INDENTATION CRACK INITIATION AND PROPAGATION IN TEMPERED GLASS

Indentation crack initiation and propagation in tempered glass surfaces are examined and the results compared to those in annealed and in ion-exchange-strengthened glasses. The presence of surface compression due to tempering inhibits median crack initiation during the loading cycle of the indentation and depresses the radial crack initiation load during unloading. However, the extent of lateral cracking is enhanced in tempered glass surfaces. In situ crack propagation experiments reveal that the compressive stress tends to weakly stabilize crack extension prior to failure. The degree of crack stabilization is considerably lower than expected from a theoretical analysis.