Fracture initiation and fragmentation in chemically tempered glass

Residual stress profiles are introduced in an alumino-silicate glass using ion-exchange. Due to a high central tension in these glasses, fracture is often followed by fragmentation. Fracture was initiated by loading the glasses with a Vicker's indenter, either monotonically to fracture, or loading to a fixed load, and then unloading. Fracture loads in the load–unload method were ½–⅓ of that during monotonic loading. A qualitative explanation of this phenomenon is proposed by extending an analysis of stresses at the indentation site. The process of fragmentation under indentation, crack propagation and repeated multiple branching, is described. The fragmentation behavior when fracture is initiated in biaxial loading is different: a single crack creates numerous branches, which then propagate without further branching. This process is explained in terms of the stress distribution in the sample prior to fracture initiation. The implications for practical instances where fragmentation is important are discussed.