For many materials, linear data and is relatively easy to measure for various angular frequencies (rad/s). When applicable, the shear viscosity can be estimated by applying the Cox-Merz [4] empirical rule. This empirical rule states that the modulus of the complex viscosity matches the nonlinear shear viscosity, as shown in the following equation.

(9–2)

A Cox-Merz rule also exists for the first normal stress difference; it is an approximation for low shear rates and is written as follows:

(9–3)