Mechanical APDL Structural Analysis Guide

8.4.1.9. Viscoelasticity

Viscoelasticity is similar to creep, but part of the deformation is removed when the loading is taken off.
A common viscoelastic material is glass. Some plastics are also considered to be viscoelastic. One type
of viscoelastic response is illustrat ed in Figure 8.13:Viscoelastic Behavior (Maxwell Model) (p. 215).

Figure 8.13: Viscoelastic Behavior (Maxwell Model)

Time Time

Applied
Force

Resulting

Direction

Small- and large-deformation models are available to model viscoelasticity in the time domain.The
small-deformation model can be used in the harmonic domain to model the steady-state response of
viscoelastic materials.

The elasticity constants correspond to the material behavior at the fast load limit.The elastic constants
are specified as follows (assuming the use of current-technology elements):

• For small-deformation hypoelasticity, issue either the MP or TB,ELASTIC command.


• For large-deformation hyperelasticity, issue the TB,HYPER command.

Specify the bulk and shear relaxation properties via the TB,PRONY command. Specify the time-temper-
ature superposition properties via the TB,SHIFT command.

!Small Strain Viscoelasticity

mp,ex,1,20.0E5 !elastic properties

mp,nuxy,1,0.3

tb,prony,1,,2,shear !define viscosity parameters (shear)

tbdata,1,0.5,2.0,0.25,4.0

tb,prony,1,,2,bulk !define viscosity parameters (bulk)

tbdata,1,0.5,2.0,0.25,4.0

!Large Strain Viscoelasticity

tb,hyper,1,,,moon !elastic properties

tbdata,1,38.462E4,,1.2E-6

tb,prony,1,,1,shear !define viscosity parameters

tbdata,1,0.5,2.0

tb,prony,1,,1,bulk !define viscosity parameters

tbdata,1,0.5,2.0

For use in the harmonic domain, an alternative method for specifying the viscoelastic properties is
available. You can input the properties using experimental values of the complex moduli (TB,EXPE).

See Viscoelastic Material Constants and the Mechanical APDL Theory Reference for details about how to
input viscoelastic material properties using the TB family of commands.

Tools are available to help you determine the relaxation properties (defined via TB,PRONY ). The TBFT
command allows you to compare your experimental data with existing material data curves and visually
fit your curve for use in the TB command. All of the TBFT command capability (except for plotting) is
available via batch and interactive (GUI) mode. See Material Curve Fitting for more information.

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Modeling Material Nonlinearities