Search for: All records

Abstract Silica glass samples were given various heat treatments under stress at low temperatures and subsequently their residual stress distributions in terms of retardance were observed using a polarized light microscope, confirming previously reported fast surface stress relaxation while providing more detailed characterization. Retardance profiles of silica glass fibers heat‐treated under a constant bending strain in the presence of atmospheric water vapor were measured and fit to a previously developed diffusion‐based relaxation model. The retardance of a cross‐section of a silica glass rod heat‐treated at 650°C in lab air under applied torsional shear strain was also measured to confirm the presence of residual surface shear stress which was predicted by the decrease of torque with time for the rod. Together, these results confirm the low‐temperature fast surface stress relaxation which occurs when water vapor is present for both bending and shear stresses. 

Abstract A constant angle of twist was applied to silica glass rods in order to produce a torsional shear strain and a reduction in torque representative of the stress state in the glass was measured as a function of time when rods were heat‐treated in air at temperatures, 550‐700°C, far below the glass transition temperature. The monotonic decrease of torque with time was explained by surface stress relaxation, which can be described by a relaxation of stress at the surface of glass which is promoted by water. The obtained surface stress relaxation diffusion coefficients were consistent with those obtained earlier from silica glass fiber bending under a similar water vapor pressure. The observed relaxation in torsion supports the mechanism of surface stress relaxation over the swelling‐based mechanism for applications including glass fiber strengthening.