Failure due to compression set occurs in both static and dynamic seals. Compression set failure looks similar to abrasion failure, in which there is a flattening on one side of the O-ring’s cross-section. With compression set, this permanent flattening of the cross-section can be seen on both sides of the O-ring at the original points of compression. The seal usually hardens and assumes the shape of the gland. In some instances, surface cracks may also be visible.

At the most basic level, use of an elastomer with inherently poor compression set properties or a compound that has not been properly cured can doom a seal. Even if materials are initially acceptable, the aforementioned O-ring hardening (and an accompanying loss of elasticity) can be caused by temperature increases during service. These thermal increases can be frictional (i.e. as a result of the buildup of friction-generated heat), environmental (as a result of external elements), and/or systemic (as a result of system fluids).

A limited degree of fluid-induced volume swell may help offset the effects of compression set. On the other hand, some fluids may cause excessive swell, which can exacerbate compression set. The excessive squeeze generated by improperly designed glands or adjustable glands that are over-tightened can also be problematic. Figure 129 is an illustration of O-ring failure due to compression set.

Materials offering improved compression set properties include peroxide-cured nitrile, peroxide-cured EPDM, hydrogenated nitrile, ethylene acrylic (Vamac®), fluorocarbon (Viton®), fluorosilicone, silicone, and natural rubber.