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D6479-15 – Standard Test Method Technical Guide
ASTM D6479 – 15 (Reapproved 2020) defines the standard test method for evaluating the edgecomb resistance of woven fabrics used in inflatable restraints. This property is critical for airbag material design, as it quantifies the force required to cause rupture at a perforated cut edge in the absence of a sewn seam.
📐 Test Method Overview and Significance
The core principle of this test method involves clamping a conditioned fabric specimen in a Constant Rate of Extension (CRE) tensile testing machine. One end is secured in standard jaws, while a specialized fixture pierces the opposite end with a row of equally spaced needle holes. In accordance with Test Method D5035, a tensile force is applied until rupture occurs, with the peak force recorded as the measurement of edgecomb resistance.
This measured value indicates the relative tendency of a fabric to pull apart under seam stress or similar action, making it highly useful for material design evaluations in applications like airbags. A key distinction of this method is that it isolates fabric performance by eliminating the effects of a particular stitch or thread, focusing purely on the inherent structural integrity of the material at the edge of a cut part.
⚙️ Test Procedure and Key Parameters
The test must be conducted under standard atmospheric conditions for textiles, as established in Practice D1776. The specimen is carefully mounted to ensure uniform tension and needle penetration. The crosshead speed and gauge length adhere rigorously to the strip test method (D5035).
| 🟦 Parameter | ⚡ Specification / Reference |
|---|---|
| 📏 Testing Machine | CRE (Constant Rate of Extension) per D5035 |
| 🎯 Rate of Extension | 300 ± 10 mm/min (per D5035) |
| 📐 Gauge Length | 75 ± 1 mm (per D5035) |
| 🟦 Specimen Width | Per D5035 (50.0 mm commonly used) |
| ⚡ Fixture Type | Special fixture with row of equally spaced needles |
| 🌡️ Conditioning Atmosphere | Standard practice for textiles (D1776) |
⚠️ Critical Maintenance Note: The integrity of the needle fixture is paramount. Dull, bent, or unevenly spaced needles can significantly skew the edgecomb resistance data, leading to poor correlation with actual material performance in deployment.
📊 Key Measured Properties and Test Comparisons
The primary result reported is the Edgecomb Resistance, measured in Newtons (N) or pounds-force (lbf). This represents the maximum force observed during the tensile pull until the specimen ruptures at the perforation line. Understanding how this test differs from related seam evaluations is vital for material selection.
| 📏 Feature | 🎯 D6479 Edgecomb Resistance | 📐 Seam Slippage / Seam Strength |
|---|---|---|
| 🔍 Test Focus | Inherent edge tearing resistance | Integrity of a sewn seam |
| ❌ Seam Presence | No sewn seam (Absent by design) | Yes (Sewn seam is the test subject) |
| 📏 Fixture Method | Row of needles piercing the fabric | Standard clamping with stitching |
| 🎯 Measured Value | Force required to rupture the fabric edge | Force at yarn slippage or seam rupture |
The standard specifically notes that the absence of a sewn seam in this test method eliminates the effect a particular stitch type might have, allowing for a pure measurement of the fabric’s yarn slippage resistance near a cut edge.
💡 Practical Application Tip: As allowed by Section 1.2, alternative procedures or apparatus may be used by specific agreement between the purchaser and the supplier. Any deviations from the standard practice must be clearly documented in the final test report.
❓ Frequently Asked Questions
🔍 What does the Edgecomb Resistance test measure exactly?
It measures the peak force required to rupture a woven fabric specimen that has been pierced by a row of equally spaced needles. This simulates stress at the edge of a cut fabric part, providing a quantitative value for the material’s resistance to edge tearing without the influence of a sewn seam.
💡 How is this different from standard sewn seam testing?
Tests like D5822 evaluate the strength of a sewn seam or yarn slippage at stitches. Edgecomb resistance (D6479) deliberately removes the sewn seam from the evaluation. It focuses solely on the fabric’s innate behavior when subjected to stress near a cut edge, isolating the material property from the sewing process variables.
⚡ What type of tensile testing machine is specified?
The standard requires a Constant Rate of Extension (CRE) tensile testing machine, consistent with Test Method D5035. The machine must accurately record the peak breaking force as the specimen is pulled at a constant rate until rupture occurs at the needle perforation line.
📌 Can modifications to the standard test procedure be used?
Yes. Section 1.2 of the standard explicitly allows for the use of alternative procedures and apparatus provided there is a specific agreement between the purchaser and the supplier. All deviations from the standard practice must be clearly acknowledged in the final report.