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

Evaluation of residual curvature in two-point bent glass fibers

“…In fact, the residual stress development by the surface stress relaxation can be used as a means to strengthen glasses above their intrinsic strength, as will be reported in our separate paper [12]. Furthermore, it appears [4] that the surface stress relaxation and the resulting residual stress are involved in the fatigue limit of glasses [13][14][15][16][17], the lowest tensile stress which can cause the static fatigue or slow crack growth, as well as the coaxing effect of glass, strengthening by an application of sub-critical tensile stress [18,19]. Theoretically, it is easier to approximate the bending of a two-point bent fiber by a circularly bent fiber with a single radius.…”

“…It was found that surface stress relaxation of silica glasses can take place at much lower temperatures than the glass transition temperature in a moist atmosphere [1][2][3][4], while bulk stress relaxation of the glasses, in general, requires a heat-treatment at the glass transition temperature or higher temperature. The surface stress relaxation can be demonstrated by subjecting a glass fiber to a bending stress at a temperature below the glass transition temperature and observing a permanent bending of the fiber upon release of the stress at room temperature.…”

“…For these reasons the observed fiber bending was attributed to stress relaxation confined to the surface layer of the fiber [3]. The kinetics of surface stress relaxation of various glasses including silica, soda-lime, and various alkali aluminosilicate were evaluated by measuring the radius of curvature of the two-point bent fibers as a function of bending treatment time and temperature in various water vapor pressures and comparing the results with the developed theory [4].…”

“…The evaluation of the radius of curvature of the fiber was the most important step in this analysis [4]. Furthermore, in view of the importance of water vapor, two point bending [11] where the entire stressed region of fiber can be exposed a constant water vapor, is superior to the fiber wound around a cylindrical object where one side of the fiber is directly contacting the cylinder, obstructing the glass-water vapor interaction.…”

Relaxation and physical aging in network glasses: a review

“…Such stress relaxation measurements have some importance in field of ion exchange glasses (e.g. sodium borosilicates) because of the strengthening of the glass surface that is steadily improved [100,101], while it has also been considered for soda-lime [102,103,104] or borosilicate glasses [105]. For the latter [105], a long-time study has permitted the first detectable signatures of glass relaxation far below T g (T /T g ≃0.3), and the measure of strain with time, in other words, the relaxation of the glass, follows a stretched exponent with a Kohlrausch exponent β=0.43 that has been predicted from dimensional arguments [106].…”