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D6147-97 – Standard Test Method Technical Guide
ASTM D6147-97 (Reapproved 2020) defines rigorous test methods for determining the decrease in counterforce (force decay or stress relaxation) exerted by a test specimen of vulcanized rubber or thermoplastic elastomer when compressed at a constant deformation. This property is essential for predicting the long-term sealing performance and material durability in applications such as gaskets, O-rings, and cushioning components under specified conditions of time, temperature, and environment.
📐 Understanding Test Methods A and B
The standard establishes two distinct procedures to evaluate force decay, which are selected based on the intended service conditions of the material. The physical history of the specimen differs significantly between the two protocols.
- Method A (Isothermal Relaxation): The test specimen is compressed at the designated test temperature and maintained at that temperature for the entire test duration. All counterforce measurements are taken directly at this elevated temperature.
- Method B (Cycled Temperature Relaxation): The initial compression and force measurement are conducted at 23 ± 2°C (73 ± 4°F). The specimen is stored in a controlled chamber at the test temperature but is removed and allowed to equilibrate to 23 ± 2°C for each subsequent force measurement.
The selection of the method depends on the application. For fundamental studies and applications where sealing at elevated temperatures is a problem, Method A is preferred. In applications where temperature cycling from normal to an elevated temperature is a problem, Method B is preferred.
| 🟦 Parameter | 📏 Method A (Isothermal) | 📐 Method B (Cycled) |
|---|---|---|
| 🎯 Compression Temp | At test temperature | 23 ± 2°C (73 ± 4°F) |
| ⚡ Measurement Temp | At test temperature | 23 ± 2°C (73 ± 4°F) |
| 📌 Primary Use Case | High-temp sealing, fundamentals | Temperature cycling environments |
⚙️ Terminology and Referenced Documents
Definitions of Terms Specific to This Standard:
- Compressive Stress: The time-dependent force necessary to maintain a constant compressive strain, divided by the original cross-sectional area over which the force is applied.
- Force Decay (Stress Relaxation): The decrease in stress which has occurred after a specified time-interval, during application of a constant deformation, expressed as a percentage of the stress at the commencement of that time-interval.
Key Referenced Standards: The test method relies on several critical ASTM practices for consistency. These include D573 for oven aging techniques, D1349 which specifies standard conditions for testing, D3182 for preparing standard vulcanized sheets, D3767 for measuring specimen dimensions, D4483 for evaluating precision, and E145 which outlines specifications for gravity-convection and forced-ventilation ovens.
| 📘 Item | ✍️ Definition / Purpose |
|---|---|
| 📏 Compressive Stress | Time-dependent force / original cross-sectional area |
| 🎯 Force Decay (Relaxation) | Decrease in stress over time at constant strain (%) |
| ⚡ D1349 | Standard conditions for rubber testing (23 ± 2°C) |
| 📌 D3767 | Practice for measurement of dimensions |
📊 Practical Considerations and Data Integrity
This standard was developed based on testing in air and liquids. Users must note that the two methods, A and B, of carrying out the measurement do not give the same values of force decay. The standard explicitly warns that comparison of values obtained from the two methods must be avoided. Furthermore, it does not purport to address all safety concerns; the user is responsible for establishing appropriate safety, health, and environmental practices prior to use.
⚠️ Critical Restriction on Data Comparison: The standard explicitly states that direct comparison of numerical values obtained from Methods A and B must be avoided. The differing thermal histories—constant temperature in A versus cycling in B—produce unique relaxation profiles that are mathematically and physically incongruent.
💡 Guidance for Method Selection: Align the test method with the anticipated service conditions. For fundamental studies and continuous high-temperature sealing, choose Method A. For applications involving thermal cycling from ambient to an elevated temperature, choose Method B. The results are specific to the protocol and should not be interchanged.
❓ Frequently Asked Questions
🔍 Why does the standard include two different methods for force decay testing?
ASTM D6147-97 recognizes that elastomeric materials behave differently depending on their thermal exposure history. Method A (isothermal) evaluates stress relaxation purely at the test temperature, eliminating the effects of thermal cycling. Method B introduces a thermal cycle by measuring the force at ambient temperature after aging at high heat. Selecting the appropriate method ensures the test data correlates closely with the real-world thermal environment, whether continuous high heat or intermittent cycling.
💡 How is “Force Decay” technically defined in this standard?
The standard defines Force Decay as the decrease in stress which has occurred after a specified time-interval during the application of a constant deformation. It is expressed mathematically as a percentage of the stress at the commencement of that time-interval. The term “stress relaxation” is explicitly utilized as a synonym for force decay within this test method.
⚡ Why can’t the results from Method A and Method B be directly compared?
The physical mechanisms at play are fundamentally different. In Method A, all molecular relaxation and physical aging occur at the test temperature. In Method B, the specimen is allowed to cool to 23°C for measurements, which causes physical changes (such as thermal contraction) that perturb the stress state. This makes direct numerical correlation scientifically invalid, as confirmed by the standard’s explicit prohibition of such comparisons.
📌 What are the specific temperature requirements for Method B?
Method B requires that both the initial specimen compression and the subsequent force measurements be conducted at a standard laboratory temperature of 23 ± 2°C (73 ± 4°F). This condition is consistent with the recommendations found in Practice D1349, which governs standard conditions for rubber testing. The test specimen is compressed and measured only at this ambient temperature, while aging occurs at the elevated test temperature.