ASTM E2368-04
(Practice)Standard Practice for Strain Controlled Thermomechanical Fatigue Testing
Standard Practice for Strain Controlled Thermomechanical Fatigue Testing
SCOPE
1.1 This practice covers the determination of thermomechanical fatigue (TMF) properties of materials under uniaxially loaded strain-controlled conditions. A "thermomechanical" fatigue cycle is here defined as a condition where uniform temperature and strain fields over the specimen gage section are simultaneously varied and independently controlled. This practice is intended to address TMF 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 practice is specific to strain-controlled testing, many sections will provide useful information for force-controlled or stress-controlled TMF testing.
1.2 This practice allows for any maximum and minimum values of temperature and mechanical strain, and temperature-mechanical strain phasing, with the restriction being that such parameters remain cyclically constant throughout the duration of the test. No restrictions are placed on environmental factors such as pressure, humidity, environmental medium, and others, provided that they are controlled throughout the test, do not cause loss of or change in specimen dimensions in time, and are detailed in the data report.
1.3 The use of this practice is limited to specimens and does not cover testing of full-scale components, structures, or consumer products.
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Designation:E2368–04
Standard Practice for
Strain Controlled Thermomechanical Fatigue Testing
This standard is issued under the fixed designation E 2368; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 112 TestMethodforYoung’sModulus,TangentModulus,
and Chord Modulus
1.1 This practice covers the determination of thermome-
E 220 Method for Calibration of Thermocouples by Com-
chanicalfatigue(TMF)propertiesofmaterialsunderuniaxially
parison Techniques
loaded strain-controlled conditions. A “thermomechanical”
E 467 Practice for Verification of Constant Amplitude Dy-
fatigue cycle is here defined as a condition where uniform
namic Loads or Displacements in an Axial Load Fatigue
temperature and strain fields over the specimen gage section
Testing System
are simultaneously varied and independently controlled. This
E 606 Practice for Strain Controlled Fatigue Testing
practice is intended to address TMF testing performed in
E 739 Practice for Statistical Analysis of Linear or Linear-
support of such activities as materials research and develop-
ized Stress-Life (S-N) and Strain-Life (e-N) Fatigue Data
ment, mechanical design, process and quality control, product
E 1012 Practice for Verification of Specimens Alignment
performance, and failure analysis. While this practice is spe-
Under Tensile Loading
cific to strain-controlled testing, many sections will provide
E 1823 Terminology Relating to Fatigue and Fracture Test-
useful information for force-controlled or stress-controlled
ing
TMF testing.
1.2 This practice allows for any maximum and minimum
3. Terminology
values of temperature and mechanical strain, and temperature-
3.1 The definitions in this practice are in accordance with
mechanical strain phasing, with the restriction being that such
definitions given in Terminology E 1823 unless otherwise
parameters remain cyclically constant throughout the duration
stated.
of the test. No restrictions are placed on environmental factors
3.2 Additional definitions are as follows:
such as pressure, humidity, environmental medium, and others,
3.2.1 stress, s—stress is defined herein to be the engineer-
provided that they are controlled throughout the test, do not
ing stress, which is the ratio of force, P, to specimen original
cause loss of or change in specimen dimensions in time, and
cross sectional area, A:
are detailed in the data report.
s5 P/A (1)
1.3 The use of this practice is limited to specimens and does
not cover testing of full-scale components, structures, or
The area, A, is that measured in an unloaded condition at
consumer products.
room temperature. See 7.2 for temperature state implications.
3.2.2 coeffıcient of thermal expansion, a—the fractional
2. Referenced Documents
change in free expansion strain for a unit change in tempera-
2.1 ASTM Standards:
ture, as measured on the test specimen.
E 3 Methods of Preparation of Metallographic Specimens
3.2.3 total strain, e—the strain component measured on the
t
E 4 Practices for Force Verification of Testing Machines
test specimen, and is the sum of the thermal strain and the
E 83 Practice for Verification and Classification of Exten-
mechanical strain.
someters
3.2.4 thermal strain, e —the strain component resulting
th
E 111 TestMethodforYoung’sModulus,TangentModulus,
from a change in temperature under free expansion conditions
and Chord Modulus
(as measured on the test specimen).
e 5s · DT (2)
th
This practice is under the jurisdiction ofASTM Committee E08 on Fatigue and
3.2.5 mechanical strain, e —thestraincomponentresulting
m
Fracture and is the direct responsibility of Subcommittee E08.05 on Cyclic
whenthefreeexpansionthermalstrain(asmeasuredonthetest
Deformation and Fatigue Crack Formation.
specimen) is subtracted from the total strain.
Current edition approved May 1, 2004. Published June 2004.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
e 5e 2e (3)
m t th
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2368–04
3.2.6 elastic strain, e —the strain component resulting 5.2 Gripping Fixtures—Any fixture, such as those specified
el
when the stress is divided by the temperature-dependent inPracticeE 606,isacceptableprovideditmeetsthealignment
Young’s Modulus (in accordance with Test Method E 111). criteria specified in Practice E 606, and the specimen fails
withintheuniformgagesection.Specimenswiththreadedends
e 5s/E~T! (4)
el
typically tend to require more effort than those with smooth
3.2.7 inelastic strain, e —the strain component resulting
in
shank ends to meet the alignment criteria; for this reason,
whentheelasticstrainissubtractedfromthemechanicalstrain.
smooth shank specimens are preferred over specimens with
e 5e 2e (5)
in m el threaded ends. Fixtures used for gripping specimens shall be
made from a material that can withstand prolonged usage,
3.2.8 strain ratio, Re—the ratio of minimum mechanical
particularly at high temperatures. The design of the fixtures
strain to the maximum mechanical strain in a strain cycle.
largely depends upon the design of the specimen. Typically, a
Re5e /e (6)
min max
combination of hydraulically-actuated collet grips and smooth
3.2.9 mechanical strain/temperature true phase angle,
shank specimens provide good alignment and high lateral
f—for the purpose of assessing phasing accuracy, this is
stiffness.
defined as the waveform shift (expressed in degrees) between
5.3 Force Transducer—Theforcetransducershallbeplaced
the maximum temperature response as measured on the speci-
in series with the load train and shall comply with the
men and the maximum mechanical strain response. For refer-
specifications in Practices E 4 and E 467.
ence purpose, the angle f is considered positive if the
5.4 Extensometers—Axial deformation in the gage section
temperature response maximum leads the mechanical strain
ofthespecimenshouldbemeasuredwithanextensometer.The
response maximum by 180° or less, otherwise the phase angle
extensometers (including optical extensometers, using an ap-
is considered to be negative.
propriate calibration procedure) should qualify as Class B-2 or
3.2.10 in-phase TMF, (f = 0°)—a cycle where the maxi-
better in accordance with Practice E 83.
mum value of temperature and the maximum value of me-
5.5 Transducer Calibration—All transducers shall be cali-
chanical strain occur at the same time (see Fig. 1a).
brated in accordance with the recommendations of the respec-
3.2.11 out-of-phase (anti-phase) TMF, (f = 180°)—a cycle
tive manufacturers. Calibration of each transducer shall be
where the maximum value of temperature leads the maximum
traceable to the National Institute of Standards andTechnology
value of mechanical strain by a time value equal to ⁄2 the cycle
(NTIS).
period (see Fig. 1b).
5.6 Heating Device—Specimen heating can be accom-
plished by various techniques including induction, direct resis-
4. Significance and Use
tance, radiant, or forced air heating. In all such cases, active
4.1 In the utilization of structural materials in elevated
specimen cooling (for example, forced air) can be used to
temperature environments, components that are susceptible to
achieve desired cooling rates provided that the temperature
fatigue damage may experience some form of simultaneously
related specifications in 7.4 are satisfied.
varying thermal and mechanical forces throughout a given
NOTE 1—If induction is used, it is advisable to select a generator with
cycle. These conditions are often of critical concern because
a frequency sufficiently low to minimize “skin effects” (for example,
they combine temperature dependent and cycle dependent
preferential heating on the surface and near surface material with respect
(fatigue) damage mechanisms with varying severity relating to
to the bulk, that is dependent on RF generator frequency) on the specimen
the phase relationship between cyclic temperature and cyclic
during heating.
mechanical strain. Such effects can be found to influence the
5.7 Temperature Measurement System—The specimen tem-
evolution of microstructure, micromechanisms of degradation,
peratureshallbemeasuredusingthermocouplesincontactwith
and a variety of other phenomenological processes that ulti-
the specimen surface in conjunction with an appropriate
mately affect cyclic life. The strain-controlled thermomechani-
temperature indicating device or non-contacting sensors that
cal fatigue test is often used to investigate the effects of
are adjusted for emisivity changes by comparison to a refer-
simultaneouslyvaryingthermalandmechanicalloadingsunder
ence such as thermocouples.
idealized conditions, where cyclic theoretically uniform tem-
perature and strain fields are externally imposed and controlled
NOTE 2—Direct contact between the thermocouple and the specimen is
throughout the gage section of the specimen. implied and shall be achieved without affecting the test results (for
example, test data for a specimen when initiation occurred at the point of
5. Test Apparatus contact of the thermocouple shall be omitted from consideration). Com-
monly used methods of the thermocouple attachment are: resistance spot
5.1 Testing Machine—All tests shall be performed in a test
welding (outside the gage section), fixing by binding or pressure.
system with tension-compression loading capability and veri-
5.7.1 Calibration of the temperature measurement system
fied in accordance with Practices E 4 and E 467. The test
system (test frame and associated fixtures) shall be in compli- shall be in accordance with Method E 220.
ance with the bending strain criteria specified in Practices 5.8 Data Acquisition System—A computerized system ca-
E 606, E 1012, and E 467. The test system shall be able to pable of carrying out the task of collecting and processing
independently control both temperature and total strain. In force, extension, temperature, and cycle count data digitally is
addition it shall be capable of adding the measured thermal recommended. Sampling frequency of data points shall be
strain to the desired mechanical strain to obtain the total strain sufficient to ensure correct definition of the hysteresis loop
needed for the independent control. especially in the regions of reversals. Different data collection
E2368–04
FIG. 1 Schematics of Mechanical Strain and Temperature for In- and Out-of-Phase TMF Tests
strategies will affect the number of data points per cycle 5.9.2 A strip-chart recorder for several time-dependent pa-
needed, however, typically 200 points per cycle are required. rameters: force, extension and temperature,
5.9.3 A peak detector per signal, and
5.9 Alternatively, an analog system capable of measuring
5.9.4 A cycle counter.
the same data may be used and would include:
5.9.1 An X-Y-Y recorder used to record force, extension,
NOTE 3—The recorders may be replaced with storage devices capable
and temperature hysteresis loops, of reproducing the recorded signals either in photographic or analog form.
E2368–04
These devices are necessary when the rate of recorded signals is greater NOTE 5—Because of the complexity of defining a gage length on the
than the maximum slew-rate of the recorder. They allow permanent specimen due to the thermal expansion/contraction, it is recommended
records to be reproduced subsequently at a lower rate. that the gauge length be fixed to the room temperature dimension.
7.3 Specimen Loading—The specimen should be loaded
6. Specimens
into the test machine without subjecting it to any damaging
6.1 Specimen Design Considerations—All specimen de-
forces. (Forces shall not exceed the elastic limit during
signs shall be restricted to those featuring uniform axial gage
installation.) Care shall be taken not to scratch the external
sections, as these specimen designs offer a reasonable con-
(and internal in the case of a tube) gage section surface while
tinuum volume for testing. Tubular specimens are preferred to
mounting contact-type extensometers.
solid specimen designs because they will tend to facilitate
7.4 Temperature:
thermal cycling due to lower material mass and will reduce the
7.4.1 The temperature command cycle (maximums, mini-
potential for unwanted radial temperature gradients during
mums and rates) is to remain constant throughout the duration
thermal cycling (see 7.4.5).
of the test, unless the aim of the program is to examine the
6.2 Specimen Geometry—Specific geometries of tubular
effect of this parameter on the behavior of the material.
specimens will vary depending upon materials and testing
7.4.2 Through out the duration of the test, the tempera-
needs. One of the more critical dimensions is wall thickness,
ture(s) indicated by the control sensor; for example, thermo-
which should be large enough to avoid instabilities during
couple(s) shall not vary by more than 6 2°K from the initial
cyclic loading and thin enough to maintain a uniform tempera-
value(s) at any given instant in time within the cycle.
ture across the specimen wall. For polycrystalline materials, at
NOTE 6—Currently, there is no standardized method for the dynamic
least 10 to 20 grains should be present through the thickness of
calibration of temperature measurement devices. Therefore, for practical
the wall to preserve isotropy. In order to determine the grain
purposes, all temperature related requirements specified under non-static
size of the material metallographic samples should be prepared
conditions assume that the temperature measuring system is calibrated
in accordance with Methods E 3 and the average grain size
under static conditions. Further, it is assumed that the temperature
should be measured according to Test Method E 112. Repre-
measurement system being used is sufficiently responsive so as to
sentative examples of tubular specimens, which have been accurately indicate the specimen temperature under the dynamic condi-
tions selected for the thermomechanical cycle.
successfully used in TMF testing, are included in Fig. 2.
Further general guidance regarding specific geometric details
7.4.3 The maximum allowable axial temperature gradient
can be gained from the uniform gage section specimen designs
over the gage section at any given instant in time within
...
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