Non-Newtonian flow behaviour of a soda-lime silicate glass at high deformation rates

Abstract

Modelling a simplified forming process, flow behaviour of soda-lime silicate glass (float glass) was studied at high deformation rates in a parallel-plate plastometer. Measurements were conducted under isothermal conditions in the temperature range 580 to 660 °C which corresponds to Newtonian viscosities of 10^8 to 10^11 Pa s. Uniaxial deformation was carried out at various constant pressing rates until brittle fracture occurred in the high-viscosity melt. At high pressing rates, viz. at initial compression stresses of more than 50 to 100 MPa, the observed stress-strain curves exhibit a pronounced temporary stress maximum. Measured data reveal a continuous decrease in glass viscosity during pressing at high pressing rates of more than a factor of 10 compared with the Newtonian values. These data agree quantitatively very well with data on non-Newtonian behaviour from fibre elongation measurements. The enhanced fluidity of the glass melt is explained partly by a heating effect due to dissipation of work of deformation and partly by structure-viscous flow behaviour. A temperature increase within the glass specimens was detected experimentally, but was less than that calculated from total dissipation of deformation work.