Mechanical strength of optical glasses

The most important properties of optical glass types are transmission and dispersion. All other properties are second priority. Sometimes however, such glasses might have to endure high mechanical loads. Examples are with cameras subject to strong accelerations such as rocket launches loads or thermal stresses induced by temperature changes. Optical glasses also find use as windows with pressure loads. In such cases the question arises, which loads are allowable to prevent breakage and how can the strength of given optical elements be increased. In the last ten years considerable progress could be achieved in reliably predicting lifetimes of structures made from the glass ceramic ZERODUR. This work will show the transfer of the knowledge gained to optical glasses. The breakage stress threshold observed with ZERODUR exists also with optical glasses. We will show its usage for assessing short time loads and long-time loads taking into account the fatigue effect. The article will provide all available strength data and discuss how to estimate data for glass types without existing measurement results. For the lead flint glass types there is a good correlation of the breakage stress threshold with their lead content allowing predicting threshold values for all glass types in this glass type family.