Stress relaxation of glass using a parallel plate viscometer

Currently, grinding and polishing is the traditional method for manufacturing optical glass lenses. However, for a few years now, precision glass molding has gained importance for its flexibility in manufacturing aspheric lenses. With this new method, glass viscoelasticity and more specifically stress relaxation are important phenomena in the glass transition region to control the molding process and to predict the final lens shape. The goal of this research is to extract stress relaxation parameters in shear of Pyrex¨ glass by using a Parallel Plate Viscometer (PPV). The PPV machine was originally created to measure the viscosity of glass solid cylinders by compression. It is used in this study for creep-recovery tests of glass samples. With this study we want to know if we are able to measure stress relaxation of glass using a helical spring geometry since it presents pure shear stress, for a temperature near the glass transition region (i.e., 570¡C). The results obtain with the PPV are post-processed using a finite term Prony series of a Generalized Maxwell model. The additional goals of this study are to compare the results with those obtained by a previous student using a tensile creep apparatus, and to compare the experimental results with a numerical simulation using the finite element analysis software Abaqus