Experimental and analytical investigation of nonisothermal viscoelastic glass fiber drawing.

A preform glass fiber drawing apparatus is designed and built to investigate non-isothermal viscoelastic glass fiber drawing. The effect of the drawing parameters on glass fiber properties is studied experimentally and analytically. Birefringence is a measure of anisotropy in glass structure that can influence fiber properties and performance. Birefringence is produced during fiber drawing as the fiber is rapidly stretched in the viscoelastic glass transition range, and is frozen into the glass during rapid cooling. With our drawing apparatus using Borosilicate glass (Corning code 7740) preforms, we produce glass fibers for a range of process conditions and measure their as-drawn birefringence. The development of birefringence in glass fibers is found to depend on the amount of deformation, the deformation rate, and temperature. Results for various process parameters show that increasing draw ratio, increasing elongation rate, and decreasing draw temperature increase birefringence. Post-process annealing is used to examine the time and temperature dependence of glass fiber birefringence relaxation and its effect on glass fiber tensile strength. Birefringence is found to completely relax in the temperature range close to the glass transition range as expected, but it is also noted that birefringence shows substantial (although incomplete) relaxation in a temperature range well below the glass transition. This low temperature relaxation indicates that the relaxation process is due to a very wide distribution of relaxation times. A nonisothermal one-dimensional model is examined to simulate the glass fiber drawing and fiber birefringence relaxation using a generalized Jeffrey model. The relaxation time and retardation time are both functions of the temperature. The conservation equations of mass, momentum and energy are solved simultaneously with the viscoelastic constitutive equation to determine the spatial distribution of fiber dimensions, velocities, stresses and frozen in elastic strains. The effects of process parameters on the fiber properties are simulated. The model predictions are compared with the experiments and are found to be in qualitative agreement with data.Ph.D.Applied SciencesMaterials scienceMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131716/2/9929888.pd