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Compression Set - ASTM D395 Plastic Test Standard

Compression Set - ASTM D395

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.

Compression Set Specimens
Specimen Type Diameter (mm) Thickness (mm) Used for Method
1 29.0 12.9 A and B
2 13.0 6.0 B

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