Degree Candidacy: Doctor of Philosophy in Mechanical Engineering
Date: Monday, April 7, 2014
Time: 3:00 PM
Location: 132 EIB
Committee Members: Dr. Paul. F. Joseph (Chair), Dr. Lonny Thompson, Dr. Richard Miller, Dr. Xiangchun Xuan
Title: Numerical Analysis of the Extrusion of Fiber Optic and Photonic Crystal Fiber Preforms Near the Glass Transition Temperature
Conventional clad core fiber optic technology has relied on a concentric structure of glass of different refraction indices. These conventional fibers suffer from constraints and limitations related to thermal expansion compatibility between the core and the glass. The new fiber technology broadly characterized as Microstructured Optic Fibers (MOFs) is intended to lift the limitations of conventional clad core fibers and also extend the range of application of fiber optics. Photonic Cristal Fibers(PCFs) are a special family of Microstructured Optic Fibers characterized by the presence of holes in the cross section of the fiber that are organized in a hexagonal pattern. In order to manufacture these fibers, a preform with the same cross section has to be prepared which can later be drawn into fiber. For such complex geometry, glass extrusion at a viscosity higher than that for extrusion of solid glass preforms has proven to give better results.
Despite these merits, the numerical modeling of the extrusion of glass at high viscosity has not received much attention in the literature. Thus, in order to study the extrusion of PCF performs at high viscosity, investigation of the extrusion of a solid glass preform must be considered first.
To establish the valid assumptions to model glass extrusion at high viscosity, a numerical study of solid rod extrusion was performed and validated based on five experimental cases. This study highlighted the importance of including friction effects to validate both the value of ram force and die swell. Since the Navier law is the most widely used friction law in the extrusion literature and has been identified as a key parameter required in modeling, the ring compression test was adapted to obtain the Navier friction coefficient from standard ring compression test data. In addition to the identification of the essential modeling choices base on experimental data, a sensitivity analysis was performed on extrusion parameters for both viscous and viscoelastic material to establish a general idea of the effect and relationships that governs the extrusion process. The results from this study correlate with several experimental observations.
Based on the same assumptions that were validated for solid rod extrusion, the numerical modeling of extrusion of PCF preforms was performed using two blockage geometries. The model was validated for both blockages qualitatively based on a preform showing a significantly deformed cross section. To investigate the creation and the distortion of the holes of PCF preforms, a sensitivity analysis was also performed using both blockages. To quantify and interpret the distortion data the implementation of several algorithms and mathematical methods were developed. In addition, other tools were developed to provide a way to alter the blockage geometry in order to improve the geometric quality of the preform. An example of this alteration was carried out.