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

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 important 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 duration of the test, with all force measurements taking place at that temperature. 

• Method B 

At 23± 2°C, the test specimen is compressed and the initial counterforce measurement is taken. The test specimen is then stored in a chamber that is 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, do not produce the same results, hence comparing the results from the two approaches should be avoided. The method chosen between  A and B is dependent on the test’s purpose. Method A may be favored for foundational investigations and applications where sealing at elevated temperatures is a concern, whereas method B may be preferred for applications where temperature cycling from normal to elevated temperatures is an issue.

Video 01: Determining the properties of rubber through stress relaxation tests

Specimen size:

A cylindrical disc of diameter 13.0 ± 0.5 mm and a thickness of 6.3± 0.3 mm is used as test specimen for the ASTM D6147 test procedure. 

Data:

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:

ASTM D6147 is a test method for determining the force decay (stress relaxation) in vulcanized rubber and thermoplastic elastomers in compression.