MATERIAL THEORY
Creep and relaxation: two tests, different questions
Time-dependent response changes the meaning of a stiffness value.
Creep holds the load
For a constant uniaxial stress in linear viscoelasticity, strain evolves as ε(t) = J(t) σ₀, where J is creep compliance. A polymer support can therefore keep deforming even though its applied load does not change.
Relaxation holds the strain
Under a constant imposed strain, stress evolves as σ(t) = E(t) ε₀. A compressed polymer spacer may lose force with time. A common generalized-Maxwell relaxation function is E(t) = E∞ + Σ Eᵢ exp(−t/τᵢ), with positive relaxation times and physically admissible moduli.
Fit for the intended conditions
Use test durations, temperatures and strain levels relevant to service. Separate calibration data from validation data, inspect residuals and parameter sensitivity, and avoid extrapolating a short test to a long service life without supporting evidence.
Do not invert a curve point by point
In general, creep compliance is not simply the reciprocal of the relaxation modulus at each time. Their relationship involves the hereditary constitutive law. The assumed bulk and shear responses also matter when moving from a uniaxial fit to a three-dimensional model.