A New Approach to Edge Stress Measurement in Tempered Glass Panels

An important characteristic of glass panels is residual stress, especially stress at the surface. To measure this stress different devices have been developed, which use either the tunneling effect in light propagation near the surface or the scattered light method. Both devices are rather expensive. At the same time, many glass manufacturers use measurement of the stress at the edge of the panel. This measurement can be carried out with traditional methods of 2D photoelasticity. However, until now the data of edge stress measurement has not been used to the full extent. The edge stress itself is an estimation of the degree of temper of the panel. Besides, the region, where average through thickness stress is tensile, can be found. However, real stress distribution in the panel can not be determined. In this paper we show that since the edge stress and surface stress are in correlation, the edge stress permits to estimate also the value of the surface stress. Experimentally and with mathematical modelling of the tempering process it has been shown that practically edge stress equals the surface stress. Knowing the surface stress, it is possible to determine distribution of the parabolic tempering stresses through the panel thickness. That permits also to determine real stress field in the so-called tensile stress area. Thus complete analysis of residual stresses near the edge of the panel can be carried out.