Thermal stresses of tempered glass in the area of typical boreholes were estimated using Narayanaswamy's structural relaxation model implemented in a Finite-Element-Code (ANSYS 5.5.2). Surface stresses were measured for samples with holes of different float glasses (soda-lime-silica-glass, borosilicate glass) from different commercial tempering processes. The bending strength in the borehole area of the samples and of annealed (float) glass samples was determined using a modified coaxial double-ring bending test. Results are compared and evaluated on a statistical basis. It is shown that the characteristic glass strength of tempered glass in the borehole area is not lower than in the “infinite area” given in European standards. From the results it is also assumed that crack healing plays an important role for the bending strength of tempered float glass.
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