ASTM D2731-21

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Designation: D2731 − 15 D2731 − 21
Standard Test Method for
Elastic Properties of Elastomeric Yarns (CRE Type Tensile
Testing Machines)
This standard is issued under the fixed designation D2731; 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 covers the determination of elastic properties of “as produced” elastomeric yarns made from rubber, spandex
or other elastomers. Elastic properties include force at specified elongations, permanent deformation and stress decay. Other
hysteresis related properties can be calculated.
NOTE 1—For a method designed specifically for testing rubber threads, refer to Test Method D2433.
1.2 This test method is not applicable to covered, wrapped, or core-spun yarns or yarns spun from elastomeric staple.
1.3 This test method is applicable to elastomeric yarns having a range of 40 to 3200 dtex (36 to 2900 denier).
1.4 The values stated in either SI units or U.S. Customary units are to be regarded separately as standard. Within the text, the U.S.
Customary units are in parentheses. The values stated in each system are not exact equivalents; therefore, each system shall be used
independently of the other.
1.5 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 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:
D76 Specification for Tensile Testing Machines for Textiles
D123 Terminology Relating to Textiles
D1776 Practice for Conditioning and Testing Textiles
D2258 Practice for Sampling Yarn for Testing
D2433 Test Methods for Rubber Thread (Withdrawn 2012)
D2591 Test Method for Linear Density of Elastomeric Yarns (Short Length Specimens)
This test method is under the jurisdiction of ASTM Committee D13 on Textiles and is the direct responsibility of Subcommittee D13.58 on Yarns and Fibers.
Current edition approved Feb. 1, 2015July 1, 2021. Published March 2015August 2021. Originally approved in 1968. Last previous edition approved in 20122015 as
D2731–07(2012).D2731–15. DOI: 10.1520/D2731-15.10.1520/D2731-21.
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.
The last approved version of this historical standard is referenced on www.astm.org.
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D2731 − 21
D2653 Test Method for Tensile Properties of Elastomeric Yarns (CRE Type Tensile Testing Machines)
D4848 Terminology Related to Force, Deformation and Related Properties of Textiles
D4849 Terminology Related to Yarns and Fibers
D6717 Test Method for Linear Density of Elastomeric Yarns (Skein Specimens)
3. Terminology
3.1 For all terminology relating to D13.58, Yarns and Fibers, refer to Terminology D4849.
3.1.1 The following terms are relevant to this standard: deformation, elastomeric yarn, elongation, force at specified elongation,
linear density, permanent deformation, stress, stress decay.
3.2 For definitions of other terms related to force and deformation, refer to Terminology D4848. For all other terms related to
textiles, refer to Terminology D123.
4. Summary of Test Method
4.1 A specimen, mounted in a CRE-type tensile machine, is initially subjected to a series of five loading/unloading cycles in which
the specimen is extended and relaxed between zero and 75 % of the elongation at first filament break (FFB). During the fifth cycle,
the specimen is held at the maximum extension point for 30 s, then unloaded to allow a return to its original gagegauge length
position. The specimen is then subjected to a sixth load/unload cycle.
4.2 Force at specified elongations are calculated from the force-elongation curve for the first and fifth loadings and for the fifth
unloading. Stress decay is calculated on the fifth cycle. Extension at a specified force is determined on the sixth loading and is used
to calculate the permanent deformation.
5. Significance and Use
5.1 This test method is considered satisfactory for acceptance testing of commercial shipments since current estimates of
between-laboratory precision are acceptable and the method is used extensively in the trade for acceptance testing.
5.1.1 If there are differences of practical significance between reported test results for two laboratories (or more), comparative tests
should be performed to determine if there is a statistical bias between them, using competent statistical assistance. As a minimum,
use samples for such comparative tests that are as homogeneous as possible, drawn from the same lot of material as the samples
that resulted in disparate results during initial testing, and randomly assigned in equal numbers to each laboratory. The test results
from the laboratories involved should be compared using a statistical test for unpaired data, at a probability level chosen prior to
the testing series. If bias is found, either its cause must be found and corrected, or future test results for that material must be
adjusted in consideration of the known bias.
5.2 Force at Specified Elongation (FASE) is a measure of the tensile force required to extendoccurring while extending a textile
material within specified limits. This characteristic of elastomeric yarn indicates the resistance that will have to be overcome by
the wearer while putting on a garment made of the material and is also an indication of the garment’s resistance to deformation
caused by normal body movements during wear. The elongations used for these measurements are typically 100 %, 200 % and
300 %.
5.3 Permanent Deformation (set) is a measure of the increase in length of an elastomeric yarn resulting from cyclic stretching and
relaxation. The characteristic is a visible indication of the realignment of intermolecular bonds within the elastic material. As with
stress decay, the amount of set increases with yarn extension; however, for any particular extension, little or no additional set takes
place after five cycles of exercising. Generally, the characteristic set of the yarn is developed during fabric preparation and the
fabric itself shows a negligible amount of set.
5.4 Stress decay increases with yarn extension, but at any specified extension the stress decay takes place in the first 30 s with
insignificant decay after 5 min. This characteristic is caused by the gradual realignment of intermolecular bonds within the elastic
material, and helps to explain the changes in yarn properties that accompany cyclic stretching and relaxing. The realignment of
the bonds is a reversible effect. Following complete relaxation of the yarn, the molecules tend to assume their original configuration
with just about complete elimination of the previously observed strain.
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5.5 This test method was developed using elastomeric yarns in the “as-produced” condition, but may be used for treated
elastomeric yarns provided the treatment is specified. The method does not cover the removal of finish for the determination of
elastic properties of “finish-free” elastomeric yarns.
6. Apparatus
6.1 Specimen Boards, with short pile or plush surfaces of black or contrasting color, for storing specimens during conditioning.
6.2 Tensile Testing Machine, CRE-type, conforming to Specification D76 with respect to force indication, working range, capacity
and verification of recorded elongation, capable of cycling, and designed for operation at a pulling speed of 500 mm/min (20
in./min).
6.3 Clamping Assembly, pneumatically operated, with jaws as described in Test Method D2653.
6.4 Computer or Microprocessor, interfaced, with automatic data gathering system, optional.
6.5 Tensioning Weights, with various masses from 10 mg to 3 g to pretension the specimen to 30 to 50 mN/tex (0.3 to 0.5 mgf/d)
before testing.
6.6 Air Supply, capable of providing 415 kPa (60 psi) to the pneumatic clamps.
7. Sampling, Test Specimens, and Test Units
7.1 Lot Sample—As a lot sample for acceptance testing, take a random number of shipping units directed in an applicable material
specification or other agreement between the purchaser and the supplier, such as an agreement to use Practice D2258. Consider
shipping cases or other shipping units to be the primary sampling units.
NOTE 2—An adequate specification or other agreement between the purchaser and the supplier requires taking into account the variability between
shipping units, between packages or ends within a shipping unit, and between specimens from a single package to provide a sampling with a meaningful
producer’s risk, consumer’s risk, acceptable quality level and limiting quality level.
7.2 Laboratory Sample—As a laboratory sample for acceptance testing, take at random from each shipping unit in the lot sample
the number of packages directed in an applicable material specification or other agreement between the purchaser and the supplier,
such as an agreement to use Practice D2258. Preferably, take the same number of packages from each of the shipping units
selected. If differing numbers of packages are to be taken from the shipping units, determine at random which shipping units are
to have each number of packages for testing.
7.3 Test Specimens—From each package or end in the laboratory sample, take six specimens as directed in 7.3.1.
7.3.1 Remove the outer layer of yarn from the package. Avoid any damaged areas in selecting segments for testing. Carefully
unwind yarn from the package with as low as tension as possible to avoid stretching. As test specimens, cut approximately 125
mm (5 in.) long segments of yarn from each package, taking them at intervals of at least 1 m (1 yd). Three of the six specimens
are used as spare to allow for unacceptable breaks, such as caused by slippage or breaking in the clamps.
7.4 Determine the tex (denier) of the yarn for each laboratory sample using Test Method D2591 or Test Method D6717.
7.5 Determine the elongation at first filament bread (FFB) for each laboratory sample as directed in Test Method D2653.
8. Preparation of Apparatus and Calibration
8.1 Prepare and verify the calibration of the tensile testing machine as directed in the manufacturer’s instructions.
8.2 Set up and adjust the CRE-type tensile testing machine as follows:
D2731 − 21
8.2.1 Examine the acrylic clamp jaw face for wear and replace as needed. Position the jaw faces horizontally in the clamps.
8.2.2 Set the distance between the jaw faces (gage(gauge length) to 50 6 1 mm (2 6 0.05 in.).
NOTE 3—A convenient technique for checking the gagegauge length is to place a piece of carbon paper and white paper in the clamps and close the clamps.
The distance between the marks made on the white paper by the carbon paper represents the set gagegauge length.
8.2.3 Use a force measuring system such that the cycling force will be between 30 and 80 % of full scale capacity.
8.2.4 Set the Cycling Limits as follows:
8.2.4.1 Set Minimum Extension to nominal gagegauge length (zero extension).
8.2.4.2 Set Maximum Extension equivalent to 75 % of the elongation at first filament break of the elastomeric yarn (see 10.2) or
300 % of gauge length.
8.2.5 Set the crosshead speed to 500 mm/min (20 in./min) or 1000 % extension per min.
8.2.6 Set the extension measuring system as follows:
8.2.6.1 When using a chart recorder, set the chart speed to 500 mm/min (20 in./min).
8.2.6.2 When using an interfaced computer or microprocessor, set parameters to obtain selected properties using supplier’s
directions and Specification D76.
8.2.7 Set the air pressure for pneumatic clamps to 415 kPa (60 psi). At this pressure, the clamping force is approximately 450 N
(100 lb).
9. Conditioning
9.1 Condition the specimens relaxed on specimen boards in th
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