Compression Set Under Constant Deflection ASTM D395 B

Compression set testing is used to determine the ability of elastomeric materials to maintain elastic properties after prolonged compressive stress. The test measures the somewhat permanent deformation of the specimen after it has been exposed to compressive stress for a set time period. This test is particularly useful for applications in which elastomers would be in a constant pressure/release state.

Test Procedure:
The thickness of the original specimen is measured. The specimen is then placed between spacers and in the compression device. The specimen is compressed to 25% of its original height, using spacers to accurately measure the compression. Within two hours of assembly, the compression device is placed in an oven at a specified temperature for the suggested time periods of 22 hours and 70 hours. After removing the sample from the oven, the specimen is allowed to cool for 30 minutes before measuring the final thickness.

Specimen size:
There are 2 cylindrical disk specimen sizes:

Type 1Type 2
Thickness is 12.5mm +/- 0.5mmThickness is 6.0mm +/- 0.2mm
Diameter is 29.0mm +/- 0.5mm Diameter is 13.0mm +/- 0.2mm

Compression set is expressed as a percentage of the original deflection.

CB = [(to-ti)/(to-tn)] x100

CB = Compression set
to = Original thickness of the specimen
ti = Final thickness of the specimen
tn = thickness of the space bars used

**Please note that this test description is intentionally generic in nature and aimed at providing a descriptive summary to enhance test understanding.  Due to copyright restrictions, we are not able to provide copies of standards. Standards can be obtained from appropriate standards authorities.


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