Mechanical strength of float glass: test results analysis and the nature of differences

Mechanical strength of float glasses manufactured by different enterprises with the same technological regimes and by one glass plant under different technological conditions was investigated relatively to the top (atmosphere) and bottom (tin) surfaces by means of the ring-on-ring bending method. The lack of statistical methods for estimating the technical strength of float glass was considered. It was shown that to obtain useful information about the mechanical properties of industrial sheet glass it is necessary to use a statistical interval of distribution of not more than 25 MPa with testing not less than 25 to 30 specimens. This will provide the means to analyze the information regarding the statistical levels of sheet glass strength. The levels of strength with average stresses at failure 56, 81, 138, 188 and 262 MPa were defined. The first two connected with the different abrasive actions can be eliminated by glass strengthening or glass surface protection. The existence of the last three levels of strength does not depend on conditions of manufacturing and methods of treatment (strengthening). The nature of these levels was connected with the existence of the layers (fields) of chemical inhomogeneity characterized by different thickness. The disposition of initial microcracks inside the fields with different thickness can determine the glass specimens' failure stresses related to the different levels of mechanical strength.