Measuring the 2D Residual Surface Stress Mapping in Tempered Glass under the Cooling Jets: the Influence of Process Parameters on the Stress Homogeneity and Isotropy

In glass products, thermally tempered on industrial lines for automotive and architectural industries, the distribution of residual stress is highly inhomogeneous and, in certain situations, can produce optical defects that are more visible in sunlight because of the impact of the cooling jets on the hot glass during the tempering process. To analyse the influence of the process parameters on stress inhomogeneity, flat square glass samples were tempered with PRELCO tempering production unit without oscillation and with oscillation under cooling jets, thus reproducing the current industrial practice for the latter. To measure the residual stress distributions in the tempered glass samples, we developed a fully automatic system that combines the scattered light polariscope (SCALP) and a four-axis machine, with three linear axes and one rotational one. Using the SCALP manually with the dedicated software makes it possible to access the glass surface at the laser beams entering point. With a four-axis Computer Numerical Control (CNC) machine, we determined the 2D residual stress distribution under the cooling jets and analysed the influence of the cooling rate and oscillation on the homogeneity and the isotropy of the residual surface stress in the tempered glass samples.