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Abstract. A portable scattered light polariscope SCALP has been developed, which permits measurement of the residual stress profile through the thickness of glass panels. At a glass factory strength assessment of glass panels of different thermal treatment was carried out using both residual stress measurement with SCALP and the traditional four-point bending tests. Linear correlation between the residual surface stress and the bending strength was observed. At another glass factory residual stress in glass panels was measured before performing the traditional fragmentation test. The results of the fragmentation test were extremely scattered and had almost no correlation with the values of the residual stress. It is concluded that sufficiently reliable assessment of the strength of glass panels is obtained by measuring the residual stress at the surface.
Key words: glass, residual stress, photoelastic measurements, quality control.
(ProQuest: ... denotes formulae omitted.)
1. INTRODUCTION
According to the European Standard EN 12150-2:2004, to establish if a product conforms to the definition of thermally toughened soda lime silicate safety glass, initial type testing shall consist of a) mechanical strength measurement and b) fragmentation test, in accordance with EN 12150-1:2000. Thus European standards prescribe application of the four-point bending test and the fragmentation test. Both of these tests are destructive, time-consuming and expensive.
At the same time, during recent years photoelastic residual stress measurement methods have been considerably developed and applied. In [1] data is described about testing of 360 glass panels by first measuring the residual surface stress with the surface polariscope GASP* [2]. Paper [1] shows that a good correlation exists between surface compressive stress data and mechanical strength according to EN 1288-3 (Fig. 1). Similar results have been obtained by several other authors [3-5].
In a somewhat simplified form the relationship between the bending strength bs σ and surface stress σ^sub sf^ can be expressed as
... (1)
where a σ is the strength of the annealed float glass and k is an empirical coefficient (k [congruent with]1). Since a σ is known, measurement of the surface residual stress determines the bending strength of the glass.
At the same time, when GASP permits measurement of the surface stress at the tin side of glass panels [2], the scattered light polariscope SCALP permits measurement of...