ASTM E2448-22

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Designation: E2448 − 18 E2448 − 22
Standard Test Method for
Determining the Superplastic Properties of Metallic Sheet
Materials
This standard is issued under the fixed designation E2448; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method describes the procedure for determining the superplastic forming properties (SPF) of a metallic sheet
material. It includes tests both for the basic SPF properties and also for derived SPF properties. The test for basic properties
encompasses effects due to strain hardening or softening.
1.2 This test method covers sheet materials with thicknesses of at least 0.5 mm but not greater than 6 mm. It characterizes the
material under a uni-axial tensile stress condition.
NOTE 1—Most industrial applications of superplastic forming involve a multi-axial stress condition in a sheet; however it is more convenient to
characterize a material under a uni-axial tensile stress condition. Tests should be performed in different orientations to the rolling direction of the sheet
to ascertain initial anisotropy.
-5 -1
1.3 This method has been used successfully between strain rates of 10 mm ⁄mm ⁄s to 10 mm ⁄mm per ⁄s second.
1.4 This method has been used successfully on Aluminum and Titanium alloys. The use of the method with other metals should
be 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, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E4 Practices for Force Calibration and Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
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 and
Formability.
Current edition approved June 1, 2018June 15, 2022. Published September 2018January 2023. Originally approved in 2005. Last previous edition approved in 20112018
ɛ1
as E2448–11–18. . DOI: 10.1520/E2448-18.10.1520/E2448-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2448 − 22
E21 Test Methods for Elevated Temperature Tension Tests of Metallic Materials
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E646 Test Method for Tensile Strain-Hardening Exponents (n -Values) of Metallic Sheet Materials
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions:Terms common to mechanical testing. Definitions such as true stress (σ), true strain (ε), normal engineering
stress (S), and engineering strain (e) are defined in Terminology E6. Thus,
ε5 ln L/L
~ !
σ5 S 11e
~ !
3.1.1 indicated temperature, engineering strain, e, n—the temperature indicated by aa dimensionless value that is the change in
length (ΔL temperature measuring device using good pyrometric practice. ) per unit length of original linear dimension (L ) along
the loading axis of the specimen; that is, e = (ΔL) ⁄L .
-2
3.1.2 nominal temperature, engineering stress, S [FL ],n—the intended test temperature.normal stress, expressed in units of
applied force, F, per unit of original cross-sectional area, A ; that is, S = F⁄A .
0 0
3.1.3 true strain, ε, n—the natural logarithm of the ratio of instantaneous gauge length, L, to the original gauge length, L ; that
is, ε = ln (L ⁄L ) or ε = ln (1+e).
-2
3.1.4 true stress, σ [FL ],n—the instantaneous normal stress, calculated on the basis of the instantaneous cross-sectional area, A;
that is, σ = F/A; if no necking has occurred, σ = S(1+e).
3.1.5 Refer to Terminology E6 for the definitions of the terms extensometer system, indicated temperature, necking, specified
temperature, strain hardening, and stress-strain diagram.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 gauge length, L, [L],n—the instantaneous distance between the shoulders of the test couponspecimen during the test
3.2.2 testing machine crosshead velocity, V, [L/T],n—the velocity of the traveling member of the testing machine to which one of
the test specimen clamps is attached.
V
3.2.3 strain rate, ε˙, [1/T],n—the time rate of change of the true strain in the test specimen, measured as:
L 11e
~ !
where:
L = original gauge length
V = machine crosshead velocity
3.2.3.1 Discussion—
This is an operational definition of strain rate.
3.2.4 strain rate sensitivity, m, n—(ln Δσ)/ (ln Δε˙)ԑ˙.
3.2.4.1 Discussion—
ln σ ⁄ σ
~ !
2 1
In practical terms, m5 under stated test conditions, see 7.2.1.
ln ε˙ ⁄ ε˙
~ !
2 1
3.3 Symbols Specific To This Standard: V = machine crosshead velocity, the velocity of the traveling member of the test machine
to which one of the coupon clamps is attached
ε = strain rate, measured as: V/ L 11e
˙ @ ~ !#
NOTE 2—This is an operational definition of strain rate.
m = strain rate sensitivity, defined as (ln Δ σ)/ (ln Δ ε˙). In practical terms, m = log (σ /σ )/log (ε˙ /ε˙ ) under stated test conditions,
2 1 2 1
see 7.2.1.
E2448 − 22
4. Significance and Use
4.1 The determination of the superplastic properties of a metallic sheet material is important for the observation, development and
comparison of superplastic materials. It is also necessary to predict the correct forming parameters during an SPF process. SPF
tensile 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 conventional
mechanical test methods cannot be used. This test method addresses those characteristics by optimizing the shape of the test
couponspecimen 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) properties
of the material are measured using the four parameters of stress, temperature, strain, and strain rate. These are obtained using
conversions from the raw data of a tensile test. Secondly, derived properties useful to define an SPF material are obtained from
the basic properties using specific equations.
4.3 The test couponspecimen undergoes an essentially uniform and constant necknecking along its length, and S and e are assumed
in this standard to be valid. However at the junction to the clamp sections of the test couponspecimen 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 the calculated values of ε and σ. In the absence of currently
available extensometers that could operate in the high temperature elevated-temperature environment of an SPF test, ε and σ are
to be inferred from crosshead extension and force.
4.4 The derived term m is widely used 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 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 Test
Method E646 is not valid for superplastic materials as strain hardening in the latter is usually a coefficient of strain, rather than
an exponent. The mechanism of strain hardening in superplastic flow is essentially due to grain growth, and although the
stress/strain relationship is often linear, it is not universal for all superplastic materials. Consequently, there is no simple definition
of a strain hardening coefficient and this standard does not define one. Consideration of strain hardening in superplastic
deformation is discussed in Ghosh and Hamilton’s, “Influences of Material Parameters and Microstructure on Superplastic
Forming.”
4.6 It is assumed no local necking takes place and the cross section of the test couponspecimen 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
true cross-sectional area has to be compensated for any strain. 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 test coupon where the gauge section transitions to the original width at the clamp section. This effect
is small and can usually be ignored.
NOTE 2—For some materials, cavitation inside the material increases the volume of the gauge section as the test progresses and changes the true
cross-sectional area. For other materials, the test specimen develops a ribbed or other local texture, and changes the minimum cross section.
4.7 It is assumed that the increasingly non uniform cross section that develops at each end of the test specimen where the gauge
section transitions to the original width at the clamp section is small and can be ignored.
5. Apparatus
5.1 The accuracy force-measuring system of the testing machine shall be within the permissible variation specified in conform to
the requirements of Practices E4.
5.2 The apparatus shall be calibrated according to appropriate standards or manufacturer instructions.
Ghosh, A. K., and Hamilton, C. H., “Influences of Material Parameters and Microstructure on Superplastic Forming,” Met Trans A, Vol 13A, May 1982, pp. 733-742.
E2448 − 22
5.3 No extensometer system is used in this test method, and the extension of the test couponspecimen is measured at the testing
machine crosshead. The accuracy of the recorded crosshead position shouldshall be better than 0.25 mm. The testing machine
compliance shall be determined before testing, and the amount of compliance subtracted from the crosshead position if it exceeds
1 % of the original gauge length of the test coupon.specimen.
NOTE 3—One method of determining compliance is to mount a 6 mm thick test couponspecimen in the clamps without heating, then load the testing
machine to the estimated maximum force of the test and measure the movement of the crosshead. Due to the low loadsforces of these tests (typically
100 N maximum) compliance is likely to be small.
5.4 The tensile testtesting machine shall be computer controlled and capable of varying the testing machine crosshead
speedvelocity in order to maintain a near constant strain rate. The testing machine crosshead speedvelocity may be increased in
steps. The instantaneous strain rate may vary up to 1 % from nominal strain rate.
5.5 The tensile testtesting machine shall be provided with clamps that hold the test couponspecimen at and under the shoulders
adjacent to the gagegauge section. The test couponspecimen shall not be compressed by the clamps, as this will induce superplastic
flow out of the clamp area during the test. Clamp design should follow that shown in Fig. 2.
5.6 The apparatus shall be provided with a furnace that shall maintain the test couponspecimen at a constant temperature
throughout the test. Test equipment shall meet the requirements of Test Methods E21 for temperature measuring, calibration, and
standardization.
6. Procedure
6.1 Test couponsspecimens shall be made to the dimensions shown in Fig. 1. The test couponspecimen width and gage
thicknessgauge thickness, t, shall be measured and recorded at a minimum of four places in the gagegauge section, to a tolerance
of 1 % of reading, or 12 μm, whichever is greater.
6.2 If material oxidation affects the superplastic behavior of the material, the furnace may be flooded with argon or other inert gas
to reduce the effects of oxidation.
Dimensions in mm. Tolerance 60.25 mm except where noted.
FIG. 1 Dimensions of Test CouponSpecimen
E2448 − 22
FIG. 2 Test CouponSpecimen Grip Configuration
E2448 − 22
6.3 Before starting the test, bring the furnace up to the nominalspecified temperature and stabilize the temperature. LoadInstall the
test couponspecimen into the clamps. During the heat up of the test coupon, important to specimen, minimize external stressforce
from the testing machine to the test coupon.specimen.
NOTE 4—Many testtesting machines incorporate a “protect specimen” or “load control” option during the heating phase to accommodate the thermal
expansion of the test coupon/gripspecimen/grip assembly inside the furnace and to prevent buckling of the test coupon.specimen. This control option
ensures “almost” zero loadingforce on the test specimen during heating through the movement of the cross-head beam.
6.4 The test should not commence until the test coupon has reached thermal equilibrium. This will be reached when the cross-head
beam ceases to move under the “protect specimen” control, indicating that no more thermal expansion is taking place. However
this time can be long enough to allow grain growth in the test coupon, which distorts the superplastic properties being evalu
...