You may also want to calculate the response of a viscoelastic material to one or more instantaneous variations of strain rate. The main types of transient extensional flows correspond to the types of transient shear flows described in Transient Shear Flow and illustrated by Figure 7.5: Transient Shear Flows (with the extensional strain rate substituted for the shear rate ). The velocity field for a transient extensional flow is as defined for the steady extensional flow in Steady Extensional Flow.

In a transient uniaxial extensional flow, the stress difference is

(7–23)

In a biaxial or planar extensional flow, the stress difference is

(7–24)

For transient extensional flow fields, the transient property curves for the properties defined in Steady Extensional Flow can be computed. Select Transient Extensional Rate, Uniaxial Extensional Stress vs. Strain [ln(l/lo)], Uniaxial Extensional Stress vs. Time, Uniaxial Extensional Viscosity vs. Time, Biaxial Extensional Stress vs. Strain [ln(l/lo)], Biaxial Extensional Stress vs. Time, Biaxial Extensional Viscosity vs. time, and/or Planar Extensional Stress vs. Strain [ln(l/lo)], Planar Extensional Stress vs. Time, Planar Extensional Viscosity vs. Time in the Load Curves (Part II) menu if you want Ansys Polymat to compute these curves.

To compute each of these curves, you will need to define the desired number of time intervals during which a constant extensional strain rate is applied. The time interval is bounded by the time values and . All times must be included between the specified minimum and maximum times, ( and ). The number of sampling points per time interval must also be specified. See Defining Numerical Parameters and Defining Numerical Parameters for information about specifying numerical parameters for viscometric property curves. See Specifying the Curves to be Calculated for information about specifying which curves you want to compute and plot. (Note that, if you use the automatic fitting method, Ansys Polymat will automatically compute and plot the curves for all properties for which experimental data curves have been defined.)