ASTM E2448-2011 Standard Test Method for Determining the Superplastic Properties of Metallic Sheet Materials《测定金属板材料的超塑料性的标准试验方法》.pdf

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1、Designation: E2448 11Standard 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.1. Scope1.1 This test method describes the procedure for determin-ing the superplastic forming properties (SPF) of a metallicsheet

3、 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 chara

4、cterizesthe 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 in

5、 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 metalsshoul

6、d 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 to

7、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 TestingE21 Test Methods for El

8、evated Temperature Tension Testsof Metallic 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 Conducting an Interlaboratory Study toDetermine the Precision

9、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 poin

10、tof 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 the

11、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 in t

12、he 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, measured

13、 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. It

14、should be used with caution, as it is dependenton strain, strain rate and temperature. Many references in the literature do1This test method is under the jurisdiction of ASTM Committee E28 onMechanical Testing and is the direct responsibility of Subcommittee E28.02 onDuctility and Formability.Curren

15、t edition approved June 1, 2011. Published July 2011. Originally approvedin 2005. Last previous edition approved in 2008 as E244808. DOI: 10.1520/E2448-11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTM

16、Standards 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.not identify the strain condition at which the readings were taken, or allowmultiple strain

17、s 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 latteris usually a coefficient of strain, rather than a

18、n 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 definition ofa strain hardening coefficient and this st

19、andard 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 the instantaneousdistance between the shoulders of the

20、 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 test progresses, and the true cross-sectional area has

21、 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 each end of the coupon where the gage section transitionst

22、o 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 comparison of superplastic materials. It is alsonecessa

23、ry 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 atthe very large elongations encountered in an SPF pull te

24、st,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 be dividedinto two parts. Firstly, the basic superplasti

25、c-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 define an SPF materialare obtained from the basic properti

26、es 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 instructions.5.3 No extensometer is used in this test method, a

27、nd 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 subtracted from the crosshead position if it exceeds1 %

28、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 crosshead. Due to the low loads of these tests(typically 100 N

29、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 variation of 1 % from nominal strain rate ispermitted.5.5 The

30、 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 during the test. Clamp design should follow that shown inFi

31、g. 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, andstandardization.6. Procedure6.1 Test coupons shall be made to the dim

32、ensions 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 superplastic behavior ofthe material, the furnace can be fl

33、ooded 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 important tominimize external stress from the machine to

34、 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 controloption ensures “almost” zero loading on the test speci

35、menduring 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 thecross-head beam ceases to move under the “protect specimen”control, indicating that no more thermal expansion is taking

36、place. However 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 extensionsh

37、all then be “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 eff

38、ect of grain growth on SPF properties. For the duration3Ghosh, A. K., and Hamilton, C. H., “Influences of Material Parameters andMicrostructure on Superplastic Forming,” Met Trans A, Vol 13A, May 1982, pp.733-742.E2448 112of the test, defined as the time from initiation of loading untilthe terminati

39、on 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 between crosshead andclamp. This error in reading is sma

40、ll compared to 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 o

41、ccurs 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 conclusion of the test, a measurement of height,width and thick

42、ness 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 derived “m” value, a step test can beemployed, in which

43、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 test machine are converted to true stress s 5 S1 1 e!# and

44、 true strain 5 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 reco

45、rds 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 basic prop-erties, the superplastic behavior of a material can been de-scribed by con

46、stitutive equations, 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 linesar

47、e extrapolated to the step, thus the two stress levels at thepoint of change are known.m 5 log s2/s1!/log 2/1! (2)7.3 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

48、 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 Am example of m value calculation is as follows. Asample of 26 data points around the step at 0.650 strain graphDimensions in mm. Tol

49、erance 60.25 mm except where noted.FIG. 1 Dimensions of Test CouponE2448 113FIG. 2 Test Coupon Grip ConfigurationE2448 1144 is shown in the following table.FIG. 3 Basic SPF Properties 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 115Data 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-