Fig 9. Schematic and illustration of the constitutive model.

A: Rheological representation of the viscoelastic model, where the material consists of parallel chains, and each chain consists of an elastic component (denoted by a spring) and a viscous component (denoted by a dashpot). Usually, the steady-state response of a viscoelastic solid is represented by a chain with no viscous component (i.e., τ = ∞). Here, in addition to one solid chain, models including one and two chains with viscous components are evaluated and denoted as one-term and two-term models. B: Illustration of how stimuli with a low strain-rate may lose information from low time-constant QLV chains. The solid line is the response of a typical two-term viscoelastic solid with time constants τ ₁ and τ ₂ under a step load. Two dashed lines represent the response of the material under slower strain-rates. For the slowest strain-rate, ${\dot{\epsilon }}_2$, stress relaxation properties of the faster chain (τ ₁) may not show up because its relaxation for the faster chain takes place within its ramp phase. Therefore, this will not be captured by curve fitting, which simply characterizes the material as a one-term QLV solid and only calculates the slower chain (with time constant τ ₂). Thus, we can eliminate the two extra parameters (G ₁, τ ₁) if we only care about low strain-rate situations. In other words, for low strain-rate cases, the single term model is sufficient and therefore more appropriate than the two-term models because of the reduced number of free parameters.