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. 
C_{A} = [(t_{o}  t_{i}) / t_{o}] * 100
where t_{o} is the original specimen thickness and t_{i} 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. 
C_{B} = [(t_{o}  t_{i}) / (t_{o}  t_{n})] * 100
where t_{o} is the original specimen thickness, t_{i} is the specimen thickness after testing, and t_{n} 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 
Similar Standards
