Relaxation behaviour, high-temperature tensile strength and brittleness of glass melts

Abstract

The intention of this paper is to investigate the frequently in practice used conception "brittleness of a glass melt" and to find a method for quantitative measurements of this kind of property. The simple compression of cylindrical glass samples has been applied as a measuring technique, by which information can be obtained on the high-temperature (ht)-fracture strength, the stiffness and the relaxation behaviour of glass melts. In this way the brittleness of a glass melt can be defined in a quantitative manner. A clear correlation has been found between the ht-fracture tensile strength and the relaxation modulus for some industrial commercial glasses and for various laboratory glasses with successive substitution of CaO by MgO in such a way that a high relaxation modulus is connected to a low ht-fracture tensile strength and vice versa at comparable (Newtonian) viscosities and deformation rates. Differences up to more than 300 % have been measured for silicate glass melts of various compositions. Thus, a powerful method has been developed which allows to understand and to measure the isothermal workability of a glass melt and to investigate the effect of various parameters on it. In contrast to the pure elastic and viscoelastic range the brittle fracture behaviour within the viscous range is a volume property.