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Compression Set - ISO 815

The permanent deformation remaining after release of a compressive stress. Compression set is expressed as the percentage of the original deflection. Compression set is an important property for elastomers and cushioning materials.

The test specimen is compressed for a specified time at a specified temperature. Compression set is calculated as the percentage of the original deflection after the material is allowed to recover at standard conditions for 30 minutes. Compression set is calculated using the formula:
C = [(ho - hi) / (ho - hn)] * 100
where ho is the original specimen thickness, hi is the specimen thickness after testing, and hn is the spacer thickness.

Compression Set Testing

Three different strains may be used for testing. The strain used depends upon the hardness of the material. IRHD stands for International Rubber Hardness Degrees (see ISO 48).

Compression Strains
% Strain Usage
25 Preferred
15 80-89 IRHD
10 90-95 IRHD

Two different specimens may be used for testing. Test specimens are cylindrical disks cut from a slab of the testing material.

Compression Set Specimens
Specimen Type Diameter (mm) Thickness (mm)
A 29.0 12.5
B 13.0 6.3

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