Fig. 1.

(a) Creep curves for a second-order (L=2) Voigt model and a step stress stimulus are illustrated. Curve a is drawn directly from Eq. (8) with finite η₀; its slope at t ≫ T₂ is σ_a/η₀. Curve b is from the same equation where η₀=∞. In both cases, ϵ₂/ϵ₁ = 2.5, T₁=3 s, and T₂=100 s. (b) The corresponding Fourier spectra D̆(ω) are from Eq. (10). Spectra from a step and 1 s ramp stress stimulus are compared.