How does a stress-relaxation test measure the behavior of a viscoelastic material?

A viscoelastic material is characterized by its ability to yield continuously under stress while retaining the capacity to return to its original shape once that stress is removed. To measure this specific behavior, a stress-relaxation test is employed.1

The test functions through the following process:

• Induction: A prescribed displacement or constant strain is applied to the material specimen.1
• Observation: Because the material is viscoelastic, the internal stress required to maintain that constant deformation will decrease over time.1
• Measurement: The load drop-off is measured as a function of time.1

By tracking how the stress relaxes while the strain remains fixed, researchers can quantify the material's time-dependent response. Furthermore, various viscoelastic theories allow this test data to be translated into predictions regarding the material's creep behavior, which is the complementary measurement of deformation under a constant load.1