Meaning
Mechanical behaviors describe the time-dependent decrease in the internal stress of a polymer or elastomer kept under a constant strain. Viscoelastic stress relaxation occurs because the material’s molecular chains gradually rearrange themselves over time to relieve the applied force. This decay in force is a critical consideration for seal and gasket materials, which must maintain a minimum sealing force to prevent leaks.
Material Deformation
Polymeric seals are compressed during installation to create a tight joint between two metal flanges. Under viscoelastic stress relaxation, the force exerted by the seal against the flanges decreases even though the compression remains constant. This gradual loss of force is faster at higher temperatures, which increase molecular mobility.
Force Decay
Seal performance will degrade if the internal stress drops below the pressure of the fluid being contained. Engineers must select seal materials with low relaxation rates to ensure long-term performance. This selection process relies on long-term test data to predict the seal’s behavior over its intended service life.
Operational Life
Maintenance schedules must be planned to replace compressed seals before the sealing force drops to a critical level. If the seals are not replaced, the joint will eventually begin to leak, leading to equipment downtime or environmental hazards. Accurate prediction of the relaxation rate of the material prevents these unexpected failures.
