low volumes. The physical sealing performance of square rings in static applications is superior to that of O-rings, however in dynamic applications it is inferior to that of O-rings. Square rings are usually used only in dynamic applications as energizers in cap seal assemblies. Square rings can also be more difficult to install than O-rings. Similar devices with a non-round cross-sections are called seals, packings or gaskets. See also washers. Automotive cylinder heads are typically sealed by flat gaskets faced with copper. Knife edges pressed into copper gaskets are used for high vacuum. Elastomers or soft metals that solidify in place are used as seals. Failure modes O-ring materials may be subjected to high or low temperatures, chemical attack, vibration, abrasion, and movement. Elastomers are selected according to the situation. There are O-ring materials which can tolerate temperatures as low as or as high as . At the low end, nearly all engineering materials become rigid and fail to seal; at the high end, the materials often burn or decompose. Chemical attack can degrade the material, start brittle cracks or cause it to swell. For example, NBR seals can crack when exposed to ozone gas at very low concentrations, unless protected. Swelling by contact with a low viscosity fluid causes an increase in dimensions, and also lowers the tensile strength of the rubber. Other failures can be caused by using the wrong size of ring for a specific recess, which may cause extrusion of the rubber. Elastomers are sensitive to ionizing radiation. In typical applications, O-rings are well-protected from less-penetrating radiation such as ultraviolet and soft X-rays, but more-penetrating radiation such as neutrons may cause rapid deterioration. In such environments, soft metal seals are used. There are a few common reasons for O-ring failure: Installation damageThis is caused by improper installation of the O-ring. Spiral failureFound on long-stroke piston seals and – to a lesser degree – on rod seals. The seal gets “hung up” at one point on its diameter (against the cylinder wall) and slides and rolls at the same time. This twists the O-ring as the sealed device is cycled and finally causes a series of deep spiral cuts (typically at a 45 degree angle) on the surface of the seal. Explosive decompressionAn O-ring embolism, also called gas expansion rupture, occurs when high pressure gas becomes trapped inside the elastomeric seal element. This expansion causes blisters and