Strength of glass influenced by shear stresses at the surface due to the forming process

Abstract

During the processing of glass melts shear stresses can arise within the contact layer with the shaping tools due to wall friction, which above a critical value may lead to defects on the product surface. These processes can be simulated and registered in a simple manner by the isothermally controlled extrusion process. For their investigation cylindric rods of an optical lead silicate glass have been extruded and subsequently their strength has been determined by means of static four-point bending tests. An influence of the die material on the flexure strength is observed. Glass rods extruded through stainless steel dies show a lower flexure strength than those being pressed through dies of electrographite. Moreover, their flexure strength decreases with increasing extrusion velocity. This can be explained by the formation of smallest imperfections on the glass surface during the flow of the glass melt along the die-channel wall. In contrast to this the flexure strength of the glass samples extruded through electrographite dies is independent of the extrusion rate. The annealing of the extruded rods slightly above the transition temperature leads to an increase of the flexure strengths.