Quality control of tempered glass panels with photoelasticity

The quality of prestressed glass components can be evaluated in terms of technical quality – which means sufficient compliance with the product standards – as well as in terms of optical quality – which means a minimum of optical distortions or thermal anisotropies. Both requirements – technical quality and optical quality – become more and more important for both current and future glass applications. Structural applications, such as glass beams or columns, can only be realised with high-quality glass that completely fulfils the product standards. Challenging architectural solutions demand a minimum of anisotropic effects. Until now the prestressing has been evaluated by destructive breakage tests, which give only random information about the glass quality. In recent years, however, a very simple non-destructive method has been developed using polarising filters. Appropriate evaluation criteria have been defined so that a screening almost over the entire surface (related to the area of a glass panel and the number of glass panels produced) gives full information about the homogeneity of the prestressing. This method has the further advantage that all possible glass geometries (for instance glass products such as curved glass panels) can easily be checked in a non-destructive way.