Mechanisms for the Relaxation Theory of Viscosity

A RECENT communication by D. D. Eley and D. C. Pepper1 describes experiments on the plastic flow of plasticized cellulose derivatives. They find that the flow velocity of compression of cylinders and extension of rods depends exponentially on stress at moderate stresses as the simple relaxation theory predicts2,3, but at higher stresses flow velocity approaches linearity with stress. This behaviour at high stress leads them to question the applicability of the relaxation theory in general to flow problems.