Influence of Cooling Jets on Stress Pattern and Anisotropy in Tempered Glass

In a glass tempering process, residual stresses to glass are made with heat treatment. The process includes heating and cooling, and they should be equal on both sides of the glass pane. In addition, heat transfer should be locally uniform on both sides of the glass, which is difficult in practice to fulfill. Cooling is the most critical phase: it gives compression in the surface and tension in the mid-region. Arrays of impinging jets are usually utilized for cooling. With this arrangement, the local heat transfer depends on the location of jets and distances between them. Because heat transfer is not uniform over the surface it can cause notable stress differences which are the main reasons for visual faults in glass called anisotropy.

In the paper, the local residual stresses created by impinging jets are solved numerically using the commercial Ansys software. Distributions of heat transfer coefficients are solved with OpenFOAM CFD software. Stress distributions of glasses with different thickness are also compared to measured values of the optical measurement device SCALP in order to check the accuracy of the calculation model. We have noticed that the stress distribution of tempered glass has a similar type of distribution as optical anisotropies stripes or spots. In order to study this connection in more detail anisotropy
patterns of a plane polariscope are presented and compared with local retardation values which are calculated by using numerically calculated residual stresses.