ASTM E328-86(1996)e1
(Test Method)Standard Test Methods for Stress Relaxation Tests for Materials and Structures
Standard Test Methods for Stress Relaxation Tests for Materials and Structures
SCOPE
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 environment, and negligible vibration. In the procedures recommended, 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 loads 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 It is recognized that 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 the conditions enumerated, and in investigations of the fundamental behavior of materials.
1.4 This standard does not purport to address the safety problems 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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Standards Content (Sample)
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e1
Designation: E 328 – 86 (Reapproved 1996)
Standard Test Methods for
Stress Relaxation for Materials and Structures
This standard is issued under the fixed designation E 328; 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 (e) indicates an editorial change since the last revision or reapproval.
e NOTE—The title was changed editorially in January 1996.
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
loading, the following 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 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 relaxation tests have the advantage that the stress can be reported
simply and unequivocally. During bending relaxation tests, the state of stress is complex, but can be
accurately determined when the initial strains are elastic. If plastic strains occur on loading, stresses
can usually be determined within a bounded range only. Tension relaxation tests, when compared to
compression 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 testing is
recommended.
(3) Bending tests for relaxation, when compared to tension and compression 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 the bending
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 external force necessary to maintain this constraint is deter-
mined as a function of time.
NOTE 1—The method of testing for the stress relaxation of plastics has
1.2 Specific methods for conducting stress relaxation tests
been withdrawn from this standard, and the responsibility has been
on materials subjected to tension, compression, bending and
transferred to Practice D 2991.
torsion loads are described in Parts A, B, C, and D, respec-
1.1 These test methods cover the determination of the time
tively. These test methods also include recommendations for
dependence of stress (stress relaxation) in materials and
the necessary testing equipment and for the analysis of the test
structures under conditions of approximately constant con-
data.
straint, constant environment, and negligible vibration. In the
1.3 It is recognized that the long time periods required for
procedures recommended, the material or structure is initially
these types of tests are often unsuited for routine testing or for
constrained by externally applied forces, and the change in the
specification in the purchase of material. However, these tests
are valuable tools in obtaining practical design information on
These test methods are under the jurisdiction of ASTM Committee E28 on
the stress relaxation of materials subjected to the conditions
Mechanical Testing and is the direct responsibility of Subcommittee E28.10 on
enumerated, and in investigations of the fundamental behavior
Effect of Elevated Temperature on Properties.
of materials.
Current edition approved Feb. 28, 1986. Published May 1986. Originally
published as E 328 – 67 T. Last previous edition E 328 – 78.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 328
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 appro-
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D 2991 Practice for Testing Stress-Relaxation of Plastics
E 4 Practices for Force Verification of Testing Machines
E 8 Test Methods for Tension Testing of Metallic Materials
E 9 Test Methods of Compression Testing of Metallic Ma-
terials at Room Temperature
E 83 Practice for Verification and Classification of Exten-
someters
E 139 Practice for Conducting Creep, Creep-Rupture, and
Stress-Rupture Tests of Metallic Materials
FIG. 1 Characteristic Behavior During Loading Period in a
Relaxation Test
3. Terminology
3.1 Definitions:
constant constraint is based. Selection of this time does not
3.1.1 stress relaxation—the time-dependent decrease in
imply that the loading procedure or period, or both, are not
stress in a solid under given constraint conditions.
significant test parameters. These must always be considered in
3.1.1.1 Discussion—The general stress relaxation test is
the application of the data.
performed by isothermally loading a specimen to a fixed value −2
3.1.4 remaining stress [FL ]—the stress remaining at a
of constraint. The constraint is maintained constant and the
given time during a stress relaxation test.
constraining force is determined as a function of time. The
3.1.5 relaxed stress—the initial stress minus the remaining
major problem in the stress relaxation test is that constant
stress at a given time during a stress relaxation test.
constraint is virtually impossible to maintain. The effects on
3.1.6 stress relaxation curve—a plot of the remaining or
test results are very significant and considerable attention must
relaxed stress as a function of time.
be given to minimize the constraint variation. Also, experi-
3.1.6.1 Discussion—A curve to demonstrate that the stress
menters should determine and report the extent of variation in
relaxation behavior can be obtained by plotting either the
each stress relaxation test so that this factor can be taken into
remaining stress or the relaxed stress as a function of time (see
consideration.
Fig. 2). The remaining stress will, of course, decrease with
−2
3.1.2 initial stress [FL ]—the stress introduced into a
time, and the relaxed stress will start at zero and increase with
specimen by imposing the given constraint conditions before
time as seen in Fig. 2(b).
stress relaxation begins.
3.1.7 relaxation rate—the absolute value of the slope of the
3.1.2.1 Discussion—There are many methods of performing
relaxation curve at a given time.
the stress relaxation test, each with a different starting proce-
dure. However, the constraint is usually obtained initially by
the application of the external load at either a specific load rate
or a specific strain rate. The two methods will produce the
characteristic behavior shown in Fig. 1 when the initial stress,
s , exceeds the proportional limit. Most testing machines,
while reaching the constraint value, do not produce either a
constant load rate or constant strain rate, but something in
between. However, the general characteristics of the data will
be similar to those indicated. The rate of loading in either case
should be reasonably rapid, but without impact or vibration, so
that any relaxation during the loading period will be small.
3.1.3 zero time, t —the time when the given loading or
constraint conditions are initially obtained in a stress relaxation
test.
3.1.3.1 Discussion—The stress relaxation test is considered
to have started at zero time, t in Fig. 1. This is the reference
time from which the observed reduction in load to maintain
Annual Book of ASTM Standards, Vol 08.02.
Annual Book of ASTM Standards, Vol 03.01. FIG. 2 Typical Relaxation Curves
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E 328
3.1.8 spherometer—an instrument used to measure circular
or spherical curvature.
3.1.9 indicated nominal temperature or indicated
temperature—the temperature that is indicated by the
temperature-measuring device.
4. Summary of Test Methods
4.1 In each of the various methods of loading described in
the applicable specific sections, the specimen is subjected to an
increasing load until the specified initial strain is attained (see
zero time in 3.1.3 and in Fig. 1). For the duration of the test, the
specimen constraint is maintained constant. The initial stress is
calculated from the initial load (moment, torque) as measured
at zero time, the specimen geometry, and the appropriate elastic
constants, often using simple elastic theory. The remaining
stress may be calculated from the load (moment or torque)
determined under constraint conditions either continuously
(4.1.1), periodically (4.1.2), or by elastic springback at the end
of the test period [4.1.3 (see Fig. 3)].
4.1.1 Readings are taken continuously from a force indica-
tor while the apparatus adjusts the force to maintain constraint
within specified bounds.
NOTE 2—Most load, moment, or torque measuring devices depend on
the devices’ elasticity to measure the quantities involved. Therefore, it is
FIG. 4 Derivation of Stress-Relaxation Curve from Continuous
necessary that when using such devices, to maintain the total strain
Unloading Technique
constant within an upper and lower bound as shown in Fig. 4(a).
4.1.2 The force required to lift the specimen just free of one
5.2 The ability of a material to relax at high-stress concen-
or more constraints during the test period is measured.
trations such as are present at notches, inclusions, cracks,
4.1.3 The elastic springback is measured after unloading at
holes, fillets, etc., may be predicted from stress relaxation data.
the end of the test period.
Such test data are also useful to judge the heat-treatment
4.2 With 4.1.1 and 4.1.2, a single specimen can be used to
condition necessary for the thermal relief of residual internal
obtain data for a curve of stress versus time. With 4.1.3, the
stresses in forgings, castings, weldments, machined or cold-
same specimens may be used to determine the remaining or
worked surfaces, etc. The tests outlined in these methods are
relaxed stress after various time intervals, if it can be demon-
limited to conditions of approximately constant constraint and
strated for a given material that identical results are obtained in
environment.
either using virgin or reloaded specimens. Otherwise, indi-
5.3 The test results are highly sensitive to small changes in
vidual specimens must be used for each point on the curve.
environmental conditions and thus require precise control of
test conditions and methods.
5. Significance and Use
5.4 The reproducibility of data will depend on the manner
5.1 Relaxation test data are necessary when designing most
with which all test conditions are controlled. The effects of
mechanically fastened joints to assure the permanent tightness
aging or residual stress may significantly affect results, as may
of bolted or riveted assemblies, press or shrink-fit components,
variations in material composition.
rolled-in tubes, etc. Other applications include predicting the
decrease in the tightness of gaskets, in the hoop stress of
6. Apparatus
solderless wrapped connections, in the constraining force of
6.1 See the appropriate paragraph under each section.
springs, and the stability of wire tendons in prestressed
6.2 It is recommended that the equipment be located in a
concrete.
draft-free, constant-temperature environment, 65°F (63°C).
7. Temperature Control and Measurement
7.1 The test space (controlled temperature room, furnace, or
cold box) should be capable of being maintained at a constant
temperature by a suitable automatic device. This is the most
important single factor in a stress relaxation test since the stress
relaxation rate, dimensions, and constraint conditions of the
specimen are dependent upon the test temperature. Any type of
heating or cooling which permits close temperature control of
the test space environment is satisfactory.
7.2 The temperature should be recorded, preferably continu-
FIG. 3 Stress-Strain Diagram for Determining Relaxation in
Stress ously or at least periodically. Temperature variations of the
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E 328
specimens from the indicated nominal test temperature due to 12.1.1.2 Microstructure,
all causes, including cycling of the controller or position along 12.1.1.3 Mechanical properties,
the specimen gage length, should not exceed6 5°F (3°C) or 12.1.2 Specimen geometry,
61/2 %, whichever is greater. These limits should apply 12.1.3 Testing machine or apparatus,
initially and for the duration of the test. 12.1.4 Strain measurement method,
7.3 The combined strain resulting from differential thermal 12.1.5 Temperature measurement method,
expansion (associated with normal temperature variation of the 12.1.6 Atmosphere.
environment) between the test specimen and the constraint and 12.1.7 Relaxation Test Data:
other variations in the constraint (such as elastic follow up) 12.1.7.1 Initial stress and strain data,
should not exceed 60.000025 in./in. (mm/mm). 12.1.7.2 Final stress and strain data,
7.4 Temperature measurement should be made in accor- 12.1.7.3 Plot of data.
dance with Practice E 139.
A. METHOD FOR CONDUCTING STRESS
RELAXATION TENSION TESTS
8. Vibration Control
8.1 Since stress relaxation tests are quite sensitive to shock
13. Scope
and vibration, the test equipment and mounting should be
13.1 This test method covers the determination of the
located so that the
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