Fracture Analysis of Chemically Strengthened Glass Disks

The increase in fracture strength in chemically strengthened, soda-lime glass disks compared with nonstrengthened ones is investigated analytically and experimentally. An empirical stress-relaxation function is proposed to predict the stress profile through the disk as a result of ion exchange, with parameters determined from experimental data. The function accounts for stress increase due to concentration rise and decrease caused by stress relaxation, which depends on concentration, treatment time, and exchange temperature. The surface-flaw closure distance caused by the compressive stress profile is computed using an existing model. The size of the most severe surface flaw for chemically strengthened disks is computed using fracture mechanics, assuming knowledge of the experimental failure strength. The predicted flaw size is slightly smaller than that calculated for a similar sample of nonstrengthened disks and the strengthening is underpredicted by a factor of 2. Reasons for the discrepancies are discussed.