Photoelastic analysis of residual stress arising from water interaction at glass surfaces

Abstract

A high-resolution characterization technique was developed based on existing polarized light microscopy techniques in concert with digital photography. This technique was employed to measure the residual stress near glass surfaces in response to environmental water and various forms of mechanical deformation and thermal processing. Techniques were developed with the goal of clarifying aspects of the behavior regarding three cases in particular: 1.) surface stress relaxation in silica glass fibers; 2.) anomalous water diffusion and stress generation in low-water silica; 3.) the connection between stress and case II diffusion in sodium silicate glass. In each case it was found that residual stress near the glass surface contributes significantly to aspects of the observed material properties. This is seen specifically in 1.) the strengthening via surface compressive stress of glass which has undergone surface stress relaxation; 2.) the time dependence of surface compressive stress generation in low-water silica which correlates with hydroxyl concentration; 3.) the tensile stress gradient present in sodium silicate glass during case II diffusion which results in a constant flux at the diffusion interface. Explanations for these observations are discussed in light of the new findings. Results agree with what previous researchers have hypothesized but never directly observed.