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D2731-21 – Standard Test Method Technical Guide
ASTM D2731-21 provides a definitive methodology for determining the elastic properties of “as produced” elastomeric yarns, including rubber, spandex, and other elastomers. Operating specifically with Constant-Rate-of-Extension (CRE) tensile testing machines, this standard captures critical hysteresis-related data points such as force at specific elongations, stress decay, and permanent deformation.
📐 Scope and Specimen Requirements
This test method is strictly applicable to elastomeric yarns within a linear density range of 40 to 3200 dtex (36 to 2900 denier). It does not apply to covered, wrapped, or core-spun yarns, nor to yarns spun from elastomeric staple. For a method specifically designed for testing rubber threads, refer to Test Method D2433.
| 🟦 Property | 📏 Specification / Range |
|---|---|
| Yarn Types | As-produced rubber, spandex, and other elastomers |
| Linear Density | 40 – 3200 dtex (36 – 2900 denier) |
| Excluded Types | Covered, wrapped, core-spun, elastomeric staple spun |
⚙️ Test Procedure and Machine Setup
The procedure requires a CRE-type tensile testing machine conforming to Specification D76. Specimens must be conditioned according to Practice D1776 and sampled according to Practice D2258. The core of the test involves a specific cyclic loading regimen:
- Mount the specimen in the CRE machine and initially subject it to five loading/unloading cycles between zero elongation 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 seconds to measure stress decay.
- Following the hold, the specimen is unloaded and allowed to return to its original gauge length before being subjected to a sixth loading/unloading cycle to determine permanent deformation.
💡 Technical Tip for Operators: The use of a CRE machine is mandatory, not a CRT. Accurately determining the “75% of FFB” endpoint for the cyclic loading is critical for standardized hysteresis measurement without inducing premature catastrophic failure during the preconditioning cycles.
📊 Key Measured Properties and Calculations
Data from the cyclic testing is utilized to calculate several key elastic properties. Force at specified elongations is derived from the curves of the 1st and 5th loadings, as well as the 5th unloading. Stress decay is calculated specifically from the force drop during the 30-second hold on the 5th cycle. Permanent deformation is calculated based on the extension observed at a specified force during the 6th loading cycle.
| 🎯 Property | ⚡ Measurement Point / Calculation |
|---|---|
| Force at Specified Elongation | 1st Loading, 5th Loading, & 5th Unloading curves |
| Stress Decay | Force drop during the 30s hold at max extension (5th cycle) |
| Permanent Deformation | Extension at specified force during the 6th loading cycle |
⚠️ Important Note on Conditioning and Terminology: All yarn specimens must be conditioned in accordance with Practice D1776 prior to testing. For precise definitions of all related deformation, stress, and yarn terms, users should refer to Terminology D4848 and D4849.
❓ Frequently Asked Questions
🔍 What types of yarns are excluded from this test method?
This method explicitly excludes covered, wrapped, or core-spun yarns, as well as yarns spun from elastomeric staple. It is designed specifically for “as produced” continuous filament elastomeric yarns like rubber and spandex.
💡 Which tensile testing machine type is required for D2731-21?
The title explicitly specifies Constant-Rate-of-Extension (CRE) type tensile testing machines conforming to ASTM D76. Constant-rate-of-traverse (CRT) machines are not applicable to this specific standard.
⚡ What is the specific loading/unloading cycle?
The specimen undergoes five cycles between 0 and 75% of the elongation at First Filament Break (FFB). On the 5th cycle, the specimen is held at the maximum extension for 30 seconds to measure stress decay. A 6th cycle is then performed to measure permanent deformation.
📌 What is the linear density (denier/dtex) range for this test method?
The test method is applicable to elastomeric yarns having a range of 40 to 3200 dtex, which corresponds to 36 to 2900 denier.