Elastic materials having the capacity to dissipate the mechanical energy due to viscous effects are characterized as viscoelastic materials.
For multiaxial stress state, the constitutive relation can be written as:
where: e(bar) and φ are the deviatoric and volumetric strain, G(t  τ) and K(t  τ) are shear and bulk relaxation functions.
The relaxation functions can then be represented by the mechanical model which is usually referred to as a Generalized Maxwell Model having the expressions:
where: G_{0} = E / 2(1+ ν), initial shear modulus (t=0)
and K_{0}= E / 3(1 2ν), initial bulk modulus (t=0)
g_{i}, k_{i}, τ_{i}^{G}, and τ_{i}^{K} are the ith shear and bulk moduli and corresponding times.
The effect of temperature on the material behavior is introduced through the timetemperature correspondence principle. The mathematical form of the principle is:
where γt is the reduced time and γ is the shift function. The WLF (WilliamsLandelFerry) equation is used to approximate the function:
where T_{0} is the reference temperature which is usually picked as the Glass transition temperature; C1 and C2 are material dependent constants.
Parameter 
Material Property 
Linear Elastic Parameters 
Elastic modulus
in X 
Poisson's ratio
in XY 
Shear modulus
in XY 
Relaxation Function Parameters 
Shear relaxation modulus (1 to 8) (represent g1, g2, ...,g8 in the Generalized Maxwell Model equations)

Time values (Shear relaxation modulus 1 to 8) (represent τ_{1}^{g}, τ_{2}^{g},..., τ_{8}^{g} in the Generalized Maxwell Model equations) 
Bulk Relaxation Modulus (1 to 8) 
Time values (Bulk relaxation modulus 1 to 8) (represent τ_{1}^{k}, τ_{2}^{k},..., τ_{8}^{k} in the Generalized Maxwell Model equations) 
WLF Equation Parameters

Glassy Transition Temperature (represents T_{0} in the WLF equation)

First Constant for WilliamsLandelFerry equation (represents C_{1} in the WLF equation)

Second Constant for WilliamsLandelFerry equation (represents C_{2} in the WLF equation)

When defining a shear or bulk relaxation curve under the Tables & Curves tab, the first point of the curve is the G_{1} or K_{1} moduli at time t_{1}. At time t = 0, the program automatically computes G_{0} or K_{0} from the Elastic modulus and Poisson's ratio.
The viscoelastic material model can be used with the draft and high quality solid and thick shell elements.
Time has real values in nonlinear analysis when you use the viscoelastic material model.