Surface stress relaxation and resulting residual stress in glass fibers: A new mechanical strengthening mechanism of glasses

Glass surfaces exhibit faster relaxation than the bulk at a constant temperature in the presence of either a liquid water or water vapor atmosphere. Permanent bending of glass fibers at temperatures lower than their glass transition temperatures in an atmosphere containing water vapor or in liquid water was attributed to and analyzed in terms of surface stress relaxation and the resulting surface residual stress. It was found that the fiber bending kinetics are controlled by water diffusion into the glass surface and that glasses with a faster transition crack velocity from region I (linear relation between logarithm of crack velocity and stress intensity factor) to the fatigue limit exhibited a faster rate of surface stress relaxation. It was suggested that this surface stress relaxation and resulting residual stress is the cause of various strengthening phenomena of glasses such as static fatigue limit and coaxing.