Non-Newtonian flow behaviour of glass melts as a consequence of viscoelasticity and anisotropic flow

Abstract

A new flow equation is derived for glass forming melts starting from very high deformation rates, ε dot , where the fluid behaves totally unrelaxed like a brittle body with exclusively elastic components. With decreasing ϵ dot flow occurs with a finite ultimate viscosity connected with a dynamic relaxation process which is characterized by the ‘dynamic relaxation rate’. This partly relaxed condition turns to a totally relaxed fluid with decreasing ϵ dot resulting in a Newtonian flow regime. In order to get the real flow curve, stress σ versus ϵ dot , the experimentally obtained values have to be corrected by a heat belance between dissipated heat and heat loss during the mechanical deformation. From those thermally corrected flow curves, expressed by the new flow equation, the true and apparent non-Newtonian viscosity are obtained as a function of ϵ dot and of log ϵ dot , respectively. In this way it could be shown that the non-Newtonian flow behaviour is a consequence of viscoelasticity. This viscoelasticity is, however, not the only one origin of non-Newtonian flow behaviour because additionally the orientability or at least the anisotropy of the flow units play an important part, which is shown by the different character of the flow curves and of the true and apparent viscosity of a silicate glass melt with a three-dimensionally connected dynamic network and of an alkali metaphosphate glass melt with a one-dimensionally connected dynamic network. Additional evidence about the anisotropic flow behaviour is obtained directly from literature on flow birefringence of single-phase glass melts and on the frozen-in birefringence of drawn glass fibres, glass rods and compressed glass cylinders above T g .