Fracture Mechanics of Chemically Strengthened Glass: Experiment and Modelling

The ion-exchange process creates a residual stress profile along the depth direction with compression near the two surfaces transitioning to tensile stress in the mid-depth. The existence of compressive stress near-surface helps mitigate the impact of pre-existing cracks on strength reduction. The key parameters used to define an ion-exchange residual stress profile are compressive stress (), depth of Layer (DOL), and central tension (). A theoretical model such as Green’s theorem has been used to predict the best combination of residual stress profile parameters ( and DOL) to produce glass with optimal strength and high fracture toughness. This model is based on the hypothesis of crack geometry conservation, which assumes that the geometry (width and depth) of a pre-ion-exchange crack (pre-existing crack) remains constant post-ion-exchange. The objectives of this study are to investigate experimentally, for a chemically strengthened glass with various severity of initial cracks/notches, the relationship between the residual stress parameters (, DOL) and the resulting fracture resistance as a function of applied fracture stress/fracture toughness and compare with analytical model predictions. The effect of the residual stress on the geometry of pre-existing crack and its efficacy in mitigating crack initiation and resisting crack (pre-ion-exchange or post-ion-exchange) propagation was also investigated. The results show that for a given crack depth, the optimal fracture resistance for a chemically strengthened glass requires a proper combination of the chemical strengthening residual stress parameters. The present study establishes that, unlike glass, the fracture toughness of a chemically strengthened glass () is not a constant material parameter but depends on the initial crack/notch depth. It was observed that the geometry of a pre-existing crack (width and depth) changes following the ion-exchange process. It was also determined that for a similar sharp contact event, ion-exchange-induced residual stresses help minimize the initiation of new cracks, and it is more effective in preventing the propagation from those cracks introduced post-ion-exchange compared to those presented before the ion-exchange process. These findings provide a novel understanding of the fracture toughness improvement of chemically strengthened glass, which should facilitate their widespread engineering applications.