1、Designation: E2448 111Standard 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 year of
2、 last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEFootnote 4 was editorially corrected in October 2015.1. Scope1.1 This test method describes the procedure for determin-ing t
3、he superplastic forming properties (SPF) of a metallicsheet material. It includes tests both for the basic SPF proper-ties and also for derived SPF properties. The test for basicproperties encompasses effects due to strain hardening orsoftening.1.2 This test method covers sheet materials with thickn
4、essesof at least 0.5 mm but not greater than 6 mm. It characterizesthe material under a uni-axial tensile stress condition.NOTE 1Most industrial applications of superplastic forming involve amulti-axial stress condition in a sheet; however it is more convenient tocharacterize a material under a uni-
5、axial tensile stress condition. Testsshould be performed in different orientations to the rolling direction of thesheet to ascertain initial anisotropy.1.3 This method has been used successfully between strainrates of 10-5to 10-1per second.1.4 This method has been used successfully on Aluminumand Ti
6、tanium alloys. The use of the method with other metalsshould be verified.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its us
7、e. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E4 Practices for Force Verification of Testing MachinesE6 Terminology Relat
8、ing to Methods of Mechanical TestingE21 Test Methods for Elevated Temperature Tension Tests ofMetallic MaterialsE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE646 Test Method for Tensile Strain-Hardening Exponents(n -Values) of Metallic Sheet MaterialsE691 Practice for Con
9、ducting an Interlaboratory Study toDetermine 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 (e) are defined in Terminology E6. Thus, 5 lnL/L0! 5 S11e!NOTE 2Engi
10、neering stress S and strain e are only valid up to the pointof necking or instability of cross section. For superplastic deformation, thecoupon undergoes an essentially uniform and constant neck along itslength, and S and e are assumed in this standard to be valid. However atthe junction to the clam
11、p sections of the coupon the cross section reducesfrom the original value to the final value, over a length of approximately4 % at each end. Also, there are local small instabilities of cross sectionover the gauge length. These contribute to an error in the calculated valuesof and . In the absence o
12、f currently available extensometers that couldoperate in the high temperature environment of an SPF test, and are tobe inferred from crosshead extension and force.1This test method is under the jurisdiction of ASTM Committee E28 onMechanical Testing and is the direct responsibility of Subcommittee E
13、28.02 onDuctility and Formability.Current edition approved June 1, 2011. Published July 2011. Originally approvedin 2005. Last previous edition approved in 2008 as E244808. DOI: 10.1520/E2448-11E01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service
14、at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2 Symbols Specific To This Standard: V = machin
15、e cross-head velocity, the velocity of the traveling member of the testmachine to which one of the coupon clamps is attached = strain rate, measured as: V/L011e!#NOTE 3This is an operational definition of strain rate.m = strain rate sensitivity, defined as (ln )/ (ln ). Inpractical terms, m = log (2
16、/1)/log (2/1) under stated testconditions, see 7.2.1.NOTE 4The derived term m is widely used to describe the SPFproperties of a material. It should be used with caution, as it is dependenton strain, strain rate and temperature. Many references in the literature donot identify the strain condition at
17、 which the readings were taken, or allowmultiple strains to be used in the determination of m.NOTE 5Many superplastic alloys exhibit strain hardening. Howeverthe conventional strain hardening exponent n as defined in Test MethodE646 is not valid for superplastic materials as strain hardening in the
18、latteris usually a coefficient of strain, rather than an exponent. The mechanismof strain hardening in superplastic flow is essentially due to grain growth,and although the stress/strain relationship is often linear, it is not universalfor all superplastic materials. Consequently there is no simple
19、definition ofa strain hardening coefficient and this standard does not define one.Consideration of strain hardening in superplastic deformation is discussedin Ghosh and Hamiltons, “Influences of Material Parameters and Micro-structure on Superplastic Forming.”33.2.1 The gage length (L) is defined as
20、 the instantaneousdistance between the shoulders of the coupon during the test.NOTE 6It is assumed no local necking takes place and the crosssection of the coupon is constant over the entire gage length. For somematerials, cavitation inside the material increases the volume of the gagesection as the
21、 test progresses, and the true cross-sectional area has to becompensated for any strain. For other materials, the coupon can developa ribbed or other local texture, and in this case, the minimum cross sectionhas to be measured. During the test there is an increasingly non uniformcross section at eac
22、h end of the coupon where the gage section transitionsto the original width at the clamp section. This effect is small and canusually be ignored.4. Significance and Use4.1 The determination of the superplastic properties of ametallic sheet material is important for the observation, devel-opment and
23、comparison of superplastic materials. It is alsonecessary to predict the correct forming parameters during anSPF process. SPF tensile testing has peculiar characteristicscompared to conventional mechanical testing, which distort thetrue values of stress, strain, strain hardening, and strain rate att
24、he very large elongations encountered in an SPF pull test,consequently conventional mechanical test methods cannot beused. This test method addresses those characteristics byoptimizing the shape of the test coupon and specifying a newtest procedure.4.2 The evaluation of a superplastic material can b
25、e dividedinto two parts. Firstly, the basic superplastic-forming (SPF)properties of the material are measured using the four param-eters of stress, temperature, strain, and strain rate. These areobtained using conversions from the raw data of a tensile test.Secondly, derived properties useful to def
26、ine an SPF materialare obtained from the basic properties using specific equations.5. Apparatus5.1 The accuracy of the testing machine shall be within thepermissible variation specified in Practices E4.5.2 The apparatus shall be calibrated according to appropri-ate standards or manufacturer instruct
27、ions.5.3 No extensometer is used in this test method, and theextension of the test coupon is measured at the machinecrosshead. The accuracy of the recorded crosshead positionshould be better than 0.25 mm. The machine compliance shallbe determined before testing coupons, and the amount ofcompliance s
28、ubtracted from the crosshead position if it exceeds1 % of the original gauge length of the coupon. A method ofdetermining compliance would be to mounta6mmthickcoupon in the clamps without heating, then load the machine tothe estimated maximum force of the test and measure themovement of the crosshea
29、d. 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 controlledand capable of varying the crosshead speed in order to maintaina near constant strain rate. Step increases in crosshead speedare allowed, a variat
30、ion of 1 % from nominal strain rate ispermitted.5.5 The tensile test machine shall be provided with clampsthat hold the test coupon at and under the shoulders adjacent tothe gage section. The coupon is not to be compressed by theclamps, as this will induce superplastic flow out of the clamparea duri
31、ng the test. Clamp design should follow that shown inFig. 2.5.6 The apparatus is provided with a furnace that shallmaintain the coupon at a constant temperature throughout thetest. Test equipment shall meet the requirements of TestMethods E21 for temperature measuring, calibration, andstandardizatio
32、n.6. Procedure6.1 Test coupons shall be made to the dimensions shown inFig. 1. The coupon width and gage thickness t shall bemeasured and recorded at a minimum of four places in the gagesection, to a tolerance of 1 % of reading, or 12 m, whicheveris greater.6.2 If material oxidation affects the supe
33、rplastic behavior ofthe material, the furnace can be flooded with argon or otherinert gas to reduce the effects of oxidation.6.3 Before starting the test, the furnace is bought up to thedesired temperature and stabilized. The coupon is loaded intothe clamps. During the heat up of the coupon, it is i
34、mportant tominimize external stress from the machine to the coupon.Many test machines incorporate a “protect specimen” or “loadcontrol” option during the heating phase to accommodate thethermal expansion of the coupon/grip assembly inside thefurnace and to prevent buckling of the coupon. This contro
35、loption ensures “almost” zero loading on the test specimenduring heating through the movement of the cross-head beam.6.4 Ideally the test should not commence until the couponhas reached thermal equilibrium. This will be reached when the3Ghosh, A. K., and Hamilton, C. H., “Influences of Material Para
36、meters andMicrostructure on Superplastic Forming,” Met Trans A, Vol 13A, May 1982, pp.733-742.E2448 1112cross-head beam ceases to move under the “protect specimen”control, indicating that no more thermal expansion is takingplace. However this time can be long enough to allow graingrowth in the coupo
37、n, which distorts the superplastic propertiesbeing evaluated. Therefore the time taken for the thermo-couples to come within tolerance can be used instead if graingrowth is considered significant. The cross-head extensionshall then be “zeroed.” At this point, any movement of thecrosshead is assumed
38、to be the same as the moving clamp onthe coupon, and is equivalent to the extension of the coupon.6.5 Loading shall start as soon as the coolest thermocouplereaches the minimum specified temperature range to minimizethe effect of grain growth on SPF properties. For the durationof the test, defined a
39、s the time from initiation of loading untilthe termination of test or fracture, the allowed tolerancebetween indicated and nominal test temperature is 63C up to700C and 66C above 700C.NOTE 7As the clamp extension rod is pulled out of the furnace, itcools and contracts, thereby altering the distance
40、between crosshead andclamp. This error in reading is small compared to the coupon length L andcan be ignored for most testing.6.6 The machine crosshead velocity is increased accordingto the equation V5 L011e!# to an accuracy of 61% tomaintain a constant true strain rate until a predetermined strainv
41、alue is reached or until fracture. (If early fracture occurs at theinterface between clamp and gauge section, then the material isunlikely to be superplastic).6.7 Force and crosshead extension shall be recorded at leasttwice per second to an accuracy of 61 % of the recorded value.6.8 At the conclusi
42、on of the test, a measurement of height,width and thickness should be taken in the clamp area tomeasure any superplastic flow in that section; this value shallbe recorded.6.9 To determine the basic SPF properties, a constant truestrain rate test as described above is employed.6.10 To determine the d
43、erived “m” value, a step test can beemployed, in which the true strain rate is periodically steppedto 20 % above nominal, then back to nominal, starting at a truestrain of 0.15 and stepping up and down every 0.1 strain.7. Analysis7.1 Basic SPF PropertiesForce and extension measure-ments from the tes
44、t machine are converted to true stress 5S11e!# and true strain 5lnL/L0!# . The basic SPF prop-erties of a material at a specified strain rate and temperatureshall be presented as a graph of true stress versus true strain asshown in Fig. 3. Several strain rates can be plotted on the samegraph.NOTE 8T
45、he usual presentation of stress/strain data records engineer-ing stress on the Y-axis. This is not applicable for an SPF test due to thesignificant elongation, and subsequent cross section area reduction, of thecoupon.7.2 Derived SPF PropertiesIn addition to the basicproperties, the superplastic beh
46、avior of a material can beendescribed by constitutive equations, generally of the form: 5 k11k2m(1)where:m = superplastic strain rate sensitivity exponent.Dimensions in mm. Tolerance 60.25 mm except where noted.FIG. 1 Dimensions of Test CouponE2448 1113FIG. 2 Test Coupon Grip ConfigurationE2448 1114
47、7.2.1 The m value is determined from the test described in6.10. The result of such a test is shown in Fig. 4. A number ofpoints (usually 10) on either side of the step are taken and linesare extrapolated to the step, thus the two stress levels at thepoint of change are known.FIG. 3 Basic SPF Propert
48、ies for Fine Grain Ti-6Al-4V Alloy at 775C, Transverse DirectionFIG. 4 Derived SPF Property “m” Value Determination for Fine Grain Ti-6Al-4V Alloy at 775C, Transverse DirectionE2448 1115m 5 log2/1!/log2/1! (2)7.3 The value of m varies both with strain and strain rate.Therefore a quoted value of m mu
49、st include the correspondingtemperature, strain, and strain rate.7.4 The default strain rate is that for maximum m, and thedefault strain is 0.693 (100 % engineering strain). Values of mfor different strain rates and strains may be quoted in particularcases.7.5 An example of m value calculation is as follows. Asample of 26 data points around the step at 0.650 strain graph4 is shown in the following table.Data point Strain rate Stress MPa Strain1 3.60E-04 30.076 0.6462 3.60E-04 29.941 0.6463 3.60E-04 29
