Computational modeling of precision molding of aspheric glass optics

In this dissertation, research in two parallel directions is presented; the first involves the prediction of the final size and shape of a glass lens during a precision glass lens molding process and the second introduces a method to compute and quantify the importance of higher order terms in fracture mechanics for different modes of fracture.;In this research, ABAQUS, a commercial FEM solver, is used to simulate the process and predict the final size/shape of the lens. The computational study of final size and shape includes a sensitivity analysis of the various material and process parameters. The material parameters include viscoelasticity, structural relaxation and the thermo-rheological behavior of the glass; friction and gap dependent heat transfer at the interface; and the thermo-mechanical properties of the molds. This comprehensive study will not only eliminate some of the parameters which have the least effect on the final size/shape, but also identify the key material properties and substantiate the need to obtain them more accurately through experimentation. At this time it should be mentioned that the material properties of the molding glasses considered are not available.;Friction coefficient at the mold/glass interface is one of the important input parameters in the model. A ring compression test was used in the current research to find the friction coefficient. In this test, a "washer" or a ring shaped specimen is compressed between two flat dies at the molding temperature and the change in internal diameter is correlated to a friction coefficient. The main strength of this test is the sensitive nature of the inner diameter change during pressing for different friction conditions at the interface. In addition to friction coefficient, approximate viscoelastic material properties and the TRS behavior were also found out using this test from the experimental force and displacement data.;After validating the model to well within one micron, it was determined that the deviation of the lens profile with respect to the molds is primarily caused by structural relaxation of glass, thermal expansion behavior of the molds, friction at the glass/mold interface and time-temperature dependence of the viscoelastic material behavior of glass. Several practical examples/numerical studies that clearly show the cause for the deviation are presented. It is also shown that the deviation in the molded lens is affected by its location with respect to the molds. Finally the process of mold compensation is demonstrated using the computational tool.;In the other parallel direction, a method to determine higher order coefficients in fracture mechanics from the solution of a singular integral equation is presented. In the asymptotic series the stress intensity factor, k0 is the first coefficient, and the T-stress, T0 is the second coefficient. For the example of an edge crack in a half space, converged values of the first twelve mode I coefficients (kn and Tn, n=0,...,5) have been determined, and for an edge crack in a finite width strip, the first six coefficients are presented. Coefficients for an internal crack in a half space are also presented. Results for an edge crack in a finite width strip are used to quantify the size of the k-dominant zone, the kT-dominant zone and the zones associated with three and four terms, taking into account the entire region around the crack tip. Finally, this method was also applied to fracture problems with Mode-II loading. (Abstract shortened by UMI.)...