Boltzmann superposition ($\sigma (t)={\int }_{-\infty }^{t}G(t-s)\dot{\gamma }(s)ds$)

In time-translation-invariant linear response, each infinitesimal past strain step contributes its amplitude times the elapsed-time relaxation modulus. Adding those causal contributions gives the displayed stress-history integral for shear. An equivalent formulation convolves the creep compliance with stress increments. This constitutive law assumes sufficiently small strains about a specified equilibrium or relaxed state; nonlinear deformation, aging and evolving material structure can invalidate that simple superposition.