Fracture phenomena and strength properties of chemically and physically strengthened glass II. Strength and fracture behaviour of chemically strengthened glass in connection with the stress profile

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.