Glass plays an important role in many engineering applications including transparent armour. Determining the actual strength of a glass is problematic because typical glass strength numbers are presented in the literature as ranges. Minor changes in surface finish, composition, test method employed, and processing all influence the mechanical strength of glass. A standardised method needs to be utilised to ensure that strength data can be compared universally. The use of a standard methodology is vital when comparing glass compositions or multiple sample lots of the same composition in order to attribute any real strength variations to the glass itself. While the procedures established in ASTM C1499 provide a good framework for glass strength measurement, flexibility in the experimental parameters of the standard allow for substantial strength variations when testing glass materials. The ratio between the load and support ring diameters, as well as the specimen dimensions, is shown to lead to variations in both the measured strength, as well as the number of valid/invalid tests based on where fracture initiates. The goal of this study is to quantify these effects in order to establish optimised testing conditions for determining the strength of float glass.
