constant force tests
ing at K levels higher than 60 % of K and to follow a involving ambient-temperature solutions of sodium chloride,
max EAC
K-decreasing test program in fatigue cracking, as described in including natural and ASTM substitute seawater (see Practice
Test Method E647. K-decreasing test procedures provide an D1141), the guideline test durations listed below are consid-
alternative means of achieving the final critical increment of ered long enough to ensure that a valid threshold has been
precracking at adequately low K (no more than 60 % of measured, but the actual times could be much shorter and need
max
K ). to be determined empirically. For constant displacement tests
EAC
7.3.5 Care should be taken to prevent the contamination of with relatively non-aggressive environments, the guideline test
the crack after precracking and before testing. durations listed below may not be long enough to ensure that
´1
E1681 − 23
a valid threshold has been measured. The actual times could be Laboratory studies on steels have supported this hypothesis by
longer and need to be determined empirically by using one or demonstrating a lack of response to changes in bulk solution
more trial samples. From this result, the test duration can be
dissolved oxygen content in K tests on a steel in a sodium
EAC
more accurately determined for the remainder of the tests. Use chloride solution (20). However, this may not be the case for
of techniques capable of detecting crack growth (acoustic
titanium alloys in which deaeration has been demonstrated to
emission) and of quantifying crack growth (d.c. potential drop)
have an effect on K values. Also, note that aeration
EAC
can be very helpful in establishing if a valid threshold has been
increases dissolved oxygen and, thus, may lower the pH, raise
reached.
the corrosivity of the solution, and make the free corrosion
steels ( < 1,200 MPa) 10 000 hours potential more anodic. For some solutions, oxygen gradients
YS
steels ( > 1,200 MPa) 5 000 hours
YS
along the crack length can establish potential gradients that
aluminum alloys 10 000 hours
assist ion migration into or out of the crack, thus, influencing
titanium alloys 1 000 hours
the K measurement.
EAC
The large differences in guideline test durations among
8.6.3 For tests in solutions other than sodium chloride, care
various alloys reflect inherent differences in incubation periods
should be taken to refresh the solution at regular intervals, if
and in crack growth kinetics. In some instances, it may be
required, to maintain the desired environmental conditions.
impractical or impossible to achieve test durations as long as
The frequency of refreshment required will depend on many
these. Under such circumstances, all data used in a K or
EAC
variables and should be determined for the particular
K determination should be qualified as to test duration (see
IEAC
environment/test material combination being studied.
10.1.8). Adequate test durations could be much shorter in
environments that are more aggressive than sodium chloride 8.6.4 For tests that require polarizing the specimen to a
solutions, such as aqueous solutions of hydrogen sulfide, potential other than the free corrosion potential (Reference
caustics, or ammonia.
Method G5), several recommendations are offered. The use of
a potentiostat is recommended rather than coupling the speci-
8.5 Force Bracketing—The interval in applied K levels
men to a dissimilar metal. However, when a potentiostat is
between specimens depends on the desired accuracy of the
used, appropriate care must be given to specimen grounding.
K or K value and the number of specimens to be tested.
EAC IEAC
For tests involving cathodic polarization with sacrificial
The interval should be in the range from 10 % to 20 % of the
anodes, periodic cleaning of the anodes and the specimen may
estimated K or K value.
EAC IEAC
be necessary if significant corrosion or calcareous deposits are
8.6 Environmental Monitoring or Control—Environmental
observed. It is further recommended that, when using sacrifi-
parameters are of vital importance in K or K testing;
EAC IEAC cial anodes, the surface area of the anode should be no less than
therefore, careful monitoring and control of the solution is
25 % of the specimen surface in contact with the solution. It is
required. Temperature, pH, conductivity, dissolved oxygen
essential that the anodes be located so that the specimen is
content, and electrode potential are variables that can affect
polarized uniformly throughout the test area. In this regard,
environment-assisted cracking processes. Among these
adequate spacing between the specimen and anodes is neces-
parameters, it is important to note that the electrode potential
sary. Cathodic or anodic polarization of the sample may
can exert a very strong influence on K or K . It is
EAC IEAC
promote changes in the solution chemistry particularly the
especially important that this parameter be carefully monitored
solution pH. As a result, when polarizing currents are applied,
or controlled either continuously or at regular intervals
the pH should be checked more frequently and precautions not
throughout the test, or both. Every chamber opening, specimen
required for open circuit potential experiment should be
inspection, and environment refreshing may result in a swing
considered.
of the potential.
8.6.5 For bolt-load compact tests, remove the force at the
8.6.1 It is necessary to maintain enough solution in the
end of the test while measuring the CMOD. The change in
environmental chamber to ensure that the crack-tip region of
CMOD upon unloading may be less than that of the original
the specimen is immersed in the corrosive environment at all
bolt-loading of the specimen because of the presence of
times and to ensure that the concentration of the electrolyte is
corrosion products on the crack surfaces or force relaxation. If
not increased by evaporation. Long-term testing is conducive
the change in CMOD upon unloading is less than 90 % of that
to the development of leaks at sites of contact between the
of the loading, check for presence of corrosion products and for
environmental chamber and the specimen; thus, seals between
evidence of force relaxation (see 5.1.7). If no reason can be
the chamber and the specimen should be inspected regularly
found for a change in CMOD due to unloading that is less than
for leakage.
90 % of that due to loading, then the constant displacement test
8.6.2 For tests involving sodium chloride solutions, replace
method may not be suitable for these test conditions, and a
the test solution at least weekly. It may be desirable to provide
constant force test should be considered.
a circulation system to ensure a constant level of aeration of the
8.7 Post-Test Examination—Specimen fracture surfaces
bulk solution. The effects, if any, of aeration on K mea-
EAC
surements are complex and not completely understood. Theo- must be visually examined after testing. The fracture surfaces
of specimens that did not fail shall be examined for evidence of
retical modeling studies have indicated, at least in steels, that
the crack-tip region is completely deoxygenated regardless of environment-assisted crack growth. Evidence of crack growth
the dissolved oxygen concentration in the bulk solution (19). In is taken as proof that the specimen was loaded at a K level
addition the CO from the air may play an important role. higher than K or K .
2 EAC IEAC
´1
E1681 − 23
8.7.1 Break the specimen to expose the crack, taking care to 110.1081 a /W 2 17.9415 a /W
~ ! ~ !
o o
3 4
minimize deformation. Cooling ferritic steel specimens enough
116.8282 ~a /W! 2 6.2241 ~a /W! #%
o o
to ensure brittle behavior may be helpful. Advancing the crack
where:
by fatigue may be needed in more ductile materials.
α = 1 – (a /W),
8.7.2 Inspect the tip of the initial fatigue precrack, looking
o
M = bending moment on the crack plane,
for evidence of crack extension. Characterize the fracture
M = W L + W L,
a a t
surface of the crack extension in comparison with the fracture
W = weight of arm,
a
surface formed by breaking the specimen to expose the crack.
L = distance from notch plane to center of gravity of arm,
a
This inspection must be made with an instrument capable of
W = total weight of platen, platen support, and added
t
resolving 0.025 mm (0.001 in.)
...