ASTM D395B Compression Set Testing

Compression Set Testing Under Constant Deflection by ASTM D395 method B measuring the capacity of an elastomeric material to get back to its original thickness or shape after compression stress is applied at a certain temperature. This test helps in identifying the best elastomer for industrial use.

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    Compression Set Testing by ASTM D395 method B is the process of measuring the capacity of an elastomeric material to get back to its original thickness or shape after compression stress is applied at a certain temperature. Elastomers lose the ability to return to their original thickness when compressed for a prolonged period. Results obtained from compression set testing by ASTM D395 are critical for determining and producing the best elastomeric materials for industrial application. As an example, a compression set is essential for sealing operations.

    Test Procedure:

    ASTM D395 test specimen with known thickness is deformed (compressed) up to 25% of its size using a compressing machine for a given time. Spacers are used while compressing for precise measurements, which have their own thickness. A recovery time of 30 minutes is given before the sample is measured again. The value of percentage decrease thus derived indicates the percentage a material sample fails to recover of its original size.

    Specimen size:

    Cylindrical disc specimen sizes categorized into Type 1 & Type 2

    In type 1, thickness is 12.5 mm +/- 0.5 mm and diameter is 29.0 mm +/- 0.5 mm

    In type 2, thickness is 6.0 mm +/- 0.2 mm and diameter is 13.0 mm +/- 0.2 mm


    The compression set is stated as a percentage of the original deflection.

    CB = [(To-Ti)/(To-Tn)] x100


    CB = Compression Set

    To = Original thickness of the sample

    Ti = Final thickness of the sampleTn = Thickness of the spacers used


    ASTM D395 method B is a standard test method for rubber compression set under constant deflection in air.


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