In this example, the automatic method is used to perform fitting for a differential viscoelastic model with experimental data curves of shear viscosity vs. shear rate, data curves for the storage and loss moduli, plus curves of stress vs. Hencky strain. In order to obtain these curves, measure the forces and deformations of a sample stretched with a constant pulling velocity in a uniaxial mode. In this set-up, the strain rate diminishes with time as shown in the following equation:

(10–1)

where , is the pulling velocity and , the initial length of the sample.

The Hencky strain is defined as:

(10–2)

where is the length of the sample at time .

The Giesekus model with 4 relaxation times is used. For the fitting calculation, the range of relaxation times is limited to between 0.01 and 100 s.