Study of edge strength of load bearing glasses

Abstract

A single glass layer can be considered as safety glass if tempered or reinforced with wire mesh. When tempered glass fractures it shatters into tiny pieces with blunt edges. Heator chemically strengthened glasses can not be considered as safety glass owing to the fracture pattern (fractures in shards), unless they are laminated. Bending tests were carried out (test arrangement according to standard EN 1288-3:2000) and the bending strength of single layer tempered and float glasses were determined with use of the well-known formulas. The calculated values for surface strength with use of ultimate strain were compared to the bending strength results in the case of float and tempered glasses. The question arise: the strains measured in the centre of the pane and near the edge are equally? Strain measurements of the surface of loaded specimens – at the middle of the pane and near the edge – were also investigated. The failure of glass originates from cracks with microscopic sharp tips. In spite of the careful manufacture and handling of glass panes, impacts by sharp particles or environmental impacts can cause defects on the surface. The glass manufacture e.g. edge finishing techniques influence the glass strength and can cause flaws which can propagate during the lifetime of the glass. Defects of edge surfaces caused by different edge work techniques were shown with scanning electron microscopy. Most of the glass strengthening methods are used to introduce residual compressive stresses into the outer layers by physical or chemical tempering. The compressed resulting layer helps to close cracks initiated on the surface, can stop crack propagation and can also increase the bending strength. Recent studies [6, 14] have shown that surface strengthening can lead to substantial improvements in degradation resistance. Therefore, in outdoor conditions, when the glass surface is exposed to humidity etc., tempered or heat strengthened glass should be used.