Time-dependent,non-Newtonian behavior of viscoelastic materials

Summary The present paper is a further development in our effort to describe non-Newtonian flow behavior. As in the past, the thixotropic shear stress or shear rate is characterized by the kinetic theory and currently a modified form ofOldroyds model provides the elastic contribution. Presented here are further modifications of the model in order to predict the first normal stress difference and logical changes to make the model more internally consistent.In order to test the approach, transient and steadystate data have been obtained with aWeissenberg rheogoniometer on a solution of polymethylmethacrylate in diethylphthalate. Both constant stress and constant shear rate data have been taken over a broad range of the parameters involved. To facilitate data acquisition, a PDP-15, model 30 computer was programmed to gather data at rates of 1000 points per second. Of the data obtained there is a great deal of uncertainty about the normal stress results.By optimization methods, the constants of the model were evaluated from steady-state data and transient results on stress growth and shear rate growth. Comparisons of the model with stress growth results as well as the prediction of the first normal stress difference are given. The inadequacies of the analysis are emphasized and possible points for improvement are made.With 8 figures