The apparent increase in the tensile strength of glass plates observed when measurements are made by pressing small spherical indenters on the surfaces of plates is discussed in terms of the statistics of the surface flaw distribution. An analytical flaw distribution fi=unction is derived from the experimentally determined relation between radius of indenter and apparent tensile strength, and this function is used to forecast the scatter of observations about their mean values. For indenters of large radius a purely statistical explanation of the observations applies, but for indenters with small radii of curvature, other mechanisms come into play. Some measurements have also been carried out to compare the values of the tensile strength found in quasi-static tests with those obtained in impact experiments.
