On the destructive and non-destructive assessment of strength of thermally toughended glass panels.

Siim Hödemann*, Hillar Aben, Andrei Errapart, Johan Anton, Jaak Kikas

University of Tartu

Assessment of the strength of glass is one of the key problems in quality control of architectural glass panels and automotive glazing. For that nowadays the fragmentation test (EN 121150-1) and the four point bending test (EN 1288-3:2000) are mostly used. Both of these methods are destructive and time-consuming. The strength of glass is to a great extent determined by its residual stresses, especially by stresses at the surface. During recent years a portable apparatus SCALP has been developed by GlasStress Ltd for complete stress measurement in glass panels. The apparatus uses registration of the intensity of the scattered light when a laser beam passes through the panel. It gives the residual stress profile through all the thickness of the panel. The results do not depend on which side of the panel is the tin side. At a glass factory strength assessment of glass panels of different thermal treatment was carried out using both residual stress measurement with SCALP and four point bending test afterwards. Correlation of the results was excellent, practically always the bending strength was equal to the surface residual stress. Stress distribution measurements were done near edge and in centre of glass in both x and y directions in many points. Correlation between of x and y stress components is found. Residual stress measurement with SCALP is non-destructive and takes about 10 seconds and it could replace cumbersome destructive methods of glass stress assessment. Since SCALP is portable (it has the size of a mobile phone), it can be used (and has been already used) to measure residual stress also in glass panels of existing buildings.