ASTM E328-21
(Test Method)Standard Test Methods for Stress Relaxation for Materials and Structures
Standard Test Methods for Stress Relaxation for Materials and Structures
SIGNIFICANCE AND USE
5.1 Stress-relaxation test data are necessary when designing most mechanically fastened joints to ensure the permanent tightness of bolted or riveted assemblies, press or shrink-fit components, rolled-in tubes, etc. Other applications include predicting the decrease in the tightness of gaskets, in the hoop stress of solderless wrapped connections, in the constraining force of springs, and in the stability of wire tendons in prestressed concrete.
5.2 The ability of a material to relax at high-stress concentrations such as are present at notches, inclusions, cracks, holes, and fillets can be predicted from stress-relaxation data. Such test data are also useful to judge the heat-treatment condition necessary for the thermal relief of residual internal stresses in forgings, castings, weldments, machined or cold-worked surfaces, etc. The tests outlined in these methods are limited to conditions of approximately constant constraint and test environment.
5.3 The general stress-relaxation test is performed by isothermally applying a force to a specimen with fixed value of constraint. The constraint is maintained constant, and the constraining force is determined as a function of time. The major problem in the stress-relaxation test is that constant constraint can be very difficult to maintain. The effects on test results are very significant, and considerable attention shall be given to minimize the constraint variation. Also, experimenters should determine and report the extent of variation in each stress-relaxation test so that this factor can be taken into consideration.
5.4 There are many methods of performing the stress-relaxation test, each with a different starting procedure. However, the constraint is usually obtained initially by the application of an external force at either a specific force-application rate or a specific strain rate. The two methods will produce the characteristic behavior shown in Fig. 1 when the initial stress, σ0, exceeds the proporti...
SCOPE
Note 1: The method of testing for the stress relaxation of plastics has been withdrawn from this standard, and the responsibility has been transferred to Practice D2991.
1.1 These test methods cover the determination of the time dependence of stress (stress relaxation) in materials and structures under conditions of approximately constant constraint, constant test environment, and negligible vibration. In the procedures, the material or structure is initially constrained by externally applied forces, and the change in the external force necessary to maintain this constraint is determined as a function of time.
1.2 Specific methods for conducting stress-relaxation tests on materials subjected to tension, compression, bending and torsion stresses are described in Parts A, B, C, and D, respectively. These test methods also include recommendations for the necessary testing equipment and for the analysis of the test data.
1.3 Bending stress-relaxation tests to determine relaxation properties by using ring-shaped specimens machined from bulk material have been thoroughly developed and widely used to determine stress-relaxation properties (1).2 These tests are outside the scope of these test methods.
1.4 The long time periods required for these types of tests are often unsuited for routine testing or for specification in the purchase of material. However, these tests are valuable tools in obtaining practical design information on the stress relaxation of materials subjected to constant constraint, constant test environment, and negligible vibration, and in investigations of the fundamental behavior of materials.
1.5 Units—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.
1.6 This standard does not purport to address all of the safety concerns, if a...
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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: E328 − 21
Standard Test Methods for
1
Stress Relaxation for Materials and Structures
This standard is issued under the fixed designation E328; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
These test methods cover a broad range of testing activities. To aid in locating the subject matter
pertinent to a particular test, the standard is divided into a general section, which applies to all
stress-relaxationtestsformaterialsandstructures.Thisgeneralsectionisfollowedbyletter-designated
parts that apply to tests for material characteristics when subjected to specific, simple stresses, such
as uniform tension, uniform compression, bending or torsion. To choose from among these types of
stress, three factors should be considered:
(1)Whenthematerialdataaretobeappliedtothedesignofaparticularclassofcomponent,thestress
during the stress-relaxation test should be similar to that imposed on the component. For example,
tension tests are suitable for bolting applications and bending tests for leaf springs.
(2) Tension and compression stress-relaxation tests have the advantage that the stress can be reported
simply and unequivocally. During bending stress-relaxation tests, the state of stress is complex, but
can be accurately determined when the initial strains are elastic. If plastic strains occur on application
of force, stresses can usually be determined within a bounded range only. Tension stress-relaxation
tests, when compared to compression stress-relaxation tests, have the advantage that it is unnecessary
toguardagainstbuckling.Therefore,whenthetestmethodisnotrestrictedbythetypeofstressinthe
component, tension stress-relaxation testing should be used.
(3)Bendingstress-relaxationtests,whencomparedtotensionandcompressionstress-relaxationtests,
have the advantage of using lighter and simpler apparatus for specimens of the same cross-sectional
area. Strains are usually calculated from deflection or curvature measurements. Since the specimens
canusuallybedesignedsothatthesequantitiesaremuchgreaterthantheaxialdeformationinadirect
stress test, strain is more easily measured and more readily used for machine control in bending
stress-relaxation tests. Due to the small forces normally required and the simplicity of the apparatus
whenstaticfixturesaresufficient,manyspecimenscanbeplacedinasingleovenorfurnacewhentests
are made at elevated temperatures.
1. Scope* external force necessary to maintain this constraint is deter-
NOTE 1—The method of testing for the stress relaxation of plastics has mined as a function of time.
been withdrawn from this standard, and the responsibility has been
1.2 Specific methods for conducting stress-relaxation tests
transferred to Practice D2991.
on materials subjected to tension, compression, bending and
1.1 These test methods cover the determination of the time
torsion stresses are described in Parts A, B, C, and D,
dependence of stress (stress relaxation) in materials and
respectively.Thesetestmethodsalsoincluderecommendations
structures under conditions of approximately constant
for the necessary testing equipment and for the analysis of the
constraint, constant test environment, and negligible vibration.
test data.
In the procedures, the material or structure is initially con-
1.3 Bending stress-relaxation tests to determine relaxation
strained by externally applied forces, and the change in the
propertiesbyusingring-shapedspecimensmachinedfrombulk
material have been thoroughly developed and widely used to
2
determine stress-relaxation properties (1). These tests are
1
These test methods are under the jurisdiction of ASTM Committee E28 on
outside the scope of these test methods.
Mechanical Testing and is the direct responsibility of Subcommittee E28.04 on
Uniaxial Testing.
Current edition approved Feb. 1, 2021. Published February 2021. Originally
2
approved in 1967. Last previous approved in 2020 as E328–20. DOI: 10.1520/ The boldface numbers in parentheses refer to a list of references at the end of
E0328-21. this standard.
*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 ----------------------
E328 − 21
−2
1.4 The long time periods required for these types of tests 3.2.1 initial stress, σ , [FL ],n—the stress introduced into
0
are often unsu
...
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: E328 − 20 E328 − 21
Standard Test Methods for
1
Stress Relaxation for Materials and Structures
This standard is issued under the fixed designation E328; 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
These test methods cover a broad range of testing activities. To aid in locating the subject matter
pertinent to a particular test, the standard is divided into a general section, which applies to all
stress-relaxation tests for materials and structures. This general section is followed by letter-designated
parts that apply to tests for material characteristics when subjected to specific, simple stresses, such
as uniform tension, uniform compression, bending or torsion. To choose from among these types of
stress, three factors should be considered:
(1) When the material data are to be applied to the design of a particular class of component, the stress
during the stress-relaxation test should be similar to that imposed on the component. For example,
tension tests are suitable for bolting applications and bending tests for leaf springs.
(2) Tension and compression stress-relaxation tests have the advantage that the stress can be reported
simply and unequivocally. During bending stress-relaxation tests, the state of stress is complex, but
can be accurately determined when the initial strains are elastic. If plastic strains occur on application
of force, stresses can usually be determined within a bounded range only. Tension stress-relaxation
tests, when compared to compression stress-relaxation tests, have the advantage that it is unnecessary
to guard against buckling. Therefore, when the test method is not restricted by the type of stress in the
component, tension stress-relaxation testing should be used.
(3) Bending stress-relaxation tests, when compared to tension and compression stress-relaxation tests,
have the advantage of using lighter and simpler apparatus for specimens of the same cross-sectional
area. Strains are usually calculated from deflection or curvature measurements. Since the specimens
can usually be designed so that these quantities are much greater than the axial deformation in a direct
stress test, strain is more easily measured and more readily used for machine control in bending
stress-relaxation tests. Due to the small forces normally required and the simplicity of the apparatus
when static fixtures are sufficient, many specimens can be placed in a single oven or furnace when tests
are made at elevated temperatures.
1. Scope*
NOTE 1—The method of testing for the stress relaxation of plastics has been withdrawn from this standard, and the responsibility has been transferred
to Practice D2991.
1.1 These test methods cover the determination of the time dependence of stress (stress relaxation) in materials and structures
under conditions of approximately constant constraint, constant test environment, and negligible vibration. In the procedures, the
material or structure is initially constrained by externally applied forces, and the change in the external force necessary to maintain
this constraint is determined as a function of time.
1
These test methods are under the jurisdiction of ASTM Committee E28 on Mechanical Testing and is the direct responsibility of Subcommittee E28.04 on Uniaxial
Testing.
Current edition approved Dec. 1, 2020Feb. 1, 2021. Published February 2021. Originally approved in 1967. Last previous approved in 20132020 as E328–13.–20. DOI:
10.1520/E0328-20.10.1520/E0328-21.
*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 ----------------------
E328 − 21
1.2 Specific methods for conducting stress-relaxation tests on materials subjected to tension, compression, bending and torsion
stresses are described in Parts A, B, C, and D, respectively. These test methods also include recommendations for the necessary
testing equipment and for the analysis of the test data.
1.3 Bending stress-relaxation tests to determine relaxation properties by using ring-shaped specimens machined from bulk
2
material have been thoroughly developed and widely used to determine stress-relaxation properties (1). These tests are outs
...
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