Automated Apparatus for Measuring the Frangibilityand Fragmentation of Strengthened Glass

The surface compressive stress developed during thermal or chemical strengthening in glass is balanced by tensile stress throughout the central region of the glass, known as central tension (CT). Greater CT can lead to a higher degree of frangibility and fragmentation. Three automatic methods are developed to measure the frangibility and fragmentation behavior of the strengthened glass. Method A uses free gravity drop of a stylus with magnetic braking to deliver consistent impact force. Method B adds an air bearing to provide accurate alignment of the stylus while reducing friction. Method C uses a flex hinge design to deliver impact force. After fine tuning of the apparatus, a delayed fracture rate of ~80 % can be achieved. Delayed fracture is a necessary condition for evaluating the intrinsic frangibility and fragmentation behavior of a strengthened glass sample as a result of the stored internal stress. On the other hand, in an immediate fracture event, externally applied energy exerted on the glass can affect the fragmentation pattern and the perceived frangibility. Use of a single strike, as opposed to multiple strikes, to fracture the sample will also prevent unwanted damage to the glass that could influence the final result.