Thermo-mechanical characterization of glass at high temperature using the cylinder compression test. Part I: Viscoelasticity, friction, and PPV

Computational mechanics simulations of glass forming processes such as precision lens molding require accurate thermo-mechanical glass property data as inputs. In the transition temperature range where glass behavior is viscoelastic, the important properties are the viscosity, the shear relaxation function, the bulk relaxation function, and the temperature dependence of these properties. The availability of such data is unfortunately limited, which in turn is the most limiting factor in applying computational mechanics to these processes. In this two-part study, computational and experimental mechanics are used to evaluate the use of the cylinder compression test for the precise determination of these properties. This test is convenient due to the simple sample geometry and availability of high temperature testing machines that can achieve accurate and uniform temperatures, such as a parallel plate viscometer (PPV) or a lens molding machine. The computational results show that deformation of the glass cylinder is strongly dependent on both frictional slip at the interface and the viscosity of the glass, which is the key material property that enables shape change in forming processes. The accuracy of viscosity predictions made using standard PPV formulas is quantified in terms of frictional slip, viscoelastic material behavior, and cylinder geometry within the transition temperature range where PPV measurements are thought not to apply and processes such as lens molding occur. The results extend PPV to log of viscosity values above 8 using units of Pa.s. Given accurate viscosity, it is possible to obtain additional properties. (C) 2013 The Society of Rheology.