Compression set

The compression set (ASTM D395) of a material is the permanent deformation remaining when a force (that was applied to it) is removed. The term is normally applied to soft materials such as elastomers. There are two ways this is normally measured; compression set A and compression set B.^[1]

Compression Set A

This has the formal name compression set under constant force in air. In compression set A a force of 1.8 kN is applied to the specimen for a set time at a set temperature. Compression set A is defined as the percentage of original specimen thickness after the specimen has been left in normal conditions for 30 minutes (therefore C_A, the compression set A is given by 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).^[2]

A diagram of the compression set A experiment (under constant force in air).

Compression Set B

This has the formal name compression set under constant deflection in air. The specimen is compressed 25% of its original height for a set time and at a set temperature. Compression set B is (like Compression set A) defined as the percentage of original specimen thickness after it has been left in normal conditions for 30 minutes (therefore C_B, the compression set B is given by 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 or the specimen thickness during the test).^[3]

A diagram of the compression set B experiment (under constant deflection in air).

References

1. Compression Set - ASTM D395 Plastic Test Standard, http://www.ides.com/property_descriptions/ASTMD395.asp, 15 June 2012
2. Compression Set - ASTM D395 Plastic Test Standard, http://www.ides.com/property_descriptions/ASTMD395.asp, 15 June 2012
3. Compression Set - ASTM D395 Plastic Test Standard, http://www.ides.com/property_descriptions/ASTMD395.asp, 15 June 2012