Elongation is the percentage increase in original length (strain) of a rubber specimen as a result of tensile force (stress) being applied to the specimen. Elongation is inversely proportional to hardness, tensile strength, and modulus. That is, the greater a material’s hardness, tensile strength, and modulus, the less it will elongate under stress. It takes more force to stretch a hard material having high tensile strength and high modulus than to stretch a soft material with low tensile strength and low modulus.

Ultimate elongation is the elongation at the moment the specimen breaks. Per ASTM D 412, ultimate elongation is generally noted along with tensile strength and modulus during tensile testing. Some elastomeric materials are much more forgiving in this area than others. Natural rubber can often stretch up to 700% before breaking. Fluorocarbons typically rupture at about 300%. Keep in mind that these figures highlight relative failure modes only and are not acceptable seal installation values.

Overstretching can doom an O-ring, so elongation is an important installation factor, especially as gland and seal dimensions decrease. What might be a small percentage increase in a larger seal can be a large increase in a smaller seal. For example, an OR-150 seal has a nominal inside diameter (I.D.) of 2 7/8". If stretched 1/16", it is elongated roughly 2%. On the other hand, an OR-102 seal has a 1/16" nominal I.D. If stretched 1/16", it is elongated 100%. Since a given amount of elongation can mean vastly different things, elongation is truly relative to a seal’s initial size. Though elongation is seldom a problem, installing small diameter, high durometer, and low elongation seals can be problematic in some instances.

PHYSICAL PROPERTIES MAIN PAGE