Perhaps the best single gauge of a compound’s overall toughness and extrusion resistance, modulus is the force (stress) in pounds per square inch (psi) required to produce a certain elongation (strain). This elongation might be 50%, 100%, or even 300%, though 100% is the most widely used figure for testing and comparison purposes. Industry literature typically refers to 100% elongation as “M100” (or modulus 100). Compounds with a higher modulus are more resilient and more resistant to extrusion. Generally speaking, the harder a compound, the higher its modulus. Because it is basically a measure of tensile strength at a particular elongation (rather than at rupture), modulus is also known as tensile modulus or tensile stress.

As described in ASTM D 412, modulus is typically gauged simultaneously with tensile strength on the same dumbbell specimen shown in Figure 19. As the specimen is being stretched, the tester records the psi (for example, 836.7) needed to achieve a given elongation (for example, 100%). This figure in psi is considered to be the sample’s modulus at that elongation. Minimum modulus is typically used as a qualification criterion when specifying a new material. Keep in mind that the elastomeric lip of a shaft seal will never undergo 100%, or even 50%, elongation. Rather, elongation (actually, expansion) of the sealing lip as it is placed onto the shaft is typically no more than 5%. For this reason, it’s not generally possible to make a direct correlation between modulus data for a given material and actual performance of a sealing lip molded from that material.

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