The elongational viscosity is important, perhaps even more so than the shear viscosity. This property can be measured for moderate strain rates (usually up to 10 s^-1 using an elongational rheometer equipped with the EVF device), although the material is often processed at a much higher strain rate. This apparent difficulty can be overcome by considering the following heuristic argument. For a given melt, all curves of transient uniaxial elongational viscosity follow the same lower linear envelope, up to a Hencky strain of 1 or 2. It is therefore reasonable to believe that a similar behavior occurs at higher strain rates, regardless of whether the melt is strain-hardening or strain-thinning. This is a qualitative extrapolation, which results from a speculative extension of the Gleissle mirror relationship to the transient elongational viscosity.

The minimum experimental data needed for successful fitting are the linear properties and the nonlinear shear viscosity (possibly obtained from the Cox-Merz rule [4] or the Gleissle mirror relationship [6]; see Empirical Rules and Principles for details). If data for the transient elongational viscosity are available, they should also be used. When all the properties are available, you can allocate a low weighting to the shear viscosity and a high weighting to the elongational properties (as described in Defining Numerical Parameters), since the elongational component in the fiber is more important.