1、Designation: E2448 111E2448 18Standard Test Method forDetermining the Superplastic Properties of Metallic SheetMaterials1This standard is issued under the fixed designation E2448; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1 NOTEFootnote 4 was editorially corrected in October 2015.1. Scope1.1 This test method describes the procedure for deter
3、mining the superplastic forming properties (SPF) of a metallic sheetmaterial. It includes tests both for the basic SPF properties and also for derived SPF properties. The test for basic propertiesencompasses effects due to strain hardening or softening.1.2 This test method covers sheet materials wit
4、h thicknesses of at least 0.5 mm but not greater than 6 mm. It characterizes thematerial under a uni-axial tensile stress condition.NOTE 1Most industrial applications of superplastic forming involve a multi-axial stress condition in a sheet; however it is more convenient tocharacterize a material un
5、der a uni-axial tensile stress condition. Tests should be performed in different orientations to the rolling direction of the sheetto ascertain initial anisotropy.1.3 This method has been used successfully between strain rates of 10-5 to 10-1 per second.1.4 This method has been used successfully onA
6、luminum and Titanium alloys. The use of the method with other metals shouldbe verified.1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.6 This standard does not purport to address all of the safety concerns, if any, assoc
7、iated with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulatory limitations prior to use.1.7 This international standard was developed in accordance with internatio
8、nally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E4 Practices
9、 for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical TestingE21 Test Methods for Elevated Temperature Tension Tests of Metallic MaterialsE177 Practice for Use of the Terms Precision and Bias in ASTM Test MethodsE646 Test Method for Tensile Strain-Hardening Expo
10、nents (n -Values) of Metallic Sheet MaterialsE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method3. Terminology3.1 DefinitionsDefinitions such as gage length (L and L0), true stress (), true strain (), normal engineering stress (S), andengineering strain
11、(e) are defined in Terminology E6. Thus,5lnL/L0!5S11e!1 This test method is under the jurisdiction of ASTM Committee E28 on Mechanical Testing and is the direct responsibility of Subcommittee E28.02 on Ductility andFormability.Current edition approved June 1, 2011June 1, 2018. Published July 2011Sep
12、tember 2018. Originally approved in 2005. Last previous edition approved in 20082011 asE244808.111. DOI: 10.1520/E2448-11E01.10.1520/E2448-18.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolu
13、me information, refer to the standards Document Summary page on the ASTM website.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. Becauseit may not be technically possible to adequ
14、ately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohock
15、en, PA 19428-2959. United States1NOTE 2Engineering stress S and strain e are only valid up to the point of necking or instability of cross section. For superplastic deformation, thecoupon undergoes an essentially uniform and constant neck along its length, and S and e are assumed in this standard to
16、 be valid. However at the junctionto the clamp sections of the coupon the cross section reduces from the original value to the final value, over a length of approximately 4 % at each end.Also, there are local small instabilities of cross section over the gauge length. These contribute to an error in
17、 the calculated values of and . In theabsence of currently available extensometers that could operate in the high temperature environment of an SPF test, and are to be inferred fromcrosshead extension and force.3.1 Definitions:Terms common to mechanical testing. Definitions such as true stress (), t
18、rue strain (), normal engineeringstress (S), and engineering strain (e) are defined in Terminology E6. Thus,5lnL/L0!5S11e!3.1.1 indicated temperature, nthe temperature indicated by a temperature measuring device using good pyrometric practice.3.1.2 nominal temperature, nthe intended test temperature
19、.3.2 Definitions of Terms Specific to This Standard:3.2.1 gauge length, L, nthe instantaneous distance between the shoulders of the test coupon during the test3.3 Symbols Specific To This Standard: V = machine crosshead velocity, the velocity of the traveling member of the test machineto which one o
20、f the coupon clamps is attached = strain rate, measured as: V/L0 11e!#NOTE 2This is an operational definition of strain rate.m = strain rate sensitivity, defined as (ln )/ (ln ). In practical terms, m = log (2/1)/log (2/1) under stated test conditions,see 7.2.1.NOTE 4The derived term m is widely use
21、d to describe the SPF properties of a material. It should be used with caution, as it is dependent on strain,strain rate and temperature. Many references in the literature do not identify the strain condition at which the readings were taken, or allow multiple strainsto be used in the determination
22、of m.NOTE 5Many superplastic alloys exhibit strain hardening. However the conventional strain hardening exponent n as defined in Test Method E646is not valid for superplastic materials as strain hardening in the latter is usually a coefficient of strain, rather than an exponent. The mechanism of str
23、ainhardening in superplastic flow is essentially due to grain growth, and although the stress/strain relationship is often linear, it is not universal for allsuperplastic materials. Consequently there is no simple definition of a strain hardening coefficient and this standard does not define one. Co
24、nsiderationof strain hardening in superplastic deformation is discussed in Ghosh and Hamiltons, “Influences of Material Parameters and Microstructure onSuperplastic Forming.”33.2.1 The gage length (L) is defined as the instantaneous distance between the shoulders of the coupon during the test.NOTE 6
25、It is assumed no local necking takes place and the cross section of the coupon is constant over the entire gage length. For some materials,cavitation inside the material increases the volume of the gage section as the test progresses, and the true cross-sectional area has to be compensated forany st
26、rain. For other materials, the coupon can develop a ribbed or other local texture, and in this case, the minimum cross section has to be measured.During the test there is an increasingly non uniform cross section at each end of the coupon where the gage section transitions to the original width atth
27、e clamp section. This effect is small and can usually be ignored.4. Significance and Use4.1 The determination of the superplastic properties of a metallic sheet material is important for the observation, developmentand comparison of superplastic materials. It is also necessary to predict the correct
28、 forming parameters during an SPF process. SPFtensile testing has peculiar characteristics compared to conventional mechanical testing, which distort the true values of stress,strain, strain hardening, and strain rate at the very large elongations encountered in an SPF pull test, consequently conven
29、tionalmechanical test methods cannot be used. This test method addresses those characteristics by optimizing the shape of the testcoupon and specifying a new test procedure.4.2 The evaluation of a superplastic material can be divided into two parts. Firstly, the basic superplastic-forming (SPF)prope
30、rties of the material are measured using the four parameters of stress, temperature, strain, and strain rate. These are obtainedusing conversions from the raw data of a tensile test. Secondly, derived properties useful to define an SPF material are obtainedfrom the basic properties using specific eq
31、uations.4.3 The test coupon undergoes an essentially uniform and constant neck along its length, and S and e are assumed in thisstandard to be valid. However at the junction to the clamp sections of the test coupon the cross section reduces from the originalvalue to the final value, over a length of
32、 approximately 4 % at each end. Also, there are local small instabilities of cross sectionover the gauge length. These contribute to an error in the calculated values of and . In the absence of currently availableextensometers that could operate in the high temperature environment of an SPF test, an
33、d are to be inferred from crossheadextension and force.E2448 1824.4 The derived term m is widely used to describe the SPF properties of a material. It should be used with caution, as it isdependent on strain, strain rate and temperature. Many references in the literature do not identify the strain c
34、ondition at which thereadings were taken, or allow multiple strains to be used in the determination of m.4.5 Many superplastic alloys exhibit strain hardening. However, the conventional strain hardening exponent n as defined in TestMethod E646 is not valid for superplastic materials as strain harden
35、ing in the latter is usually a coefficient of strain, rather thanan exponent. The mechanism of strain hardening in superplastic flow is essentially due to grain growth, and although thestress/strain relationship is often linear, it is not universal for all superplastic materials. Consequently, there
36、 is no simple definitionof a strain hardening coefficient and this standard does not define one. Consideration of strain hardening in superplasticdeformation is discussed in Ghosh and Hamiltons, “Influences of Material Parameters and Microstructure on SuperplasticForming.”34.6 It is assumed no local
37、 necking takes place and the cross section of the test coupon is constant over the entire gauge length.For some materials, cavitation inside the material increases the volume of the gauge section as the test progresses, and the truecross-sectional area has to be compensated for any strain. For other
38、 materials, the coupon can develop a ribbed or other localtexture, and in this case, the minimum cross section has to be measured. During the test there is an increasingly non uniform crosssection at each end of the test coupon where the gauge section transitions to the original width at the clamp s
39、ection. This effectis small and can usually be ignored.5. Apparatus5.1 The accuracy of the testing machine shall be within the permissible variation specified in Practices E4.5.2 The apparatus shall be calibrated according to appropriate standards or manufacturer instructions.5.3 No extensometer is
40、used in this test method, and the extension of the test coupon is measured at the machine crosshead. Theaccuracy of the recorded crosshead position should be better than 0.25 mm. The machine compliance shall be determined beforetesting coupons, testing, and the amount of compliance subtracted from t
41、he crosshead position if it exceeds 1 % of the originalgauge length of the coupon. A method of determining compliance would be to mount a 6 mm thick coupon in the clamps withoutheating, then load the machine to the estimated maximum force of the test and measure the movement of the crosshead. Due to
42、the low loads of these tests (typically 100 N maximum) compliance is likely to be small.test coupon.NOTE 3One method of determining compliance is to mount a 6 mm thick test coupon in the clamps without heating, then load the machine to theestimated maximum force of the test and measure the movement
43、of the crosshead. Due to the low loads of these tests (typically 100 N maximum)compliance is likely to be small.5.4 The tensile test machine shall be computer controlled and capable of varying the crosshead speed in order to maintain a nearconstant strain rate. Step increases in crosshead speed are
44、allowed, a variation of The crosshead speed may be increased in steps.The instantaneous strain rate may vary up to 1 % from nominal strain rate is permitted.rate.5.5 The tensile test machine shall be provided with clamps that hold the test coupon at and under the shoulders adjacent to thegage sectio
45、n. The test coupon isshall not to be compressed by the clamps, as this will induce superplastic flow out of the clamparea during the test. Clamp design should follow that shown in Fig. 2.5.6 The apparatus is shall be provided with a furnace that shall maintain the test coupon at a constant temperatu
46、re throughoutthe test. Test equipment shall meet the requirements of Test Methods E21 for temperature measuring, calibration, andstandardization.6. Procedure6.1 Test coupons shall be made to the dimensions shown in Fig. 1. The test coupon width and gage thickness t shall be measuredand recorded at a
47、 minimum of four places in the gage section, to a tolerance of 1 % of reading, or 12 m, whichever is greater.6.2 If material oxidation affects the superplastic behavior of the material, the furnace canmay be flooded with argon or otherinert gas to reduce the effects of oxidation.6.3 Before starting
48、the test, bring the furnace is bought up to the desired temperature and stabilized. The coupon is loadednominal temperature and stabilize the temperature. Load the test coupon into the clamps. During the heat up of the test coupon,it is important to minimize external stress from the machine to the c
49、oupon. Many test machines incorporate a “protect specimen”or “load control” option during the heating phase to accommodate the thermal expansion of the coupon/grip assembly inside thefurnace and to prevent buckling of the coupon. This control option ensures “almost” zero loading on the test specimen duringheating through the movement of the cross-head beam.test coupon.NOTE 4Many test machines incorporate a “protect specimen” or “load control” option during the heating phase to accommodate the thermalexpansion of the tes
