Material parameters are specified within a range of acceptable values. For example, viscosity factors must be positive. If the automatic fitting method is used, each material parameter can have three types of status: subject to fitting, initial value, or fixed value. By default, all parameters are subject to fitting, meaning that Ansys Polymat will compute the best value based on the experimental data. If you have a priori knowledge of the value of a parameter, you can speed up the fitting calculation by assigning an initial value for it.

Since there may be many parameters for a model, and there may be only a limited amount of experimental data available, it can be difficult to compute the best set of parameters. In this case, it is preferable to assign a fixed value to one or more parameters, so that they will not change during the fitting calculation.

If the model involves only one relaxation time, its value can be either assigned or computed by Ansys Polymat. For a multi-mode model, however, you will need to specify the spectrum of relaxation times yourself, or let it computed by Ansys Polymat. In that last case, you have just to specify the minimum and maximum possible relaxation times.

Next, there are nonlinear parameters for the models. For the PTT model (described in Phan-Thien-Tanner Model), the parameters ε and ξ control the elongational viscosity and viscometric properties, respectively: an increasing ε reduces or even cancels the strain hardening, while ξ affects shear-thinning properties as well as the amount of second normal-stress difference. Based on your needs, knowledge, or available experimental data, you may want to fix these values. Typically, strain hardening occurs for to , and disappears for . For example, low values for ε should be specified for a LDPE, while moderate values are appropriate for a LLDPE or a HDPE. On the other hand, shear thinning occurs for nonzero values of ξ, which usually is set to about 0.2. For practical purposes, it can be given a value of 0.5 or even as high as 1.

The Giesekus model (described in Giesekus Model) involves the parameter α, which simultaneously increases shear thinning and the second normal-stress difference while it reduces the strain-hardening property. Again, based on your needs, knowledge, or available experimental data, you can fix the value of α. When viscometric properties are relevant for the flow, values of α ranging from 0.2 to 0.8 are common. If elongational properties are needed, α plays a role similar to ε in the PTT model, and very low values (10^-3 to 10^-2) should be considered if strain hardening is needed.

For a PTT model with a nonzero value of ξ or a Giesekus model with α>0.5, as well as for the DCPP and the Leonov models, in both single- and multi-mode models, it is important to check whether the shear stress remains a monotonically increasing function of shear rate. A non-increasing shear stress can be corrected by adding a purely Newtonian component to the stress tensor. For single mode PTT model with a zero value of ε and for single mode Giesekus model with α = 1, the viscosity of this component is at least 1/9 of the zero-shear-rate viscosity. For single mode PTT model with a non-zero value of ε, for single mode Giesekus model with α less than 1, as well as for multi-mode models, the viscosity of this component can be lower.

The DCPP model (described in Differential Viscoelastic Models) involves the parameter ξ, which simultaneously increases shear thinning and the second normal-stress difference while the parameter q increases the strain-hardening property. You can fix the value of ξ and q, based on your requirements, knowledge, or available experimental data. When viscometric properties are relevant for the flow, values of ξ ranging around 0.2 are reasonable. If elongational properties are needed, and in particular if strain hardening is needed, the parameter q should be increased; it reflects the number of branches, and therefore affects the behavior in elongation.

The Leonov model (described in Differential Viscoelastic Models) involves several nonlinear parameters, affecting either the viscometric behavior or the elongation properties. You assign values to some of these nonlinear parameters, based on your requirements, knowledge, or available experimental data. Parameters q and affect the transition from trapped to free configuration of macromolecular chains. When viscometric properties are relevant for the flow, it is interesting to note that enhances the shear thinning property, while increases the viscosity. has no effect on the shear viscosity, while it contributes to a decrease of the elongational viscosity. If elongational properties are needed it can be noted that n increases the strain hardening, while b and m decrease it.

It is possible that the fitting calculation may yield values for nonlinear parameters that are unusual, although within the limits of accuracy. In this case, you should set these parameters to more appropriate fixed values, and rerun the fitting calculation. This will yield another set of parameters with the expected properties.

In general, the fitting calculation will determine parameter values on the basis of the available experimental data. However, the available data do not necessarily include the operating conditions, as measurement techniques do not always allow for reaching the conditions present in the actual process. Fiber spinning is a typical example, where the melt is processed at strain rates much higher than those available for rheometric measurement. For such cases, you can extrapolate from available data.