When a constant strain is imposed on rubber, the force necessary to maintain that strain is not constant but decreases with time; this phenomenon is called force decay (stress relaxation). Conversely, when rubber is subjected to a constant stress, an increase in the deformation takes place in time; this behavior is called creep. These phenomena are of practical significance in rubber articles, such as seals and gaskets.

The processes responsible for force decay may be either physical or chemical in nature, and under all normal conditions both processes will occur simultaneously. However, at ambient or low temperatures and/or short times, force decay is dominated by physical processes, while at elevated temperatures and/or long times, chemical processes are dominant. Hence, it is neither safe to extrapolate time/force decay curves in order to predict force decay after periods considerably longer than those covered by the test, nor to use tests at higher temperatures as accelerated tests to give information on force decay at lower temperatures.

In addition to the need to specify the temperature and time-intervals in a force decay test, it is also necessary to specify the initial stress and the previous mechanical history of the test specimen, since these may also influence the measured force decay, particularly in rubbers containing fillers.

1.1 This standard specifies two methods for determining the decrease in counterforce exerted by a test specimen of vulcanized rubber or thermoplastic elastomer which has been compressed at a constant deformation under specified conditions of time and temperature.

1.2 This document was developed based on testing in air and liquids.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.