ASTM E606/E606M-21

Standard Test Method for Strain-Controlled Fatigue Testing

SIGNIFICANCE AND USE
4.1 Strain-controlled fatigue is a phenomenon that is influenced by the same variables that influence force-controlled fatigue. The nature of strain-controlled fatigue imposes distinctive requirements on fatigue testing methods. In particular, cyclic total strain should be measured and cyclic plastic strain should be determined. Furthermore, either of these strains typically is used to establish cyclic limits; total strain usually is controlled throughout the cycle. The uniqueness of this test method and the results it yields are the determination of cyclic stresses and strains at any time during the tests. Differences in strain histories other than constant-amplitude alter fatigue life as compared with the constant amplitude results (for example, periodic overstrains and block or spectrum histories). Likewise, the presence of nonzero mean strains and varying environmental conditions may alter fatigue life as compared with the constant-amplitude, fully reversed fatigue tests. Care must be exercised in analyzing and interpreting data for such cases. In the case of variable amplitude or spectrum strain histories, cycle counting can be performed with Practice E1049.  
4.2 Strain-controlled fatigue can be an important consideration in the design of industrial products. It is important for situations in which components or portions of components undergo either mechanically or thermally induced cyclic plastic strains that cause failure within relatively few (that is, approximately 5) cycles. Information obtained from strain-controlled fatigue testing may be an important element in the establishment of design criteria to protect against component failure by fatigue.  
4.3 Strain-controlled fatigue test results are useful in the areas of mechanical design as well as materials research and development, process and quality control, product performance, and failure analysis. Results of a strain-controlled fatigue test program may be used in the formulation of empirical relat...
SCOPE
1.1 This test method covers the determination of fatigue properties of nominally homogeneous materials by the use of test specimens subjected to uniaxial forces. It is intended as a guide for fatigue testing performed in support of such activities as materials research and development, mechanical design, process and quality control, product performance, and failure analysis. While this test method is intended primarily for strain-controlled fatigue testing, some sections may provide useful information for force-controlled or stress-controlled testing.  
1.2 The use of this test method is limited to specimens and does not cover testing of full-scale components, structures, or consumer products.  
1.3 This test method is applicable to temperatures and strain rates for which the magnitudes of time-dependent inelastic strains are on the same order or less than the magnitudes of time-independent inelastic strains. No restrictions are placed on environmental factors such as temperature, pressure, humidity, medium, and others, provided they are controlled throughout the test, do not cause loss of or change in dimension with time, and are detailed in the data report.  
Note 1: The term inelastic is used herein to refer to all nonelastic strains. The term plastic is used herein to refer only to the time-independent (that is, noncreep) component of inelastic strain. To truly determine a time-independent strain the force would have to be applied instantaneously, which is not possible. A useful engineering estimate of time-independent strain can be obtained when the strain rate exceeds some value. For example, a strain rate of 1 × 10−3 sec−1  is often used for this purpose. This value should increase with increasing test temperature.  
1.4 This test method is restricted to the testing of uniform gage section test specimens subjected to axial forces as shown in Fig. 1(a). Testing is limited to strain-controlled cycling. The test ...

General Information

• Status: Published
• Publication Date: 31-May-2021
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: E08 - Fatigue and Fracture
• Drafting Committee: E08.05 - Cyclic Deformation and Fatigue Crack Formation

Relations

• Refers: ASTM A370-24 - Standard Test Methods and Definitions for Mechanical Testing of Steel Products
• Effective Date: 01-Mar-2024
• 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 E8/E8M-24 - Standard Test Methods for Tension Testing of Metallic Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM E1049-85(2023) - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 01-Nov-2023
• Refers: ASTM E1823-20 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Feb-2020
• Refers: ASTM A370-19 - Standard Test Methods and Definitions for Mechanical Testing of Steel Products
• Effective Date: 01-Jul-2019
• Refers: ASTM E209-18 - Standard Practice for Compression Tests of Metallic Materials at Elevated Temperatures with Conventional or Rapid Heating Rates and Strain Rates
• Effective Date: 01-Feb-2018
• Refers: ASTM A370-17a - Standard Test Methods and Definitions for Mechanical Testing of Steel Products
• Effective Date: 15-Nov-2017
• Refers: ASTM E132-17 - Standard Test Method for Poisson’s Ratio at Room Temperature
• Effective Date: 15-Jul-2017
• Refers: ASTM E1049-85(2017) - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 01-Jun-2017
• Refers: ASTM A370-17 - Standard Test Methods and Definitions for Mechanical Testing of Steel Products
• Effective Date: 01-Jan-2017
• Refers: ASTM E8/E8M-16 - Standard Test Methods for Tension Testing of Metallic Materials
• Effective Date: 15-Jul-2016
• Refers: ASTM A370-15 - Standard Test Methods and Definitions for Mechanical Testing of Steel Products
• Effective Date: 01-Nov-2015
• Refers: ASTM E8/E8M-15 - Standard Test Methods for Tension Testing of Metallic Materials
• Effective Date: 01-Feb-2015

Overview

ASTM E606/E606M-21 is the internationally recognized Standard Test Method for Strain-Controlled Fatigue Testing developed by ASTM International. This test method provides procedures to determine the fatigue properties of nominally homogeneous materials by subjecting test specimens to uniaxial, strain-controlled cyclic loading. Strain-controlled fatigue testing is essential for evaluating how mechanical and thermal cyclic strains affect material performance, particularly in engineering components that may fail due to fatigue after relatively few cycles.

This standard supports a wide range of activities including materials research and development, mechanical design, process and quality control, product performance evaluation, and failure analysis. Although primarily intended for strain-controlled testing, it also provides useful guidance for force- or stress-controlled fatigue assessments.

Key Topics

• Fatigue Phenomena and Variables:

  • Strain-controlled fatigue is influenced by similar variables as force-controlled fatigue, including total and plastic strain measurements.
  • Cyclic total strain and plastic strain are critical, and their control and measurement throughout testing are unique aspects of this method.

• Distinct Test Requirements:

  • The method details preparation and setup of uniform-gage section test specimens exposed to axial forces.
  • It defines procedures for consistent alignment and monitoring of specimen fixtures to minimize bending and ensure test result repeatability.
  • The use of extensometers, force transducers, and accurate data recording systems is emphasized for precise measurement and data acquisition.

• Cyclic Stress and Strain Analysis:

  • The test provides insights into the evolution of cyclic stresses and strains over time.
  • Non-constant amplitude, mean strain, and variable environmental conditions are addressed as factors that can significantly affect fatigue life results.

• Data Interpretation and Reporting:

  • Empirical relationships built from cyclic stress, strain, and fatigue life can inform material selection, component design, and life prediction.
  • Careful analysis and reporting are encouraged, particularly when tests deviate from constant amplitude cycling.

Applications

• Mechanical Design:

  • Results inform the design of industrial and consumer products, especially those subjected to repeated mechanical or thermal cycling.
  • Data from strain-controlled fatigue testing is vital for establishing robust design criteria to prevent premature component failure.

• Materials Development and Research:

  • Useful for developing new alloys and materials with improved fatigue performance.
  • Enables researchers to compare the cyclic stability and fatigue resistance of various materials under controlled conditions.

• Quality Control and Product Performance:

  • Manufacturers leverage this method during process and product validation to ensure materials meet fatigue performance specifications.
  • Practical insights from testing enable better prediction and management of product lifecycle and durability.

• Failure Analysis:

  • Assists in root cause identification of component failures where cyclic loading is suspected.
  • Provides input for updating or refining fatigue models and improving future product designs.

Related Standards

ASTM E606/E606M-21 references and aligns with several other ASTM standards relevant to fatigue testing and material characterization, including:

• ASTM A370: Mechanical Testing of Steel Products
• ASTM E3: Preparation of Metallographic Specimens
• ASTM E8/E8M: Tension Testing of Metallic Materials
• ASTM E466: Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials
• ASTM E1049: Practices for Cycle Counting in Fatigue Analysis
• ASTM E1012: Specimen Alignment Under Axial Force
• ASTM E1823: Terminology Relating to Fatigue and Fracture Testing

Consulting these related documents provides broader context and ensures comprehensive adherence to best practices in fatigue testing and materials analysis.

---

By following ASTM E606/E606M-21, engineers and materials scientists can ensure reliable, repeatable assessment of strain-controlled fatigue properties, supporting safer, longer-lasting product and structural designs across industries.

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: E606/E606M − 21
Standard Test Method for
Strain-Controlled Fatigue Testing
This standard is issued under the fixed designation E606/E606M; 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.
1. Scope times at repeated intervals.The test method may be adapted to
guide testing for more general cases where strain or tempera-
1.1 This test method covers the determination of fatigue
ture may vary according to application specific histories. Data
properties of nominally homogeneous materials by the use of
analysis may not follow this test method in such cases.
test specimens subjected to uniaxial forces. It is intended as a
guideforfatiguetestingperformedinsupportofsuchactivities 1.5 The values stated in either SI units or inch-pound units
as materials research and development, mechanical design, are to be regarded separately as standard. The values stated in
process and quality control, product performance, and failure each system may not be exact equivalents; therefore, each
analysis. While this test method is intended primarily for system shall be used independently of the other. Combining
strain-controlled fatigue testing, some sections may provide values from the two systems may result in non-conformance
useful information for force-controlled or stress-controlled with the standard.
testing.
1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.2 The use of this test method is limited to specimens and
ization established in the Decision on Principles for the
does not cover testing of full-scale components, structures, or
Development of International Standards, Guides and Recom-
consumer products.
mendations issued by the World Trade Organization Technical
1.3 Thistestmethodisapplicabletotemperaturesandstrain
Barriers to Trade (TBT) Committee.
rates for which the magnitudes of time-dependent inelastic
strains are on the same order or less than the magnitudes of
2. Referenced Documents
time-independent inelastic strains. No restrictions are placed
2.1 ASTM Standards:
on environmental factors such as temperature, pressure,
A370Test Methods and Definitions for Mechanical Testing
humidity, medium, and others, provided they are controlled
of Steel Products
throughoutthetest,donotcauselossoforchangeindimension
E3Guide for Preparation of Metallographic Specimens
with time, and are detailed in the data report.
E4Practices for Force Verification of Testing Machines
NOTE 1—The term inelastic is used herein to refer to all nonelastic
E8/E8MTest Methods for Tension Testing of Metallic Ma-
strains. The term plastic is used herein to refer only to the time-
terials
independent (that is, noncreep) component of inelastic strain. To truly
E9Test Methods of Compression Testing of Metallic Mate-
determine a time-independent strain the force would have to be applied
instantaneously, which is not possible. A useful engineering estimate of
rials at Room Temperature
time-independentstraincanbeobtainedwhenthestrainrateexceedssome
E83Practice for Verification and Classification of Exten-
−3 −1
value. For example, a strain rate of 1×10 sec is often used for this
someter Systems
purpose. This value should increase with increasing test temperature.
E111Test Method for Young’s Modulus, Tangent Modulus,
1.4 This test method is restricted to the testing of uniform
and Chord Modulus
gage section test specimens subjected to axial forces as shown
E112Test Methods for Determining Average Grain Size
in Fig. 1(a).Testing is limited to strain-controlled cycling.The
E132Test Method for Poisson’s Ratio at RoomTemperature
test method may be applied to hourglass specimens, see Fig.
E177Practice for Use of the Terms Precision and Bias in
1(b), but the user is cautioned about uncertainties in data
ASTM Test Methods
analysis and interpretation. Testing is done primarily under
E209PracticeforCompressionTestsofMetallicMaterialsat
constant amplitude cycling and may contain interspersed hold
Elevated Temperatures with Conventional or Rapid Heat-
ing Rates and Strain Rates
This test method is under the jurisdiction ofASTM Committee E08 on Fatigue
and Fracture and is the direct responsibility of Subcommittee E08.05 on Cyclic
Deformation and Fatigue Crack Formation. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2021.PublishedJuly2021.Originallyapproved contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
ɛ1
in 1977. Last previous edition approved in 2019 as E606/E606M–19 . DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0606_E0606M-21. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E606/E606M − 21
NOTE 1—*Dimension d is recommended to be 6.35 mm [0.25 in.]. See 7.1. Centers permissible. **This diameter may be made greater or less than
2d depending on material hardness. In typically ductile materials diameters less than 2d are often employed and in typically brittle materials diameters
greater than 2d may be found desirable.
NOTE 2—Threaded connections are more prone to inferior axial alignment and have greater potential for backlash, particularly if the connection with
the grip is not properly designed.
FIG. 1 Recommended Low-Cycle Fatigue Specimens
E337Test Method for Measuring Humidity with a Psy- E1245Practice for Determining the Inclusion or Second-
chrometer (the Measurement of Wet- and Dry-Bulb Tem- Phase Constituent Content of Metals byAutomatic Image
peratures) Analysis
E384Test Method for Microindentation Hardness of Mate- E1823TerminologyRelatingtoFatigueandFractureTesting
rials
E399Test Method for Linear-Elastic Plane-Strain Fracture 3. Terminology
Toughness of Metallic Materials
3.1 The definitions in this test method are in accordance
E466Practice for Conducting Force Controlled Constant
with Terminology E1823.
Amplitude Axial Fatigue Tests of Metallic Materials
3.2 Definitions:Additional definitions associated with time-
E467Practice for Verification of Constant Amplitude Dy-
dependent deformation behavior observed in tests at elevated
namic Forces in an Axial Fatigue Testing System
homologous temperatures are as follows:
E468Practice for Presentation of Constant Amplitude Fa-
tigue Test Results for Metallic Materials
3.2.1 hold period, τ —the time interval within a cycle
h
E691Practice for Conducting an Interlaboratory Study to during which the stress or strain is held constant.
Determine the Precision of a Test Method
3.2.2 inelastic strain, ε —the strain that is not elastic.
in
E739PracticeforStatisticalAnalysisofLinearorLinearized
3.2.2.1 Discussion—For isothermal conditions, ε is calcu-
in
Stress-Life (S-N) and Strain-Life (ε-N) Fatigue Data
lated by subtracting the elastic strain from the total strain.
E1012Practice for Verification of Testing Frame and Speci-
men Alignment Under Tensile and Compressive Axial 3.2.3 total cycle period, τ—the time for the completion of
t
Force Application one cycle. The parameter τ can be separated into hold and
t
E1049Practices for Cycle Counting in Fatigue Analysis non-hold (that is, steady and dynamic) components:
E606/E606M − 21
sively in isothermal, constant-rate testing, in the analysis of hysteresis
τ 5 τ 1 τ (1)
t ( h ( nh
loops. In such cases, a value for E* can best be determined by cycling the
specimen prior to the test at stress or strain levels below the elastic limit.
where:
E* is NOT the monotonic Young’s modulus.
∑τ = sum of all the hold portions of the cycle and
h
∑τ = sum of all the nonhold portions of the cycle.
nh
4. Significance and Use
τ also is equal to the reciprocal of the overall frequency
t
4.1 Strain-controlled fatigue is a phenomenon that is influ-
when the frequency is held constant.
enced by the same variables that influence force-controlled
The following equations are often used to define the instan-
fatigue.Thenatureofstrain-controlledfatigueimposesdistinc-
taneous stress and strain relationships for many metals and
tive requirements on fatigue testing methods. In particular,
alloys:
cyclic total strain should be measured and cyclic plastic strain
ε 5 ε 1ε (2)
in e should be determined. Furthermore, either of these strains
typicallyisusedtoestablishcycliclimits;totalstrainusuallyis
σ
controlled throughout the cycle. The uniqueness of this test
ε 5 seeNote2
~ !
e
E*
method and the results it yields are the determination of cyclic
and the change in strain from any point (1) to any other stresses and strains at any time during the tests. Differences in
strain histories other than constant-amplitude alter fatigue life
point (3), as illustrated in Fig. 2, can be calculated as fol-
lows: as compared with the constant amplitude results (for example,
periodic overstrains and block or spectrum histories).
σ σ
3 1
ε 2 ε 5 ε 1 2 ε 1 (3)
S D S D Likewise, the presence of nonzero mean strains and varying
3 1 3in 1in
E* E*
environmental conditions may alter fatigue life as compared
All strain points to the right of and all stress points above
with the constant-amplitude, fully reversed fatigue tests. Care
the origin are positive. The equation would then show an
must be exercised in analyzing and interpreting data for such
increase in inelastic strain from 1 to 3 or:
cases. In the case of variable amplitude or spectrum strain
histories, cycle counting can be performed with Practice
σ σ
1 3
ε 2 ε 5 ε 2 ε 1 2 (4)
3in 1in 3 1
E1049.
E* E*
4.2 Strain-controlled fatigue can be an important consider-
Similarly, during the strain hold period, the change in the
ation in the design of industrial products. It is important for
inelastic strain will be equal to the change in the stress di-
situations in which components or portions of components
vided by E*, or:
undergo either mechanically or thermally induced cyclic plas-
σ 2 σ
2 3
tic strains that cause failure within relatively few (that is,
ε 2 ε 5 (5)
3in 2in
E* 5
approximately <10 ) cycles. Information obtained from strain-
NOTE 2—E* represents a material parameter that may be a function of
controlled fatigue testing may be an important element in the
environment and test conditions. It also may vary during a test as a result
establishment of design criteria to protect against component
of metallurgical or physical changes in the specimen. In many instances,
however, E* is practically a constant quantity and is used rather exten- failure by fatigue.
4.3 Strain-controlled fatigue test results are useful in the
areas of mechanical design as well as materials research and
development, process and quality control, product
performance,andfailureanalysis.Resultsofastrain-controlled
fatigue test program may be used in the formulation of
empirical relationships between the cyclic variables of stress,
total strain, plastic strain, and fatigue life. They are commonly
usedindatacorrelationssuchascurvesofcyclicstressorstrain
versuslifeandcyclicstressversuscyclicplasticstrainobtained
from hysteresis loops at some fraction (often half) of material
life. Examination of the cyclic stress–strain curve and its
comparison with monotonic stress–strain curves gives useful
information regarding the cyclic stability of a material, for
example, whether the values of hardness, yield strength,
ultimate strength, strain-hardening exponent, and strength
coefficient will increase, decrease, or remain unchanged (that
is,whetheramaterialwillharden,soften,orbestable)because
ofcyclicplasticstraining (1). Thepresenceoftime-dependent
inelastic strains during elevated temperature testing provides
the opportunity to study the effects of these strains on fatigue
FIG. 2 Analyses of a Total Strain versus Stress Hysteresis Loop Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
Containing a Hold Period this standard.
E606/E606M − 21
life and on the cyclic stress-strain response of the material. 6.3 Fixtures:
Information about strain rate effects, relaxation behavior, and
6.3.1 Tominimizebendingstrains,specimenfixturesshould
creep also may be available from these tests. Results of the
be aligned such that the major axis of the specimen closely
uniaxial tests on specimens of simple geometry can be applied
coincides with the force axis throughout each cycle. It is
to the design of components with notches or other complex
important that the accuracy of alignment be kept consistent
shapes, provided that the strains can be determined and
from specimen to specimen. Accuracy and repeatability of
multiaxial states of stress or strain and their gradients are
alignment are functions of the load frame alignment and
correctly correlated with the uniaxial strain data.
specimen-to-specimen geometric variability. Alignment shall
be checked by means of a trial test specimen with longitudinal
5. Functional Relationships
strain gages placed at four equidistant locations around the
5.1 Empirical relationships that have been commonly used minimum diameter. The trial test specimen shall be turned
for description of strain-controlled fatigue data are given in
about its axis, installed, and checked for each of four orienta-
AppendixX1.Theserelationshipsmaynotbevalidwhenlarge tions within the fixtures. The maximum bending strains so
time-dependent inelastic strains occur. For this reason, original
determined shall not exceed 5% of the minimum axial strain
data should be reported to the greatest extent possible. Data
range imposed during any test program. For specimens having
reduction methods should be detailed along with assumptions.
a uniform gage length, it is advisable to place a similar set of
Sufficient information should be developed and reported to
gages at two or three axial positions within the gage section.
permit analysis, interpretation, and comparison with results for
One set of strain gages should be placed at the center of the
other materials analyzed using currently popular methods.
gagelengthtodetectmisalignmentthatcausesrelativerotation
ofthespecimenendsaboutaxesperpendiculartothespecimen
5.2 If use is made of hourglass geometries, original data
axis.Anadditionalsetofgagesshouldbeplacedawayfromthe
should be reported along with results analyzed using the
gage-lengthcentertodetectrelativelateraldisplacementofthe
relationships in Appendix X2.
specimen ends. The lower the bending strain, the more repeat-
6. Methodology
able the test results will be from specimen to specimen.This is
especially important for materials with low ductility where
6.1 Testing Machine—Testing should be conducted with a
much better alignment may be needed.
tension-compression fatigue testing machine that has been
verifiedinaccordancewithPracticesE4andE467,unlessmore
NOTE 6—This section refers to Practice E1012 Type A tests.
stringent requirements are called for in this specification. The
NOTE 7—Four strain measurements, 90° opposed to each other, are
testing machine, together with any fixtures used in the test requiredtoensurethatbendingstrainsarenotlarge.Utilizationofasingle
extensometer with dual axial outputs will allow for only two specimen
program, must meet the bending strain criteria in 6.3.1. The
loadings to gather the required four strain readings, without the necessity
machine should be one in which specific measures have been
of strain gaging specimens.
taken to minimize backlash in the loading train.
6.3.2 Several commonly used fixturing techniques are
NOTE 3—Force measuring capability of 45 kN [approximately 10 kips]
shown schematically in Fig. 3. The selection of any one
orgreaterwouldbesufficientfortherecommendedspecimens(Section7)
fixturing technique depends primarily upon the user’s speci-
and most test materials. The machine force capacity used for these
specimens would not be required to exceed 110 kN [approximately 25 men design. Fixtures should be constructed of hardened steel
kips];however,large-capacityfatiguemachinesmaybebeneficialbecause
for high strength and abrasion resistance. The collet type grip
of increased axial stiffness and decreased lateral deflection of these
shown, or another fixturing technique that provides high
systems.Achievingachangeinaxialconcentricityoflessthanorequalto
precision lateral stiffness to hold precise alignment is accept-
0.05 mm [0.002 in.] TIR (total indicator reading), as measured between
able. Fixtures not capable of high alignment may be coupled
the top and bottom specimen fixture under cyclic force, is a measure of
success with respect to minimizing lateral deflection of the loading train. with the Field’s metal pot (2, 3) of Fig. 4 or a similar device.
Such a device may help to compensate for misalignment in the
6.2 Strain Control—Testingmachinecontrolsshouldpermit
loadingtrainthatwouldinducebendingstrainsinthespecimen
cycling between constant strain limits. If material behavior
duringfixturing.Placementofthefixtureswithindie-setorflex
permits (for example, aging effects do not hinder), control
bars reduces relative lateral motion of specimen ends and
stabilityshouldbesuchthatthestrainmaximumandminimum
increases lateral stiffness that is important in machines that do
limits are repeatable over the test duration to within 1% of the
not provide adequate safeguards against compressive buckling
range between maximum and minimum control limits.
of the test specimen.
NOTE 4—See 6.4.1 and 6.5 on use of force and strain transducers in
6.3.3 For elevated-temperature testing it is usually neces-
relation to repeatability requirements.
sary to provide some means for cooling the fixtures to prevent
NOTE 5—For strain control under long-life conditions it is sometimes
damage to other loading-train components such as force
advantageous to run a pseudostrain control test under force control. The
test could be started in strain control and switched to force control after
tranducers. One method commonly used employs water-
cyclic stabilization of the stress response occurs. In these cases strain
cooling coils attached to the fixtures or to other appropriate
shouldbemonitored(directlyorindirectly)andadjustmentsmadeinforce
locations in the loading train. Care must be taken to avoid
control to maintain strain limits within an envelope of 2% of the desired
affecting the force tranducer calibration or the loading-train
strain amplitude of the specified waveform. Practice E466 provides
additional details on force controlled axial fatigue testing. alignment by the addition of cooling coils.
E606/E606M − 21
NOTE 1—The clamping force should be greater than the cyclic force to avoid backlash within the specimen fixture.
FIG. 3 Schematic Examples of Fixturing Techniques For Various Specimen Designs
6.4 Extensometers—Extensometers should be employed for shouldbesuitableforcontrolpurposes,readout,andrecording.
the purpose of measuring deformation in the gage section. The extensometers should qualify as Class B-2 or better in
They should be suitable for dynamic measurements over long
accordance with Practice E83.
periods of time.
NOTE 8—For best results, the extensometer system (mechanical and
6.4.1 The non-self contained extensometer may be of two
electrical) should have a maximum nonlinearity of 0.3% of full-scale
major types: Contacting (for example, the more frequently
range. Thus, the extensometer design should minimize sources of me-
used strain gage or LVDT type as shown in Fig. 5)or
chanicalhysteresis.Themoreeffectivedesignshavealowactivationforce
noncontacting (for example, optical types). The output of the
that eliminates slippage of the contacts and a low mass to provide high
extensometer or auxiliary device of the extensometer system natural frequency for improved dynamic response characteristics.
E606/E606M − 21
minimum diameter. Calibration of extensometers should be
conducted before and after each test program.
NOTE10—Careshouldbetakeninthemeasurementofdiametralstrains
for materials such as cast materials that possess large grains or a large
degree of preferred orientation. These, as well as hexagonal close-packed
materials, tend to be anisotropic and therefore may require special
methods of strain measurement and interpretation because Poisson’s ratio
changessubstantiallywiththeorientationoftheextensometerwithrespect
to the crystallographic orientation of the specimen. Cyclic hardening or
softening also might alter the apparent value of Poisson’s ratio, thereby
complicating data analyses and interpretation.
6.5 Force Transducers—A force transducer should be
placed in series with the test specimen for the purpose of
measuring magnitude and sense of the axial force transmitted
through the specimen. Force transducer capacity should be
selectedtoadequatelycovertherangeofforcestobemeasured
in the test being conducted, but not so large as to render larger
errors (that is, greater than 1% of the difference between
maximum and minimum control limits). Force transducer
calibration should be verified in accordance with Practices E4
and E467.
NOTE 11—The force transducer should be designed specifically for
NOTE 1—Field’s metal pot is used to provide initially zero stress in the
fatiguetestingandpossessthefollowingcharacteristics:highresistanceto
specimen during fixturing. This pot may be within a die-set to combine
bending; high axial stiffness; high linearity; accuracy and sensitivity; low
zero fixturing stress with rigid alignment. Field’s metal is a non-toxic
hysteresis; high overturning moment stiffness; and high lateral stiffness.
alternative to Wood’s metal, which was referred to in earlier versions of
For best results, it is recommended that the maximum force transducer
this standard.
nonlinearity and hysteresis should not exceed 0.5% and 0.3% of
FIG. 4 Schematic of Field’s Metal Pot Showing Principle of Op-
full-scale range, respectively.
eration
6.6 Data Recording Systems—Analog strip chart and X-Y
recorders or their digital equivalent should be considered a
6.4.2 Extensometers should measure longitudinal deforma- minimum requirement for data collection.
tion when a uniform-gage specimen, such as shown in Fig.
NOTE12—Accuracyofrecordingsystemsshouldbekeptwithin1%of
1(a), is tested. Generally, these extensometers are attached as
full scale.Analog/digital devices are available that include maximum and
shown in Fig. 5(a).
minimum limit detection, maximum-minimum memory, and underpeak
detection.
NOTE 9—Care should be exercised when installing the longitudinal
NOTE 13—Data acquisition system characteristics such as sampling
extensometer so as to prevent damage to the specimen surface and
frequencyanddataskewbetweenforceanddeformation(stressandstrain)
consequential premature fatigue failure at the contact points. It is very
channels can affect hysteresis loop presentation on an X-Y recorder used
important to ensure a secure attachment of the extensometer to the test
in digital recording systems. It is recommended that these characteristics
specimen. Damaged or worn contacts or flexure in the attachment
be taken into consideration along with the strain rate or frequency of
apparatus can induce significant hysteresis errors in the measurement.
cycling to determine that the hysteresis plots are within the required error
Often, small strips of transparent tape, copper bondable strain gage
limits.
terminals, or other such protective tabs are adhered to the specimen’s
6.6.1 X-YRecording—SomemeansofX-Yrecordingshould
uniformsectionatthelocationswhereextensometertipswouldcontactthe
material. Use of the tape or tabs tends to “cushion” the attachment. be used for the purpose of recording hysteresis loops of force
Another alternative is the use of quick-drying epoxy. Light force springs
versus deformation or stress versus strain. A potentiometric
or small rubber bands are often employed to hold the extensometer to the
X-Y recorder or an oscilloscope equipped with a camera or
specimen. Dulling the tips for softer material is also commonly done.
data storage capability is an acceptable alternative. The poten-
Extensometer slippage can be observed after the first several cycles from
tiometric X-Y recorder should be used only when the rate of
X-Y traces or strip chart recordings by observing the stress-strain
response. Unusual shifts in mean values of stress in response to imposed
cyclingresultsinapenvelocitythatislessthanone-halfofthe
strain ranges are an indication of such slippage.
recorder’s slewing speed. At higher frequencies, the oscillo-
6.4.3 Extensometers should measure diametral deforma- scope may be used. Alternative devices include: digital X-Y
tions when specimens having hourglass profiles are tested. A plotters for real time recording or to plot stored data and data
typical method of diametral displacement measurement is logging devices that store data in a host computer system or
shown schematically in Fig. 5(b). Curved extensometer tips, transmit data to a printer.
convex in the longitudinal plane, can provide point contact 6.6.2 If digital-type recording devices are used, it is recom-
during testing. Care should be exercised during installation of mended that a sufficient number of simultaneous data pairs
the diametral extensometer to prevent damage to the test (such as stress and strain) be taken for both the ascending
specimen surface. Extensometer tips should be adjusted prop- segment and descending segment of the hysteresis loop to
erly to minimize the force they impose on the specimen.When adequately determine the shape of the loop.
installing the extensometer, gently move its tip longitudinally 6.6.3 Strip Chart Recording—Strip chart recorders may be
alongthespecimenwhilewatchingthegagereadouttofindthe used to monitor force (or strain). If used, the frequency of the
E606/E606M − 21
FIG. 5 Extensometer Schematic
E606/E606M − 21
test should be such that the recording pen velocity never
exceeds one-half of the recorder’s slewing speed. It is recom-
mended that these recorders be calibrated at the testing
frequencies used. Storage oscilloscopes also may be used to
record the force versus strain loops. Force or strain peaks also
may be monitored by devices that detect, display, and retain
maximums and minimums in memory or that reproduce these
data at predetermined periods.
6.7 Cycle Counter—A cycle counter shall be used to indi-
cate total accumulated cycles of loading or straining. An
elapsed time indicator is a desirable adjunct to the cycle
FIG. 6 Block Diagram of Strain Computer (See Appendix X2 for
countertoprovideanexcellentcheckofbothfrequencyandthe
Discussion of Mathematical Relationship)
current cycle count. Two types of counters are generally
available,mechanicalorelectronic.Aminimumrequirementis
that a counter have typically five or six digits and×10,×100,
(see Note 10 and Note 14). Both of these recommended
and×1000 range multipliers. Digital counters with 1 count
specimens possess a solid circular cross section and minimum
resolution with 1 count resolution (no multipliers) are avail-
diameters of 6.35 mm [0.25 in.] in the test section. Specific
able.Countersareoftenequippedwitha“presetcount”feature
cross-sectional dimensions are listed here only because they
thatmaybeusedtostopatestforexaminationofthespecimen,
have been dominant in the generation of the low-cycle fatigue
to command a recorder to take data, or to end a test after a
database that exists in the open literature. Specimens possess-
specific number of cycles.
ing other diameters or tubular cross sections may be tested
6.8 Calibration—The calibration interval of all electronic
successfully within the scope of this test method; however,
recording and transducer systems should be performed in
crack growth rate, specimen grain size, and other consider-
accordance with the manufacturer’s recommendations; in the
ations might preclude direct comparison with test results from
absence of these, the interval shall be no greater than one year
the recommended specimens (see Note 15). While design of
and even more frequently if necessary to maintain required
specimen end connections is primarily dependent upon user
accuracy. Calibration should be checked whenever accuracy is
preference (see Note 16), a number of commonly used con-
in doubt. All calibrations should be traceable to the Interna-
figurations are shown in Fig. 1(c), 1(d), 1(e) and 1(f). Care
tional System of Units (SI) through a National Metrology
mustbeexercisedinthemachiningofuniform-gagespecimens
Institute (NMI) or an International Metrology Institute. When
to blend the shoulder radius at the specimen ends with
calibrating a transducer system, it is important that it be
minimum diameter so as to avoid undercutting. So that stress
performed using the same setup and arrangement of compo-
concentrations are minimized, the shoulder radius should be as
nents as used in the test. As an example, when calibrating a
large as possible, consistent with limitations on specimen
force transducer used on an automated system, it is necessary
length.
to calibrate the output from the computer, not from any
NOTE14—Livesdeterminedusingtubularspecimensarelessthanthose
intermediary electronics.
forsolidspecimens,theextentofwhichdependsonthefailurecriteriaand
specimen configuration. Differences in excess of a factor of two are not
6.9 Strain Computer—An analog (or digital) computer is
unusualforfailurecriteriabasedonseparation,whereasforfailuredefined
recommended for use in low-cycle fatigue tests of hourglass
by crack size contained within the tube wall there will be much less
specimens whenever appreciable cyclic hardening and soften-
difference.
ingoccursduringthetest.Suchacomputerisusefulwhenused
NOTE 15—Selection of either the uniform-gage section or hourglass
profile is commonly based upon the magnitude of strain range to be
in the real-time mode with servocontrolled testing machines
imposed.Therecommendeduniformgagespecimenisfrequentlysuitable
and can be used for limit control of screw-driven machines.
forstrainrangesuptoabout2%.Above2%hourglassspecimensmaybe
The computer should be designed to convert diametral strain
necessary.Softmaterialsorelevatedtemperaturesmaydictatelowerstrain
andaxialforcesignalsintoanaxialstrainsignal.SeeAppendix
ranges.Themaximumstrainrangemaybeincreasedbyappropriatelateral
X2 for conversion relations. In the case of servocontrolled
restraints and through the use of short loading trains. Options to increase
stiffness to avoid the use of hourglass specimens should be exhausted
machines, this axial strain signal may be used as a feedback
before adopting the configuration shown in Fig. 1(b). If these options fail,
signalforcontrolpurposes,thussimulatingaxialstraincontrol.
the recommended hourglass specimen possesses a profile ratio of 12:1 for
Ablock diagram for the analog (or digital) computer program
radius-of-curvature to minimum radius-of-specimen. If the user wishes,
is shown in Fig. 6.
different ratios between the limits of 8:1 and 16:1 may be employed.
Lowerlimitswillincreasestressconcentrationandmayaffectfatiguelife;
higher ratios limit the specimen’s buckling resistance. For some materials
7. Specimens
tested in the low-life range, hourglass specimens might give different
7.1 Specimen Design—Fig. 1 shows two basic specimen results from similarly stressed uniform-gage specimens. It is very difficult
to determine axial strains from measurements of diametral strain in
configurations. Fig. 1(a) shows a recommended uniform-gage
hourglass specimens for many anisotropic as well as cast materials.
specimen. When the choice of an hourglass configuration is
NOTE 16—Design of specimen end connections is dependent upon user
deemednecessary,theprofilerecommendedisasshowninFig.
preference, fixturing, or availability of material, or a combination of all
1(b). Use of Fig. 1(b) should follow careful consideration of
three; it is constrained principally by proper considerations of axial
problems of data interpretation, and anisotropy and buckling alignment and backlash. Button-head end connections, such as those
E606/E606M − 21
showninFig.1(d)and1(e),permitprecisealignmentwithaspecimenend
section hourglass specimen in Fig. 7(b), see Ref (4) for other
clampingpreload(toavoidbacklashinthegrip).Thethreadedconnection,
designs. The geometries that are adequate for resisting buck-
shown in Fig. 1(c), is useful where the available material is not thick
ling and/or incremental bending collapse at short lives often
enough to provide for the larger diameter button-head ends. As a
willleadtogripfailuresatlonglives.Theinvestigatormayfind
cautionary note, threaded connections are more prone to inferior axial
it convenient to employ two geometrically similar specimen
alignment and have greater potential for backlash particularly if the
connection with the grip is not properly designed. The efficiency button-
designs for development of a strain-life curve.
head connection, shown in Fig. 1(e), provides the button-head preloading
feature without requiring larger diameter ends. The button-head design is 7.2 Specimen Preparation—Specimens should be prepared
useful at elevated temperatures, as it does not suffer the “oxidation-
by a specific set of procedures that is known to provide
sticking” experienced with threaded ends, but it may produce some
consistent test results. Agreement between the testing organi-
specimen failures in the fixture when used at room temperatures. The
zation and the user of the test results concerning preparation
design shown in Fig. 1(f) is convenient for use in collet-type hydraulic
procedures should be obtained.The following provides recom-
grips. This configuration eliminates long life thread failures often associ-
ated with Fig. 1(c) type specimens. mended guidelines.
7.2.1 Specimen Coupons and Materials—Coupons from
7.1.1 Alternative Specimen Design for Sheet Specimens—
which specimens are machined should either be nominally
Often, it is desirable to obtain test specimens from sheet
homogeneous or sampled from the source material, or both, so
materialthatislessthan6.0mm[0.24in.]thick.Ingeneral,the
as to be representative of the properties sought in the applica-
considerations discussed in other sections apply equally to
tion of the material to its end use. Thus, when material
sheet testing. However, special specimen geometries and
requirements allow, specimens should be removed from the
gripping arrangements, as well as more sensitive force and
same material and product form that will be used in the
strain transducers, are necessary. It is strongly recommended
fabricated component of interest. Any material orientations,
that torques introduced by actuator rod rotations be eliminated
such as rolling direction or casting direction, should be
by use of rotational restraints or similar devices. Typical
identified with respect to the orientation of the specimen axes.
specimen designs that have been used successfully are shown
Orientation notation used in accordance with Test Method
in Fig. 7. The specimens in Fig. 7(a) have a rectangular cross
E399 is acceptable such as L, T, S, LT, TL, ST, and the like.
section and are suitable up to strain amplitudes of at least 1%
appliedtosheetsasthinas2.54mm[0.10in.].Forhigherstrain 7.2.2 Specimen Surface Preparation—Specimens prepared
amplitudes, antibuckling restraints can be adapted to the from coupons will possess a “surface preparation history” as a
specific geometry and extensometer used. In using such consequence of machining operations, heat treatments, and the
restraints, care must be taken to avoid increased resistance to effects of environment during the storage period prior to
axial force influenced by the restraints.When restraints cannot testing. Unless the purpose of testing is to determine the
be adopted, it may be necessary to use the cylindrical cross influence of specific surface conditions on fatigue life, it is
FIG. 7 Sheet Fatigue Specimens—Alternative to Fig. 1 Specimens
E606/E606M − 21
recommended that specimen surface preparation be performed 8.1.1.1 For materials that are fatigue tested at temperatures
in a manner that will have a minimum influence upon the other than ambient, all temperatures throughout the gage
variability in fatigue lives exhibited by the specimen group section (for uniform gage specimens this is the region with
tested. Ordinarily, this would be accomplished by: constant cross-sectional area) shall be:
7.2.2.1 consistently machining specimens to be as smooth
T 6∆T (6)
n
anduniforminsurfacefinish(inthegageregion)asfeasiblefor
where:
the subject material and the machining techniques available,
T = nominal test temperature in °C and
and by employing as a final operation a machining or other
n
∆T = 2°C [3.6 °F] or 1% × T , °C, whichever is greater.
“finishing” procedure that would introduce minimal surface n
NOTE 18—The temperature variability in the gage section can become
metal distortion (see Note 17), and by
a critical issue, particularly if material properties (for example, major
7.2.2.2 ensuring, through the use of protective atmospheres,
alterations of strength, modulus of elasticity, ductility, etc.) or metallur-
that surface attack, such as oxidation and corrosion, does not
gical stability (for example, microstructure, crystal structure, etc.) are
occur, either during heat treatments or during specimen
affected significantly. For these reasons as well as others, the temperature
storage, for all specimens within a program. variability within the gage section should be maintained as small as
possible. Because temperature effects can be significant, the actual
NOTE17—AppendixX3presentsanexampleofamachiningprocedure
temperature variability should be reported with the test results, as should
that has been employed on some metals to minimize variability of
the heating method (induction heating, resistance heating, infrared lamp,
machining and heat treatment influences upon fatigue life.
etc.).
The exact procedure of specimen preparation and handling
8.1.1.2 For the duration of the test, the controlled tempera-
should be clearly and carefully documented. It also would be
ture of the specimen should be T 6 2°C [3.6 °F].
n
prudent to determine and record the surface residual stresses
NOTE 19—If the temperature cannot be maintained within limits
and the residual stress profile of at least one exemplary
mentioned above, then temperature deviations should be reported. If
specimen.
possible, the effect of temperature should be demonstrated throughout the
range of test temperatures.
7.3 Specimen Storage—Test specimens that may be suscep-
tible to corrosion in moist room-temperature air should be
8.1.2 Elevated temperatures may be imposed by any of
protected immediately after preparation and stored until they
several methods: (1) high-frequency induction (Note 20), (2)
are tested. Specimens may be stored in a suitable protective
resistance or radiant furnace, or (3) immersion in an inert
environment, such as dry inert gas (as might be conveniently
heated gas or liquid. In (1) and (2) above, an enclosure is
employedinalaboratorydesiccator)oravacuum.Themethod
recommended to prevent air currents in the vicinity of the
of storage should be clearly and carefully documented.
specimen from causing undesirable temperature gradients.
7.4 Materials Description—Acompletematerialdescription Specimens tested at room temperature also should be in
draft-freesurroundings.Temperaturesbelowroomtemperature
is desirable. It is recommended that the following microstruc-
tural and mechanical properties be obtained. may be imposed by placing the specimen and gripping appa-
ratus in a refrigerated chamber that may be either of the liquid
7.4.1 Microstructural Characteristics—Composition, grain
or gaseous type, depending on temperature requirements and
size (see Test Methods E112), crystallographic structure, pre-
other possible environmental considerations. Liquefied gases,
ferred orientation if present, general shape of grains (that is,
such as liquid nitrogen, or solidified gases, such as dry ice
equiaxed or elongated), second-phase particles (see Practice
placed in a liquid medium, provide possible means for low-
E177), heat treatment (whether at the mill, during fabrication,
temperature testing.
in the laboratory, or a combination of all three), position in
ingot or sheet roll, and specification designation (ASTM,
NOTE 20—When inductively heating magnetic materials (those mate-
ASME, AISI, Military, SAE, etc.).
rials having relative permeabilities significantly greater than unity), it
7.4.2 Mechanical Properties—For purposes of performing
should be recognized that a varying stress in the specimen can affect the
distribution of eddy currents in the specimen and may change the
the test and calculating results it is desirable to have available
temperature profile. This effect is influenced by the specimen material,
thefollowingrepresentativemechanicalproperties,obtainedat
design and heat transfer characteristics, the temperature magnitude, the
the appropriate temperature and measured in accordance with
stress magnitude and distribution, the cyclic waveform, and the testing
the applicable standards such as Test Methods E8/E8M, E9,
frequency (strain rate). The most pronounced effect is generally produced
E111, E132 and Practice E209; tensile or compressive yield when conducting tests at low frequencies or with tests containing hold
periods.Inanycase,thetemperatureprofileofmagneticspecimensshould
strength or yield point, or both; ultimate tensile strength;
be evaluated throughout the straining cycle. When the effect is severe, it
percent elongation; percent reduction of area; Poisson’s ratio;
may be necessary to use a susceptor with the induction coil or to use an
and Young’s modulus. The following true stress-strain proper-
alternate heating method.
ties also may be desirable: true fracture strength, true fracture
NOTE 21—Use of glass insulation may avoid difficulty with wires
ductility, strain hardening exponent, and strength coefficient.
submerged in a cooling solvent.
Hardness also may be determined in accordance with Test
8.1.3 Iftestingisperformedinair,relativehumiditymaybe
Methods A370 or E384, or both.
measured in accordance with Test Method E337, unless it has
already been determined that moisture has little or no effect on
8. Procedure
fatigue life for the material under test. If an effect is present,
8.1 Test Environment:
relative humidity should be controlled; when uncontrolled it
8.1.1 Temperature: should be carefully monitored and reported.
E606/E606M − 21
8.2 Measurement of Test Specimen Dimensions—For the a hold on diametral strain will permit the total axial strain to
purpose of making an accurate determination of specimen change during each cycle and will not produce correct relax-
ation information.
cross-sectional area, measure the reduced section as follows:
8.3.3 Other Control Methods—Fatigue testing machines
8.2.1 Measure the diameter at the center of the gage section
that do not provide continuous closed loop control of either
bymeansofanopticalcomparatororotheropticalmeanstoan
specimen force or specimen displacement generally have the
accuracy of 0.0125 mm [0.0005 in.] or better. A precision
capability to impose limits on the chosen test variable.
micrometer may be used in place of the optical comparator if
However, they do not control that variable throughout the
its use does not damage the gage section surface in a way as to
fatigue cycle. Limit control is a special case of closed loop
affect specimen performance. For uniform-gage specimens,
control. Thus, force and displacement signals may be handled
check diameters for at least two other positions within the
in a manner similar to that of 8.3.2 to determine strain limits.
specimen gage length. The minimum cross-sectional areas
It is not necessary to use a computer for limit control of
should be used for computing the stresses in the specimen
hourglass speci
...

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.
´1
Designation: E606/E606M − 19 E606/E606M − 21
Standard Test Method for
Strain-Controlled Fatigue Testing
This standard is issued under the fixed designation E606/E606M; 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—The published year date in footnote 1 was editorially corrected in April 2020.
1. Scope
1.1 This test method covers the determination of fatigue properties of nominally homogeneous materials by the use of test
specimens subjected to uniaxial forces. It is intended as a guide for fatigue testing performed in support of such activities as
materials research and development, mechanical design, process and quality control, product performance, and failure analysis.
While this test method is intended primarily for strain-controlled fatigue testing, some sections may provide useful information for
force-controlled or stress-controlled testing.
1.2 The use of this test method is limited to specimens and does not cover testing of full-scale components, structures, or consumer
products.
1.3 This test method is applicable to temperatures and strain rates for which the magnitudes of time-dependent inelastic strains
are on the same order or less than the magnitudes of time-independent inelastic strains. No restrictions are placed on environmental
factors such as temperature, pressure, humidity, medium, and others, provided they are controlled throughout the test, do not cause
loss of or change in dimension with time, and are detailed in the data report.
NOTE 1—The term inelastic is used herein to refer to all nonelastic strains. The term plastic is used herein to refer only to the time-independent (that is,
noncreep) component of inelastic strain. To truly determine a time-independent strain the force would have to be applied instantaneously, which is not
possible. A useful engineering estimate of time-independent strain can be obtained when the strain rate exceeds some value. For example, a strain rate
−3 −1
of 1 × 10 sec is often used for this purpose. This value should increase with increasing test temperature.
1.4 This test method is restricted to the testing of uniform gage section test specimens subjected to axial forces as shown in Fig.
1(a). Testing is limited to strain-controlled cycling. The test method may be applied to hourglass specimens, see Fig. 1(b), but the
user is cautioned about uncertainties in data analysis and interpretation. Testing is done primarily under constant amplitude cycling
and may contain interspersed hold times at repeated intervals. The test method may be adapted to guide testing for more general
cases where strain or temperature may vary according to application specific histories. Data analysis may not follow this test
method in such cases.
1.5 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the
two systems may result in non-conformance with the standard.
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue and Fracture and is the direct responsibility of Subcommittee E08.05 on Cyclic
Deformation and Fatigue Crack Formation.
Current edition approved Nov. 1, 2019June 1, 2021. Published February 2020July 2021. Originally approved in 1977. Last previous edition approved in 20122019 as
ɛ1
E606/E606M – 12.E606/E606M – 19 . DOI: 10.1520/E0606_E0606M-19E01.10.1520/E0606_E0606M-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E606/E606M − 21
NOTE 1—* Dimension d is recommended to be 6.35 mm [0.25 in.]. See 7.1. Centers permissible. ** This diameter may be made greater or less than
2d depending on material hardness. In typically ductile materials diameters less than 2d are often employed and in typically brittle materials diameters
greater than 2d may be found desirable.
NOTE 2—Threaded connections are more prone to inferior axial alignment and have greater potential for backlash, particularly if the connection with
the grip is not properly designed.
FIG. 1 Recommended Low-Cycle Fatigue Specimens
1.6 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:
A370 Test Methods and Definitions for Mechanical Testing of Steel Products
E3 Guide for Preparation of Metallographic Specimens
E4 Practices for Force Verification of Testing Machines
E8/E8M Test Methods for Tension Testing of Metallic Materials
E9 Test Methods of Compression Testing of Metallic Materials at Room Temperature
E83 Practice for Verification and Classification of Extensometer Systems
E111 Test Method for Young’s Modulus, Tangent Modulus, and Chord Modulus
E112 Test Methods for Determining Average Grain Size
E132 Test Method for Poisson’s Ratio at Room Temperature
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
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.
E606/E606M − 21
E209 Practice for Compression Tests of Metallic Materials at Elevated Temperatures with Conventional or Rapid Heating Rates
and Strain Rates
E337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
E384 Test Method for Microindentation Hardness of Materials
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
E466 Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials
E467 Practice for Verification of Constant Amplitude Dynamic Forces in an Axial Fatigue Testing System
E468 Practice for Presentation of Constant Amplitude Fatigue Test Results for Metallic Materials
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E739 Practice for Statistical Analysis of Linear or Linearized Stress-Life (S-N) and Strain-Life (ε-N) Fatigue Data
E1012 Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force
Application
E1049 Practices for Cycle Counting in Fatigue Analysis
E1245 Practice for Determining the Inclusion or Second-Phase Constituent Content of Metals by Automatic Image Analysis
E1823 Terminology Relating to Fatigue and Fracture Testing
3. Terminology
3.1 The definitions in this test method are in accordance with Terminology E1823.
3.2 Definitions: Additional definitions associated with time-dependent deformation behavior observed in tests at elevated
homologous temperatures are as follows:
3.2.1 hold period, τ —, the time interval within a cycle during which the stress or strain is held constant.
h
3.2.2 inelastic strain, ε —, the strain that is not elastic.
in
3.2.2.1 Discussion—
For isothermal conditions, ε is calculated by subtracting the elastic strain from the total strain.
in
3.2.3 total cycle period, τ —the time for the completion of one cycle. The parameter τ can be separated into hold and non-hold
t t
(that is, steady and dynamic) components:
τ 5 τ 1 τ (1)
t ( h ( nh
where:
∑τ = sum of all the hold portions of the cycle and
h
∑τ = sum of all the nonhold portions of the cycle.
nh
τ also is equal to the reciprocal of the overall frequency when the frequency is held constant.
t
The following equations are often used to define the instantaneous stress and strain relationships for many metals and alloys:
ε5 ε 1ε (2)
in e
σ
ε 5 ~see Note 2!
e
E*
and the change in strain from any point (1) to any other point (3), as illustrated in Fig. 2, can be calculated as follows:
σ σ
3 1
ε 2 ε 5 ε 1 2 ε 1 (3)
S D S D
3 1 3in 1in
E* E*
All strain points to the right of and all stress points above the origin are positive. The equation would then show an in-
crease in inelastic strain from 1 to 3 or:
σ σ
1 3
ε 2 ε 5 ε 2 ε 1 2 (4)
3in 1in 3 1
E* E*
Similarly, during the strain hold period, the change in the inelastic strain will be equal to the change in the stress divided by
E*, or:
σ 2 σ
2 3
ε 2 ε 5 (5)
3in 2in
E*
NOTE 2—E* represents a material parameter that may be a function of environment and test conditions. It also may vary during a test as a result of
E606/E606M − 21
FIG. 2 Analyses of a Total Strain versus Stress Hysteresis Loop Containing a Hold Period
metallurgical or physical changes in the specimen. In many instances, however, E* is practically a constant quantity and is used rather extensively in
isothermal, constant-rate testing, in the analysis of hysteresis loops. In such cases, a value for E* can best be determined by cycling the specimen prior
to the test at stress or strain levels below the elastic limit. E* is NOT the monotonic Young’s modulus.
4. Significance and Use
4.1 Strain-controlled fatigue is a phenomenon that is influenced by the same variables that influence force-controlled fatigue. The
nature of strain-controlled fatigue imposes distinctive requirements on fatigue testing methods. In particular, cyclic total strain
should be measured and cyclic plastic strain should be determined. Furthermore, either of these strains typically is used to establish
cyclic limits; total strain usually is controlled throughout the cycle. The uniqueness of this test method and the results it yields are
the determination of cyclic stresses and strains at any time during the tests. Differences in strain histories other than
constant-amplitude alter fatigue life as compared with the constant amplitude results (for example, periodic overstrains and block
or spectrum histories). Likewise, the presence of nonzero mean strains and varying environmental conditions may alter fatigue life
as compared with the constant-amplitude, fully reversed fatigue tests. Care must be exercised in analyzing and interpreting data
for such cases. In the case of variable amplitude or spectrum strain histories, cycle counting can be performed with Practice E1049.
4.2 Strain-controlled fatigue can be an important consideration in the design of industrial products. It is important for situations
in which components or portions of components undergo either mechanically or thermally induced cyclic plastic strains that cause
failure within relatively few (that is, approximately <10 ) cycles. Information obtained from strain-controlled fatigue testing may
be an important element in the establishment of design criteria to protect against component failure by fatigue.
4.3 Strain-controlled fatigue test results are useful in the areas of mechanical design as well as materials research and
development, process and quality control, product performance, and failure analysis. Results of a strain-controlled fatigue test
program may be used in the formulation of empirical relationships between the cyclic variables of stress, total strain, plastic strain,
and fatigue life. They are commonly used in data correlations such as curves of cyclic stress or strain versus life and cyclic stress
versus cyclic plastic strain obtained from hysteresis loops at some fraction (often half) of material life. Examination of the cyclic
stress–strain curve and its comparison with monotonic stress–strain curves gives useful information regarding the cyclic stability
of a material, for example, whether the values of hardness, yield strength, ultimate strength, strain-hardening exponent, and
strength coefficient will increase, decrease, or remain unchanged (that is, whether a material will harden, soften, or be stable)
because of cyclic plastic straining (1). The presence of time-dependent inelastic strains during elevated temperature testing
provides the opportunity to study the effects of these strains on fatigue life and on the cyclic stress-strain response of the material.
Information about strain rate effects, relaxation behavior, and creep also may be available from these tests. Results of the uniaxial
The boldface numbers in parentheses refer to the list of references at the end of this standard.
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tests on specimens of simple geometry can be applied to the design of components with notches or other complex shapes, provided
that the strains can be determined and multiaxial states of stress or strain and their gradients are correctly correlated with the
uniaxial strain data.
5. Functional Relationships
5.1 Empirical relationships that have been commonly used for description of strain-controlled fatigue data are given in Appendix
X1. These relationships may not be valid when large time-dependent inelastic strains occur. For this reason, original data should
be reported to the greatest extent possible. Data reduction methods should be detailed along with assumptions. Sufficient
information should be developed and reported to permit analysis, interpretation, and comparison with results for other materials
analyzed using currently popular methods.
5.2 If use is made of hourglass geometries, original data should be reported along with results analyzed using the relationships
in Appendix X2.
6. Methodology
6.1 Testing Machine—Testing should be conducted with a tension-compression fatigue testing machine that has been verified in
accordance with Practices E4 and E467, unless more stringent requirements are called for in this specification. The testing machine,
together with any fixtures used in the test program, must meet the bending strain criteria in 6.3.1. The machine should be one in
which specific measures have been taken to minimize backlash in the loading train.
NOTE 3—Force measuring capability of 45 kN [approximately 10 kips] or greater would be sufficient for the recommended specimens (Section 7) and
most test materials. The machine force capacity used for these specimens would not be required to exceed 110 kN [approximately 25 kips]; however,
large-capacity fatigue machines may be beneficial because of increased axial stiffness and decreased lateral deflection of these systems. Achieving a
change in axial concentricity of less than or equal to 0.05 mm [0.002 in.] TIR (total indicator reading), as measured between the top and bottom specimen
fixture under cyclic force, is a measure of success with respect to minimizing lateral deflection of the loading train.
6.2 Strain Control—Testing machine controls should permit cycling between constant strain limits. If material behavior permits
(for example, aging effects do not hinder), control stability should be such that the strain maximum and minimum limits are
repeatable over the test duration to within 1 % of the range between maximum and minimum control limits.
NOTE 4—See 6.4.1 and 6.5 on use of force and strain transducers in relation to repeatability requirements.
NOTE 5—For strain control under long-life conditions it is sometimes advantageous to run a pseudostrain control test under force control. The test could
be started in strain control and switched to force control after cyclic stabilization of the stress response occurs. In these cases strain should be monitored
(directly or indirectly) and adjustments made in force control to maintain strain limits within 1 % of the range between maximum and minimum limits.
an envelope of 2 % of the desired strain amplitude of the specified waveform. Practice E466 provides additional details on force controlled axial fatigue
testing.
6.3 Fixtures:
6.3.1 To minimize bending strains, specimen fixtures should be aligned such that the major axis of the specimen closely coincides
with the force axis throughout each cycle. It is important that the accuracy of alignment be kept consistent from specimen to
specimen. Accuracy and repeatability of alignment are functions of the load frame alignment and specimen-to-specimen geometric
variability. Alignment shall be checked by means of a trial test specimen with longitudinal strain gages placed at four equidistant
locations around the minimum diameter. The trial test specimen shall be turned about its axis, installed, and checked for each of
four orientations within the fixtures. The maximum bending strains so determined shall not exceed 5 % of the minimum axial strain
range imposed during any test program. For specimens having a uniform gage length, it is advisable to place a similar set of gages
at two or three axial positions within the gage section. One set of strain gages should be placed at the center of the gage length
to detect misalignment that causes relative rotation of the specimen ends about axes perpendicular to the specimen axis. An
additional set of gages should be placed away from the gage-length center to detect relative lateral displacement of the specimen
ends. The lower the bending strain, the more repeatable the test results will be from specimen to specimen. This is especially
important for materials with low ductility where much better alignment may be needed.
NOTE 6—This section refers to Practice E1012 Type A tests.
NOTE 7—Four strain measurements, 90° opposed to each other, are required to ensure that bending strains are not large. Utilization of a single
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extensometer with dual axial outputs will allow for only two specimen loadings to gather the required four strain readings, without the necessity of strain
gaging specimens.
6.3.2 Several commonly used fixturing techniques are shown schematically in Fig. 3. The selection of any one fixturing technique
depends primarily upon the user’s specimen design. Fixtures should be constructed of hardened steel for high strength and abrasion
resistance. The collet type grip shown, or another fixturing technique that provides high precision lateral stiffness to hold precise
alignment is acceptable. Fixtures not capable of high alignment may be coupled with the Field’s metal pot (2, 3) of Fig. 4 or a
similar device. Such a device may help to compensate for misalignment in the loading train that would induce bending strains in
the specimen during fixturing. Placement of the fixtures within die-set or flex bars reduces relative lateral motion of specimen ends
and increases lateral stiffness that is important in machines that do not provide adequate safeguards against compressive buckling
of the test specimen.
NOTE 1—The clamping force should be greater than the cyclic force to avoid backlash within the specimen fixture.
FIG. 3 Schematic Examples of Fixturing Techniques For Various Specimen Designs
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NOTE 1—Field’s metal pot is used to provide initially zero stress in the specimen during fixturing. This pot may be within a die-set to combine zero
fixturing stress with rigid alignment. Field’s metal is a non-toxic alternative to Wood’s metal, which was referred to in earlier versions of this standard.
FIG. 4 Schematic of Field’s Metal Pot Showing Principle of Operation
6.3.3 For elevated-temperature testing it is usually necessary to provide some means for cooling the fixtures to prevent damage
to other loading-train components such as force tranducers. One method commonly used employs water-cooling coils attached to
the fixtures or to other appropriate locations in the loading train. Care must be taken to avoid affecting the force tranducer
calibration or the loading-train alignment by the addition of cooling coils.
6.4 Extensometers—Extensometers should be employed for the purpose of measuring deformation in the gage section. They
should be suitable for dynamic measurements over long periods of time.
6.4.1 The non-self contained extensometer may be of two major types: Contacting (for example, the more frequently used strain
gage or LVDT type as shown in Fig. 5) or noncontacting (for example, optical types). The output of the extensometer or auxiliary
device of the extensometer system should be suitable for control purposes, readout, and recording. The extensometers should
qualify as Class B-2 or better in accordance with Practice E83.
NOTE 8—For best results, the extensometer system (mechanical and electrical) should have a maximum nonlinearity of 0.3 % of full-scale range. Thus,
the extensometer design should minimize sources of mechanical hysteresis. The more effective designs have a low activation force that eliminates slippage
of the contacts and a low mass to provide high natural frequency for improved dynamic response characteristics.
6.4.2 Extensometers should measure longitudinal deformation when a uniform-gage specimen, such as shown in Fig. 1(a), is
tested. Generally, these extensometers are attached as shown in Fig. 5(a).
NOTE 9—Care should be exercised when installing the longitudinal extensometer so as to prevent damage to the specimen surface and consequential
premature fatigue failure at the contact points. It is very important to ensure a secure attachment of the extensometer to the test specimen. Damaged or
worn contacts or flexure in the attachment apparatus can induce significant hysteresis errors in the measurement. Often, small strips of transparent tape,
copper bondable strain gage terminals, or other such protective tabs are adhered to the specimen’s uniform section at the locations where extensometer
tips would contact the material. Use of the tape or tabs tends to “cushion” the attachment. Another alternative is the use of quick-drying epoxy. Light
force springs or small rubber bands are often employed to hold the extensometer to the specimen. Dulling the tips for softer material is also commonly
done. Extensometer slippage can be observed after the first several cycles from X-Y traces or strip chart recordings by observing the stress-strain response.
Unusual shifts in mean values of stress in response to imposed strain ranges are an indication of such slippage.
6.4.3 Extensometers should measure diametral deformations when specimens having hourglass profiles are tested. A typical
method of diametral displacement measurement is shown schematically in Fig. 5(b). Curved extensometer tips, convex in the
longitudinal plane, can provide point contact during testing. Care should be exercised during installation of the diametral
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FIG. 5 Extensometer Schematic
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extensometer to prevent damage to the test specimen surface. Extensometer tips should be adjusted properly to minimize the force
they impose on the specimen. When installing the extensometer, gently move its tip longitudinally along the specimen while
watching the gage readout to find the minimum diameter. Calibration of extensometers should be conducted before and after each
test program.
NOTE 10—Care should be taken in the measurement of diametral strains for materials such as cast materials that possess large grains or a large degree
of preferred orientation. These, as well as hexagonal close-packed materials, tend to be anisotropic and therefore may require special methods of strain
measurement and interpretation because Poisson’s ratio changes substantially with the orientation of the extensometer with respect to the crystallographic
orientation of the specimen. Cyclic hardening or softening also might alter the apparent value of Poisson’s ratio, thereby complicating data analyses and
interpretation.
6.5 Force Transducers—A force transducer should be placed in series with the test specimen for the purpose of measuring
magnitude and sense of the axial force transmitted through the specimen. Force transducer capacity should be selected to
adequately cover the range of forces to be measured in the test being conducted, but not so large as to render larger errors (that
is, greater than 1 % of the difference between maximum and minimum control limits). Force transducer calibration should be
verified in accordance with Practices E4 and E467.
NOTE 11—The force transducer should be designed specifically for fatigue testing and possess the following characteristics: high resistance to bending;
high axial stiffness; high linearity; accuracy and sensitivity; low hysteresis; high overturning moment stiffness; and high lateral stiffness. For best results,
it is recommended that the maximum force transducer nonlinearity and hysteresis should not exceed 0.5 % and 0.3 % of full-scale range, respectively.
6.6 Data Recording Systems—Analog strip chart and X-Y recorders or their digital equivalent should be considered a minimum
requirement for data collection.
NOTE 12—Accuracy of recording systems should be kept within 1 % of full scale. Analog/digital devices are available that include maximum and
minimum limit detection, maximum-minimum memory, and underpeak detection.
NOTE 13—Data acquisition system characteristics such as sampling frequency and data skew between force and deformation (stress and strain) channels
can affect hysteresis loop presentation on an X-Y recorder used in digital recording systems. It is recommended that these characteristics be taken into
consideration along with the strain rate or frequency of cycling to determine that the hysteresis plots are within the required error limits.
6.6.1 X-Y Recording—Some means of X-Y recording should be used for the purpose of recording hysteresis loops of force versus
deformation or stress versus strain. A potentiometric X-Y recorder or an oscilloscope equipped with a camera or data storage
capability is an acceptable alternative. The potentiometric X-Y recorder should be used only when the rate of cycling results in
a pen velocity that is less than one-half of the recorder’s slewing speed. At higher frequencies, the oscilloscope may be used.
Alternative devices include: digital X-Y plotters for real time recording or to plot stored data and data logging devices that store
data in a host computer system or transmit data to a printer.
6.6.2 If digital-type recording devices are used, it is recommended that a sufficient number of simultaneous data pairs (such as
stress and strain) be taken for both the ascending segment and descending segment of the hysteresis loop to adequately determine
the shape of the loop.
6.6.3 Strip Chart Recording—Strip chart recorders may be used to monitor force (or strain). If used, the frequency of the test
should be such that the recording pen velocity never exceeds one-half of the recorder’s slewing speed. It is recommended that these
recorders be calibrated at the testing frequencies used. Storage oscilloscopes also may be used to record the force versus strain
loops. Force or strain peaks also may be monitored by devices that detect, display, and retain maximums and minimums in memory
or that reproduce these data at predetermined periods.
6.7 Cycle Counter—A cycle counter shall be used to indicate total accumulated cycles of loading or straining. An elapsed time
indicator is a desirable adjunct to the cycle counter to provide an excellent check of both frequency and the current cycle count.
Two types of counters are generally available, mechanical or electronic. A minimum requirement is that a counter have typically
five or six digits and × 10, × 100, and × 1000 range multipliers. Digital counters with 1 count resolution with 1 count resolution
(no multipliers) are available. Counters are often equipped with a “preset count” feature that may be used to stop a test for
examination of the specimen, to command a recorder to take data, or to end a test after a specific number of cycles.
6.8 Calibration—The calibration interval of all electronic recording and transducer systems should be performed in accordance
with the manufacturer’s recommendations; in the absence of these, the interval shall be no greater than one year and even more
frequently if necessary to maintain required accuracy. Calibration should be checked whenever accuracy is in doubt. All
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calibrations should be traceable to the International System of Units (SI) through a National Metrology Institute (NMI) or an
International Metrology Institute. When calibrating a transducer system, it is important that it be performed using the same setup
and arrangement of components as used in the test. As an example, when calibrating a force transducer used on an automated
system, it is necessary to calibrate the output from the computer, not from any intermediary electronics.
6.9 Strain Computer—An analog (or digital) computer is recommended for use in low-cycle fatigue tests of hourglass specimens
whenever appreciable cyclic hardening and softening occurs during the test. Such a computer is useful when used in the real-time
mode with servocontrolled testing machines and can be used for limit control of screw-driven machines. The computer should be
designed to convert diametral strain and axial force signals into an axial strain signal. See Appendix X2 for conversion relations.
In the case of servocontrolled machines, this axial strain signal may be used as a feedback signal for control purposes, thus
simulating axial strain control. A block diagram for the analog (or digital) computer program is shown in Fig. 6.
7. Specimens
7.1 Specimen Design—Fig. 1 shows two basic specimen configurations. Fig. 1(a) shows a recommended uniform-gage specimen.
When the choice of an hourglass configuration is deemed necessary, the profile recommended is as shown in Fig. 1(b). Use of Fig.
1(b) should follow careful consideration of problems of data interpretation, and anisotropy and buckling (see Note 10 and Note
14). Both of these recommended specimens possess a solid circular cross section and minimum diameters of 6.35 mm [0.25 in.]
in the test section. Specific cross-sectional dimensions are listed here only because they have been dominant in the generation of
the low-cycle fatigue database that exists in the open literature. Specimens possessing other diameters or tubular cross sections may
be tested successfully within the scope of this test method; however, crack growth rate, specimen grain size, and other
considerations might preclude direct comparison with test results from the recommended specimens (see Note 15). While design
of specimen end connections is primarily dependent upon user preference (see Note 16), a number of commonly used
configurations are shown in Fig. 1(c), 1(d), 1(e) and 1(f). Care must be exercised in the machining of uniform-gage specimens to
blend the shoulder radius at the specimen ends with minimum diameter so as to avoid undercutting. So that stress concentrations
are minimized, the shoulder radius should be as large as possible, consistent with limitations on specimen length.
NOTE 14—Lives determined using tubular specimens are less than those for solid specimens, the extent of which depends on the failure criteria and
specimen configuration. Differences in excess of a factor of two are not unusual for failure criteria based on separation, whereas for failure defined by
crack size contained within the tube wall there will be much less difference.
NOTE 15—Selection of either the uniform-gage section or hourglass profile is commonly based upon the magnitude of strain range to be imposed. The
recommended uniform gage specimen is frequently suitable for strain ranges up to about 2 %. Above 2 % hourglass specimens may be necessary. Soft
materials or elevated temperatures may dictate lower strain ranges. The maximum strain range may be increased by appropriate lateral restraints and
through the use of short loading trains. Options to increase stiffness to avoid the use of hourglass specimens should be exhausted before adopting the
configuration shown in Fig. 1(b). If these options fail, the recommended hourglass specimen possesses a profile ratio of 12:1 for radius-of-curvature to
minimum radius-of-specimen. If the user wishes, different ratios between the limits of 8:1 and 16:1 may be employed. Lower limits will increase stress
concentration and may affect fatigue life; higher ratios limit the specimen’s buckling resistance. For some materials tested in the low-life range, hourglass
specimens might give different results from similarly stressed uniform-gage specimens. It is very difficult to determine axial strains from measurements
of diametral strain in hourglass specimens for many anisotropic as well as cast materials.
NOTE 16—Design of specimen end connections is dependent upon user preference, fixturing, or availability of material, or a combination of all three; it
is constrained principally by proper considerations of axial alignment and backlash. Button-head end connections, such as those shown in Fig. 1(d) and
1(e), permit precise alignment with a specimen end clamping preload (to avoid backlash in the grip). The threaded connection, shown in Fig. 1(c), is useful
where the available material is not thick enough to provide for the larger diameter button-head ends. As a cautionary note, threaded connections are more
prone to inferior axial alignment and have greater potential for backlash particularly if the connection with the grip is not properly designed. The efficiency
button-head connection, shown in Fig. 1(e), provides the button-head preloading feature without requiring larger diameter ends. The button-head design
is useful at elevated temperatures, as it does not suffer the“ oxidation-sticking”the “oxidation-sticking” experienced with threaded ends, but it may
FIG. 6 Block Diagram of Strain Computer (See Appendix X2 for Discussion of Mathematical Relationship)
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produce some specimen failures in the fixture when used at room temperatures. The design shown in Fig. 1(f) is convenient for use in collet-type hydraulic
grips. This configuration eliminates long life thread failures often associated with Fig. 1(c) type specimens.
7.1.1 Alternative Specimen Design for Sheet Specimens—Often, it is desirable to obtain test specimens from sheet material that
is less than 6.0 mm [0.24 in.] thick. In general, the considerations discussed in other sections apply equally to sheet testing.
However, special specimen geometries and gripping arrangements, as well as more sensitive force and strain transducers, are
necessary. It is strongly recommended that torques introduced by actuator rod rotations be eliminated by use of rotational restraints
or similar devices. Typical specimen designs that have been used successfully are shown in Fig. 7. The specimens in Fig. 7(a) have
a rectangular cross section and are suitable up to strain amplitudes of at least 1 % applied to sheets as thin as 2.54 mm [0.10 in.].
For higher strain amplitudes, antibuckling restraints can be adapted to the specific geometry and extensometer used. In using such
restraints, care must be taken to avoid increased resistance to axial force influenced by the restraints. When restraints cannot be
adopted, it may be necessary to use the cylindrical cross section hourglass specimen in Fig. 7(b), see Ref (4) for other designs.
The geometries that are adequate for resisting buckling and/or incremental bending collapse at short lives often will lead to grip
failures at long lives. The investigator may find it convenient to employ two geometrically similar specimen designs for
development of a strain-life curve.
7.2 Specimen Preparation—Specimens should be prepared by a specific set of procedures that is known to provide consistent test
results. Agreement between the testing organization and the user of the test results concerning preparation procedures should be
obtained. The following provides recommended guidelines.
7.2.1 Specimen Coupons and Materials—Coupons from which specimens are machined should either be nominally homogeneous
or sampled from the source material, or both, so as to be representative of the properties sought in the application of the material
to its end use. Thus, when material requirements allow, specimens should be removed from the same material and product form
that will be used in the fabricated component of interest. Any material orientations, such as rolling direction or casting direction,
should be identified with respect to the orientation of the specimen axes. Orientation notation used in accordance with Test Method
E399 is acceptable such as L, T, S, LT, TL, ST, and the like.
7.2.2 Specimen Surface Preparation—Specimens prepared from coupons will possess a “surface preparation history” as a
consequence of machining operations, heat treatments, and the effects of environment during the storage period prior to testing.
Unless the purpose of testing is to determine the influence of specific surface conditions on fatigue life, it is recommended that
FIG. 7 Sheet Fatigue Specimens—Alternative to Fig. 1 Specimens
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specimen surface preparation be performed in a manner that will have a minimum influence upon the variability in fatigue lives
exhibited by the specimen group tested. Ordinarily, this would be accomplished by:
7.2.2.1 consistently machining specimens to be as smooth and uniform in surface finish (in the gage region) as feasible for the
subject material and the machining techniques available, and by employing as a final operation a machining or other “finishing”
procedure that would introduce minimal surface metal distortion (see Note 17), and by
7.2.2.2 ensuring, through the use of protective atmospheres, that surface attack, such as oxidation and corrosion, does not occur,
either during heat treatments or during specimen storage, for all specimens within a program.
NOTE 17—Appendix X3 presents an example of a machining procedure that has been employed on some metals to minimize variability of machining and
heat treatment influences upon fatigue life.
The exact procedure of specimen preparation and handling should be clearly and carefully documented. It also would be prudent
to determine and record the surface residual stresses and the residual stress profile of at least one exemplary specimen.
7.3 Specimen Storage—Test specimens that may be susceptible to corrosion in moist room-temperature air should be protected
immediately after preparation and stored until they are tested. Specimens may be stored in a suitable protective environment, such
as dry inert gas (as might be conveniently employed in a laboratory desiccator) or a vacuum. The method of storage should be
clearly and carefully documented.
7.4 Materials Description—A complete material description is desirable. It is recommended that the following microstructural and
mechanical properties be obtained.
7.4.1 Microstructural Characteristics—Composition, grain size (see Test Methods E112), crystallographic structure, preferred
orientation if present, general shape of grains (that is, equiaxed or elongated), second-phase particles (see Practice E177), heat
treatment (whether at the mill, during fabrication, in the laboratory, or a combination of all three), position in ingot or sheet roll,
and specification designation (ASTM, ASME, AISI, Military, SAE, etc.).
7.4.2 Mechanical Properties—For purposes of performing the test and calculating results it is desirable to have available the
following representative mechanical properties, obtained at the appropriate temperature and measured in accordance with the
applicable standards such as Test Methods E8/E8M, E9, E111, E132 and Practice E209; tensile or compressive yield strength or
yield point, or both; ultimate tensile strength; percent elongation; percent reduction of area; Poisson’s ratio; and Young’s modulus.
The following true stress-strain properties also may be desirable: true fracture strength, true fracture ductility, strain hardening
exponent, and strength coefficient. Hardness also may be determined in accordance with Test Methods A370 or E384, or both.
8. Procedure
8.1 Test Environment:
8.1.1 Temperature:
8.1.1.1 For materials that are fatigue tested at temperatures other than ambient, all temperatures throughout the gage section (for
uniform gage specimens this is the region with constant cross-sectional area) shall be:
T 6ΔT (6)
n
where:
T = nominal test temperature in °C and
n
ΔT = 2 °C [35.6 °F] or 1 % × T , °C, whichever is greater.
n
ΔT = 2 °C [3.6 °F] or 1 % × T , °C, whichever is greater.
n
NOTE 18—The temperature variability in the gage section can become a critical issue, particularly if material properties (for example, major alterations
of strength, modulus of elasticity, ductility, etc.) or metallurgical stability (for example, microstructure, crystal structure, etc.) are affected significantly.
For these reasons as well as others, the temperature variability within the gage section should be maintained as small as possible. Because temperature
effects can be significant, the actual temperature variability should be reported with the test results, as should the heating method (induction heating,
resistance heating, infrared lamp, etc.).
8.1.1.2 For the duration of the test, the controlled temperature of the specimen should be T 6 2°C [35.62 °C [3.6 °F].
n
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NOTE 19—If the temperature cannot be maintained within limits mentioned above, then temperature deviations should be reported. If possible, the effect
of temperature should be demonstrated throughout the range of test temperatures.
8.1.2 Elevated temperatures may be imposed by any of several methods: (1) high-frequency induction (Note 20), (2) resistance
or radiant furnace, or (3) immersion in an inert heated gas or liquid. In (1) and (2) above, an enclosure is recommended to prevent
air currents in the vicinity of the specimen from causing undesirable temperature gradients. Specimens tested at room temperature
also should be in draft-free surroundings. Temperatures below room temperature may be imposed by placing the specimen and
gripping apparatus in a refrigerated chamber that may be either of the liquid or gaseous type, depending on temperature
requirements and other possible environmental considerations. Liquefied gases, such as liquid nitrogen, or solidified gases, such
as dry ice placed in a liquid medium, provide possible means for low-temperature testing.
NOTE 20—When inductively heating magnetic materials (those materials having relative permeabilities significantly greater than unity), it should be
recognized that a varying stress in the specimen can affect the distribution of eddy currents in the specimen and may change the temperature profile. This
effect is influenced by the specimen material, design and heat transfer characteristics, the temperature magnitude, the stress magnitude and distribution,
the cyclic waveform, and the testing frequency (strain rate). The most pronounced effect is generally produced when conducting tests at low frequencies
or with tests containing hold periods. In any case, the temperature profile of magnetic specimens should be evaluated throughout the straining cycle. When
the effect is severe, it may be necessary to use a susceptor with the induction coil or to use an alternate heating method.
NOTE 21—Use of glass insulation may avoid difficulty with wires submerged in a cooling solvent.
8.1.3 If testing is performed in air, relative humidity may be measured in accordance with Test Method E337, unless it has already
been determined that moisture has little or no effect on fatigue life for the material under test. If an effect is present, relative
humidity should be controlled; when uncontrolled it should be carefully monitored and reported.
8.2 Measurement of Test Specimen Dimensions—For the purpose of making an accurate determination of specimen cross-sectional
area, measure the reduced section a
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