ASTM E646-16
(Test Method)Standard Test Method for Tensile Strain-Hardening Exponents (n -Values) of Metallic Sheet Materials
Standard Test Method for Tensile Strain-Hardening Exponents (<emph type="bdit">n</emph > -Values) of Metallic Sheet Materials
SIGNIFICANCE AND USE
5.1 This test method is useful for estimating the strain at the onset of necking in a uniaxial tension test (1). Practically, it provides an empirical parameter for appraising the relative stretch formability of similar metallic systems. The strain-hardening exponent is also a measure of the increase in strength of a material due to plastic deformation.
5.2 The strain-hardening exponent may be determined over the entire plastic stress-strain curve or any portion(s) of the stress-strain curve specified in a product specification.
Note 4: The engineering strain interval 10–20% is commonly used for determining the strain-hardening exponent, n, of formable low-carbon steel products
5.3 This test method is not intended to apply to any portion of the true stress versus true strain curve that exhibits discontinuous behavior; however, the method may be applied by curve-smoothing techniques as agreed upon.
Note 5: For example, those portions of the stress-strain curves for mild steel, aluminum, or other alloys that exhibit yield point and Lüders band elongation, twinning, or Portevin–Le Chatelier effect (PLC) may be characterized as behaving discontinuously.
Note 6: Caution should be observed in the use of curve-smoothing techniques as they may affect the n-value.
5.4 This test method is suitable for determining the tensile stress-strain response of metallic sheet materials in the plastic region prior to the onset of necking.
5.5 The n-value may vary with the displacement rate or strain rate used, depending on the metal and test temperature.
SCOPE
1.1 This test method covers the determination of a strain-hardening exponent by tension testing of metallic sheet materials for which plastic-flow behavior obeys the power curve given in the Introduction.
Note 1: A single power curve may not be a satisfactory fit to the entire stress-strain curve between yield and necking. If such is the case, more than one value of the strain-hardening exponent may be obtained (2) by agreement using this test method.
1.2 This test method is specifically for metallic sheet materials with thicknesses of at least 0.005 in. (0.13 mm) but not greater than 0.25 in. (6.4 mm). The method has successfully been and may be applied to other forms and thicknesses by agreement
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
Note 2: The value of the strain-hardening exponent, n, has no units and is independent of the units used in its determination
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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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: E646 − 16
Standard Test Method for
Tensile Strain-Hardening Exponents (n -Values) of Metallic
1
Sheet Materials
This standard is issued under the fixed designation E646; 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.
INTRODUCTION
This test method for determining tensile strain-hardening exponents n utilizes stress-stain data
obtained in a uniaxial tension test. Tensile data are obtained in a continuous and rate-controlled
manner via displacement or strain control. The strain-hardening exponents are determined from an
empirical representation over the range of interest of the true-stress versus true-strain curve. The
2
mathematical representation used in this method is a power curve (Note 1) of the form (1) :
n
σ = Kε
where:
σ = true stress,
ε = plastic component of true strain, but in special cases may be the total true strain. (See 10.2),
K = is a constant, often called the strength coefficient having the units of stress, and
n = strain-hardening exponent
1. Scope* 1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This test method covers the determination of a strain-
responsibility of the user of this standard to establish appro-
hardening exponent by tension testing of metallic sheet mate-
priate safety and health practices and determine the applica-
rials for which plastic-flow behavior obeys the power curve
bility of regulatory limitations prior to use.
given in the Introduction.
NOTE1—Asinglepowercurvemaynotbeasatisfactoryfittotheentire
2. Referenced Documents
stress-strain curve between yield and necking. If such is the case, more
3
than one value of the strain-hardening exponent may be obtained (2) by 2.1 ASTM Standards:
agreement using this test method.
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
1.2 This test method is specifically for metallic sheet mate-
E8/E8M Test Methods for Tension Testing of Metallic Ma-
rials with thicknesses of at least 0.005 in. (0.13 mm) but not
terials
greater than 0.25 in. (6.4 mm). The method has successfully
E29 Practice for Using Significant Digits in Test Data to
been and may be applied to other forms and thicknesses by
Determine Conformance with Specifications
agreement
E83 Practice for Verification and Classification of Exten-
1.3 The values stated in inch-pound units are to be regarded
someter Systems
as standard. The values given in parentheses are mathematical
E111 Test Method for Young’s Modulus, Tangent Modulus,
conversions to SI units that are provided for information only
and Chord Modulus
and are not considered standard.
E177 Practice for Use of the Terms Precision and Bias in
ASTM Test Methods
NOTE 2—The value of the strain-hardening exponent, n, has no units
and is independent of the units used in its determination
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
2.2 ISO Standard
ISO 10275:2007 Metallic materials -- Sheet and strip --
1
This test method is under the jurisdiction of ASTM Committee E28 on
Mechanical Testing and is the direct responsibility of Subcommittee E28.02 on Determination of tensile strain hardening exponent
Ductility and Formability.
Current edition approved Feb. 1, 2016. Published March 2016. Originally
3
approved in 1978. Last previous edition approved in 2015 as E646 - 15. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/E0646-16 contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
2
The boldface numbers in parentheses refer to the list of references appended to Standards volume information, refer to the standard’s Document Summary page on
this method. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1
---------------------- Page: 1 ----------------------
E646 − 16
range for metallic sheet material for which the true-stress true-strain data
3. Terminology
follow the stated relationship.
3.1 For definitions of other terms used in this test method,
5. Significance and Use
refer to E6 (Standard Terminology Relating to Methods of
Mechanical Testing).
5.1 This test method is useful for estimating the strain at the
onset of necking in a uniaxial tension test (1). Practica
...
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: E646 − 15 E646 − 16
Standard Test Method for
Tensile Strain-Hardening Exponents (n -Values) of Metallic
1
Sheet Materials
This standard is issued under the fixed designation E646; 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.
INTRODUCTION
This test method for determining tensile strain-hardening exponents n utilizes stress-stain data
obtained in a uniaxial tension test. Tensile data are obtained in a continuous and rate-controlled
manner via displacement or strain control. The strain-hardening exponents are determined from an
empirical representation over the range of interest of the true-stress versus true-strain curve. The
2
mathematical representation used in this method is a power curve (Note 1) of the form (1) :
n
σ = Kε
where:
σ = true stress,
ε = plastic component of true strain, but in special cases may be the total true strain. (See 10.2),
K = is a constant, often called the strength coefficient having the units of stress, and
n = strain-hardening exponent
1. Scope Scope*
1.1 This test method covers the determination of a strain-hardening exponent by tension testing of metallic sheet materials for
which plastic-flow behavior obeys the power curve given in the Introduction.
NOTE 1—A single power curve may not be a satisfactory fit to the entire stress-strain curve between yield and necking. If such is the case, more than
one value of the strain-hardening exponent may be obtained (2) by agreement using this test method.
1.2 This test method is specifically for metallic sheet materials with thicknesses of at least 0.005 in. (0.13 mm) but not greater
than 0.25 in. (6.4 mm). The method has successfully been and may be applied to other forms and thicknesses by agreement
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
NOTE 2—The value of the strain-hardening exponent, n, is has no units and is independent of the units used in its determination
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
3
2.1 ASTM Standards:
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E8/E8M Test Methods for Tension Testing of Metallic Materials
E29 Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
E83 Practice for Verification and Classification of Extensometer Systems
E111 Test Method for Young’s Modulus, Tangent Modulus, and Chord Modulus
1
This test method is under the jurisdiction of ASTM Committee E28 on Mechanical Testing and is the direct responsibility of Subcommittee E28.02 on Ductility and
Formability.
Current edition approved Nov. 15, 2015Feb. 1, 2016. Published February 2016March 2016. Originally approved in 1978. Last previous edition approved in 20072015 as
ɛ1
E646 - 07E646 - 15. . DOI: 10.1520/E0646-15.10.1520/E0646-16
2
The boldface numbers in parentheses refer to the list of references appended to this method.
3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1
---------------------- Page: 1 ----------------------
E646 − 16
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
2.2 ISO Standard
ISO 10275:2007 Metallic materials -- Sheet and strip -- Determination of tensile strain hardening exponent
3. Terminology
3.1 For definitions of other terms used in this test method, refer to E6 (Standard Terminology Relating to Methods of
Mechanical Testing).
3.2 Definitions:
3.2.1 elastic true strain, ɛ , n—el
...
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