The spontaneous cracking of thermally toughened glasses connected with the occurrence of nickel sulfide inclusions is well-known. Sudden fracture is known to be brought about by a volume change of the inclusion as the result of a retarded phase transformation from hexagonal NiS at high temperatures into hexagonal-rhombohedral for at low temperatures. As the latter configuration is stable at room temperature, the gradual transformation takes place with an accompanying increase in volume and a redistribution of the state of stresses in the neighbourhood of the inclusion. When such an inclusion occurs in the high tension field, localised fractures develop. In time, a critical condition is reached, spontaneous fracture of tempered glass results. This paper deals with such a situation with interpretations centred on the mechanics of the behaviour of an inclusion in glass.
