Strengthening of E-glass fibers by surface stress relaxation

Earlier, a new glass strengthening method was demonstrated using silica glass fiber. The method involved heat-treatment of a glass while held under a sub-critical tensile stress. The added strength was attributed to the formation of a compressive stress layer on the surface created by a surface stress relaxation process. This new glass strengthening method does not require glass of finite thickness, as in tempering, or glass containing alkali ions, as in ion-exchange. The same method was applied to E-glass fibers by treating the fibers under 1 and 2 GPa stresses at 200 °C for 60 s in air and a strength increase of ~ 0.6 GPa was achieved. The extent of the surface residual stress formation was estimated from the permanent bending kinetics of glass fibers heated under bending stresses and released. In the present work, the origin of the strengthening is attributed to the surface compressive stress rather than an anisotropic structure of the glass. The stability of the residual surface stress produced was estimated by measuring the kinetics of unbending of bent fibers at 325-425 °C in air. The relaxation time of the surface stress release at room temperature in air was estimated to be at least 1 to 3 years.