Stress relaxation of a compound occurs when a constant stress creates physical and chemical changes, thus reducing the force that the compound exerts over a specified period of time.
 
The first observed effects generally include the relaxation of polymer chains and fillers.  Because the changes that occur are generally chemical reactions, the effects tend to be long term and irreversible.
Accelerated aging can also be added to a stress relaxation test using a forced air oven and a wide variety of fluids that the material would normally come in contact with in service to better simulate real life conditions.  

Common Standards

  • ASTM D6147—Standard Test Method for Vulcanized Rubber and Thermoplastic Elastomer—Determination of Force Decay (Stress Relaxation) in Compression
  • ISO 3384—Rubber, vulcanized or thermoplastic -- Determination of stress relaxation in compression -- Part 1: Testing at constant temperature