1、Designation: E2368 10Standard Practice forStrain Controlled Thermomechanical Fatigue Testing1This standard is issued under the fixed designation E2368; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A nu
2、mber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the determination of thermome-chanical fatigue (TMF) properties of materials under uniaxiallyloaded strain-controlle
3、d 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 address TMF testing performed insupport of such activities as ma
4、terials 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 force-controlled or stress-controlledTMF testing.1.2 This pract
5、ice 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 restrictions are placed on environmental factorssuch as press
6、ure, 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 limited to specimens and doesnot cover testing of full-scale compone
7、nts, structures, orconsumer products.1.4 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.2. Referenced Documents2.1 ASTM Standards:2E3 Guide for Preparation of Metallographic SpecimensE4 Practices for Force Verification of Testi
8、ng MachinesE83 Practice for Verification and Classification of Exten-someter SystemsE111 Test Method for Youngs Modulus, Tangent Modulus,and Chord ModulusE112 Test Methods for Determining Average Grain SizeE220 Test Method for Calibration of Thermocouples ByComparison TechniquesE337 Test Method for
9、Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)E467 Practice for Verification of Constant Amplitude Dy-namic Forces in an Axial Fatigue Testing SystemE606 Practice for Strain-Controlled Fatigue TestingE1012 Practice for Verification of Test Frame and Spec
10、imenAlignment Under Tensile and Compressive Axial ForceApplicationE1823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 The definitions in this practice are in accordance withdefinitions given in Terminology E1823 unless otherwisestated.3.2 Additional definitions are as follow
11、s:3.2.1 stress, sstress is defined herein to be the engineer-ing stress, 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 ther
12、mal expansion, athe fractionalchange in free expansion strain for a unit change in tempera-ture, as measured on the test specimen.3.2.3 total strain, tthe strain component measured on thetest specimen, and is the sum of the thermal strain and themechanical strain.3.2.4 thermal strain, ththe strain c
13、omponent resultingfrom a change in temperature under free expansion conditions(as measured on the test specimen).th5a DT (2)1This practice is under the jurisdiction of ASTM Committee E08 on Fatigue andFracture and is the direct responsibility of Subcommittee E08.05 on CyclicDeformation and Fatigue C
14、rack Formation.Current edition approved May 1, 2010. Published June 2010. Originallyapproved in 2004. Last previous edition approved in 2004 as E2368041. DOI:10.1520/E2368-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For
15、 Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.NOTE 1For some materials, a may be nonlinear over the temperaturerange
16、of interest.3.2.5 mechanical strain, mthe strain component resultingwhen the free expansion thermal strain (as measured on the testspecimen) is subtracted from the total strain.m5t2th(3)3.2.6 elastic strain, elthe strain component resultingwhen the stress is divided by the temperature-dependentYoung
17、s Modulus (in accordance with Test Method E111).el5s/ET! (4)3.2.7 inelastic strain, inthe strain component resultingwhen the elastic strain is subtracted from the mechanical strain.in5m2el(5)3.2.8 strain ratio, Rthe ratio of minimum mechanicalstrain to the maximum mechanical strain in a strain cycle
18、.R5min/max(6)3.2.9 mechanical strain/temperature true phase angle,ffor the purpose of assessing phasing accuracy, this isdefined as the waveform shift (expressed in degrees) betweenthe maximum temperature response as measured on the speci-men and the maximum mechanical strain response. For refer-enc
19、e purpose, the angle f is considered positive if thetemperature response maximum leads the mechanical strainresponse maximum by 180 or less, otherwise 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-c
20、hanical 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 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 struc
21、tural materials in elevatedtemperature environments, components that are susceptible tofatigue damage may experience some form of simultaneouslyvarying thermal and mechanical forces throughout a givencycle. These conditions are often of critical concern becausethey combine temperature dependent and
22、cycle dependent(fatigue) damage mechanisms with varying severity relating tothe phase relationship between cyclic temperature and cyclicmechanical strain. Such effects can be found to influence theevolution of microstructure, micromechanisms of degradation,and a variety of other phenomenological pro
23、cesses that ulti-mately affect cyclic life. The strain-controlled thermomechani-cal fatigue test is often used to investigate the effects ofsimultaneously varying thermal and mechanical loadings underidealized conditions, where cyclic theoretically uniform tem-perature and strain fields are external
24、ly imposed and controlledthroughout the gage section of the specimen.5. Test Apparatus5.1 Testing MachineAll tests shall be performed in a testsystem with tension-compression loading capability and veri-fied in accordance with Practices E4 and E467. The test system(test frame and associated fixtures
25、) shall be in compliance withthe bending strain criteria specified in Practices E606, E1012,and E467. The test system shall be able to independentlycontrol both temperature and total strain. In addition it shall becapable of adding the measured thermal strain to the desiredmechanical strain to obtai
26、n the total strain needed for theindependent control.5.2 Gripping FixturesAny fixture, such as those specifiedin Practice E606, is acceptable provided it meets the alignmentcriteria specified in Practice E606, and the specimen failswithin the uniform gage section. Specimens with threaded endstypical
27、ly tend to require more effort than those with smoothshank ends to meet the alignment criteria; for this reason,smooth shank specimens are preferred over specimens withthreaded ends. Fixtures used for gripping specimens shall bemade from a material that can withstand prolonged usage,particularly at
28、high temperatures. The design of the fixtureslargely depends upon the design of the specimen. Typically, acombination of hydraulically-actuated collet grips and smoothshank specimens provide good alignment and high lateralstiffness.5.3 Force TransducerThe force transducer shall be placedin series wi
29、th the load train and shall comply with thespecifications in Practices E4 and E467.5.4 ExtensometersAxial deformation in the gage sectionof the specimen should be measured with an extensometer. Theextensometers (including optical extensometers, using an ap-propriate calibration procedure) should qua
30、lify as Class B-2 orbetter in accordance with Practice E83.5.5 Transducer CalibrationAll transducers shall be cali-brated in accordance with the recommendations of the respec-tive manufacturers. Calibration of each transducer shall betraceable to the National Institute of Standards and Technology(NI
31、ST).5.6 Heating DeviceSpecimen heating can be accom-plished by various techniques including induction, direct resis-tance, radiant, or forced air heating. In all such cases, activespecimen cooling (for example, forced air) can be used toachieve desired cooling rates provided that the temperaturerela
32、ted specifications in 7.4 are satisfied.NOTE 2If induction is used, it is advisable to select a generator witha frequency sufficiently low to minimize “skin effects” (for example,preferential heating on the surface and near surface material with respectto the bulk, that is dependent on RF generator
33、frequency) on the specimenduring heating.5.7 Temperature Measurement SystemThe specimen tem-perature shall be measured using thermocouples in contact withthe specimen surface in conjunction with an appropriatetemperature indicating device or non-contacting sensors thatare adjusted for emisivity chan
34、ges by comparison to a refer-ence such as thermocouples.NOTE 3Direct contact between the thermocouple and the specimen isimplied and shall be achieved without affecting the test results (forexample, test data for a specimen when initiation occurred at the point ofcontact of the thermocouple shall be
35、 omitted from consideration). Com-monly used methods of the thermocouple attachment are: resistance spotwelding (outside the gage section), fixing by binding or pressure.NOTE 4Under inductive heating, thermocouple wires may act as heatsinks, and can thus lower the local specimen surface temperature.
36、 ThisE2368 102effect may be substantial at high temperatures. (1)5.7.1 Calibration of the temperature measurement systemshall be in accordance with Method E220.5.8 Data Acquisition SystemA computerized system ca-pable of carrying out the task of collecting and processingforce, extension, temperature
37、, and cycle count data digitally isrecommended. Sampling frequency of data points shall besufficient to ensure correct definition of the hysteresis loopespecially in the regions of reversals. Different data collectionstrategies will affect the number of data points per cycleneeded, however, typicall
38、y 200 points per cycle are required.5.9 Alternatively, an analog system capable of measuringthe same data may be used and would include:FIG. 1 Schematics of Mechanical Strain and Temperature for In- and Out-of-Phase TMF TestsE2368 1035.9.1 An X-Y-Y recorder used to record force, extension,and temper
39、ature hysteresis loops,5.9.2 A strip-chart recorder for several time-dependent pa-rameters: force, extension and temperature,5.9.3 A peak 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 photog
40、raphic or analog form.These devices are necessary when the rate of recorded signals is greaterthan the maximum slew-rate of the recorder. They allow permanentrecords to be reproduced subsequently at a lower rate.6. Specimens6.1 Specimen Design ConsiderationsAll specimen de-signs shall be restricted
41、to those featuring uniform axial gagesections, as these specimen designs offer a reasonable con-tinuum volume for testing. Tubular specimens are preferred tosolid specimen designs because they will tend to facilitatethermal cycling due to lower material mass and will reduce thepotential for unwanted
42、 radial temperature gradients duringthermal cycling (see 7.4.5).6.2 Specimen GeometrySpecific geometries of tubularspecimens will vary depending upon materials and testingneeds. One of the more critical dimensions is wall thickness,which should be large enough to avoid instabilities duringcyclic loa
43、ding and thin enough to maintain a uniform tempera-ture across the specimen wall. For polycrystalline materials, atleast 10 to 20 grains should be present through the thickness ofthe wall to preserve isotropy. In order to determine the grainsize of the material metallographic samples should be prepa
44、redin accordance with Methods E3 and the average grain sizeshould be measured according to Test Method E112. Repre-sentative examples of tubular specimens, which have beensuccessfully used in TMF testing, are included in Fig. 2.Further general guidance regarding specific geometric detailscan be gain
45、ed from the uniform gage section specimen designspresented in E606. Solid specimen designs such as thosepresented in Practice E606 are also permitted. However, careshall be taken to ensure that radial temperature gradients duringthermal cycling are not excessive; see 7.4.4 and associatednote.NOTE 6F
46、or tubular specimens, wall thicknesses (WT) and outerdiameters (OD) that fall in the following range are often found acceptable:5 0), the mechanical componentof loading should be gradually ramped to its minimum absolutevalue such that this value is reached at the appropriatetemperature in the therma
47、l cycle; at this point, the properlyphased TMF cycle is immediately commenced. For large totalstrain amplitude tests, the mechanical component of strain maybe increased to its final amplitude over multiple cycles toprevent overshoot. Such a gradual increase may also berequired for materials that exh
48、ibit a serrated yielding phenom-enon.7.10 Monitoring the TestThe specimen temperature andtotal strain shall be monitored during the course of the test. Themechanical strain shall be maintained to the criteria set forth in7.5, and the specimen temperature condition shall be main-tained to the criteri
49、a set forth in 7.4.3.7.11 Failure CriteriaThe failure criterion used in the testseries should be reported and may consist of one of thefollowing:7.11.1 Specimen SeparationTotal separation or fracture ofthe specimen into two parts at some location within theuniform gage section. Total separation of the specimen outsidethe gage section shall be reported, but not as a specimenseparation failure.7.11.2 Tensile Force DropWith this method, the specimenis considered to have failed when there is some specifiedamount (typically betwe
