A new description and interpretation of the flow behaviour of glass forming melts

Abstract

Abstract Experimental flow curves (stress-deformation rate diagrams) are obtained at various Newtonian viscosities, η 0 , and at various degrees of axial deformation of cylindrical samples Δh , by means of the cylinder compression method. Similar flow curves can be obtained by the rod elongation method. They represent the stress-dependent flow behaviour of the glass melt (Newtonian and non-Newtonian) and the stress-induced heat of dissipation as well. The experimental flow curves have to be corrected by means of eliminating the influence of heat dissipation of the σ−ϵ flow diagrams. These corrected curves represent the real or realistic flow behaviour which can be described very well by a new general flow equation with a clear physical significance. The linear range of the low deformation rates (stable Newtonian flow). the non-linear transition range (non-Newtonian) and the linear range of high deformation rates (metastable Bingham flow) are connected to the change of the dynamic network from an isotropic to an anisotropic microstructure. From the new flow equation an equation for the true and for the apparent viscosity as well can be developed from which one can calculate the Newtonian equilibrium viscosity, η 0 , the true and apparent non-Newtonian viscosities ( η tru and η app and the metastable Bingham viscosity, η ∞ , at large deformation rates ϵ. The calculated apparent viscosities agree well with the values calculated from the Gent equation. A constant final value of the axial compressive or tensile stress σ limit , is reached at high ϵ and Δh , only if the heat of dissipation is not eliminated. It depends on η 0 , Δh , h and on composition. If, however, the effect of heat of dissipation is eliminated, no such stress limit is found.