The computation of viscosity and relaxation time of doughs from biaxial extension data

Stress (T) versus time (t) data were obtained for wheat flour doughs in lubricated, constant crosshead speed, uniaxial compression experiments. These data were then analyzed by using the Upper Convected Maxwell (UCM) Model with two parameters, a relaxation time (Λ) and a shear viscosity (η). The numerical solutions to the differential equation governing the behavior of the UCM model in this experiment can be very sensitive to the initial slope of the stress/time plots. This region of the curve, near t = 0, is most sensitive to experimental error. Best-fit values of Λ and η were thus computed for a given curve at a series of initial starting points, ti, which were sequentially taken up the stress/time plots. There was considerable fluctuation in the best-fit parameters but, nevertheless, values of relaxation time and shear viscosity could be obtained that gave good agreement between computed and experimental stress/strain curves.