Elongation is defined as the increase in length, expressed numerically, as a percent of initial length. It is generally reported as ultimate elongation, the increase over the original dimension at break.
This property primarily determines the stretch which can be tolerated during the installation of an O-ring. Elongation, like tensile strength, is used throughout the industry as a quality assurance measure on production batches of elastomer materials. Elongation increases in importance as the diameters of a gland become smaller. It is also a measure of the ability of a compound to recover from peak overload, or a force localized in one small area of a seal, when considered in conjunction with tensile strength. An adverse change in the elongation of a compound after exposure to a fluid is a definite sign of degradation of the material.