Voigt model nonlinearity

Example 19.1 Nonlinear Models Show that the equation for the impedance of a Voigt element is nonlinear with respect to parameters. 

In flow situations where the elastic properties play a role, viscoelastic fluid models are generally needed. Such models may be linear (e.g., Voigt, Maxwell) or nonlinear (e.g., Oldroyd). In general they are quite complex and will not be treated in this chapter. For further details, interested readers are referred to the textbooks by Bird et al. [6] and Barnes et al. [25],... 

To account for the fact that neither ideal solids nor ideal liquids exist in the real word, the rheological behavior of real materials can be approximated by a combination of the individual model elements, either in Unear two-element models of MaxweU and Voigt-Kelvin types, or in linear three-element models. In many real cases, nonlinear models have to be invoked (see Section 2.4.1.3). 

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