Relaxation modulus

Summary

The relaxation modulus $G(t)$ of a linear viscoelastic material is its stress at time $t$ after a unit strain step at time $0$. It typically decays in time, from an instantaneous modulus $G(0^+)$ to an equilibrium modulus $G(\infty)$, which is zero for a fluid-like material. By the Boltzmann superposition principle, this one function determines the response to every strain history. Its companion is the creep compliance $J(t)$, the strain produced by a unit stress step.