Thermo-mechanical characterization of glass at high temperature using the cylinder compression test. Part II: No-slip experiments, viscoelastic constants, and sensitivity.

The limited availability of viscoelastic material constants for moldable glasses in the transition temperature range makes it difficult to apply computational mechanics to processes such as precision lens molding. In Part I of this two-part study, the cylinder compression test and standard parallel plate viscometer procedures were analyzed, since this test is convenient and relatively easy to conduct. The computational results showed 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 for shape change in forming processes. Motivated by these results, in the current study, a specimen was designed that did not allow slip, which results in a very accurate determination of viscosity from the cylinder compression test data and in turn enables a sufficient characterization of the viscoelastic component of the creep curve. The resulting combined experimental and computational procedure was validated with N-BK7 glass using accurate values of viscosity provided by the supplier and then this method was applied to L-BAL35 glass. Given accurate creep data and an accurate viscosity, the shear relaxation functions for the two glasses were determined. In order understand the requirements for a valid test and to best take advantage of the procedure, sensitivity analysis is performed on the height to diameter ratio of the cylinder, the degree of compressibility, imperfections in specimen geometry, and the effects of nonuniform temperature. [ABSTRACT FROM AUTHOR]