ASTM B909-21a

Standard Guide for Plane Strain Fracture Toughness Testing of Non-Stress Relieved Aluminum Products

SIGNIFICANCE AND USE
4.1 The property KIc, determined by Test Method E399 or ISO 12135, characterizes a material's resistance to fracture in a neutral environment and in the presence of a sharp crack subjected to an applied opening force or moment within a field of high constraint to lateral plastic flow (plane strain condition). A KIc value is considered to be a lower limiting value of fracture toughness associated with the plane strain state.  
4.1.1 Thermal quenching processes used with precipitation hardened aluminum alloy products can introduce significant residual stresses.5 Mechanical stress relief procedures (stretching, compression) are commonly used to relieve these residual stresses in products with simple shapes. However, in the case of mill products with thick cross-sections (for example, heavy gauge plate or large hand forgings) or complex shapes (for example, closed die forgings, complex open die forgings, stepped extrusions, castings), complete mechanical stress relief is not always possible. In other instances residual stresses may be introduced into a product during fabrication operations such as straightening, forming, or welding operations.
Note 1: For the purposes of this guide, only bulk residual stress is considered (that is, of the type typically created during a quench process for thermal heat treatment) and not engineered residual stress, such as from shot peening or cold hole expansion.  
4.1.2 Specimens taken from such products that contain residual stress will likewise themselves contain residual stress. While the act of specimen extraction in itself partially relieves and redistributes the pattern of original stress, the remaining magnitude can still be appreciable enough to cause significant error in the test result.  
4.1.3 Residual stress is a non-proportional internal stress that is superimposed on the applied stress and results in an actual crack-tip stress-intensity factor that is different from one based solely on externally applied forces or di...
SCOPE
1.1 This guide covers supplementary guidelines for plane-strain fracture toughness testing of aluminum products for which complete stress relief is not practicable. Guidelines for recognizing when residual stresses may be significantly biasing test results are presented, as well as methods for minimizing the effects of residual stress during testing. This guide also provides guidelines for an empirical correction as well as interpretation of data produced during the testing of these products. Test Method E399 is the standard test method to be used for plane-strain fracture toughness testing of aluminum alloys.  
1.2 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.  
1.3 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-Nov-2021
• ICS: 77.150.99 - Other products of non-ferrous metals
• Technical Committee: B07 - Light Metals and Alloys
• Drafting Committee: B07.05 - Testing

Relations

• Refers: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Feb-2024
• Refers: ASTM E1823-24 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Feb-2024
• Refers: ASTM E1823-20 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Feb-2020
• Refers: ASTM E561-19e1 - Standard Test Method for <emph type="bdit">K<inf>R</inf></emph> Curve Determination
• Effective Date: 15-Jul-2019
• Refers: ASTM E561-19 - Standard Test Method for <emph type="bdit">K<inf>R</inf></emph> Curve Determination
• Effective Date: 15-Jul-2019
• Refers: ASTM E561-15a - Standard Test Method for <emph type="bdit">K<inf>R</inf></emph> Curve Determination
• Effective Date: 01-Dec-2015
• Refers: ASTM E561-15 - Standard Test Method for <emph type="bdit">K<inf>R</inf></emph> Curve Determination
• Effective Date: 15-Oct-2015
• Refers: ASTM E1823-12e - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Dec-2012
• Refers: ASTM E399-12e3 - Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness K<inf>Ic</inf > of Metallic Materials
• Effective Date: 15-Nov-2012
• Refers: ASTM E399-12 - Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness <emph type="bdit" >K<inf> Ic</inf></emph> of Metallic Materials
• Effective Date: 15-Nov-2012
• Refers: ASTM E399-12e1 - Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness K<inf>Ic</inf > of Metallic Materials
• Effective Date: 15-Nov-2012
• Refers: ASTM E1823-12d - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Nov-2012
• Refers: ASTM E399-12e2 - Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness K<inf>Ic</inf > of Metallic Materials
• Effective Date: 15-Nov-2012
• Refers: ASTM E1823-12c - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Sep-2012
• Refers: ASTM E1823-12b - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Aug-2012

Overview

ASTM B909-21a: Standard Guide for Plane Strain Fracture Toughness Testing of Non-Stress Relieved Aluminum Products is a specialized guideline developed by ASTM International. This standard provides supplementary direction for the fracture toughness testing of aluminum alloys and products that have not been fully stress relieved, often due to their thickness or geometric complexity.

When aluminum products such as heavy gauge plates, large hand forgings, closed die forgings, extrusions, or castings cannot be fully relieved of residual stresses through mechanical means, accurate assessment of their plane strain fracture toughness becomes challenging. ASTM B909-21a outlines procedures to identify, minimize, and correct for residual stress effects during plane strain fracture toughness testing, ensuring more reliable and meaningful test results.

Key Topics

• Fracture Toughness and KIc: The guide references Test Method E399 and ISO 12135 for determining plane strain fracture toughness (KIc), an indicator of resistance to fracture in the presence of a sharp crack.
• Residual Stress in Aluminum: Addresses how thermal quenching and fabrication processes (such as straightening, forming, or welding) can introduce residual stresses in aluminum products.
• Test Specimen Considerations: Discusses the impact of residual stress on test specimens, particularly in extraction and preparation, potentially leading to significant measurement errors.
• Recognition of Bias: Offers guidelines for detecting signs of residual stress bias during testing, including specimen distortion, excessive saw drag, abnormal precrack curvature, and non-linear load-COD traces.
• Minimizing and Correcting Effects:
  • Experimental techniques to reduce residual stresses in specimens, such as optimizing specimen dimensions and using specific precrack fatigue protocols.
  • Post-test empirical correction methods, including use of average surface crack lengths for fracture toughness calculation.
• Reporting: Details requirements for documenting both uncorrected and corrected fracture toughness values, and the specific correction methods used.

Applications

ASTM B909-21a is highly relevant for industries and professionals working with aluminum alloys where achieving complete stress relief is impracticable. This includes:

• Aerospace and Defense: Heavy aluminum forgings and thick plates used in structural applications.
• Automotive and Transportation: Complex extrusion profiles or castings requiring certification of material performance.
• Manufacturing and Fabrication: Any situation where post-fabrication forming, welding, or straightening may introduce significant residual stress.
• Quality Assurance and Materials Testing Laboratories: Implementing reliable fracture toughness data adjusted for residual stress, leading to more accurate material selection and design decisions.
• Research and Development: Supporting the evaluation and development of new aluminum alloys and processing techniques by enabling better understanding of the material’s true toughness properties.

Related Standards

For comprehensive fracture toughness assessment and residual stress management in aluminum products, consider the following standards alongside ASTM B909-21a:

• ASTM E399: Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
• ASTM E561: Test Method for K Curve Determination
• ASTM E1823: Terminology Relating to Fatigue and Fracture Testing
• ISO 12135: Unified method for the determination of quasistatic fracture toughness
• ANSI H35.1: Alloy and temper designations for aluminum

Keywords: aluminum, fracture toughness, residual stress, aluminum alloys, KIc testing, ASTM B909-21a, aluminum plate, hand forgings, material testing, standard guide, mechanical testing, non-stress relieved aluminum

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: B909 − 21a
Standard Guide for
Plane Strain Fracture Toughness Testing of Non-Stress
Relieved Aluminum Products
This standard is issued under the fixed designation B909; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.3 ISO Standard:
ISO 12135 Unified method of test for the determination of
1.1 This guide covers supplementary guidelines for plane-
quasistatic fracture toughness
strain fracture toughness testing of aluminum products for
which complete stress relief is not practicable. Guidelines for
3. Terminology
recognizingwhenresidualstressesmaybesignificantlybiasing
3.1 Definitions:
test results are presented, as well as methods for minimizing
3.1.1 Terms in Test Method E399 and Terminology E1823
the effects of residual stress during testing. This guide also
are applicable herein.
provides guidelines for an empirical correction as well as
3.2 Definitions of Terms Specific to This Standard:
interpretation of data produced during the testing of these
3.2.1 corrected plane-strain fracture toughness—a test
products. Test Method E399 is the standard test method to be
result, designated K (corrected), which has been corrected for
Q
used for plane-strain fracture toughness testing of aluminum
residual stress bias by one of the methods outlined in this
alloys.
guide.
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 3.2.1.1 Discussion—The corrected result is an estimation of
the K or K thatwouldhavebeenobtainedinaresidualstress
responsibility of the user of this standard to establish appro-
Q Ic
priate safety, health, and environmental practices and deter- freespecimen.Thecorrectedresultmaybeobtainedfromatest
record which yielded either an invalid K or valid K , but for
mine the applicability of regulatory limitations prior to use.
Q Ic
1.3 This international standard was developed in accor- which there is evidence that significant residual stress is
present in the test coupon.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the 3.2.2 invalid plane-strain fracture toughness—a test result,
Development of International Standards, Guides and Recom- designated K , that does not meet one or more validity
Q
mendations issued by the World Trade Organization Technical requirements in Test Method E399 or ISO 12135 and may or
Barriers to Trade (TBT) Committee. may not be significantly influenced by residual stress.
3.2.3 valid plane-strain fracture toughness—a test result,
2. Referenced Documents
designated K , meeting the validity requirements in Test
Ic
2.1 ASTM Standards:
Method E399 or ISO 12135 that may or may not be signifi-
E399 Test Method for Linear-Elastic Plane-Strain Fracture
cantly influenced by residual stress.
Toughness of Metallic Materials
4. Significance and Use
E561 Test Method forK Curve Determination
R
E1823 TerminologyRelatingtoFatigueandFractureTesting
4.1 The property K , determined by Test Method E399 or
Ic
2.2 ANSI Standard:
ISO 12135, characterizes a material’s resistance to fracture in
ANSI H35.1 Alloy and Temper Designations for Aluminum
a neutral environment and in the presence of a sharp crack
subjected to an applied opening force or moment within a field
1 of high constraint to lateral plastic flow (plane strain condi-
This guide is under the jurisdiction ofASTM Committee B07 on Light Metals
and Alloys and is the direct responsibility of Subcommittee B07.05 on Testing.
tion).A K value is considered to be a lower limiting value of
Ic
Current edition approved Dec. 1, 2021. Published December 2021. Originally
fracture toughness associated with the plane strain state.
approved in 2000. Last previous edition approved in 2021 as B909 – 21. DOI:
4.1.1 Thermal quenching processes used with precipitation
10.1520/B0909-21A.
hardened aluminum alloy products can introduce significant
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. Available from International Organization for Standardization (ISO), ISO
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St., Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
4th Floor, New York, NY 10036, http://www.ansi.org. Switzerland, https://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B909 − 21a
residual stresses. Mechanical stress relief procedures
(stretching, compression) are commonly used to relieve these
residual stresses in products with simple shapes. However, in
the case of mill products with thick cross-sections (for
example,heavygaugeplateorlargehandforgings)orcomplex
shapes (for example, closed die forgings, complex open die
forgings, stepped extrusions, castings), complete mechanical
stress relief is not always possible. In other instances residual
stresses may be introduced into a product during fabrication
operations such as straightening, forming, or welding opera-
tions.
NOTE 1—Measure the specimen height before and after machining the
NOTE 1—For the purposes of this guide, only bulk residual stress is
crack starter notch.
considered (that is, of the type typically created during a quench process
FIG. 1 Residual Stress Distortion Characterization for K Testing
Ic
for thermal heat treatment) and not engineered residual stress, such as
of C(T) Specimens
from shot peening or cold hole expansion.
4.1.2 Specimens taken from such products that contain
residual stress will likewise themselves contain residual stress.
4.2.2 Provide experimental methods that can be used to
While the act of specimen extraction in itself partially relieves
minimize the effect of residual stress on measured fracture
and redistributes the pattern of original stress, the remaining
toughness values.
magnitude can still be appreciable enough to cause significant
4.2.3 Suggest methods that can be used to correct residual
error in the test result.
stress influenced values of fracture toughness to values that
4.1.3 Residual stress is a non-proportional internal stress
approximate a fracture toughness value representative of a test
that is superimposed on the applied stress and results in an
performed without residual stress bias.
actual crack-tip stress-intensity factor that is different from one
based solely on externally applied forces or displacements, and
5. Interferences
residual stress can bias the toughness measurement.
5.1 There are a number of warning signs that test measure-
Conceptually, compressive residual stress in the region of the
ments are or might be biased by the presence of residual stress.
crack tip must be overcome by the applied force before the
If any one or more of the following conditions exist, residual
cracktipexperiencestensilestresses,thusbiasingthe K or K
Q Ic
stress bias of the ensuing plane strain fracture toughness test
measurement to a higher value, potentially producing a non-
resultshouldbesuspected.Thelikelihoodthatresidualstresses
lower-bound toughness value. Quantitatively, the effect de-
are biasing test results increases as the number of warning
pends on stress equilibrium for the continuously varying
signs increase.
residual stress field and the associated crack tip response.
5.1.1 A temper designation of a heat treatable aluminum
Conversely, a tensile residual stress is additive to the applied
product that does not indicate that it was stress relieved. Stress
forceandbiasesthemeasured K or K resulttoalowervalue,
Q ic
relief is indicated by any of the following temper designations:
potentially under-representing the material “true” toughness
T_51, T_510, T_511, T_52, or T_54, as described in ANSI
capability.
H35.1.
4.1.4 Teststhatutilizedeepedge-notchedspecimenssuchas
5.1.2 Machining distortion during specimen preparation.An
thecompacttensionC(T)areparticularlysensitivetodistortion
effective method to characterize distortion of a C(T) specimen
during specimen machining when substantial residual stress is
is to measure the specimen height directly above the knife
present.Ingeneral,forthosecaseswheresuchresidualstresses
edges (typically at the front face for specimen designs with
are thermal quench induced, the resulting K or K result is
Ic Q
integralknifeedges)priortoandaftermachiningthenotch(see
typically biased upward (that is, K is higher than that which
Q
Fig. 1). Experience has shown that for an aluminum C(T)
would have been achieved in a residual stress-free specimen).
specimen with a notch length to width ratio (a /W) of 0.45, a
The inflated values result from the redistribution of residual
o
difference in the height measured before and after machining
stress during specimen machining and excessive fatigue pre-
the notch equal to or greater than 0.003 in. (0.076 mm) is an
crack front curvature caused by variable residual stresses
indicator that the ensuing test result will be significantly
across the crack front.
influenced by residual stress (for example, for a specimen size
4.2 This guide can serve the following purposes:
of nominally W = 2 or 3 in. (50 or 75 mm) with W/B = 2).
4.2.1 Provide warning signs that the measured value of K
Ic
NOTE 2—Often the first indication of residual stresses is when there is
has been biased by residual stresses and may not be a lower
difficulty sawing the specimen notch due to excessive drag on the
limit value of fracture toughness.
sawblade.This is caused by the release of compressive residual stresses at
the front face causing the specimen to clamp down on the sawblade,
creating excessive vibration and noise. Incremental sawing, where the
sawblade is backed out periodically, usually solves this problem.
Prime, M. B. and Hill, M. R., “Residual stress, stress relief, and inhomogeneity
in aluminum plate,” Scripta Materialia, Vol 46, 2002, pp. 77–82.
5.1.3 Excessive fatigue precrack front curvature not meet-
Bucci, R.J., “Effect of Residual Stress on Fatigue Crack Growth Rate
ing the crack-front straightness requirements in Test Method
Measurement,” Fracture Mechanics: Thirteenth Conference, ASTM STP 743,
American Society for Testing and Materials, 1981, pp. 28–47. E399 or ISO 12135.
B909 − 21a
5.1.4 Unusually high loads or number of cycles required for Moreover, the straighter crack fronts that result from precrack-
precracking relative to the same or similar alloy/products. ing at higher stress ratios have been shown to reduce the error
5.1.5 A significant change in fracture toughness that is in the ensuing fracture toughness measurement by up to 75 %.
greater than that typically observed upon changing specimen
NOTE 4—Test Method E399 requires precracking to be performed at
configuration (for example
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: B909 − 21 B909 − 21a
Standard Guide for
Plane Strain Fracture Toughness Testing of Non-Stress
Relieved Aluminum Products
This standard is issued under the fixed designation B909; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide covers supplementary guidelines for plane-strain fracture toughness testing of aluminum products for which
complete stress relief is not practicable. Guidelines for recognizing when residual stresses may be significantly biasing test results
are presented, as well as methods for minimizing the effects of residual stress during testing. This guide also provides guidelines
for an empirical correction as well as interpretation of data produced during the testing of these products. Test Method E399 is the
standard test method to be used for plane-strain fracture toughness testing of aluminum alloys.
1.2 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.
1.3 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:
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
E561 Test Method forK Curve Determination
R
E1823 Terminology Relating to Fatigue and Fracture Testing
2.2 ANSI Standard:
ANSI H35.1 Alloy and Temper Designations for Aluminum
2.3 ISO Standard:
ISO 12135 Unified method of test for the determination of quasistatic fracture toughness
3. Terminology
3.1 Definitions:
3.1.1 Terms in Test Method E399 and Terminology E1823 are applicable herein.
3.2 Definitions of Terms Specific to This Standard:
This guide is under the jurisdiction of ASTM Committee B07 on Light Metals and Alloys and is the direct responsibility of Subcommittee B07.05 on Testing.
Current edition approved May 1, 2021Dec. 1, 2021. Published June 2021December 2021. Originally approved in 2000. Last previous edition approved in 20172021 as
B909 – 17.21. DOI: 10.1520/B0909-21.10.1520/B0909-21A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from International Organization for Standardization (ISO), ISO Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva, Switzerland,
https://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B909 − 21a
3.2.1 corrected plane-strain fracture toughness—a test result, designated K (corrected), which has been corrected for residual
Q
stress bias by one of the methods outlined in this guide.
3.2.1.1 Discussion—
The corrected result is an estimation of the K or K that would have been obtained in a residual stress free specimen. The
Q Ic
corrected result may be obtained from a test record which yielded either an invalid K or valid K , but for which there is evidence
Q Ic
that significant residual stress is present in the test coupon.
3.2.2 invalid plane-strain fracture toughness—a test result, designated K , that does not meet one or more validity requirements
Q
in Test Method E399 or ISO 12135 and may or may not be significantly influenced by residual stress.
3.2.3 valid plane-strain fracture toughness—a test result, designated K , meeting the validity requirements in Test Method E399
Ic
or ISO 12135 that may or may not be significantly influenced by residual stress.
4. Significance and Use
4.1 The property K , determined by Test Method E399 or ISO 12135, characterizes a material’s resistance to fracture in a neutral
Ic
environment and in the presence of a sharp crack subjected to an applied opening force or moment within a field of high constraint
to lateral plastic flow (plane strain condition). A K value is considered to be a lower limiting value of fracture toughness
Ic
associated with the plane strain state.
4.1.1 Thermal quenching processes used with precipitation hardened aluminum alloy products can introduce significant residual
stresses into the product.stresses. Mechanical stress relief procedures (stretching, compression) are commonly used to relieve
these residual stresses in products with simple shapes. However, in the case of mill products with thick cross-sections (for example,
heavy gauge plate or large hand forgings) or complex shapes (for example, closed die forgings, complex open die forgings, stepped
extrusions, castings), complete mechanical stress relief is not always possible. In other instances residual stresses may be
unintentionally introduced into a product during fabrication operations such as straightening, forming, or welding operations.
NOTE 1—For the purposes of this guide, only bulk residual stress is considered (that is, of the type typically created during a quench process for thermal
heat treatment) and not engineered residual stress, such as from shot peening or cold hole expansion.
4.1.2 Specimens taken from such products that contain residual stress will likewise themselves contain residual stress. While the
act of specimen extraction in itself partially relieves and redistributes the pattern of original stress, the remaining magnitude can
still be appreciable enough to cause significant error in the test result.
4.1.3 Residual stress is a non-proportional internal stress that is superimposed on the applied stress and results in an actual
crack-tip stress-intensity factor that is different from one based solely on externally applied forces or displacements, and residual
stress can bias the toughness measurement. Conceptually, compressive residual stress in the region of the crack tip must be
overcome by the applied force before the crack tip experiences tensile stresses, thus biasing the K or K measurement to a higher
Q Ic
value, potentially producing a non-lower-bound toughness value. Quantitatively, the effect depends on stress equilibrium for the
continuously varying residual stress field and the associated crack tip response. Conversely, a tensile residual stress is additive to
the applied force and biases the measured K or K result to a lower value, potentially under-representing the material “true”
Q ic
toughness capability.
4.1.4 Tests that utilize deep edge-notched specimens such as the compact tension C(T) are particularly sensitive to distortion
during specimen machining when substantial residual stress is present. In general, for those cases where such residual stresses are
thermal quench induced, the resulting K or K result is typically biased upward (that is, K is higher than that which would have
Ic Q Q
been achieved in a residual stress-free specimen). The inflated values result from the redistribution of residual stress during
specimen machining and excessive fatigue precrack front curvature caused by variable residual stresses across the crack front.
4.2 This guide can serve the following purposes:
Prime, M. B. and Hill, M. R., “Residual stress, stress relief, and inhomogeneity in aluminum plate,” Scripta Materialia, 46, 2002, pp. 77–82. Prime, M. B. and Hill, M.
R., “Residual stress, stress relief, and inhomogeneity in aluminum plate,” Scripta Materialia, Vol 46, 2002, pp. 77–82.
Bucci, R.J., “Effect of Residual Stress on Fatigue Crack Growth Rate Measurement,” Fracture Mechanics: Thirteenth Conference, ASTM STP 743, American Society for
Testing and Materials, 1981, pp. 28–47.
B909 − 21a
NOTE 1—Measure the specimen height before and after machining the crack starter notch.
FIG. 1 Residual Stress Distortion Characterization for K Testing of C(T) Specimens
Ic
4.2.1 Provide warning signs that the measured value of K has been biased by residual stresses and may not be a lower limit value
Ic
of fracture toughness.
4.2.2 Provide experimental methods that can be used to minimize the effect of residual stress on measured fracture toughness
values.
4.2.3 Suggest methods that can be used to correct residual stress influenced values of fracture toughness to values that approximate
a fracture toughness value representative of a test performed without residual stress bias.
5. Interferences
5.1 There are a number of warning signs that test measurements are or might be biased by the presence of residual stress. If any
one or more of the following conditions exist, residual stress bias of the ensuing plane strain fracture toughness test result should
be suspected. The likelihood that residual stresses are biasing test results increases as the number of warning signs increase.
5.1.1 A temper designation of a heat treatable aluminum product that does not indicate that it was stress relieved. Stress relief is
indicated by any of the following temper designations: T_51, T_510, T_511, T_52, or T_54, as described in ANSI H35.1.
5.1.2 Machining distortion during specimen preparation. An effective method to characterize distortion of a C(T) specimen is to
measure the specimen height directly above the knife edges (typically at the front face for specimen designs with integral knife
edges) prior to and after machining the notch (see Fig. 1). Experience has shown that for an aluminum C(T) specimen with a notch
length to width ratio (a /W) of 0.45, a difference in the height measured before and after machining the notch equal to or greater
o
than 0.003 in. (0.076 mm) is an indicator that the ensuing test result will be significantly influenced by residual stress (for example,
for a specimen size of nominally W = 2 or 3 in. (50 or 75 mm) with W/B = 2).
NOTE 2—Often the first indication of residual stresses is when there is difficulty sawing the specimen notch due to excessive drag on the sawblade. This
is caused by the release of compressive residual stresses at the front face causing the specimen to clamp down on the sawblade, which creates creating
excessive vibration and noise. Incremental sawing, where the sawblade is backed out periodically, is usually the solution to usually solves this problem.
5.1.3 Excessive fatigue precrack front curvature not meeting the crack-front straightness requirements in Test Method E399 or ISO
12135.
5.1.4 Unusually high loads or number of cycles required for precracking relative to the same or similar alloy/products.
5.1.5 A significant change in fracture toughness that is greater than that typically observed upon changing specimen configuration
(for example, from C(T) to three point bend bar) or upon changing specimen’s W dimension that cannot be explained by other
means. For example, if residual stress is biasing fracture toughness tests results, then increasing the specimen’s W dimension may
result in increasing K values because the larger specimen will intersect a larger portion of the stress field in the host material.
Q
NOTE 3—Other factors, such as a steeply rising R-curve (see Type I force-displacement (CMOD) record in Test Method E399) in high toughness
alloy/products, may also be responsible for K values increasing
...