ASTM D6147 Determination of Force Decay in Vulcanized Rubber and Thermoplastic Elastomer in Compression

Introduction

The standard testing method, by ASTM D6147, provides a controlled assessment of the force decay, otherwise termed stress relaxation, in compressed rubber and thermoplastic elastomers. This evaluation is beneficial for those applications where all rubbers, such as seals and gaskets, are required to maintain a certain level of closure force over time while being subjected to varied temperatures. In evaluating the reduction of counterforce over time, for instance, ASTM D6147 aids manufacturers in ascertaining the suitability of the material to its long-term performance in elasticity and compression durability, thus sustaining the integrity of all products exposed to compression and environmental elements.

Scope

ASTM D6147 is a test method for determining the force decay (stress relaxation) in vulcanized rubber and thermoplastic elastomers in compression. The ASTM D6147 standard was designed based on testing in air and liquids, and this concept is essential in rubber products like seals and gaskets. This test defines two procedures (A and B) for determining a test specimen of vulcanized rubber or thermoplastic elastomer’s decrease in counterforce.

Test Procedure

According to ASTM D6147, there are two test procedures:

• Method A 

The test specimen is compressed at the test temperature and kept there for the test duration, with all force measurements taking place at that temperature. 

• Method B 

At 23± two °C, the test specimen is compressed, and the initial counterforce measurement is taken. The test specimen is then stored in a chamber kept at the test temperature, but it is removed from the chamber for each of the subsequent force measurements, which are also taken at 23± 2°C. 

The two ways of measuring force decay, A and B, produce different results; hence, comparing the results from the two approaches should be avoided. The method chosen between  A and B depends on the test’s purpose. Method A may be favored for foundational investigations and applications where sealing at elevated temperatures is a concern. In contrast, method B may be preferred for applications where temperature cycling from ordinary to elevated temperatures is an issue.

Specimen Size

The ASTM D6147 test procedure uses a cylindrical disc with a diameter of 13.0 ± 0.5 mm and a thickness of 6.3± 0.3 mm as the test specimen. 

Result

As per the ASTM D6147, the compression force decay (stress relaxation), R(t) after a specified  duration of the test, t, expressed as a percentage of the initial counterforce, is calculated by: 

Where:  

Fo = initial counterforce measured 30 min. after compression of the test specimen 

Ft = counterforce measured after the specified duration of the test

Conclusion

In compressed conditions, the stress relaxation quality evaluation of elastomers, especially rubber and thermoplastic elastomers, is carried out by ASTM D6147. Such Understanding is crucial because these materials are often used in compressed conditions. This test also helps the manufacturer estimate the material’s real-life behavior, especially in constant force applications, which occur frequently at higher temperatures, by looking at how the force is reduced over time. Performance data reported by ASTM D6147 results aids in making design choices in sectors where elastomers need to remain under compression, such as in automotive gaskets, industrial seals, and even consumer products, improving the durability of the products.

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