Stress relaxation is an engineering term used to describe the reduction in internal stress that occurs over time. When an object is deformed, stress will develop inside the material. If the material is elastic and the stress is below the elastic limit, the material will return to its original shape when the object is once again freed.
This behavior is commonly observed when displacements are held for a short period of time. Unlike elastic behavior, stress relaxation occurs over time and is characterized as viscoelastic behavior. When a viscoelastic material is displaced, stress will develop initially in the material that pushes against the displacing object. Over time, molecular rearrangements occur that reduce the stress within the material. Given enough time, the stress could reduce to zero and the deformed configuration is the new natural state. When the displacing object is removed, no elastic spring back occurs because the material is no longer stressed.
The rate of stress relaxation is different for different materials. Spring steel at room temperature has negligible stress relaxation. Thus, metal springs are often used in applications that require a spring to remain in a displaced or loaded condition for long periods of time. On the other hand, because plastics tend to have significant stress relaxation even at room temperature, it is not good practice to use a plastic spring in applications that requires continuous loading or displacement. Biological materials are also viscoelastic and have similar limitations (or benefits, depending on the perspective).
Stress relaxation is a quantifiable material property. Once determined, it may be used in engineering design. Because stress relaxation is caused by molecular rearrangements, the rate of stress relaxation depends on temperature. Higher temperatures generally increase the rate of relaxation. At elevated temperatures, stress relaxation can occur in materials that are essentially immune to its effect at room temperature.
