ASTM D395B Compression Set Testing

Scope:

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

Data:

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

C_B = [(To-T_i)/(T_o-T_n)] x100

Where,

C_B = Compression Set

T_o = Original thickness of the sample

T_i = Final thickness of the sampleT_n = Thickness of the spacers used

Conclusion:

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