Compression Set - ASTM D395

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The permanent deformation remaining after release of a compressive stress. Compression set is expressed as the percentage of the original specimen thickness for Method A (constant force) or the percentage of the original deflection for Method B (constant deflection). Compression set is an important property for elastomers and cushioning materials.

Compression Set Methods

Method	Name	Test Description	Calculation	Diagram
A	Compression Set Under Constant Force in Air	A constant compressive force of 1.8 kN (400 lbf) is applied to the specimen for a specified time at a specified temperature. Compression set is taken as the percentage of the original specimen thickness after the material is allowed to recover at standard conditions for 30 minutes.	CA = [(to - ti) / to] * 100 where to is the original specimen thickness and ti is the specimen thickness after testing.
B	Compression Set Under Constant Deflection in Air	Specimen is compressed 25% for a specified time at a specified temperature. Compression set is taken as the percentage of the original deflection after the material is allowed to recover at standard conditions for 30 minutes.	CB = [(to - ti) / (to - tn)] * 100 where to is the original specimen thickness, ti is the specimen thickness after testing, and tn is the spacer thickness.

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

Similar Standards

• ISO 815