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SAE J855: Test Method for Stretch and Set of Textiles and Plastics
SAE J855, reaffirmed in 2022, provides a standardized procedure for evaluating the elastic and recovery properties of textiles and flexible plastics under a low-static load. This test method is essential for assessing material behavior in applications such as automotive seating, interior trim, and other formed components where stretch and set properties affect performance and appearance.
Test Overview and Scope
The test measures two key parameters: percent stretch (elastic extension under load) and percent set (permanent deformation after recovery). The method applies a 12.25 kg (27 lb) load to a conditioned specimen for five minutes, records the stretched length, then allows a five-minute recovery period and records the final length.
Specimens are cut in three directions to capture directional behavior:
- Machine direction (MD) – parallel to the material’s manufacturing direction
- Across machine direction (AMD) – perpendicular to MD
- Bias direction – at a 45° angle, including both directions where applicable
Each specimen is 76 mm x 229 mm (3 in x 9 in), and three specimens per direction are tested for statistical reliability.
🛠️ Key Apparatus: The test uses vice grip clamps and a combined clamp/weight assembly totaling exactly 12.25 kg. The clamp design allows quick, secure fastening without damaging the specimen.
Test Procedure and Calculations
The procedure follows a strict sequence to ensure repeatability:
- Condition specimens at 23°C ± 2°C and 50% ± 5% relative humidity for at least 24 hours prior to testing.
- Mark two parallel benchmark lines 76 mm (3 in) apart, centered lengthwise on each specimen. Record as L₁.
- Secure the specimen in the clamps and attach the weight gently to avoid shock loading.
- Suspend the assembly vertically for exactly 5 minutes, then measure the distance between bench marks under load to the nearest 0.40 mm (1/64 in). Record as L₂.
- Remove the weight and clamps, lay the specimen flat, and allow it to recover for 5 minutes.
- Measure the final distance between bench marks. Record as L₃.
Calculate the results using the following formulas:
| Parameter | Formula |
|---|---|
| Percent Stretch | %Stretch = ((L₂ – L₁) / L₁) × 100 |
| Percent Set | %Set = ((L₃ – L₁) / L₁) × 100 |
Report the average of three specimens for each direction (MD, AMD, and bias). These averages represent the material’s stretch and set behavior in that orientation.
⚠️ Important Considerations for Accurate Testing:
- Ensure specimens are fully conditioned to the specified temperature and humidity before testing – variations can significantly affect results.
- Apply the load slowly and steadily to prevent initial overshoot or damage to the specimen.
- Measure L₂ and L₃ at the same points marked by the bench marks, using a device capable of reading to 0.40 mm.
- Do not skip the bias direction test for materials with orientation-specific behavior; bias data can reveal asymmetric elastic properties.
Engineering Design Insights and FAQs
Understanding stretch and set is critical for materials used in applications that undergo repeated deformation. For example, automotive seat coverings must stretch during installation and return to shape without permanent distortion. The test method provides:
- Consistency through tightly controlled conditions (temperature, humidity, load mass, and time).
- Directional data that enables engineers to optimize material orientation in the product.
- Quantitative metrics for comparing materials or qualifying new suppliers.
Common mistakes in applying this method include inadequate conditioning, misreading the bench marks, and failing to average results from all three specimens. Adhering to the standard’s specifications eliminates these pitfalls and ensures reliable data for design decisions.
Why is conditioning at 23°C and 50% RH required?
Elastic properties of textiles and plastics are sensitive to temperature and humidity. Conditioning ensures that all specimens are tested under the same standard atmospheric conditions, making results comparable across labs and production batches.
What is the difference between stretch and set?
Stretch is the immediate elongation under load, expressed as a percentage of original length. Set is the permanent elongation remaining after the specimen has recovered for 5 minutes, also expressed as a percentage. Set indicates how much the material permanently deforms.
Why are three directions tested separately?
Textiles and plastics often have different elastic behavior in their machine, cross-machine, and bias orientations. Testing in all three directions captures the material’s anisotropy, which is crucial for products that experience multi-directional stresses.
Can I use a different weight or clamp?
The standard specifies a total weight of 12.25 kg and specifically preferred clamps. Any deviation must be agreed upon between contractual parties and should be noted in the report. For standard compliance, use exactly the specified apparatus.
For further information, refer to the full SAE J855 document (SAE International).
