Comparison of the Residual Stresses at the Edge and Surface of Thermally Toughened Glass

The strength of thermally toughened glass is regulated in EN 12150-1 for fully tempered glass (FTG) and EN 1863-1 for heat-strengthened glass (HSG). The manufacturer has to prove the strength by four-point bending tests based on EN 1288-3. A measurement of the residual stresses of thermally toughened glass is not demanded. Consequently, the determination of the strength is containing the amount of residual stresses. As residual stress is depending on the manufacturing process, the amount and distribution can change within one glass pane and between different glass panes. A research project determined the residual stresses at the surfaces and the edges of HSG and FTG with varying glass thicknesses using photoelastic measurements. Thereby, FTG showed lower values of the residual stresses at the edge compared to the surface. Looking at HSG, the results of the residual stresses at the edge and surface were nearly the same. To analyse the edge strength, four-point bending tests were conducted. Thereby, the load was introduced into the strong axis of the specimens to implement constant tensile stresses along the glass edge. The comparison of the edge strengths showed that the edge strength of FTG is in the same range than HSG. Therefore, the lower residual stress at the edge of FTG reflects in significantly lower edge strengths. The paper includes a detailed presentation of the photoelastic measurements at the edge and the surface as well as the determined residual stresses of more than 80 specimens. In addition, the conducted four-point bending tests are described and the obtained edge strengths are correlated to the measured residual stresses. Based on that, the paper contains a discussion of the standardisations, the process of thermally prestressing and the consequences for the final glass strengths