O-rings are elastomer rings with a round section, they are used as mechanic anti-leakage or sealing devices. O-rings are designed to be inserted into specific housings and be compressed during the assembly of two or more parts, in so doing they work as sealing gaskets.

Their sealing can be static, when both the o-rings and the parts by which the o-ring is compressed do not move or dynamic when they do.
O-rings are quite common sealing devices; their employment is widespread as they are not expensive and they resist to tens of MPa pressure.

Their being organic elastomers (quite simply: rubber) and therefore organic derivatives, limit the time span over which they retain their characteristics ( elasticity, structural integrity ). Their resistance over time, which is nonetheless remarkable, is significantly reduced by high working temperatures, extremely low working temperatures or ionizing radiations due to the nature itself of elastomers.

The failure of a seal joint (called field-joint ) made by an O-ring lead to the disastrous accident of the Space Shuttle Challenger STS-51-L on the 28th of January 1986. The working temperature was different from that calculated in the project. This hypothesis was confirmed by the physics Richard Feynman of Caltech; he demonstrated that by placing a small o-ring in icy water and showing that it lost flexibility.

The material of the o-ring was based on a FKM copolymer compound, generally known as Viton by the producer Morton-Thiokol based in Utah. Standard FKM, such as the one used in this case, is not suitable for applications at very low working temperatures.

In a cold environment the o-ring may reach a threshold temperature, the Tg temperature ( glass transition temperature), below which the material looses elasticity. Even when the o-ring does not reach the glass transition temperature, at a low temperature, it takes more time to regain its initial shape after being compressed.

O-rings (as any sealing device) create a positive pressure against the surface in order to prevent leakage.