A standard linear solid model has a finite instantaneous stiffness, a nonzero long-time stiffness and one exponential stress-relaxation mode. A Kelvin-Voigt model with spring and viscosity in series with spring gives
These expressions follow by adding series strains under common stress. Its storage modulus is and its loss modulus is for . Its relaxation modulus is , while its creep compliance is . Thus creep retardation and stress-relaxation times differ. This solid has bounded creep and cannot represent terminal fluid flow.
A spring in series with a parallel spring-dashpot branch has creep compliance . It includes immediate and delayed recoverable strain, but no unbounded viscous flow. Large strain, damage or anisotropy require additional constitutive structure.
Apply constant stress at and remove it at . For series spring and Voigt-branch creep, the series spring recovers its strain immediately on unloading. The Kelvin-Voigt model branch has retardation time and must keep continuous strain because an instantaneous dashpot displacement would require an impulse. Its post-unloading equation is . Solving with the branch's strain at gives the title formula. Thus all remaining strain eventually recovers: this standard linear solid model has no permanent set.
For a standard linear solid model with , put . Its storage modulus is and its loss modulus is . The latter peaks at , whereas its loss tangent is
Differentiation puts the loss-tangent peak at , with height . A loss-modulus peak and a loss-tangent peak therefore need not occur at the same frequency. For weak relaxation, is close to one and their frequencies are approximately equal.

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The Standard Linear Solid (SLS) model is a mathematical model used to describe the viscoelastic behavior of materials. It is particularly effective in capturing the time-dependent strain response of materials that exhibit both elastic (instantaneous) and viscous (time-dependent) behaviors when subjected to stress. ### Components of the SLS Model The SLS model combines two main elements: 1. **Spring Element (Elastic Component):** This represents the elastic behavior of the material.