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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.
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.
Specimen Type | Diameter (mm) | Thickness (mm) | Used for Method |
---|---|---|---|
1 | 29.0 | 12.9 | A and B |
2 | 13.0 | 6.0 | B |