Kelvin-Voigt harmonic compliance ($J^{\ast }(\omega )=1/(E+i\omega \eta )$)

Under harmonic stress, strain amplitude is ${\sigma }_0/\sqrt{E^2+{\omega }^2{\eta }^2}$ and its lag is $\arctan (\omega \eta /E)$. Low frequencies are nearly elastic; high-frequency stress-controlled deformation is small and nearly a quarter cycle behind stress.