Meaning
A time-dependent deformation occurs when a polymeric material is subjected to a constant mechanical load over an extended period. In structural applications, polymer viscoelastic creep causes plastic parts to slowly deform even when the applied stresses are well below the material’s yield strength. This behavior arises from the gradual uncoiling and sliding of long polymer chains.
Designers must account for this when specifying load-bearing parts.
Stress Relaxation
When a constant strain is applied instead of a constant load, the material undergoes stress relaxation, where the force required to maintain that deformation decreases over time. Under constant stress, the strain continues to increase, progressing through primary, secondary, and tertiary stages. This continuous movement can lead to the eventual failure of the part.
Reinforcing fibers are often added to suppress this deformation.
Temperature Influence
Higher operating temperatures accelerate the rate of chain movement and increase the creep rate. Near the glass transition temperature, the material’s resistance to deformation drops sharply. This limits the safe temperature range for loaded parts.
Design Limit
Engineers use long-term creep modulus values rather than short-term tensile modulus values from datasheets to design plastic parts. This ensures the part remains functional over its intended service life. Failure to do so leads to premature structural collapse.
