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Chemical strengthening methods, for example those using the ion stuffing technique, often provide a greater degree of reinforcement than that obtainable by conventional thermal toughening methods. However, on fracturing, not very thin-walled objects disintegrate into fairly large, sharp and therefore dangerous splinters. A method is described by means of which the fracture behaviour can be improved without adversely affecting the high strength. The fragments produced on breakage of an object strengthened by ion stuffing can be rendered harmless by causing the sharp edges and points to powder finely and thus become blunt. This can be achieved by forming such stress profiles in the glass that the maximum compressive stress is found not at the surface but in a sub-surface layer. The requirements to be satisfied by such a stress profile and the methods of producing it are discussed.

Nonlinear deflections and radial surface stresses in thin elastic circular plates laterally deformed into symmetrical concave shapes are analyzed. The deformations are induced by loading each plate with a small center ring while the plate is resting on a ring of nearly the same diameter as the plate. The rings and plate are coaxial, i.e. concentric. Center deflections up to 3 and 4.5 times the plate thickness were predicted for ring-diameter ratios of 0.5 and 0.2, respectively. The predicted deflection profiles at various loads agreed quite well with those which were determined experimentally on a chemically strengthened glass plate. This analysis provides a new criterion for using the coaxial-ring loading method for flexural strength testing of brittle materials.