1、Designation: E 2448 08Standard Test Method forDetermining the Superplastic Properties of Metallic SheetMaterials1This standard is issued under the fixed designation E 2448; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o
2、f 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. Scope1.1 This test method describes the procedure for determin-ing the superplastic forming properties (SPF) of a metallicshe
3、et 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 thicknessesof at least 0.5 mm but not greater than 6 mm. It cha
4、racterizesthe 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-axial tensile stress condition. Testsshould be performed
5、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 Titanium alloys. The use of the method with other metalssho
6、uld 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 use. It is theresponsibility of the user of this standard t
7、o 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 Relating to Methods of Mechanical Test-ingE21 Test Methods for
8、 Elevated Temperature Tension Testsof Metallic MaterialsE 177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE 646 Test Method for Tensile Strain-Hardening Exponents(n -Values) of Metallic Sheet MaterialsE 691 Practice for Conducting an Interlaboratory Study toDetermine the Prec
9、ision of a Test Method3. Terminology3.1 DefinitionsDefinitions such as gage length (L and L0),true stress (s), true strain (), normal engineering stress (S),and engineering strain (e) are defined in Terminology E6.Thus,5lnL/L0!s5S1 1 e!NOTE 2Engineering stress S and strain e are only valid up to the
10、 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 clamp sections of the coupon the cross section reducesfrom
11、 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 s. In the absence of currently available extensometers that couldoperate
12、 in the high temperature environment of an SPF test, and s areto be inferred from crosshead extension and force.3.2 Symbols Specific To This Standard:V = machine crosshead velocity, the velocity of the travelingmember of the test machine to which one of the coupon clampsis attached= strain rate, mea
13、sured as: V/L01 1 e!#NOTE 3This is an operational definition of strain rate.m = strain rate sensitivity, defined as (ln Ds)/ (ln D). Inpractical terms, m = log (s2/s1)/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
14、. 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 which the readings were taken, or allowmultiple strains to be used in the determination of m.1This test method is under the jurisdictio
15、n of ASTM Committee E28 onMechanical Testing and is the direct responsibility of Subcommittee E28.02 onDuctility and Formability.Current edition approved Dec. 1, 2008. Published January 2009. Originallyapproved in 2005. Last previous edition approved in 2006 as E 244806.2For referenced ASTM standard
16、s, 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 Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 1
17、9428-2959, United States.NOTE 5Many superplastic alloys exhibit strain hardening. Howeverthe conventional strain hardening exponent n as defined in Test MethodE 646 is not valid for superplastic materials as strain hardening in thelatter is usually a coefficient of strain, rather than an exponent. T
18、hemechanism of strain hardening in superplastic flow is essentially due tograin growth, and although the stress/strain relationship is often linear, itis not universal for all superplastic materials. Consequently there is nosimple definition of a strain hardening coefficient and this standard doesno
19、t define one. Consideration of strain hardening in superplastic defor-mation is discussed in Ghosh and Hamiltons, “Influences of MaterialParameters and Microstructure on Superplastic Forming.”33.2.1 The gage length (L) is defined as the instantaneousdistance between the shoulders of the coupon durin
20、g 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 test progresses, and the true cross-sectional area has to becompens
21、ated 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 each end of the coupon where the gage section transitionsto the origina
22、l 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 comparison of superplastic materials. It is alsonecessary to predict
23、 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 atthe very large elongations encountered in an SPF pull test,consequent
24、ly 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 be dividedinto two parts. Firstly, the basic superplastic-forming (SP
25、F)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 define an SPF materialare obtained from the basic properties using spec
26、ific 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 instructions.5.3 No extensometer is used in this test method, and theextensi
27、on 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 subtracted from the crosshead position if it exceeds1 % of the origin
28、al 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 crosshead. Due to the low loads of these tests(typically 100 N maximum) comp
29、liance 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 variation of 1 % from nominal strain rate ispermitted.5.5 The tensile test
30、 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 during the test. Clamp design should follow that shown inFig. 2.5.6 The
31、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, andstandardization.6. Procedure6.1 Test coupons shall be made to the dimensions shown
32、 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 superplastic behavior ofthe material, the furnace can be flooded with ar
33、gon 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 important tominimize external stress from the machine to the coupon.M
34、any 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 controloption ensures “almost” zero loading on the test specimenduring hea
35、ting 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 thecross-head beam ceases to move under the “protect specimen”control, indicating that no more thermal expansion is takingplace. Howeve
36、r this time can be long enough to allow graingrowth in the coupon, 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 “
37、zeroed.” At this point, any movement of thecrosshead is assumed 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
38、growth on SPF properties. For the durationof the test, defined as the time from initiation of loading untilthe termination of test or fracture, the allowed tolerance3Ghosh, A. K., and Hamilton, C. H., “Influences of Material Parameters andMicrostructure on Superplastic Forming,” Met Trans A, Vol 13A
39、, May 1982, pp.733-742.E2448082between 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 between crosshead andclamp. This error in reading is small compared to
40、 the coupon length L andcan be ignored for most testing.6.6 The machine crosshead velocity is increased accordingto the equation V 5L01 1 e!# to an accuracy of 61%tomaintain a constant true strain rate until a predetermined strainvalue is reached or until fracture. (If early fracture occurs at thein
41、terface 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 conclusion of the test, a measurement of height,width and thickness should be
42、 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 derived “m” value, a step test can beemployed, in which the true strai
43、n 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 test machine are converted to true stress s 5 S1 1 e!# and true strain 5
44、 ln L/L0!# . The basic SPFproperties of a material at a specified strain rate and tempera-ture shall be presented as a graph of true stress versus truestrain as shown in Fig. 3. Several strain rates can be plotted onthe same graph.NOTE 8The usual presentation of stress/strain data records engineer-i
45、ng 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 basic prop-erties, the superplastic behavior of a material can been de-scribed by constitutive equa
46、tions, generally of the form:s5k11 k2m(1)where:m = superplastic strain rate sensitivity exponent.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
47、 to the step, thus the two stress levels at thepoint of change are known.m 5 log s2/s1!/log 2/1! (2)FIG. 1 Dimensions of Test CouponE2448083FIG. 2 Test Coupon Grip ConfigurationE2448084FIG. 3 Basic SPF Properties for Fine Grain Ti-6Al-4V Alloy at 775C, Transverse DirectionFIG. 4 Derived SPF Property
48、 “m” Value Determination for Fine Grain Ti-6Al-4V Alloy at 775C, Transverse DirectionE24480857.3 The value of m varies both with strain and strain rate.Therefore a quoted value of m must include the correspondingtemperature, strain, and strain rate.7.4 The default strain rate is that for maximum m,
49、and thedefault strain is 0.693 (100 % engineering strain). Values of mfor different strain rates and strains may be quoted in particularcases.7.5 Am 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 table below.Data point Strain rate Stress MPa Strain1 3.60E-04 30.076 0.6462 3.60E-04 29.941 0.6463 3.60E-04 29.910 0.6474 3.60E-04 29.889 0.6475 3.60E-04 30.556 0.6486 3.60E-04 30.080 0.6487 3.60E-04 29.820 0.6498 3.60E-04 30.300 0.6499 3.60E-04 29.985 0.
