On the method for determining the true strength of inorganic glasses

The possibility of determining the true structural strength of glass by bending of glass fibers with a defect-free surface is considered. Two methods are compared, viz., the method of transverse three-point bending in which the breaking stress (strength) is determined, and the method of two-point bending in which the breaking strain is determined. In the latter case, the dependence of the elastic modulus on strain is required for determining the breaking stress (strength). The strength measured in three-point bending is compared with the strength calculated from the breaking strain measured in two-point bending. It is shown that the measurements based on these two methods give close values of strength for defect-free silica fibers used as optical waveguides. The observed difference of $\sim$ 12% in the values of strength is explained by the difference in the loading rates obtained using these two methods. The advantages and disadvantages of these two techniques are analyzed.