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 fulfil. Cooling, which is usually made with impinging jets is important for formation of residual stresses. With this arrangement the local heat transfer depends on the location of jets and distances between the jets. Because heat transfer is not uniform over the surface it can cause notable stress differences which are reasons for visual faults called anisotropy in glass.

In the paper the local residual stresses created by impinging jets are solved numerically using the CFD software [1]. Distributions of heat transfer coefficients are solved from measured correlations in the literature. 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. It has been pointed out that the stress distribution of a tempered glass has a similar type of distribution as optical anisotropies seen as stripes or spots [2]. In order to study this connection in more detail anisotropy patterns of a plane polariscope are presented.