1、Designation: E 2368 04e1Standard Practice forStrain Controlled Thermomechanical Fatigue Testing1This standard is issued under the fixed designation E 2368; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.
2、A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.e1NOTEEditorial changes were made throughout in May 2005.1. Scope1.1 This practice covers the determination of thermome-chanical fatigue (TMF) p
3、roperties of materials under uniaxiallyloaded strain-controlled conditions. A “thermomechanical”fatigue cycle is here defined as a condition where uniformtemperature and strain fields over the specimen gage sectionare simultaneously varied and independently controlled. Thispractice is intended to ad
4、dress TMF testing performed insupport of such activities as materials research and develop-ment, mechanical design, process and quality control, productperformance, and failure analysis. While this practice is spe-cific to strain-controlled testing, many sections will provideuseful information for f
5、orce-controlled or stress-controlledTMF testing.1.2 This practice allows for any maximum and minimumvalues of temperature and mechanical strain, and temperature-mechanical strain phasing, with the restriction being that suchparameters remain cyclically constant throughout the durationof the test. No
6、 restrictions are placed on environmental factorssuch as pressure, humidity, environmental medium, and others,provided that they are controlled throughout the test, do notcause loss of or change in specimen dimensions in time, andare detailed in the data report.1.3 The use of this practice is limite
7、d to specimens and doesnot cover testing of full-scale components, structures, orconsumer products.2. Referenced Documents2.1 ASTM Standards:2E3 Methods of Preparation of Metallographic SpecimensE4 Practices for Force Verification of Testing MachinesE83 Practice for Verification and Classification o
8、f Exten-sometersE 111 Test Method for Youngs Modulus, Tangent Modulus,and Chord ModulusE112 Test Method for Youngs Modulus, Tangent Modulus,and Chord ModulusE 220 Method for Calibration of Thermocouples by Com-parison TechniquesE 467 Practice for Verification of Constant Amplitude Dy-namic Loads or
9、Displacements in an Axial Load FatigueTesting SystemE 606 Practice for Strain Controlled Fatigue TestingE 739 Practice for Statistical Analysis of Linear or Linear-ized Stress-Life (S-N) and Strain-Life (e-N) Fatigue DataE 1012 Practice for Verification of Specimens AlignmentUnder Tensile LoadingE 1
10、823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 The definitions in this practice are in accordance withdefinitions given in Terminology E 1823 unless otherwisestated.3.2 Additional definitions are as follows:3.2.1 stress, sstress is defined herein to be the engineer-ing st
11、ress, which is the ratio of force, P, to specimen originalcross sectional area, A:s5P/A (1)The area, A, is that measured in an unloaded condition atroom temperature. See 7.2 for temperature state implications.3.2.2 coeffcient of thermal expansion, athe fractionalchange in free expansion strain for a
12、 unit change in tempera-ture, as measured on the test specimen.3.2.3 total strain, etthe strain component measured on thetest specimen, and is the sum of the thermal strain and themechanical strain.3.2.4 thermal strain, eththe strain component resultingfrom a change in temperature under free expansi
13、on conditions(as measured on the test specimen).eth5s DT (2)NOTE 1For some materials, s may be nonlinear over the temperaturerange of interest.1This practice is under the jurisdiction of ASTM Committee E08 on Fatigue andFracture and is the direct responsibility of Subcommittee E08.05 on CyclicDeform
14、ation and Fatigue Crack Formation.Current edition approved May 1, 2004. Published June 2004.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document S
15、ummary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.5 mechanical strain, emthe strain component resultingwhen the free expansion thermal strain (as measured on the testspecimen) is subtracted from the t
16、otal strain.em5et2eth(3)3.2.6 elastic strain, eelthe strain component resultingwhen the stress is divided by the temperature-dependentYoungs Modulus (in accordance with Test Method E 111).eel5s/ET! (4)3.2.7 inelastic strain, einthe strain component resultingwhen the elastic strain is subtracted from
17、 the mechanical strain.ein5em2eel(5)3.2.8 strain ratio, Rethe ratio of minimum mechanicalstrain to the maximum mechanical strain in a strain cycle.Re5emin/emax(6)3.2.9 mechanical strain/temperature true phase angle,ffor the purpose of assessing phasing accuracy, this isdefined as the waveform shift
18、(expressed in degrees) betweenthe maximum temperature response as measured on the speci-men and the maximum mechanical strain response. For refer-ence purpose, the angle f is considered positive if thetemperature response maximum leads the mechanical strainresponse maximum by 180 or less, otherwise
19、the phase angleis considered to be negative.3.2.10 in-phase TMF, (f = 0)a cycle where the maxi-mum value of temperature and the maximum value of me-chanical strain occur at the same time (see Fig. 1a).3.2.11 out-of-phase (anti-phase) TMF, (f = 180)a cyclewhere the maximum value of temperature leads
20、the maximumvalue of mechanical strain by a time value equal to12 the cycleperiod (see Fig. 1b).4. Significance and Use4.1 In the utilization of structural materials in elevatedtemperature environments, components that are susceptible tofatigue damage may experience some form of simultaneouslyvarying
21、 thermal and mechanical forces throughout a givencycle. These conditions are often of critical concern becausethey combine temperature dependent and cycle dependent(fatigue) damage mechanisms with varying severity relating tothe phase relationship between cyclic temperature and cyclicmechanical stra
22、in. Such effects can be found to influence theevolution of microstructure, micromechanisms of degradation,and a variety of other phenomenological processes that ulti-mately affect cyclic life. The strain-controlled thermomechani-cal fatigue test is often used to investigate the effects ofsimultaneou
23、sly varying thermal and mechanical loadings underidealized conditions, where cyclic theoretically uniform tem-perature and strain fields are externally imposed and controlledthroughout the gage section of the specimen.5. Test Apparatus5.1 Testing MachineAll tests shall be performed in a testsystem w
24、ith tension-compression loading capability and veri-fied in accordance with Practices E4 and E 467. The testsystem (test frame and associated fixtures) shall be in compli-ance with the bending strain criteria specified in PracticesE 606, E 1012, and E 467. The test system shall be able toindependent
25、ly control both temperature and total strain. Inaddition it shall be capable of adding the measured thermalstrain to the desired mechanical strain to obtain the total strainneeded for the independent control.5.2 Gripping FixturesAny fixture, such as those specifiedin Practice E 606, is acceptable pr
26、ovided it meets the alignmentcriteria specified in Practice E 606, and the specimen failswithin the uniform gage section. Specimens with threaded endstypically tend to require more effort than those with smoothshank ends to meet the alignment criteria; for this reason,smooth shank specimens are pref
27、erred over specimens withthreaded ends. Fixtures used for gripping specimens shall bemade from a material that can withstand prolonged usage,particularly at high temperatures. The design of the fixtureslargely depends upon the design of the specimen. Typically, acombination of hydraulically-actuated
28、 collet grips and smoothshank specimens provide good alignment and high lateralstiffness.5.3 Force TransducerThe force transducer shall be placedin series with the load train and shall comply with thespecifications in Practices E 4 and E 467.5.4 ExtensometersAxial deformation in the gage sectionof t
29、he specimen should be measured with an extensometer. Theextensometers (including optical extensometers, using an ap-propriate calibration procedure) should qualify as Class B-2 orbetter in accordance with Practice E83.5.5 Transducer CalibrationAll transducers shall be cali-brated in accordance with
30、the recommendations of the respec-tive manufacturers. Calibration of each transducer shall betraceable to the National Institute of Standards and Technology(NTIS).5.6 Heating DeviceSpecimen heating can be accom-plished by various techniques including induction, direct resis-tance, radiant, or forced
31、 air heating. In all such cases, activespecimen cooling (for example, forced air) can be used toachieve desired cooling rates provided that the temperaturerelated specifications in 7.4 are satisfied.NOTE 2If induction is used, it is advisable to select a generator witha frequency sufficiently low to
32、 minimize “skin effects” (for example,preferential heating on the surface and near surface material with respectto the bulk, that is dependent on RF generator frequency) on the specimenduring heating.5.7 Temperature Measurement SystemThe specimen tem-perature shall be measured using thermocouples in
33、 contact withthe specimen surface in conjunction with an appropriatetemperature indicating device or non-contacting sensors thatare adjusted for emisivity changes by comparison to a refer-ence such as thermocouples.NOTE 3Direct contact between the thermocouple and the specimen isimplied and shall be
34、 achieved without affecting the test results (forexample, test data for a specimen when initiation occurred at the point ofcontact of the thermocouple shall be omitted from consideration). Com-monly used methods of the thermocouple attachment are: resistance spotwelding (outside the gage section), f
35、ixing by binding or pressure.NOTE 4Under inductive heating, thermocouple wires may act as heatsinks, and can thus lower the local specimen surface temperature. Thiseffect may be substantial at high temperatures. (1)E236804e125.7.1 Calibration of the temperature measurement systemshall be in accordan
36、ce with Method E 220.5.8 Data Acquisition SystemA computerized system ca-pable of carrying out the task of collecting and processingforce, extension, temperature, and cycle count data digitally isrecommended. Sampling frequency of data points shall besufficient to ensure correct definition of the hy
37、steresis loopespecially in the regions of reversals. Different data collectionstrategies will affect the number of data points per cycleneeded, however, typically 200 points per cycle are required.5.9 Alternatively, an analog system capable of measuringthe same data may be used and would include:5.9
38、.1 An X-Y-Y recorder used to record force, extension,and temperature hysteresis loops,5.9.2 A strip-chart recorder for several time-dependent pa-rameters: force, extension and temperature,FIG. 1 Schematics of Mechanical Strain and Temperature for In- and Out-of-Phase TMF TestsE236804e135.9.3 A peak
39、detector per signal, and5.9.4 A cycle counter.NOTE 5The recorders may be replaced with storage devices capableof reproducing the recorded signals either in photographic or analog form.These devices are necessary when the rate of recorded signals is greaterthan the maximum slew-rate of the recorder.
40、They allow permanentrecords to be reproduced subsequently at a lower rate.6. Specimens6.1 Specimen Design ConsiderationsAll specimen de-signs shall be restricted to those featuring uniform axial gagesections, as these specimen designs offer a reasonable con-tinuum volume for testing. Tubular specime
41、ns are preferred tosolid specimen designs because they will tend to facilitatethermal cycling due to lower material mass and will reduce thepotential for unwanted radial temperature gradients duringthermal cycling (see 7.4.5).6.2 Specimen GeometrySpecific geometries of tubularspecimens will vary dep
42、ending upon materials and testingneeds. One of the more critical dimensions is wall thickness,which should be large enough to avoid instabilities duringcyclic loading and thin enough to maintain a uniform tempera-ture across the specimen wall. For polycrystalline materials, atleast 10 to 20 grains s
43、hould be present through the thickness ofthe wall to preserve isotropy. In order to determine the grainsize of the material metallographic samples should be preparedin accordance with Methods E3and the average grain sizeshould be measured according to Test Method E112. Repre-sentative examples of tu
44、bular specimens, which have beensuccessfully used in TMF testing, are included in Fig. 2.Further general guidance regarding specific geometric detailscan be gained from the uniform gage section specimen designspresented in E 606. Solid specimen designs such as thosepresented in Practice E 606 are al
45、so permitted. However, careshall be taken to ensure that radial temperature gradients duringthermal cycling are not excessive; see 7.4.4 and associatednote.6.3 Specimen FabricationThe procedure used for ma-chining solid and tubular specimens shall meet all the specifi-cations documented in Appendix
46、X3 of Practice E 606.Inaddition, the bore of the tubular specimen should be honed toinhibit fatigue crack nucleation from machining anomalies onthe inner surface of the specimen.7. Test Procedure7.1 Laboratory EnvironmentAll tests should be per-formed under a well-controlled laboratory environment.
47、Iftesting is performed in air, uniform ambient temperatureconditions should be maintained throughout the duration of thetest. Relative humidity may be measured in accordance withE 337 unless it has already been determined to have little or noeffect on thermomechanical fatigue life. If an effect is p
48、resent,relative humidity should be controlled. In either situation itshould be carefully monitored and reported.NOTE 6It is strongly recommended that the relatively humidity iscontrolled within the laboratory environment because of its potential toaffect strain gage based extensometry devices.7.2 Me
49、asurement of Test Specimen DimensionsThe diam-eter(s) of the gage section (or width and thickness for the caseof a rectangular cross section) should be measured in at leastthree different locations to an accuracy of 0.0125 mm (0.0005in.) or better. Use the minimum of the values to compute thecross-sectional area.NOTE 7Because of the complexity of defining a gage length on thespecimen due to the thermal expansion/contraction, it is recommendedthat the gauge length be fixed to the room temperature dimension. Theerror introduced by this definition
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